organization of direct hippocampal efferent projections to the cerebral cortex of the rhesus monkey:...

Organization of Direct HippocampalEfferent Projections to the Cerebral

Cortex of the Rhesus Monkey:Projections From CA1, Prosubiculum,and Subiculum to the Temporal Lobe

GENE J. BLATT* AND DOUGLAS L. ROSENE

Department of Anatomy and Neurobiology, Boston University School of Medicine,Boston, Massachusetts 02118

ABSTRACTThis study investigates direct hippocampal efferent projections to the temporal lobe of the

rhesus monkey. Tritiated amino acid injections were placed into the hippocampal formation toidentify terminal fields, and complementary fluorescent retrograde tracer injections wereplaced into the cortex to identify the cells of origin. Tritiated amino acid injections into CA1,prosubicular, or subicular subfields produced anterograde label over parts of the parahippo-campal gyrus and temporal pole. Injections of fluorescent retrograde tracers demonstratedthat these projections originate from longitudinal strips of neurons that occupy part of theCA1 subfield as well as from strips of neurons in adjacent prosubicular and subicularsubfields. Thus, an injection into area TH of the posterior parahippocampal gyrus labeledneurons in a longitudinal strip of proximal CA1 (i.e., near CA2) as well as a strip in thesubiculum; injections into areas TF, TL, 35, or Pro labeled neurons in a longitudinal strip ofdistal CA1 (i.e., near the prosubiculum) as well as one in the prosubiculum; and an injectioninto area TFO labeled neurons in a longitudinal strip in the middle of CA1. These strips ofneurons extended longitudinally throughout the entire rostrocaudal length of the hippocam-pus. These results demonstrate that, in the monkey, CA1 projections to cortex arise topographicallyfrom longitudinally oriented strips of neurons that occupy only a part of the transverse extent of CA1but that cover most of the anteroposterior extent of the hippocampus. Thus, in the monkey, CA1 isnot a single uniform entity and may have a unique role as a source of direct hippocampal projectionsto the cerebral cortex. J. Comp. Neurol. 392:92–114, 1998. r 1998 Wiley-Liss, Inc.

Indexing terms: hippocampal formation; parahippocampal gyrus; memory; entorhinal cortex;

temporal pole

In man, numerous studies have indicated that thehippocampal formation (HF; the dentate gyrus, ammonicsubfields CA1–CA4, prosubiculum, and the subiculum)plays an important role in learning and memory (Scoville,1954, 1957; Penfield and Milner, 1958; Penfield and Mathie-sen, 1974). The first of these studies in humans evaluatedpatient H.M. and demonstrated a profound and enduringanterograde amnesia as a result of a large bilateral lesionthat destroyed almost the entire HF and as well as theadjacent entorhinal, amygdalar, and parahippocampal ar-eas, as reported by the surgeon (Scoville, 1954) andrecently verified by magnetic resonance imaging (Corkinet al., 1997). Subsequent studies (Penfield and Mathiesen,1974) confirmed that large but more limited lesions of thehippocampal formation also produced profound amnesia.

Subsequent studies in monkeys have indicated that dam-age to the amygdala, entorhinal cortex, perirhinal cortex,parahippocampal cortex, or subcortical white matter(McLardy, 1970; Horel, 1978; Mishkin, 1978; Saunders etal., 1984; Murray and Mishkin, 1986; Zola-Morgan et al.,1989b, 1993) can also produce amnesia and/or exacerbatethe deficit following hippocampal damage. However, it has

Grant sponsor: NIH; Grant numbers: NS 19416, AG 04321 and NS 16841.*Correspondence to: Gene J. Blatt, Ph.D., Department of Anatomy and

Neurobiology, Boston University School of Medicine, 715 Albany Street,Boston, MA 02118. E-mail: [email protected]

Received 12 February 1997; Revised 15 September 1997; Accepted 4October 1997

THE JOURNAL OF COMPARATIVE NEUROLOGY 392:92–114 (1998)

r 1998 WILEY-LISS, INC.

also been shown that a lesion confined to the hippocampusalone can produce a memory deficit in monkeys (Alvarez etal., 1995). This confirms the report that a small bilateralhippocampal lesion restricted to the CA1 ammonic sub-field in human patient R.B. resulted in marked antero-grade amnesia (Zola-Morgan et al., 1986).

Interestingly, transection of the fornix, which wasthought to be the principal output pathway by which theHF could influence memory function, produces only a mildor transient anterograde amnesia in humans (Ojemann,1966). Although Gaffan and Gaffan (1991) have pointedout that many of these human cases are confounded bydisease and are compromised by meager clinical descrip-tions of both surgery and memory function, experimentalstudies in monkeys (Mahut, 1972; Mahut and Zola, 1973;Zola-Morgan et al., 1989a, 1993; Gaffan, 1994) demon-strate similar mild and transient effects. Together, theseresults suggest two things. First, they suggest that hippo-campal connections with other medial temporal lobe struc-tures, such as the amygdala and the entorhinal, perirhi-nal, and parahippocampal cortices, may be critical for themnemonic functions of the hippocampus. Second, theysuggest that hippocampal efferents that do not travel inthe fornix but project directly to the cortex may also be animportant substrate for hippocampal involvement inmemory.

Indeed, direct hippocampal cortical projections in themonkey have been identified by a number of investigators(Rosene and Van Hoesen, 1977; Schwerdtfeger, 1979;Goldman-Rakic et al., 1984; Blatt and Rosene, 1988, 1989;Iwai and Yukie, 1988; Saunders et al., 1988; Witter andAmaral, 1991; Morecraft et al., 1992; Barbas and Blatt,1995; Leonard et al., 1995) and terminate in exactly theexpected areas: the entorhinal, perirhinal, and parahippo-campal cortices of the temporal lobe as well as medialprefrontal, orbital prefrontal, and cingulate cortices. Thepresent investigation was designed to address three as-pects of hippocampal projections to the temporal lobe.First, experiments were performed to determine the totalextent of the projection field of direct hippocampal effer-ents to the temporal lobe by using tritiated amino acidinjections covering the full extent of the HF. Second, it wasimportant to determine which subfields or parts of sub-fields in the HF contribute to these direct cortical projec-tions. This was accomplished by using retrograde fluores-cent tracers injected into cortical targets that wereidentified from the anterograde cases. Third, we examinedthe topographic organization of direct hippocampal projec-tions to parts of the temporal lobe, especially with regardto the mediolateral and longitudinal extent of the HF. Anabstract of some aspects of the present study has beenpresented previously (Blatt and Rosene, 1988).

Abbreviations

17 primary visual cortex (striate cortex)18,19 peristriate cortex28 entorhinal cortex35 perirhinal cortexAB anterior body of the amygdalaal alveusANT BODY anterior body of the hippocampal formationanterior PHG anterior parahippocampal gyrus (areas TLr, 35, 28)ARG autoradiographic tracing experimentC caudalCA1–CA4 cornu ammonis 1–4 (subfields of Lorente de No, 1934)cas calcarine sulcusCE central nucleus of the amygdalaCN cortical nucleus of the amygdalacols collateral sulcusCTA corticoamygdalar transition areaDG dentate gyrusDY Diamidino yellow fluorescent retrograde tracerFB fast blue fluorescent retrograde tracerfm fimbriaFRT fluorescent retrograde tracerGB green beads (fluorescent latex microspheres)gc granule cell layer of the dentate gyrusHATA hippocampal-amygdalar transition areahf hippocampal fissureHF hippocampal formation (dentate gyrus, CA1–CA4,

prosubiculum, and subiculum)ios inferior occipital sulcusIPa subdivision of association cortex in the caudal bank

of the superior temporal cortexJS juxtastriate areaKaL lateral auditory koniocortexKam medial auditory koniocortexL lateralLB lateral basal nucleus of the amygdalalf lateral fissureLT lateral nucleus of the amygdalaM medialMB medial basal nucleus of the amygdalaME medial nucleus of the amygdalaMID BODY middle level of the main body of the hippocampal formationOAA lower lateral bank of the superior temporal sulcus

(corresponding to area MT)

ots occipitotemporal sulcusPaAc caudal parakoniocortexPaAlt lateral parakoniocortexPaAr rostral parakoniocortexPaI parainsular cortexParaS parasubiculumPGA subdivision of association cortex in the rostral bank of the

superior temporal sulcusPHG posterior parahippocampal gyrus (areas TH, TLc, TF, THO,

TLO, and TFO)POC primary olfactory cortex (f, frontal; t, temporal)POST BODY posterior level of the main body of the hippocampal forma-

tion posteriorPr1 prorhinal 1 area of Van Hoesen and Pandya (1975)PreS presubiculumPro proisocortical area in the temporal poleProK prokoniocortexProS prosubiculumProSt prostriate cortexR rostralRB red beads (fluorescent latex microspheres)ReIt retroinsular parietotemporal cortexrs rhinal sulcusRSplg retrosplenial granular cortexsts superior temporal sulcusSub subiculumTE1–TE3, subdivisions of inferotemporal cortexTEA anterior subdivision of the inferotemporal cortexTEM middle subdivision of the inferotemporal cortexTEO occipital subdivision of the inferotemporal cortexTH,TLc,TF posterior parahippocampal gyrusTHO,TLO,

