Download - Microstimulation of monkey dorsolateral prefrontal cortex impairs antisaccade performance

Exp Brain Res (2008) 190:463–473
DOI 10.1007/s00221-008-1488-4
RESEARCH ARTICLE

Microstimulation of monkey dorsolateral prefrontal cortex impairs antisaccade performance

Stephen P. Wegener · Kevin Johnston · Stefan Everling

Received: 15 April 2008 / Accepted: 2 July 2008 / Published online: 19 July 2008© Springer-Verlag 2008

Abstract The dorsolateral prefrontal cortex (DLPFC) hasbeen implicated in various cognitive functions, includingresponse suppression. This function is frequently probedwith the antisaccade task, which requires suppression of theautomatic tendency to look toward a Xashed peripheralstimulus (prosaccade), and instead generate a voluntarysaccade to the mirror location. To test whether activity inthe DLPFC is causally linked to antisaccade performance,we applied electrical microstimulation to sites in theDLPFC of two monkeys, while they performed randomlyinterleaved pro- and antisaccade trials. Microstimulationresulted in signiWcantly longer saccadic reaction times foripsilaterally directed prosaccades and antisaccades, andincreased the error rate on ipsilateral antisaccade trials.These Wndings provide causal evidence that activity in theDLPFC inXuences saccadic eye movements.

Keywords Antisaccade · Inhibition · Prefrontal cortex · Saccade · Attention

Introduction

Top-down control in both humans and monkeys has beenextensively investigated using the antisaccade task (Hallett1978; Everling and Fischer 1998; Munoz and Everling2004). Correct performance of this task requires that the

subject suppress the automatic tendency to look toward aXashed peripheral stimulus (prosaccade), and instead usethe stimulus information to program and execute a saccadeto the diametrically opposite location. The antisaccade tasktests the subject’s ability to suppress a congruent in favor ofan incongruent stimulus-response mapping, and as such,provides an ideal paradigm to investigate executivefunction.

Evidence for a role of the DLPFC in the performanceof the antisaccade task has come from lesion (Pierrot-Deseilligny et al. 1991; Walker et al. 1998; Gaymard et al.2003; Ploner et al. 2005) and functional imaging studies inhumans (Sweeney et al. 1996; Doricchi et al. 1997; McDowellet al. 2002; Curtis and D’Esposito 2003; Desouza et al.2003; Ford et al. 2005; Brown et al. 2007), as well as singleneuron recordings in non-human primates (Funahashi et al.1993; Everling and Desouza 2005; Johnston and Everling2006b; Johnston and Everling 2006a). Recently, werecorded from DLPFC neurons that project directly to thesuperior colliculus (SC) (Johnston and Everling 2006a).These corticotectal neurons demonstrated an increasedlevel of pre-stimulus activity, and higher contralateral stim-ulus-related responses, on antisaccade trials in comparisonto prosaccade trials. We also found that corticotectal pre-stimulus activity and saccadic reaction times werenegatively correlated for antisaccade trials. Based on theseWndings, we proposed that the DLPFC provides top-downsignals to saccade-related brain areas that suppress contra-lateral automatic saccades. Support for such a conceptuali-zation is provided by the observation that patients withDLPFC lesions make performance errors in the antisaccadetask consistent with a failure of automatic saccade inhibi-tion (Pierrot-Deseilligny et al. 1991; Walker et al. 1998;Gaymard et al. 2003; Ploner et al. 2005). However, a directcausal relationship between DLPFC activity and antisaccade

S. P. Wegener · K. Johnston · S. EverlingDepartment of Physiology and Pharmacology, University of Western Ontario, London, ON, Canada

S. Everling (&)The Centre for Brain and Mind, Robarts Research Institute, 100 Perth Drive, London, ON N6A 5K8, Canadae-mail: [email protected]

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performance remains to be established, as the notion thatthe DLPFC is involved in suppression of automatic sac-cades was based on diVerences in activity between pro andantisaccades and on a correlation between corticotectal pre-stimulus activity and antisaccade reaction time (Johnstonand Everling 2006a). In fact, a recent neurophysiologicalstudy that employed chemical deactivation of the prefrontalcortex in monkeys found eVects on anti-saccades for ven-trolateral but not for dorsolateral muscimol injections(Condy et al. 2007).

To further investigate the role of the DLPFC in antisaccadetask performance, we applied electrical microstimulation tothe DLPFC of two monkeys while they performed a taskcomposed of randomly interleaved pro and antisaccade trials.Based on our previous Wndings (Johnston and Everling2006a), we hypothesized that microstimulation of the DLPFCat the time of stimulus presentation would enhance suppres-sion of reXexive saccades, and therefore improve the animalsperformance for antisaccades when the stimulus appears onthe contralateral side (i.e., ipsilateral antisaccades).

Methods

Two male rhesus monkeys (Macaca mulatta) weighing 8and 11 kg were subjects in the present experiment. Allexperimental methods described were performed in accor-dance with the guidelines of the Canadian Council on Ani-mal Care policy on the care and use of experimentalanimals, and an ethics protocol approved by the AnimalUsers Subcommittee of the University of Western OntarioCouncil on Animal Care.

Details of the surgical procedures have been describedpreviously (Desouza and Everling 2004). BrieXy, bothmonkeys were prepared for chronic electrophysiologicalexperiments by undergoing a surgical procedure to place animplant, containing both a recording chamber and a headpost, on the skull. The recording chamber was placed over a19 mm diameter craniotomy located above the principalsulcus of the right hemisphere. A preformed eye coil wasimplanted into one eye behind the conjunctiva (Judge et al.1980). The correct placement of the recording chamber wasveriWed after surgery by a high-resolution T2-weightedmagnetic resonance image (MRI) obtained in a 4 TMR-scanner.

Behavioral task

Monkeys performed an experimental task of randomlyinterleaved prosaccade and antisaccade trials (Everlinget al. 1999). Prosaccade trials required the monkeys to looktowards a Xashed peripheral stimulus, and antisaccade trialsrequired the monkeys to look away from the stimulus in a

direction opposite the stimulus (Fig. 1a). Each trial beganwith the presentation of a white Wxation point at the centerof a 21-inch CRT monitor at a viewing distance of 42 cm infront of the animals. Monkeys were required to Wxate onthis white center point within a 3 £ 3° window for a periodof 500 ms. Following this, the central Wxation pointchanged color, either to red or green, to provide the taskinstruction. For Monkey W a red Wxation point instructed aprosaccade, and a green Wxation point instructed an antisac-cade. The color of the instructional cues was reversed forMonkey R. Monkeys were required to continue Wxation onthe colored Wxation point within a 3° £ 3° window for aperiod of 500 ms, after which the Wxation point disap-peared. A 200 ms gap period followed Wxation point oVsetin which the monkey was required to maintain central Wxa-tion. The 200-ms gap was included because it is known todecrease the Wxation-related activity and increase the sac-cade-related activity in the superior colliculus (Dorris et al.1997; Everling et al. 1999), resulting in a larger proportionof erroneous prosaccade responses on antisaccade trials(Fischer and Weber 1992; Forbes and Klein 1996; Bellet al. 2000). After the gap period, a 0.15° visual stimuluswas presented pseudorandomly with equal probabilityeither 8° to the left or right of Wxation along the horizontalplane. The visual stimulus was red for Monkey W andgreen for Monkey R. Saccade endpoints were required tofall within a 8° £ 6° window centered at the saccade targetlocation. This large target window was necessary becausemonkey R generated very hypometric antisaccades towardthe right side (<5°, see Table 1). The monkeys received ajuice reward if their behavior met the following conditions:maintained central Wxation for the Wxation (1,000 ms) andgap periods (200 ms); generated a saccade in the correctdirection into the target window within 500 ms; and main-tained eccentric Wxation for at least 100 ms. Stimulus pre-sentation, administration of the behavioral paradigm,control of the microstimulator, and reward delivery werecontrolled by a CORTEX data acquisition system. Horizon-tal and vertical eye movements were recorded at 1,000 Hzusing a magnetic search coil technique (David NorthmoreInstitute, Newark, DE, USA). Eye positions and trial eventswere stored with the Plexon MAP system (Plexon Inc.,Dallas, TX, USA) and analyzed both on-line and oV-lineusing custom-designed software running in Matlab 7.3 (TheMathworks Inc., Natick, MA, USA).