TFO caudalmost part of posterior parahippocampal gyrusTLr part of anterior parahippocampal gyrus (includes areas 36r

and 36c)tma anterior middle temporal sulcustmp posterior middle temporal sulcusTpt temporoparietal cortexTPO temporal-parietal-occipital cortex in the superior temporal

sulcusTS1–TS3 subdivisions of the rostral superior temporal gyrus

HIPPOCAMPAL PROJECTIONS TO TEMPORAL CORTEX 93

MATERIALS AND METHODS

The present investigation used tritiated amino acidautoradiography (Cowan et al., 1972) to trace anterogradeprojections from the HF to the temporal lobe in 18 rhesusmonkeys (Macaca mullata). Each animal received oneinjection into different parts of the HF of between 15 µCiand 50 µCi of a mixture of tritiated lysine, leucine, andproline and, in most cases, an amino acid mixture derivedfrom algal protein hydrolysate. The latter contains amixture of all amino acids and was included to protectagainst the possibility that some populations of neuronsmay not be well labeled by the usual proline, leucine, orlysine mixtures. Based on the distribution of amino acidlabelling, in an additional five monkeys, 19 injections offluorescent retrograde tracers (FRTs) were made intodifferent parts of the temporal lobe to determine the exactcells of origin of hippocampal projections to these regions.Three monkeys each received unilateral injections of threeFRTs: Diamidino yellow (DY), fast blue (FB), and rhoda-mine-labelled latex beads (RB) in different sites in thetemporal lobe. One monkey (PGF-L) received four tracerinjections into one hemisphere, the fourth being fluorescein-labelled, green, fluorescent latex beads (GB). The fifthmonkey (case PET) received injections of three FRTs intoeach hemisphere after transection of the corpus collosum(except the rostrum), dorsal hippocampal commissure, andanterior commissure.

Surgical procedures

Surgical procedures were carried out under aseptic con-ditions. All surgical procedures were approved by IACUCcommittee at Boston University School of Medicine. Eachanimal was lightly sedated with ketamine hydrochloride(10–15 mg/kg) and deeply anesthetized by intravenousinfusion of sodium pentobarbitol. At the end of the opera-tion, the wound was closed in layers with 4-0 (dura), 3-0(muscle), and 2-0 (skin) suture. Prophylactic doses ofBicillin LA (Wyeth Laboratories, Philadelphia, PA) wereadministered at the conclusion of surgery. Each animalwas monitored closely during postoperative recovery, andtheir diet was supplemented with soft fruit.

Injections of radioactively labeledamino acids

The amino acid mixture was prepared by desiccatingstock solutions under a stream of gaseous nitrogen andreconstituting with sterile saline to a concentration of 100microCuries (µCi) per microliter (µl). In individual autora-diographic (ARG) cases, injections ranged from 0.15 µl to0.5 µl containing 15–50 µCi of tritiated amino acids. Forthese injections, the monkeys’ heads were fixed in astereotaxic machine, and a small skin incision was madeover the target area. A burr hole was made in the skull, anda small incision was made in the dura. Injections into theHF were placed stereotactically with electrophysiologicalmonitoring in specific regions of the HF in ten monkeys.These injections were made through a specially con-structed bipolar injection electrode (for details, see Saun-ders and Rosene, 1988) coupled to a 5 µl Hamilton syringe(Reno, NV) controlled by a microdrive. During each penetra-tion, the bipolar injection electrode recorded field poten-tials. Upon entering into the HF, penetration by theelectrode evoked seizure activity (Crowne et al., 1972;Saunders and Rosene, 1988), which ensured accurate

placement of the injection cannulae within the HF. Toprevent back flow of labeled amino acids up the needletrack when the injection electrode was withdrawn, it wasleft in place in the brain and cemented to the skull, whereit remained until after perfusion-fixation of the tissue.Leaving the electrode in place produced no significanttissue damage but did prevent any labeling of the needletrack (for illustration and further discussion, see Saundersand Rosene, 1988; Saunders et al., 1988).

Injections of FRTs

To expose the medial and ventral temporal cortex, themonkey’s head was placed in a specially constructed headholder, and the skin and muscle overlying the temporalbone were reflected. A frontotemporal craniotomy wasmade, and 1.25–1.50 g/kg (5 ml/kg of a 25% solution) ofmannitol was administered IV to shrink the brain. Tenminutes later, the dura was reflected to visualize thelateral temporal lobe. Cottonoids and gentle retractionwere then used to expose the ventromedial cortex. Aftervisualization of the rhinal or occipitotemporal sulci, injec-tions of FRTs were placed in the appropriate corticaltargets with the aid of an operating microscope by using a5 µl Hamilton syringe fitted with a 26-gauge needle. TheFRTs that were administered were 1) fast blue (FB), whichlabels the cytoplasm blue (Bentivoglio et al., 1980; Kuyperset al., 1980); 2) Diamidino yellow (DY), which labels thenucleus yellow (Keizer et al., 1983); 3) rhodamine-labeledlatex microspheres or red beads (RB), which producered fluorescent granules in the cytoplasm (Katz et al.,1984); and 4) fluorescein-labeled latex microspheres orgreen beads (GB), which act similar to the RB but fluorescegreen under a wide-band ultraviolet filter (Katz andIarovici, 1988). The FB and DY (Dr. Illing, Inc., Germany)were injected as 3% suspensions in distilled water, and theRB and GB were injected in the concentration supplied(LumaFluor, Inc., New York). The FRTs were injected involumes of 0.2–0.5 µl per injection and were placed be-tween 1.5 mm and 2.0 mm deep in the cortex. In somecases, multiple injections of a single tracer were madeapproximately 0.5 mm apart in volumes of 0.15–0.25 µl perinjection. Following each injection, the syringe was left inplace for 10 minutes before removal.

Fixation and histological procedures

Following a postoperative survival period of 10 days(FRT cases) or 5–10 days (ARG cases), the animals weredeeply anesthetized with sodium pentobarbitol and trans-cardially perfused with approximately 200 ml of normalsaline followed by two liters of 4% paraformaldehyde in 0.1M cacodylate buffer, pH 7.4, for the FRT cases or 10%formalin for the ARG cases. The brain was then blocked inthe coronal plane with the head held in the stereotacticapparatus. The ARG cases were stored for 2 weeks in 10%formalin and then embedded in paraffin and cut into10-µm-thick coronal sections. Sections at 200-µm intervalswere processed for autoradiography according to a methodmodified from Cowan et al. (1972). The brains with FRTinjections were processed for frozen sectioning according toa cryoprotection method that completely eliminates freez-ing artifact (Rosene et al., 1986). The brain blocks werefirst kept overnight in a cryoprotectant solution of 10%glycerol and 2% dimethylsulfoxide with 4% buffered para-formaldehyde. The blocks were transferred the next day toa solution of 20% glycerol and 2% DMSO with 4% buffered

94 G.J. BLATT AND D.L. ROSENE

paraformaldehyde. After either 3 or 4 days in this solution,the blocks were rapidly frozen by immersion in 275°Cisopentane. Frozen sections were cut at 40 µm, and threeseries of adjacent sections at 320-µm intervals (three ofevery eight sections) were mounted immediately ontosubbed slides, dried, and stored in the dark with desiccantat 4°C. The middle series (no. 2) was charted as describedbelow and then stained with thionin. The first and thirdseries were desiccated and then coverslipped (withoutexposure to alcohol or xylene) with Fluoromount or DPX(Fisher, Philadelphia, PA).

Effective injection site

A clear determination of the ‘‘effective’’ injection site, i.e.,the area from which the quantity of tracer taken up andtransported is sufficient to produce detectable label atdistant sites, is of crucial importance in the analysis of anyneuroanatomical tracing study. In the illustrations anddescriptions in this paper, the ‘‘apparent’’ injection site isdefined for the ARG cases by the visible area where diffusetracer can be seen at low power under brightfield illumina-tion. For the FRT cases, it is defined by the circular halo offluorescence surrounding the central core of dense precipi-tate and necrosis seen under florescence illumination. Forboth types of injections, the illustrated apparent injectionsite is likely to be an overestimate of the effective area oftracer uptake. Determined by high–power examination ofthe ARG injection site, neuronal somata that are coveredby a concentration of silver grains significantly greaterthan the surrounding neuropil (and, hence, are presumedto have taken up sufficient tracer to permit anterogradetransport and detection of tracer at distant sites) occupyan area smaller than the illustrated area of brightfielddensity. Similarly, the apparent injection site in the FRTcases may be an overestimate, because a recent studyreported that the effective injection site is restricted to thecenter area of diffusion, i.e., the zone of necrosis at the siteof injection (Conde, 1987). An additional complication forthe FRT method is the evidence that injured axons take upand actively transport FRTs (Sawchencko and Swanson,1981; Illert et al., 1982). Although this is always a concern,in this investigation, FRT injections were limited to thesuperficial 1.5–2.0 mm of cortex, so the problem of uptakeby fibers of passage should be somewhat diminished.Moreover, we have largely resolved the attendant uncer-tainty of estimating injection sites in both FRT and ARGcases by using complementary anterograde and retrogradetechniques to cross validate the observations of eachmethod (for a more detailed discussion, see Saunders andRosene, 1988). Thus, the results of any apparent antero-grade projection from any hippocampal subfield can bepositively confirmed by injecting a complementary tracerinto the presumed site of termination without the prob-lems of formulating conclusions by ‘‘subtraction’’ amongoverlapping injection sites, as required when only one typeof tracer is used.