Stimulation

In each session, a single commercially available dura-punc-turing tungsten microelectrode (UEWLGDSMNN1E, FHCInc., Bowdoinham, ME, USA) was driven into the dorsalbank of the principal sulcus using either a computer-con-trolled microdrive (NAN; Plexon Inc., Dallas, TX, USA) or

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manually controlled hydraulic microdrive (Narashige, TypeMO-95, Tokyo, Japan) while multiunit extracellular activ-ity was monitored using the Plexon MAP system (Plexon

Inc., Dallas, TX, USA). Once strong single- or multi-unitactivity was observed and stable recordings were obtained,the electrode was disconnected from the recording systemand connected to a microstimulator with a photoelectricstimulus isolation unit (Grass S88 with PSIU6 photoelectricstimulus isolation unit, Astro-Med, RI, USA) and the para-digm commenced. Microstimulation was applied randomlyon 50% of the trials and consisted of a 200 ms pulse train(0.3 ms biphasic pulses, 200 Hz, 50 �A) that began coinci-dent with the time of peripheral stimulus presentation(Fig. 1b). The stimulation parameters were chosen based onseveral initial sessions in which modiWcations were made toduration (range 50–200 ms), frequency (range 100–300 Hz)and current (range 50–200 �A) in order to determinethreshold parameters for obtaining behavioral eVects. Theseparameters are similar to those recently used for obtainedbehavioral eVects in other frontal cortical areas (presupple-mentary motor area 0.2 ms, 200 Hz, 60–80 �A (Isoda andHikosaka 2007); supplementary eye Weld: 0.2 ms, 250 Hz,50–120 �A area (Histed and Miller 2006)). Microstimula-tion currents of up to 200 �A did not evoke saccades at anyof the stimulation sites (200 ms, 300 Hz, 0.3 ms biphasicpulses). The eVects of microstimulation on saccadic reac-tion times (SRTs) of pro- and anti-saccades were monitoredon-line during the stimulation session. If no eVect on SRTswas observed after 22 trials per condition (t-tests, P > 0.05for all conditions) at this stimulation site, the electrode wasmoved by at least 500 �m and a second data Wle was col-lected for a second stimulation site (same penetration, butdiVerent depth). Data from all sites (those with and withouton-line signiWcant eVects) were included in the data analy-sis. We later realized that many of the SRTs had non-nor-mal distributions and we therefore replaced the parametrict-test with non-parametric tests in the oV-line data analysis.Monkeys performed between 22 and 65 trials per condition(176–520 trials).

Fig. 1 Behavioral task and timing of microstimulation. a Monkeyswere required to generate either a saccade to a peripheral stimulus (pro-saccade) or to its diametrically opposite location (antisaccade) depend-ing on Wxation point coloring, either red or green. The paradigmincluded a gap period, i.e., the Wxation point extinguished 200 ms priorto peripheral stimulus onset. b Electrical microstimulation (200 ms,200 Hz, 0.3 ms biphasic pulses, 50 �A) was applied at the time ofperipheral stimulus presentation. Eh horizontal eye trace; Stim appliedmicrostimulation; VS peripheral target; FP Wxation point

Table 1 Percentage of exclud-ed trials (n = 56)

Control Microstimulation P value(Wilcoxon)

Contralateral prosaccades

Anticipation (SRT < 80 ms) 2.4 § 0.59 2.4 § 0.62 0.94

No response (SRT > 500 ms) 0.0 § 0.0 0.1 § 0.51 1.00

Ipsilateral prosaccades



Contralateral antisaccades



Ipsilateral antisaccades

Anticipations (SRT < 80 ms) 4.0 § 0.67 4.2 § 0.74 0.63


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Data analysis

In an oV-line analysis, saccade onsets were automaticallyidentiWed by a custom-written computer program usingMATLAB 7.3 (The Mathworks Inc., Natick, MA, USA).

Saccade onset was deWned as the time when horizontaleye velocity Wrst exceeded 30°/s following stimulus presen-tation, while saccade oVset was identiWed as the time, aftersaccade onset, when horizontal eye velocity fell below 30°/s.Trials were visually inspected to ensure accurate markingof saccade onset and oVset. Trials with saccadic reactiontimes (SRTs) less than 80 ms or greater than 500 ms wereexcluded as anticipations or no-response trials, respectively(Table 1). Trials with incorrect or broken Wxation were alsoexcluded.

For each stimulation site, non-parametric Wilcoxon ranksum tests were used to compare SRTs between stimulationand control trials for each of the four task conditions (leftprosaccades, right prosaccades, left antisaccades, right anti-saccades). A site was deWned as signiWcant if the P value ofthis test reached <0.05 for any condition at that site. Foreach stimulation site, error rate was calculated for each taskcondition by taking the number of direction errors dividedby the total number of trials performed (correct and direc-tion error trials) multiplied by 100. Across stimulation sites,mean § standard error of the mean SRTs and error ratewere computed for stimulation and control trials in the fourtask conditions from the mean SRTs and error rate, respec-tively, obtained at each site. Statistical comparisons acrossstimulation sites for SRTs and error rate were conductedusing Wilcoxon sign rank tests evaluated at P < 0.05.

Although we focus on SRTs and error rates in this study,we also computed saccade durations, peak saccade veloci-ties, and horizontal saccade amplitudes for microstimula-tion and control trials for the four task conditions. Weperformed this analysis analogous to the analysis of SRTs

and error rates by Wrst calculating the means of theseparameters for each microstimulation site and then by com-paring the means of microstimulation and control trialsfrom these sites with non-parametric Wilcoxon rank sumtests across stimulation sites for each condition separatelyfor the two animals.

Results

We applied stimulation at a total of 56 DLPFC sites (34 inMonkey W; 22 in Monkey R; Fig. 2), of which 57.1% weredeWned as signiWcant sites (32 total; 18 (53%) from Mon-key W; 14 (64%) from Monkey R).

Saccadic reaction times

SRTs for all 56 stimulation sites are depicted in Fig. 3.Across all stimulation sites, the mean SRT for contralateralprosaccades was 150.6 § 2.8 ms on microstimulation trialsand 154.1 § 3.1 ms on control trials (Fig. 3a, top panel).Although this diVerence was signiWcant, closer inspectionshowed that this eVect was carried by monkey W, in which9 sites (Fig. 3a, top panel, Wlled circles) displayed signiW-cantly shorter SRTs with microstimulation. In contrast, wedid not obtain any signiWcantly shorter SRT eVects withmicrostimulation in monkey R sites (Fig. 3a, top panel,squares). In fact, at one site, microstimulation prolongedSRTs for contralateral prosaccades (Wlled square). Overall,the eVects for contralateral prosaccades are inconclusiveand prevent us from drawing any Wrm conclusions.

The eVects of microstimulation on SRTs were moreclear for ipsilateral prosaccades. Across all 56 stimulationsites, the SRT for ipsilateral prosaccade trials was signiW-cantly longer on microstimulation trials than on control trials(164.3 § 3.9 vs. 158.9 § 3.5 ms, respectively) (P < 0.005,

Fig. 2 Stimulation locations. Stimulation locations in Mon-keys W (top) and R (bottom), reconstructed from MRI ana-tomical images. Slices are sepa-rated by 1 mm. White circles represent stimulation locations showing no signiWcant change in ipsilateral antisaccadic reaction time with DLPFC stimulation (P > 0.05, n = 38), and black cir-cles those locations which dem-onstrated a signiWcant stimulated change in ipsilateral antisaccadic reaction time (P < 0.05; n = 18). PS principal sulcus; iAS inferior arcuate sulcus; A Anterior; P posterior; M medial; L lateral

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Fig. 3a, bottom panel). This eVect was also consistentacross all eight signiWcant sites (four from Monkey R; fourfrom Monkey W). When mean SRTs were determined fromonly the signiWcant sites, monkey W demonstrated anincrease from 136.0 § 2.9 ms on control trials to161.3 § 4.2 ms on microstimulation trials (P < 0.001).Similarly, monkey R showed an increase in mean SRTfrom 179.0 § 3.5 ms on control trials to 204.3 § 3.6 ms onmicrostimulation trials.

Next, we investigated the SRT for contralateral antisac-cades (i.e., stimulus on the ipsilateral side). We found thatthe mean SRT during stimulation trials (176.9 § 4.1 ms)did not signiWcantly diVer from that during control trials(177.1 § 3.7 ms; p > 0.90; Figure 3b, top panel).