Data analysis

The distribution of both anterograde label and retro-gradely labeled cells was charted with an X-Y plottercoupled to the stage of a microscope equipped with bright-field, darkfield, and epifluorescence illumination. The sameseries that was charted was also used to derive thecytoarchitectural boundaries of the target areas in themedial temporal cortex and HF. For both areas, two-

dimensional flattened maps were constructed according tothe method of Van Essen and Maunsell (1980). However,for the HF, to simulate layer IV of the cortex, which wasused in the Van Essen and Maunsell paper, a line wasdrawn through the middle of the pyramidal cell layerthroughout the HF. The central location of this line in thepyramidal cell layer acts to minimize distortion in themap. For both the HF and the temporal lobe maps, threedifferent cases were unrolled. Comparison revealed thatthe overall shape of the flattened subfields and architec-tonic areas was very similar, and the profile lines wererelatively invariant as well. This consistency probablyreflects 1) our strict blocking and consistent processing ofevery brain in an identical fashion and 2) adherence to thesame flattening routine (e.g., beginning at the most rostralsection). Because of this, we selected one map for the HFand one for the ventral temporal lobe and plotted injectionsites and retrogradely filled cells or anterograde label fromall cases onto these standard maps by identifying thecorresponding profile line for each case. Figure 2A,Billustrates the flattened temporal lobe and the location ofFRT injection sites on it. Figure 3A,B illustrates thetwo-dimensional plot of the amino acid injection sites onthe flattened HF and the composite anterograde terminalsonto the flattened temporal lobe.

RESULTS

General description and nomenclature

HF. The term HF is applied to the three-layered allo-cortex of the dentate gyrus, the CA1–CA4 ammonic sub-fields, the prosubiculum, and the subiculum. In Figure 1,the cytoarchitectonic subdivisions of the HF in the rhesusmonkey are shown in coronal sections through severalrostrocaudal levels. These longitudinal levels are desig-nated the genu (Fig. 1A), uncus, anterior body (Fig. 1B),midbody (Fig. 1C), and posterior body (Fig. 1D). The cyto-architectonic nomenclature follows the general descriptionof Lorente de No (1934), as recently illustrated in detail forthe rhesus monkey (Rosene and Van Hoesen, 1987).

Posterior parahippocampal gyrus. The posteriorparahippocampal gyrus (PHG; Van Hoesen, 1982) is aheterogeneous area that begins caudal to the rhinal sulcusand is bounded laterally by the occipitotemporal sulcus.Medially, it includes and is bounded by the caudal exten-sion of the periallocortical presubiculum. The cortex be-tween the presubiculum and the occipitotemporal sulcuswas divided into areas TH and TF by von Bonin and Bailey(1947). Area TF is a typical isocortex with a densely packedlayer III and well-developed layer IV. In contrast, area THis a proisocortex with little or no layer IV and very littledifferentiation between layers II and III and betweenlayers V and VI. Between these areas, there is a distinctlydifferent cortex that was not recognized by von Bonin andBailey that has an incipient layer IV and a strong layer Vbut little separation between layers II and III or withinlayer V. In keeping with the style of von Bonin and Bailey,we have designated this cortex as area TL (Rosene andPandya, 1983; Pandya et al., 1988; Demeter et al., 1990;Barnes and Pandya, 1992; Schmahmann and Pandya,1992, 1993). This reflects the fact that it is an independenttemporal lobe area with distinct connections with thelimbic system. Amaral et al. (1987) apparently recognizedsome aspects of this area, because they designated part ofthis as a caudal extension of Brodmann’s area 36 back


Fig

ure

1

between TF and TH but did not recognize or illustrate itsfull extent. Caudally, these cytoarchitectonic areas (TH,TL, and TF) become modified before they are replaced byarea OA (area 19). The most striking feature of thismodification is the appearance of a crisp layer IV, even asthe other feature of layers II/III and V/VI remain constant.Therefore, as indicated on Figure 1D, we have designatedthese as areas THO, TLO, and TFO (Pandya et al., 1988).Nevertheless, these areas retain many of the same connec-tional features of the more rostral posterior PHG, even asthey also develop stronger connections with the occipitallobe.

Anterior PHG. The anterior PHG, as defined by VanHoesen (1982), includes all of the rostral temporal lobemedial to the rhinal sulcus (the periallocortical entorhinalcortex, presubiculum, and parasubiculum). Lateral to therhinal sulcus, there are two proisocortical areas, area 35 inthe lateral bank of the rhinal sulcus and a more lateralarea that Amaral et al. (1987) have designated area 36after Brodmann (1909), who limited this ‘‘entorhinal’’ areato the immediate bank of the rhinal sulcus but that Amaralet al. have extended out onto the ventral temporal lobe.Based on both connections and cytoarchitecture, we thinkthat this bears more similarity to our area TL of theposterior PHG and have designated it as TLr (Rosene andPandya, 1983; Pandya et al., 1988) and designated thecaudal extension of it between TH and TF as TLc. Theperirhinal cortex (area 35) is characterized by a denselayer V and the absence of a layer IV, whereas TLr ischaracterized by dense and homogeneous supragranularand infragranular layers separated by a thin dysgranularlayer IV (Rosene and Pandya, 1983). Area TLr is boundedlaterally first by neocortical area TE1, which usuallybegins in or near the anterior middle temporal sulcus andis regarded as part of the inferotemporal cortex or gyrus.Farther rostral on the temporal pole area, TLr is boundedlaterally by another proisocortex, designated Pro (Pandyaand Sanides, 1973; Galaburda and Pandya, 1983).

Inferotemporal cortex and the temporal pole. Theneocortex of the inferotemporal area has a thicker layer IVthan any adjacent areas of the anterior PHG (TLr) orposterior PHG (TF; Fig. 1A–C) and consists entirely ofarea TE of von Bonin and Bailey (1947). Area TE has beenfurther subdivided into many areas, including TE1, TE2,TE3, TEA, TEM, and TEO, based on myeloarchitecture,cytoarchitecture, cortical inputs (for review, see Pandyaand Yeterian, 1985) and physiology (see, e.g., Bayliss et al.,1987). In the present study, the area of inferotemporalcortex that received injections of retrograde tracers wasarea TE1 in the ventral part of inferotemporal cortex onthe lateral border of area TLr (Fig. 1A). At the tip of thetemporal pole, there is an additional area, area Pro, a bandof proisocortex that is bordered by areas TS1 and TS2laterally and by areas TLr and 35 medially (Fig. 2; see alsoPandya et al., 1988; Gower, 1989).

Anterograde tracer experiments

To understand the overall direct hippocampal projec-tions to the temporal lobe, tritiated amino acid injectionswere made into different parts of the HF in one hemi-sphere in each of 18 rhesus monkeys. The injection sitesfrom ten cases showing temporal lobe projections aremapped on a flattened reconstruction of the HF in Figure3A. These positive injections covered different rostrocau-dal levels of CA18, CA1, prosubiculum, and subiculum. Insome of these cases, the injection also involved the overly-ing DG, CA4, or CA3 subfields, but this is not illustrated onthis two-dimensional map, because, in the other eightcases, in which the isotope injections were limited to partsof the DG, CA4, CA3, or CA2 subfields, no temporal lobecortical projections were observed.

Each of the positive injection sites shown in Figure 3Agave rise to direct hippocampal projections to widespreadareas of the temporal lobe. Although there was consider-able specificity in the total projection pattern from each ofthese cases, there was considerable overlap in the arealprojection pattern of individual cases that was difficult toaccount for on the basis of injection locus. Because ourretrograde studies resolved this ambiguity, only the com-posite projection pattern from all of the positive antero-grade cases is presented in Figure 3B. Comparison withthe temporal lobe map in Figure 2A indicates that thesedirect hippocampal efferents project to the rostromedialpart of area Pro and all of the anterior and posterior PHG.The only medial temporal lobe areas not receiving projec-tions were the primary olfactory cortex and the medialamygdaloid nuclei. The projections to entorhinal cortex(areas 28M, 28I, 28L, 28S, and Pr1), to the corticoamygda-lar transition area (CTA), and to the cortical nucleus (CN)of the amygdala have been described previously in detail(Saunders and Rosene, 1988; Saunders et al., 1988).Additional direct hippocampal efferents to parts of medialfrontal and orbitofrontal cortices (Barbas and Blatt, 1995)and cingulate cortex (Blatt and Rosene, 1989) have alsobeen demonstrated. Five of the ten tritiated amino acidinjections that showed the most specific temporal lobeprojections are described below.

Anterograde cases involving rostral HF injections

Case PDA-L. In case PDA-L, an injection of approxi-mately 15 µCi of tritiated amino acids was centered inrostral CA1 in the lateral part of the genu and anteriorbody, as illustrated in Figures 3A and 4A. In the temporallobe, anterograde label was present over most of the

Fig. 1. A–E: Thionin-stained coronal sections through four levelsalong the rostrocaudal axis of the monkey hippocampal formation(HF), including the adjacent parahippocampal gyrus. The approxi-mate plane of section for the coronal sections in A–D are indicated byarrows on E, which is a dorsal (superior) view of the monkey HF as it isseen on the floor of the inferior horn of the lateral ventricle in the righthemisphere. The section in A was taken through the genu, the mostrostral part of the medial flexure, and illustrates the appearance ofgranule cells (gc) in the dentate gyrus, the dorsomedial CA18 subfield,which is composed of smaller and more densely packed neurons thanthose of ventrolateral CA1, as well as the insertion of the hippocampal-amygdaloid transition area (HATA) between the medial entorhinalcortex (28M) and the cortical-amygdaloid transition area (CTA) andthe rest of the amygdala dorsally. Lateral to the rhinal sulcus (rs) isthe perirhinal cortex (35) and the rostral part of area TL in theanterior parahippocampal gyrus. The section in B illustrates thedentate gyrus (DG) as it rotates medially (uncus) and in the anteriorbody. Ammonic subfields CA1–CA4, prosubiculum (ProS), and subicu-lum (Sub) are again seen, but the presubiculum (PreS) has expanded,and the parasubiculum (ParaS) is also present. The anterior parahip-pocampal gyrus is now ending as the rhinal sulcus ends caudally. Thesections in C and D illustrate the typical appearance of the subdivi-sions of the monkey HF in the midbody (C) and posterior body (D)levels. C shows the posterior parahippocampal gyrus, including areasTH, the caudal part of area TL, and TF in the occipitotemporal sulcus(ots). D demonstrates the appearance of the calcarine sulcus (cas) anda slight change in the cytoarchitecture of the caudalmost part of theposterior parahippocampal gyrus (THO, TLO, and TFO), which re-sembles the adjacent occipital cortex. For abbreviations, see list. Scalebars 5 1 mm in A–D, 5 mm in E.