For ipsilateral antisaccade trials (i.e., stimulus on the con-tralateral side), DLPFC microstimulation resulted in a sig-niWcant increase in SRT (microstimulation: 188.9 § 5.4 ms;control: 176.8 § 4.3 ms; P < 0.001; Fig. 3b, bottom panel).SigniWcant diVerences between microstimulation and con-trol trials were found for 18 sites (10 from Monkey R; 8from Monkey W). Microstimulation at each of these sitesprolonged the SRTs of ipsilateral antisaccades. In monkeyW, SRTs increased from 157.1 § 1.8 ms on control trials to

177.4 § 2.2 ms on microstimulation trials. Similarly, inmonkey R, SRTs increased from 210.6 § 2.3 ms to237.9 § 2.6 ms on microstimulation trials.

These Wndings demonstrate that microstimulation of theright DLPFC increases SRTs for ipsilateral prosaccadesand antisaccades. This eVect was stronger for antisaccadesthan prosaccades when all 56 sites from both monkeys werecompared (P < 0.01; Wilcoxon Sign Rank test) but onlymonkey R showed a signiWcant diVerence (P < 0.05;Wilcoxon Sign Rank test).

Figure 4 shows the cumulative distribution of SRTsobtained from the two monkeys for the contralateral anti-saccade condition during the experiments in which weobtained signiWcant eVects between microstimulation andcontrol trials in this task condition. Correct antisaccades onmicrostimulation trials (thick dashed lines) had longerSRTs that correct antisaccades on control trials (thick solidlines). Direction errors (thin lines), i.e., prosaccadestowards the stimulus, had shorter SRTs than correct anti-saccades (thick lines). For these sites, it also appears thatdirection errors had shorter SRTs on microstimulation trialsthan on control trials, although the low number of directionerrors prevented us from further analyzing this eVect.

Fig. 3 EVect of microstimula-tion on saccade reaction times (SRTs). Mean SRT for each DLPFC stimulation site in the control condition is plotted against mean SRT in the stimu-lation condition. Circles repre-sent data from monkey W (n = 34); squares represent data from monkey R (n = 22). Filled symbols indicate signiWcant sites. n(sig) number of signiW-cant stimulation sites; dashed line is the unity line (slope = 1). Ipsilateral and contralateral refers to the direction of the saccade. a Prosaccades; b antisaccades

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Error rates

We next investigated whether microstimulation changedthe animals’ performance in the four task conditions.Figure 5 shows error rates for monkey W (circles) andmonkey R (squares) for all 56 stimulation sites. For boththe contralateral prosaccade condition and the ipsilateralprosaccade condition, we did not Wnd any eVects of micr-ostimulation on the rate of direction errors across all sites(contralateral prosaccades: microstimulation: 2.5 § 0.5%;control: 3.5 § 0.6%; P > 0.2; Fig. 5a top panel; ipsilateralprosaccades: microstimulation: 3.4 § 0.0%; control:3.1 § 0.7%; P > 0.8; Fig. 5b bottom panel). These resultswere similar for both monkeys (monkey R: contralateralprosaccades: microstimulation: 2.3 § 0.6%; control:4.3 § 1.1%; P = 0.15; ipsilateral prosaccades: microstimu-lation: 5.7 § 2.2%; control: 5.5 § 1.5%; P = 0.64; monkeyW: contralateral prosaccades: microstimulation: 2.6 §0.7%; control: 2.9 § 0.6%; P = 0.73; ipsilateral prosac-cades: microstimulation: 1.8 § 0.5%; control: 1.5 § 1.5%;P = 0.68). These Wndings show that the number of directionerrors on prosaccade trials (i.e., antisaccades) did notchange between microstimulation and control trials.

When we compared error rates for microstimulation tri-als with control trials in the antisaccade conditions, wefound no signiWcant diVerence in error rates between themicrostimulation (4.8 § 0.7%) and control trials(6.1 § 0.7%) for the contralateral antisaccade condition(P > 0.15; Fig. 5b top panel). This Wnding was similar forboth monkeys (monkey R: microstimulation: 4.4 § 0.6%;control: 6.8 § 1.1%; P = 0.06; monkey W: microstimula-tion: 5.1 § 1.0%; control: 5.6 § 0.8%; P = 0.65).