posterior PHG and anterior PHG, where it was distributedin bands over both layers III and V but with considerableareal and laminar specificity. Thus, in area TLr, there wasdense label of approximately equal density over bothlayers III and V (Fig. 3C). In the adjacent rostral part ofTLc, label was densest over layer V (Fig. 4B,C), but, morecaudally in TLc, it was densest over layer III. In theposterior PHG, there was lighter label in both TF and TH,but, here, it was mainly in layer III. Farther caudally,there was light label over layers III and V of THO and TLO

and over layer V of TFO. In the most rostral part of theanterior PHG, there was diffuse label in TLr and inadjacent areas 35 and Pro. In summary, this case demon-strated projections to the entire anterior PHG and poste-rior PHG, but the main focus of this projection was thecentral part of area TL.

Case MNQ-R. In case MNQ-R, an injection of approxi-mately 15–20 µCi of tritiated amino acids was centered indistal CA1 and the prosubiculum of the genu (Fig. 3A).Anterograde label was present over layers III and V in

Fig. 2. A: Line drawing illustrating a flattened reconstruction ofthe right temporal lobe mapping the cytoarchitectonic borders ofcortical areas. B: Thirteen representative fluorescent retrograde tracerinjection sites from five hemispheres from four monkeys (see key) areindicated. The injections covered most of the area of the parahippocam-pal gyrus and control injections in inferotemporal cortex (TE) andentorhinal cortex (28). One monkey, case PET, received three different

tracer injections (fast blue [FB], Diamidino yellow [DY], and red beads[RB]) in each hemisphere due to a prior transection of the corpuscollosum (all levels except the rostrum), anterior commissure, anddorsal hippocampal commissure. Case PGF-L is the only case in whichgreen beads (GB) were used. Examples of retrograde tracing experi-ments are illustrated in the following six figures (Figs. 6–11). Forabbreviations, see list.


areas Pro, 35, and TLr of the anterior PHG and in areas TFand TLc of the posterior PHG. In contrast to case PDL-L,there was no projection to areas TH, THO, TLO, or TFO.

Anterograde injections into the midbody and/or pos-

terior body of the HF

Case MPG-R. In case MPG-R, an injection of 15 µCi oftritiated amino acids was centered in CA1 of the midbodyof the HF (see Figs. 3A, 5A). Anterograde label was presentin layers III and V of areas TLc, TLO, and TF of theposterior PHG (see also Fig. 5B,C). In this case, no labelwas observed over areas Pro, 35, TH, THO, or TFO.Although the apparent injection site raised the possibilitythat there may have been some spread into layer VI of areaTF in the underlying posterior PHG, examination of

cellular labeling indicated that this was not the case, andthere was no evidence of either the corticocortical orcorticothalamic projections that would be expected if theeffective injection site included area TF.

Case MMH-R. In case MMH-R, an injection of approxi-mately 15 µCi of tritiated amino acids was centered inproximal CA1 and adjacent CA2 at the midbody level ofthe HF, just proximal to the injection site from case MPG-R(Fig. 3A). Anterograde label was present in layer V of areasTH and TF, but no label was present in the interveningarea TLc or rostrally in areas TLr, 35, or Pro.

Case BMV-R. In case BMV-R, an injection of 30 µCiwas centered in the subiculum in the posterior body of theHF (Fig. 3A). Surprisingly, there was very little labeling in

Fig. 3. A: Line drawing illustrating tritiated amino acid (3H AA)injection sites in different parts of the hippocampal formation (HF)from ten cases plotted on a two-dimensional, flattened reconstructionof the right HF. In the flattened map, rostral (anterior) appears at thetop, and caudal (posterior) appears at the bottom. The thin linesrepresent profile lines drawn from individual coronal sections throughthe HF. The flattened hippocampal map in A fits together with theflattened temporal lobe map in Figure 2A (into the lower right [medial]side of Fig. 2A; indicated by hf, just inferior to HATA). B: A smallerversion of Figure 2A. The overall pattern of termination in thetemporal lobe from these ten cases is shown on the left in B, with the

matching cytoarchitectural areas defined on a two-dimensional, flat-tened reconstruction of the right temporal lobe on the right (also, forenlarged details, see Figs. 4A, 5A). C,D: Line illustrations of coronalsections from two representative cases (PDA-L and MPG-R) showingthe terminal label in the HF and parahippocampal gyrus. Labeledfibers travel from CA1 through the alveus (al) to the subiculum.Labeled terminals in the parahippocampal gyrus are especially evi-dent in layers III and V (also, for photomicrographs of the injectionsites and terminal label in these cases, see Figs. 4A–C and 5A–C). Forabbreviations, see list.


the posterior PHG except for sparse label over layers II,III, and V in caudal TH. However, there was a robustprojection to layer V of entorhinal area 28S and a modestprojection to layer V in areas 28L and Pr1.

Additional anterograde cases. Results from the fiveadditional anterograde cases confirmed the projectionsdescribed in detail in the above cases. For example, in caseMLP-L, an injection of 50 µCi of tritiated amino acids wascentered in the distal one-third of CA1 and prosubiculum

at the midbody level of the HF (Fig. 3A). This was similarto case MNQ-R, in which an injection of these samesubfields was centered at a more rostral levels in the genu(compare in Fig. 3A). In case MLP-L, there was antero-grade label over layers II, III, and V in areas TLr and 35 inthe anterior PHG and layers III and V in areas TLc and TFof the posterior PHG. However, in contrast to case MNQ-R,in case MLP-L, no anterograde label was seen in area Proin the temporal pole. The remaining cases all labeled some

Fig. 4. A–C: Photomicrographs from an anterograde case in whichan injection of tritiated amino acids was placed into the rostral level ofthe hippocampal formation (HF). A shows the injection site from casePDA-L centered in the CA1 subfield at the genu level of the HF. Notethe dense terminal labeling in the rostral subiculum. B and C show a

high-magnification view of terminal label in rostral area TL (C) andthe matching thionin-stained coronal section (B). Terminal labeling isespecially dense in layer V, and lighter label is seen in layer III.Arrowheads denote borders of cell layers. Scale bars 5 1 mm in A, 200µm in B,C.

Fig. 5. A–C: Photomicrographs from an anterograde case in whichan injection of tritiated amino acids was placed into the midbody levelof the hippocampal formation (HF). A shows the injection site fromcase MPG-R centered in the CA1 subfield at the genu level of the HF.Note the dense terminal labeling in the rostral subiculum. A shows an

injection site centered in the CA1 subfield through the midbody of theHF. B and C show the matching light- and darkfield photomicrographsof terminal label in caudal area TL at the TL/TF border. In C, theterminal label is most dense in layers III and V. Arrowheads denoteborders of cell layers. Scale bars 5 1 mm in A, 200 µm in B,C.


combination of the areas observed in the cases described.For example, in case MOL-L, an injection of 35 µCi oftritiated amino acids was centered in the subiculum of theposterior body of the HF, just caudal to the injection site incase BMV-R (Fig. 3A). Like case BMV-R, it labeled onlyarea TH (layers III and V) in the posterior PHG and layerV of entorhinal cortex.

Summary of anterograde cases. Examination of theten cases yielded the composite direct hippocampal projec-tion field that is plotted on a flat map of the temporal lobein Figure 3B. Review of the ten cases confirmed that therewas an apparent underlying but complex topography bothin differential densities of projections and in topography ofcells of origin. For example, anterograde label was found inarea TH in case MMH-R, in which the injection wascentered in proximal CA1 (i.e., near CA2) in the midbody ofthe HF, as well as in case BMV-R, in which the injectionwas centered in the subiculum of the posterior body.Another example of the complex organization of hippocam-pal efferents was the anterograde label in parts of areasTL, 35, and TF. In case MMH-R, an injection centered inthe proximal half of CA1 produced anterograde label in TFbut not in adjacent TLc or in 35. However, the injection incase PDA-L centered in CA1 at a more rostral level labeledparts of all three areas. Furthermore, in case PDA-L,anterograde label was also found in the most caudalposterior PHG in areas THO, TLO, and TFO, yet, in caseMPG-R, a larger injection that included parts of thecentral and distal two-thirds of CA1 labeled TLO but notTHO or TFO. Interestingly, the laminar pattern of labelingamong the ten cases was similar. Anterograde label inareas of the anterior and posterior PHG was seen primar-ily in layer V and, in many of the cases, was also present inlayers II and III. The obvious difficulty in extracting aspecific organizational principle from these cases couldreflect either the ever-present ambiguity in localizing theeffective injection site to a given subfield in the HF or anunderlying organization within CA1, prosubiculum, andsubiculum or a rostral-to-caudal differentiation. To resolvethese issues, injections of complementary retrograde trac-ers were placed throughout the hippocampal projectiontargets in the temporal lobe.

Fluorescent retrograde tracing experiments

To precisely identify the cells of origin of the directhippocampal projections in the rhesus monkey, 19 FRTinjections were made into six hemispheres in five mon-keys. Thirteen of these injection sites are illustrated inFigure 2A,B on a flattened reconstruction of the temporallobe. The other six injection sites (not illustrated) partiallyoverlap with one or more of these and confirm the observa-tions of those that are illustrated. Most of the posteriorPHG is covered by the injections as well as much of theanterior PHG and temporal pole. For controls, one injec-tion into TE1 as well as one injection into the entorhinalcortex (area 28) are illustrated. However, for both areas,numerous other injections were available for comparison,and the observations from the entorhinal injections werepresented earlier in a comprehensive study of hippocam-pal projections to this area (Saunders and Rosene, 1988).

Anterior PHG. Figures 2A,B, and 6A show that the FBinjection site in the right hemisphere in case PDJ-R wascentered in area TLr of the anterior PHG and also spreadinto a part of area 35. An abundance of retrogradelylabeled cells was found in the distal one-third of CA1 and

the adjacent prosubiculum in the genu of the HF, asillustrated in Figures 6A–C and 9C. Figure 6B shows thatlabeled cells occupied these same loci throughout theentire rostrocaudal extent of the HF. In this case, thesubiculum was devoid of labeled cells. The flattened map ofthe HF (Fig. 6B) shows that the labeled cells in distal CA1and the prosubiculum were organized in a continuous,longitudinal strip of projection neurons. In contrast, an RBinjection in case PGF-L was centered in area 28S, withspread into areas 35 and 28M at approximately the sameanteroposterior level as the TLr injection in case PDJ-Rbut on the opposite side of the rhinal sulcus (Figs. 2A,B,6D). This injection labeled many cells in the subiculum(Fig. 6D,F) as well as some cells in CA1 (Fig. 6F). Incontrast to case PDJ-R, these labeled cells were limited intheir rostrocaudal distribution to the rostral one-third ofthe HF (Fig. 6E). Thus, unlike the longitudinal strip ofhippocampal neurons that project to area TLr and area 35(Fig. 6B), neurons that project to the entorhinal cortexoccupy a wider mediolateral sector of the HF at a morelimited longitudinal level. Further examples of this organi-zation can be seen in Figures 15–17 of Saunders andRosene (1988), in which fluorescent dyes (DY, FB, or RB)were placed into different areas in the entorhinal cortexwith a similar pattern of results.

Another interesting comparison is shown in Figure 7. Incase PET-L (Figs. 2A,B, 7A), an injection of DY wascentered in TLr in the anterior PHG (somewhat rostral tothe similar injection in case PDJ-R-FB), whereas a morecaudal injection of RB was centered in TLc in the posteriorPHG of case PGC-R (Figs. 2A,B, 7D). The DY injection inPET-L involved areas TLr and 35 and labeled a longitudi-nal strip of neurons in distal CA1 and the prosubiculum.This pattern of labeling was almost identical to that in thesimilar but more rostral injection of case PDJ-R (Fig.6A–C), except that, in the caudal case PET-L, there wereno labeled cells in the most rostral prosubiculum or distalone-third of CA1. In contrast to these relatively rostral TLinjections, in case PGC-R, the injection of RB was localizedto the most caudal part of area TLc (Figs. 2A,B, 7D).Figure 2B shows that this injection was much smaller thaneither case PDJ-R or case PET-L, but it still labeled alongitudinal strip of cells that was localized in the centralone-third of CA1 and extended along most of the longitudi-nal axis of the HF except for the extreme rostral andcaudal levels.

Posterior PHG. In case PDJ-R, a DY injection wascentered in area TH but spread slightly into TLc and area35 (Figs. 2A,B, 8A) and labeled cells in the proximal half ofCA1 and most of the subiculum (Figs. 8A,C, 9D). Both theCA1 strip and the subicular labeling extended the entirelongitudinal extent of the HF. In another case, PGF-L, aninjection of FB (not illustrated) was centered in area THand labeled cells in the same proximal strip of CA1. Someof these labeled neurons are shown in two photomicro-graphs in Figure 9A,B. Thus, it appears that an extensivestrip of CA1 neurons that projects to TH is largely comple-mentary to the more distal strip of CA1 neurons thatprojects to all levels of TL (Figs. 6B, 7B,E). Only in thecentral part of CA1 (Fig. 7E) would these strips overlap. Amuch different result is seen when an injection is made ina more lateral location in the posterior PHG, as illustratedin case PET-R (Fig. 8D). In this case, an injection of FB wascentered in area TF in the medial bank of the occipitotem-poral sulcus (Fig. 8D) and labeled a dense strip of neurons


in the distal half of CA1 and in the prosubiculum, but nolabeled cells were found in the subiculum. The strip oflabeled CA1 cells extended the entire length of the HF and

overlapped with the distal CA1 and central CA1 sectorsthat were labeled after injections into TLr and 35 or TLc(compare with Figs. 6B, 7B,E). This strip again comple-

Fig. 6. Line illustrations from injections into the anterior parahip-pocampal gyrus on either side of the rhinal sulcus (rs). A–C: CasePDJ-R (FB). The FB injection on the lateral bank of the rhinal sulcusin areas TLr and 35 (A) results in a cluster of projection neurons in thedistal one-third of CA1 and in prosubiculum (A, C) extending theentire rostrocaudal extent of the HF. D–F: Case PGF-L (RB). The RBinjection site in D is restricted to the caudalmost part of area 28S on

the medial bank of the rhinal sulcus in entorhinal cortex. RB-labeledcells are seen only in the rostral part of the HF and include cells acrossa mediolateral extent of proximal to mid-CA1 (E,F) and the subiculum(D–F), which is in contrast to the more extensive longitudinal striporganization seen in case PDJ-R (FB) in B and in the other illustratedcases. For abbreviations, see list.


Fig. 7. Line illustrations from two retrograde fluorescent tracerinjections into the parahippocampal gyrus. A–C: Case PET-L (DY).The DY injection site in the anterior parahippocampal gyrus in rostralTL extending into perirhinal cortex (area 35) is illustrated in A. Thefull extent of the injection sites for this and other illustrated retro-grade tracer cases are shown in Figure 2. Retrogradely labeled cellsare seen in the middle part of CA1 at the genu level (A) and in a moredistal part of CA1 and in the prosubiculum in more caudal levels (e.g.,in C). The retrograde labeling from representative sections are plottedon a flattened map of the hippocampal formation (HF) in B. The levelsfrom which sections A and C were taken are indicated by arrows. The

clusters of retrogradely labeled CA1 neurons are located in a longitudi-nal strip throughout the entire rostrocaudal extent of the HF. D–F:Case PGC-R (RB). The smaller RB injection site is restricted to a morecaudal level of TL in the posterior parahippocampal gyrus andresulted in retrogradely labeled cells in the middle part of CA1 in theanterior and midbody levels of the HF (F and D are from the rostraland caudal parts of the midbody, respectively). The more limitedlongitudinal strip of labeled cells seen in E is adjacent to the strip oflabeled CA1 cells from the rostral TL injection in case PET-L in B. Forabbreviations, see list.

Fig. 8. Line illustrations from two retrograde tracer injections intothe posterior parahippocampal gyrus. A–C: Case PDJ-R (DY). In A,the DY injection site is centered in the medial posterior parahippocam-pal gyrus in area TH and resulted in labeled cells in the proximal halfof CA1 (most dense in the proximal one-third) and in the middle todistal subiculum. A similar pattern of DY-labeled cells is seen in amore rostral coronal section through the hippocampal formation (HF)(C). The clusters of DY-labeled cells form two dense, longitudinal

strips that extend throughout the entire length of the HF seen in B.D–F: Case PET-R (FB). A more lateral injection into area TF (D)resulted in a cluster of FB-labeled cells in the distal half of CA1 and inthe prosubiculum (D and F). In the flattened map in E, the FB-labeledcells extend throughout the entire rostrocaudal extent of the HF and‘‘fill in’’ the unlabeled zone from the pattern of retrograde labeled cellsin B from the medial posterior parahippocampal gyrus injection. Forabbreviations, see list.

mented the strip that projects to area TH (compare Fig. 8Bwith Fig. 8E).

Injections of FRTs were also made into the most caudalpart of the posterior PHG, as illustrated in Figure 9. Incase PGC-R, the FB injection was centered in area TLOand involved the adjacent part of area THO (Fig. 10A).Labeled cells were abundant in the distal half of CA1 andadjacent prosubiculum throughout caudal levels of the HFbut became more sparse at rostral levels. A few labeledcells were also found in the most distal subiculum (Fig.10A,C). In case PET-R, a more lateral injection of RB wascentered in area TFO in the caudal posterior PHG andspread slightly into adjacent area 19 (Figs. 2A,B, 10D).This injection labeled a cluster of neurons in the centralpart of CA1 (Fig. 10D,F). The flat map of the HF in Figure10E shows that this strip of projection neurons was limitedto the caudal two-thirds of the HF, and no labeled neuronswere present in the genu. Nevertheless, this strip oflabeled neurons overlaps with that seen from the TLOinjection (Fig. 10B) as well as with the strips of projectionneurons seen after injections into TL in previously illus-trated cases (Figs. 6B, 7B,E).

Hippocampal projections to the temporal pole. Twofluorescent retrograde injections in case PET-L were made

at a far rostral location in the left temporal pole. The FBinjection site was centered in area Pro (Fig. 11A) andlabeled cells in the prosubiculum and in the distal one-third of CA1. This strip of labeled CA1 cells extendedrostrally to include the CA18 subfield but did not extend tothe most posterior part of the HF (Fig. 11B,C). No labeledcells were found in the subiculum. In contrast, an RBinjection in the same case into area TE1 (Fig. 11D) did notlabel any neurons in the HF but did label cells in thelateral nucleus (LT) in the amygdala and in the cortical-amygdalar transition area (CTA), as illustrated in Figure11E. It is interesting that the FB injection into Pro alsolabeled cells in the LT, CTA, the accessory basal nucleus(AB), and the dorsal part of the lateral basal nucleus (LB)of the amygdala. In case PGF-L (Fig. 2B), a DY injectionwas made into a more caudal part of TE1, just medial tothe anterior middle temporal sulcus (tma). Again, nolabeled cells were found in the HF, but numerous cellswere labeled in various nuclei in the amygdala.

Summary of retrograde cases. Retrograde tracerinjections into areas 35, TLr, TLc, TLO, or TF labeled alongitudinal strip of neurons in the distal part of CA1 andthe adjacent prosubiculum throughout the entire longitudi-nal extent of the HF. An injection into area Pro of the

Fig. 9. Photomicrographs of retrogradely labeled neurons in theHF from cases in which injections of fluorescent retrograde tracerswere injected into different sites in the parahippocampal gyrus.A: Photomicrograph of FB cells through the proximal part of CA1 froman injection centered in area THO from case PGF-L. B: Higher

magnification view of a few FB-labeled cells in CA1 from the same caseshown in A. C: Photomicrograph of a cluster of FB-labeled cells in thedistal part of CA1 from an injection into rostral TL and 35 from casePDJ-R. D: A cluster of labeled cells in the subiculum from a DYinjection into area TH also from case PDJ-R. Scale bars 5 50 µm.


temporal pole also labeled cells in the distal one-third ofCA1 and the prosubiculum, but this labeling was limited tothe rostral half of the HF and also continued into theunique and smaller CA18 subfield of the genu (case PET-L;

Fig. 11B). A complementary strip of neurons extendingalmost the entire length of the HF was labeled in centralCA1 (partially overlapping with the distal CA1 sector)even after a very small injection into area TLc (PGC-R;

Fig. 10. Line illustrations from two retrograde tracer injectionsinto the caudal posterior parahippocampal gyrus. A–C: Case PGC-R(FB). The injection site in A is located in area TLO and extendsmedially into a part of THO. Retrogradely labeled FB cells are seen inthe distal half of CA1, in the prosubiculum, and in the distalsubiculum in A and C. A narrow strip of labeled cells in the genu andanterior body is illustrated on the flat map of the hippocampalformation (HF) in B, which then widens in progressively caudal levels.

D–F: Case PET-R (RB). The RB injection into the more lateral TFO atthe TLO/TFO border (D) labeled a strip of cells in the middle one-thirdof CA1 but not in the prosubiculum or subiculum (D and F). The stripof RB-labeled cells is limited to the caudal two-thirds of the HF (E) andoverlaps with labeled cells from the injection into THO/TLO in B. Thepattern of labeled cells in E from the TFO injection is very similar tothe strip of labeled cells in Figure 7E from an injection into caudal TL.For abbreviations, see list.


Fig. 11. A–E: Line illustration of two retrograde tracers placed intodifferent parts of the temporal pole in case PET-L. The FB injection,which is centered in proisocortex (Pro; A) and extends into rostral 35and TL (see Fig. 2), resulted in FB-labeled cells in the very distalone-third of CA1 and across CA18 (C), forming a continuous strip oflabeled cells limited to the rostral half of the hippocampal formation

(HF; B). In contrast, an RB injection at the same level through thetemporal pole that was restricted to area TE1 in infratemporal cortex(D) did not label any cells in the HF. RB-labeled neurons are seen inthe lateral nucleus (LT) in the amygdala and in the cortical-amygdaloid transition area (CTA). Some labeled cells are also seen inarea 35 and in rostral TL. For other abbreviations, see list.

Fig. 7E). Injections into the more caudal area TFO labeleda similar central strip of CA1 neurons, but, in this case,labeled cells were limited to the caudal two-thirds of theHF. Retrograde tracers placed into the medial posteriorPHG in area TH labeled a continuous rostrocaudal strip ofneurons in the proximal one-third of CA1 as well as in thesubiculum, strips that also extended the rostrocaudallength of the HF. It is interesting that, in all five cases,although two or three different retrograde tracers wereplaced into different parts of the posterior PHG and/or theanterior PHG in a given hemisphere, only about 2% of theretrogradely labeled cells in the HF were double or triplelabeled. In contrast to the longitudinal organization ofdirect hippocampal efferents to the anterior PHG, poste-rior PHG, and TP, an injection into 28S (as previouslyreported by Saunders et al., 1988) labeled cells in CA1 andthe subiculum at only a limited rostrocaudal level of therostral HF. Only injections into TH or entorhinal cortexlabeled the subiculum, and two injections into differentparts of area TE1 did not produce any labeled cells inthe HF.

DISCUSSION

The classic view of hippocampal outputs that couldinfluence the cerebral cortex involved the circuitous subcor-tical relay of the ‘‘Papez’’ circuit from the subiculum of theHF, through the fornix, to the mammillary bodies, then tothe anterior thalamic nuclei, and, finally, to the cingulategyrus (Rosene and Van Hoesen, 1977; Aggleton and Mish-kin, 1983). Previous anterograde studies have identifiedmany direct hippocampal efferent projections to the cere-bral cortex (Rosene and Van Hoesen, 1977; Swanson andCowan, 1977; Swanson et al., 1978; Sorensen and Shipley,1979; Swanson, 1981; Saunders and Rosene, 1988; Barbasand Blatt, 1995). From this, the flow of information wouldpass through the intrinsic circuitry of the HF (DG to CA3to CA1) to prosubiculum and subiculum, where it wouldthen project directly to the cerebral cortex (Rosene andVan Hoesen, 1977; Van Hoesen et al., 1979) as well assubcortically through the fornix. In this light, the subicu-lum was previously suggested as a final common pathwayfor the majority of hippocampal efferents to both corticaland subcortical targets in the primate brain (see, e.g.,Rosene and Van Hoesen, 1977; Schwerdtfeger, 1979; VanHoesen et al., 1979; Van Hoesen, 1982; Goldman-Rakic etal., 1984). Among the projections of the subiculum are theanterior and posterior cingulate areas 24, 23, and 29(Rosene and Van Hoesen, 1977; Vogt and Pandya, 1987;Blatt and Rosene, 1989); the medial frontal areas 25 and14 (Rosene and Van Hoesen, 1977; Blatt and Rosene, 1988;Barbas and Blatt, 1995); orbital prefrontal areas 12, 13,OLF/PAII, and Pro (Goldman-Rakic et al., 1984; Cavadaand Reinoso-Suarez, 1988; Morecraft et al., 1992; Barbasand Blatt, 1995); and entorhinal and perirhinal areas 28and 35 (Van Hoesen et al., 1972; Rosene and Van Hoesen,1977; Saunders and Rosene, 1988; Witter and Amaral,1991; Leonard et al., 1995).

However, it soon became clear that some cortical projec-tions also originated from the CA1 subfield. Thus, in therat, both the subiculum and CA1 subfields project toperirhinal cortex and entorhinal cortex (Swanson andCowan, 1977; Swanson et al., 1978) and to the prefrontalinfralimbic area (Swanson, 1981). In the monkey, CA1projections to entorhinal and perirhinal cortices have also

been demonstrated (Saunders and Rosene, 1988; Leonardet al., 1995), and dense CA1, prosubiculum, and subiculumprojections have been demonstrated to medial prefrontalareas, whereas less dense projections from these samehippocampal subfields project to orbital prefrontal cortices(Barbas and Blatt, 1995). However, because of the frequentspread of anterograde injections into both CA1 subfieldand the subiculum and prosubiculum, it was difficult todefine precisely the ‘‘output’’ role of the CA1 field incontrast to its role in the intrinsic hippocampal circuitry asthe obligatory relay to the subiculum, where hippocampaloutput that reached beyond the system to the diencepha-lon or cerebral cortex originated (Rosene and Van Hoesen,1977). The present investigation is part of a series de-signed to define the roles of these three hippocampalsubfields that send outgoing projections to cerebral cortex.In this regard, we have demonstrated that hippocampalprojections to the PHG of the temporal lobe originate in ahighly organized and topographic pattern from longitudi-nal strips of neurons in different subdivisions of CA1 aswell as from the subiculum and the prosubiculum.

Comparison of FRT and ARG cases

In general, the two tracer methods confirm the opera-tional estimates of the effective injection site, but they alsoprovide unique information. Thus, the pattern of antero-grade termination demonstrated by the tritiated aminoacid cases demonstrates that the direct hippocampal pro-jections are confined to the posterior PHG and anteriorPHG, where they typically terminate in layers III and Vrather than as ‘‘feedback’’ projections to layer I (Maunselland Van Essen, 1983). Anterograde termination in thesupra- and infragranular layers within PHG areas is inagreement with the description by Galaburda and Pandya(1983) of outgoing or ‘‘feed-forward’’ projections from oneassociation area to the next higher association area, whereit terminates in and around layer IV (layers III, IV, and V).Although there is no layer IV in area TH and a only a weaklayer IV in area TL, the hippocampal efferent projectionsoriginating in subfields CA1, prosubiculum, and/or subicu-lum generally avoid layer IV, even in area TF where it iswell developed, and target the output layers III and V.

In contrast, the retrograde tracer data demonstrate alongitudinal organization of direct hippocampal efferentsfrom CA1, the adjacent prosubiculum, and the subiculumthat was not apparent from the anterograde tracer cases.For example, two very different anterograde injectionsboth produced anterograde label in area TH. Specifically,in case MMH-R, the injection was centered rostrally inproximal CA1, and, in case BMV-R, the injection wascentered caudally in the subiculum. Injection of retrogradetracers into TH revealed both a longitudinal strip ofprojection neurons in the proximal part of CA1 as well as atopographically separate strip of projection neurons in thesubiculum. Moreover, in one of the anterograde cases(MMH-R) just described, an injection centered in theproximal half of CA1 also labeled area TF. From theretrograde experiments, it was shown that a central stripof CA1 cells projects to TF. Thus, the effective injection sitein case MMH-R included enough of the central one-third ofCA1 to also label area TF. This demonstration of theorganization of projection neurons into longitudinal stripscovering the entire anteroposterior extent of the HFexplains why injections of tritiated amino acids at verydifferent anteroposterior levels had very similar projection


patterns, whereas injections into different parts of CA1 atthe same longitudinal level had different patterns.

CA1 subfield is a heterogeneous structure

Although, in Nissl sections in the monkey, like the rat,the stratum pyramidale in the CA1 subfield appears to bea homogenous structure throughout its mediolateral androstrocaudal length, results from the above experimentsdemonstrate that different proximal-to-distal sectors withinthe CA1 subfield project to distinct parts of temporal lobecortex. The proximal one-third of CA1 projects to areas THand THO, and the central one-third of CA1 projects to TLc,TLO, TF, and TFO in the posterior PHG, whereas thedistal one-third of CA1 and prosubiculum projects to TLr,35, and Pro in the anterior PHG. The CA18 subfield in thegenu of the HF also projects to Pro in the temporal pole. Itis interesting that connectional and morphological differ-ences within the CA1 subfield were identified over 50 yearsago by Lorente de No (1934) but have been largely ignored.He divided the CA1 subfield into three parts: a distal CA1anear the prosubiculum, a central CA1b, and a proximalCA1c located adjacent to CA2. This parcellation was basedprimarily on different intrinsic features, such as dendriticbranching and whether cells in a CA1 subfield receivedbranches from a particular pathway (e.g., alvear pathway,Schaffer collaterals, etc.). Although these multiple Golgicriteria are compelling, they make it virtually impossibleto utilize this breakdown in standard experimental studiesin which Nissl stains are used; hence, they have beenlargely ignored. Although we have divided the CA1 sub-field into proximal, central, and distal one-thirds, it isinteresting to speculate that these correspond to the CA1a,CA1b, and CA1c subdivisions of Lorente de No. Additionalsupport for such a transverse heterogeneity comes fromrecent studies of Witter and colleagues. Witter and Amaral(1991) demonstrated differential entorhinal inputs to simi-lar transverse sectors of CA1 and subiculum in the mon-key, and, in the rat (Witter and Groenewegen, 1990; Witteret al., 1990), differential outputs from the subiculum to thecortex have ben described. All of these morphologicaldifferences suggest differential functions for these trans-verse strips in CA1 and the subiculum.

There is also recent evidence that CA1 contains alaminar differentiation in its projection to some corticalareas (Blatt and Rosene, 1988; Cavada and Reinoso-Suarez, 1988; Barbas and Blatt, 1995). Retrograde tracersplaced into medial ventral prefrontal cortex (area 14 andventral part of area 25) labeled cells in the deepest part ofstratum pyramidale in the distal half of CA1 and also cellsin the prosubiculum and subiculum (Blatt and Rosene,1988). Barbas and Blatt (1995) demonstrated that it ismainly the deepest part of CA1 (the portion that is closestto the alveus) that projects to prefrontal areas whereasmost labeled neurons in the prosubiculum and subiculumoriginated from an intermediate zone. This suggests thatthere may be an additional differentiation within thedifferent proximal to distal sectors, even though this doesnot appear to be the case for the ventral temporal lobeprojections described here.

A rostrocaudal topography within the HF was found forprojections to some prefrontal areas (Blatt and Rosene,1988; Cavada and Reinoso-Suarez, 1989; Barbas andBlatt, 1995). Cavada and Reinoso-Suarez found that it wasmainly the rostral one-third of the HF that projects to area14, whereas the caudal one-third of the HF mainly projects

to areas 10 and 11. This study suggested that thesehippocampal projections are limited in their rostrocaudalextent to some prefrontal areas. Barbas and Blatt demon-strated that projections from HF to medial prefrontalcortex were more abundant than to orbitofrontal cortexand that, although most labeled neurons from prefrontalretrograde injections were located rostrally within the HF,some projections extended the entire rostrocaudal lengthof the HF just as the temporal lobe projections do. Directlongitudinal projection patterns of hippocampal neuronshave also been described to cingulate and retrosplenialcortex (Blatt and Rosene, 1989).

Further evidence that CA1 is a heterogeneous structureis provided by postmortem histological analyses of braintissue from patients with Alzheimer’s disease. Many inves-tigators have demonstrated a propensity for neurofibril-lary tangles and senile plaques in the HF (Hirano andZimmerman, 1962; Dayan, 1970; Ball, 1972, 1978; Mori-matsu et al., 1975; Ball et al., 1983; Burger, 1983; Hymanet al., 1984), where they occur in greatest density in the‘‘cortical output regions’’—the prosubiculum, subiculum,and CA1 (Kemper, 1984). Furthermore, within CA1, agreater accumulation of senile plaques was seen in thedistal one-third compared with the proximal and centralsectors (Kemper, 1984). Additional criteria that suggestsdifferences within the CA1 subfield (including CA18) isprovided by recent neurochemical and receptor-bindingstudies in the rhesus monkey. For example, there is a highdensity of acetylcholinesterase (AChE) in CA2 and theadjacent proximal one-third of CA1, but this density dropsoff in the central one-third and then increases again in thedistal one-third adjacent to the prosubiculum (Bakst andAmaral, 1984; Rosene and Van Hoesen, 1987). A similarpattern of AChE staining occurs in CA1 and prosubiculumin the normal human (Rosene and Van Hoesen, 1987;Green and Mesulam, 1988). In the monkey, a very highdensity of M2 receptors (muscarinic cholinergic) has beenfound in stratum moleculare in distal CA1 and prosubicu-lum (Rhodes et al., 1990), whereas a high density ofbenzodiazipine receptors is localized in the proximal halfof CA1 (Rhodes and Rosene, unpublished results). Takentogether, these observations and the results of the presentstudy confirm that the CA1 subfield of the primate HF ishighly differentiated. These heterogeneous morphological,connectional, and neurochemical characteristics withinthe CA1 subfield clearly suggest that there is a correspond-ing functional differentiation.

Organization of direct hippocampal efferentsto the temporal lobe

The longitudinal organization of projection neuronsrevealed in the present study is in sharp contrast to theorganization of the projection from CA1, prosubiculum,and subiculum to entorhinal cortex (area 28) in the rhesusmonkey (Saunders and Rosene, 1988). In that study,injections of retrograde tracers into the entorhinal cortexlabeled cells throughout the entire proximal-to-distal ex-tent of the CA1 subfield as well as the adjacent prosubicu-lum and subiculum, and the labeling was relatively lim-ited in rostrocaudal extent. Consequently, the hippocampalprojection to the entorhinal cortex appears to be topographi-cally organized according to rostrocaudal levels of the HF(Saunders and Rosene, 1988). An example of this differ-ence was demonstrated in the present study by the injec-tion into area 28S (Fig. 6D–F). This suggests a very


Figure 12


complex organization within the HF subfields in whichneurons that give rise to the entorhinal projection origi-nate from limited sectors of hippocampal cells in contrastto projections to the proisocortical and neocortical parts ofthe anterior PHG and posterior PHG, which arise fromextensive, longitudinal strips within CA1 as well as thesubiculum. Thus, activity throughout a longitudinal stripof CA1 could influence a large part of the posterior PHG,but different longitudinal levels of that strip would influ-ence different parts of the entorhinal cortex.

To determine whether individual CA1 neurons, prosu-bicular or subicular neurons, or separate populations ofneurons project to both entorhinal cortex and to one orseveral parts of the remaining PHG, we examined ourinjections of multiple retrograde tracers for double ortriple labeling. In all of these experiments, very fewdouble-labeled or triple-labeled cells were observed. Forexample, in case PGF-L, an RB injection was made into theentorhinal cortex (Figs. 2, 6D–F), and an FB injection wasmade into area TH (not illustrated). Although neuronsretrogradely labeled with either RB or FB were presenttogether at one rostrocaudal level of the proximal one-third of CA1 and the subiculum, only about 2% of the cellswere double labeled. Obviously, more comprehensivedouble-label studies need to be performed targeting mul-tiple injections into both cortical and subcortical loci tooptimize labeling in common areas of the HF, but thepresent observations clearly suggest that there may beanother level of differentiation within CA1 where separatebut intermingled neuronal populations project to differentcortical targets.

In other control experiments, injections of retrogradetracers (DY and RB) were placed into different parts ofarea TE1, but no labeled cells in the HF were seen.Previously, Iwai and Yukie (1988) had injected HRP intodorsal and ventral area TE, and, although no retrogradelylabeled cells were found in the HF following dorsal TEinjections, a cluster of labeled cells was observed in thedistal one-third of CA1 following ventral TE injections. Inthe one case illustrated in their paper (case 2 in Iwai andYukie, 1988), the HRP injection was positioned medial tothe anterior middle temporal sulcus (tma), where spreadinto the area we have designated TLr would be likely. Inthe present study, injections into area TLr labeled neuronsin a longitudinal strip of distal CA1 and in the prosubicu-lum, whereas a DY injection placed just medial to tma in

ventral TE did not label any HF cells. This suggests thatthe HRP injection into ventral TE in Iwai and Yukie’sstudy did indeed spread into the adjacent area TLr.

Functional implications

The pattern of connections reported here raises a num-ber of interesting functional questions. First, what is thelikely functional significance of these extensive, directhippocampal projections to both the anterior and theposterior parahippocampal cortices in the well-establishedmnemonic functions of the medial temporal lobe? Second,what is the significance for information processing withinthe hippocampus and in the target areas of the parahippo-campal cortices of the origin of these projections fromtransversely discrete, longitudinal strips within the CA1subfield? These issues can be considered in light of boththe known anatomical connections of the HF and parahip-pocampal cortices with other medial temporal lobe struc-tures

First, from an anatomical perspective, the PHG is amajor source of input to the HF through projections to theentorhinal and perirhinal cortices (see, e.g., Van Hoesenand Pandya, 1975) as well as through direct projectionsinto the CA1 and subicular subfields (Van Hoesen, 1982).This input is likely a highly processed abstraction ofsensory information, because the PHG is a multimodalarea that receives afferent input from visual, auditory,somatosensory, and multimodal areas of the occipital,temporal, and parietal lobes (Seltzer and Pandya, 1976;Blatt et al., 1989; Suzuki and Amaral, 1994). Consideringthis pathway of sensory information into the HF, the directprojections back might constitute a feedback projectiononto an earlier stage of sensory processing. Such feedbackor back-going projections have been well characterized insensory association areas of the neocortex (see, e.g., Rock-land and Pandya, 1979), where they usually originate inlayer V pyramids and project to layer I of the target area inthe immediately preceding stage of sensory processing.Because the projections here terminate in deeper layers(III and V), it seems likely that this direct hippocampalprojection is acting upon the parahippocampal cortices asa feed-forward afferent input. This raises the possibilitythat this projection may be part of the hippocampalmnemonic output system; indeed, behavioral studies inthe monkey are entirely compatible with this view.

The role of the HF in memory function has been wellestablished in both the early studies of humans, beginningwith patient H.M., (Scoville, 1954, 1957) where the lesioninvolved the amygdala, hippocampus, entorhinal cortex,and parts of the PHG (Corkin, 1984; Corkin et al., 1997).This observation led to numerous attempts to replicate thesyndrome by making selective hippocampal ablations inmonkeys, but the approach to hippocampal ablation wasthrough the posterior PHG and often resulted in damageto at least the posterior half of the entorhinal and perirhi-nal cortices as well. Although such lesions did producesignificant impairments on recognition memory tasks,Mishkin and colleagues (see, e.g., Mishkin, 1978; Mishkinet al., 1982) demonstrated that adding ablation of theamygdala to the hippocampal lesion (making the totallesion quite comparable to that of patient H.M.) exacer-bated the deficit. Subsequently, there have been a varietyof attempts to identify the relative role of these differentstructures in memory function and amnesias. In thecourse of these studies, numerous claims have been madeabout the relative importance of each of these structures in

Fig. 12. A–D: Summary illustrations showing the organization ofthe total direct hippocampal projections to the posterior parahippocam-pal gyrus, to the anterior parahippocampal gyrus (except to entorhinalcortex), and to the proisocortex (Pro) of the temporal pole. A shows aflattened map of the hippocampal formation (HF; described in detail inFig. 2A) containing four longitudinal strips of cells that extend theentire length of the HF and project to specific parts of the anteriorPHG or posterior PHG in C (also see D for identification of cytoarchitec-tonic areas on the flattened map of the temporal lobe). Two longitudi-nal strips of cells, one from the proximal one-third of CA1 and one fromthe subiculum, project to areas TH and THO. Another strip of cellsoriginates in the central one-third of CA1 and projects to areas TLc,TLO, and TF. A fourth strip of cells originates from the distal one-thirdof CA1 and the adjacent prosubiculum and projects to areas TLr and35. B shows that two HF projection strips originated from limitedrostrocaudal levels. Cells in the rostral half of the HF originating fromCA18, distal CA1, and the prosubiculum project to area Pro in thetemporal pole (C), whereas a central CA1 strip limited to the caudaltwo-thirds of the HF (B) projects to area TFO in the caudal posteriorPHG (C). For abbreviations, see list.


memory function based on the presence or the absence ofdeficits after selective lesions. Thus, some investigatorssee these medial temporal lobe structures as part of aninteractive system in which the hippocampus plays apivotal role (e.g., see Zola-Morgan et al., 1992, 1994),whereas others dismiss the importance of the hippocam-pus and focus on the rhinal cortices (for review, seeMurray, 1996).

Although many procedural variables can affect theexperimental outcome, one major difference that appearsto be important and may contribute to these opposingviews of medial temporal lobe memory function is thebehavioral testing paradigm. In this regard, two verydifferent behavioral paradigms have been used. The Mish-kin group has used a ‘‘retention/reacquisition’’ procedurein which the monkeys are trained preoperatively on thememory tasks to be tested after the lesion (see, e.g.,Murray and Mishkin, 1984, 1986; Murray et al., 1988;Murray, 1996), whereas the Squire and Zola-Morgan grouphas used an ‘‘acquisition’’ procedure in which the memorytasks are presented only after the lesion has been made(see, e.g., Zola-Morgan et al., 1989a,b). Although argu-ments for the appropriateness of each paradigm can bemade, the results often differ, particularly with regard tothe severity of deficits following hippocampal lesions.Thus, it is important to consider the possibility that thepreoperative training procedure of the Mishkin group maytransfer memory processing for that specific task from acore structure, like the hippocampus, out to more periph-eral rhinal cortical areas in the temporal lobe as overtrain-ing occurs. Such a paradigm may even ‘‘recruit’’ nontempo-ral lobe systems into processing information when the taskis well learned. If this is true, then subsequent lesions ofthe hippocampus may produce less severe deficits thanthose produced when the task is acquired postoperatively.The Horel group, which cools cortical regions to produce‘‘reversible lesions’’ and uses the preoperative trainingtechnique, found that deficits in delayed-nonmatch-to-sample (DNMS) and visual discrimination tests were notcaused by cooling of the hippocampus but were significantwhen there was either cooling of the PHG alone orexacerbation by cooling both the PHG and the posteriorinferotemporal gyrus (George et al., 1989). Until thesignificance of these paradigm differences are fully estab-lished, the relative importance of different medial tempo-ral lobe structures in memory function will remain indispute.

With this in mind, the results of the Squire and Zola-Morgan group using the postoperative acquisition para-digm indicate that all of these medial temporal lobestructures (hippocampus, entorhinal cortex, perirhinalcortex, and parahippocampal cortex) are important inmemory processing. Hence, relatively small and partiallesions confined to the hippocampus by using a stereotacticapproach that spares the surrounding structures producea mild but significant and long-lasting recognition memorydeficit compared with the typical ablation procedure thatdamages the HF, entorhinal cortex, perirhinal cortex, andposterior PHG (Zola-Morgan and Squire, 1986). Thoseauthors also reported that extending the lesion forwardinto the anterior PHG (perirhinal cortex plus our TLr)produces an even greater deficit (Zola-Morgan et al., 1993).It is also worth noting that, by using their standardmemory-assessment paradigm, they demonstrated thatlesions of the fornix (the main subcortical hippocampal

projection pathway) or the mammillary bodies, a principaltarget of hippocampal projections, produce relatively mildand transient memory impairments (Zola-Morgan et al.,1989a).

One interpretation of these observations is that memorydeficits in monkeys and those in patient H.M. are largelythe result of damage to the surrounding cortices (forconvenience, designated as the ‘‘rhinal’’ cortices by Murrayand Mishkin, 1986) and not damage to the hippocampus(Murray, 1996). Although this notion can be criticized on anumber of grounds and may only apply to the preoperativetraining paradigm, the recent description of amnesia inpatient R.B. (Zola-Morgan et al., 1986), where the lesionwas restricted to the CA1 subfield, demonstrates that thehippocampus is of critical importance in this system.Hence, a more parsimonious explanation is to considerthese structures as part of a cortical-hippocampal-corticalloop in which the hippocampus is an essential participantin the early stages of memory formation and where itsprojections back to cortex are critical for two aspects ofmemory function: first, for the transfer of information to bestored to neocortical areas and second, for transport tothese areas of the rules for processing similar information.We propose that the projections from the CA1 and subicu-lum out to the PHG are essential for both functions andthat damage to the PHG will disrupt memory functionboth by cutting off the hippocampus from important multi-modal sensory input and by depriving it of its main corticaloutput target. From this perspective, the critical feature ofthe pathology in case R.B. may be the involvement of theentire rostrocaudal extent of CA1. It is interesting tospeculate that a lesion limited to one or two specificlongitudinal strips in CA1 might produce a more limited oreven a modality-specific memory deficit. The answers tosuch speculations await further explication of the entirepattern of hippocampal projections to the cerebral cortexas well as delineation of the functional attributes of thecortical targets.

ACKNOWLEDGMENTS

The authors thank Dr. Deepak Pandya for his assistancein the preparation of the flattened reconstruction of thetemporal lobe and for his unending patience and advice.We also thank Elizabeth Johnson, Helen King, Dr. AgnesVirga, and Ramy Rizkalla for their technical assistance inthe histological preparation of material and photography.

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organization of direct hippocampal efferent projections to the cerebral cortex of the rhesus monkey:...

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