For the ipsilateral antisaccade condition, microstimula-tion trials had a higher error rate than control trials across all

stimulation sites (9.4 § 1.3 and 5.9 § 0.9%, respectively;P < 0.005; Fig. 5b bottom panel). This eVect was strongerin monkey R (monkey R: microstimulation: 14.7 § 2.5%;control: 9.3 § 1.9%; P < 0.01; monkey W: microstimula-tion: 6.0 § 1.2%; control: 3.8 § 0.7%; P = 0.14).

Taken together, microstimulation increased the numberof direction errors only for the ipsilateral antisaccade condi-tion, i.e., when the stimulus was presented on the contralat-eral side.

Comparing ipsilateral antisaccade reaction times and error rates

DLPFC microstimulation aVected both SRTs and error ratefor contralateral stimulus presentations in the antisaccadetask. In order to identify whether these two eVects were cor-related, we plotted the SRT percent change [((stimulation/control) £ 100) ¡ 100] against the percent change in errorrate (stimulation minus control) (Fig. 6). Thus, every stimu-lation site is represented (n = 56), and any point located inthe top right quadrant corresponds to a site in the DLPFCwhere microstimulation increased both SRTs and error rate.We found a positive correlation between the eVect of micr-ostimulation on SRTs and its eVect on error rate (r = 0.39,P < 0.005). We also found that signiWcantly more stimula-tion sites had diVerence values falling within the top rightquadrant of this plot using a �2 test (P < 0.001).

Additional saccade parameters

We also compared saccade durations, peak velocities, andhorizontal saccade amplitudes between microstimulationand control trials for the four task conditions. The results foreach monkey are presented in Table 2. The only consistent

Fig. 4 EVect of microstimula-tion on ipsilateral antisaccade trials. a Traces show cumulative saccadic reaction time (SRT) distributions for control trials (solid lines) and microstimula-tion trials (dashed lines) for monkey W. Thick lines indicate correct trials and thin lines indi-cate error trials. SRTs are pooled for all signiWcant stimulation sites (n = 18). b Same as a, but for monkey R (n = 14)

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Wnding for both monkeys was a reduction in the amplitudeof ipsilateral antisaccades on microstimulation trials.Although signiWcant (P < 0.01), the diVerence in amplitudebetween control and stimulation trials was only »0.25° forboth animals.

Discussion

We found that microstimulation of the sites in the dorsalbank of the right principal sulcus increased SRTs for ipsi-lateral prosaccades and antisaccades, and increased the pro-portion of erroneous prosaccades on antisaccade trialswhen the stimulus is presented on the side contralateral tothe stimulated hemisphere. These Wndings are inconsistentwith our initial hypothesis which predicted that microsti-mulation would improve ipsilateral antisaccade perfor-mance due to an enhancement of DLPFC-mediatedsuppression of reXexive saccades.

There is substantial evidence that electrical microstimu-lation in gray matter primarily excites axonal elementsand only to a small degree soma and dendrites at the tip

of microelectrode (e.g. Ranck 1975; Gustafsson andJankowska 1976; McIntyre and Grill 2000) It has furtherbeen shown that the initial response following a singlemicrostimulation pulse is excitatory which is then followedby an inhibition that lasts >100 ms (Butovas and Schwarz2003). This inhibition seems to be caused by synaptic inhi-bition mediated by GABAergic interneurons since it isreduced by blockage of GABAB receptors (Butovas et al.2006). Despite this inhibition, repetitive stimulation (testedat 20 and 40 Hz) still evokes repetitive excitation against abackground of inhibited activity (Butovas and Schwarz2003). The extent of the excited and inhibited areas areremarkable large. Butovas and Schwartz showed thatalthough the responses following microstimulation with8 �A in the somatosensory cortex of ketamine anaesthe-tized rats were largest for neurons around the tip of micro-electrode, eVects could in a few cases even be observed at adistance of about 2,000 �m (Butovas and Schwarz 2003).Based on these Wndings, electrical microstimulation in ourexperiment likely triggered action potentials to orthodromi-cally and antidromically connected brain areas but it alsolikely disrupted normal neural processing at the stimulation

Fig. 5 EVect of microstimula-tion on error rates. Error rates for each DLPFC stimulation site in the control condition is plotted against error rate in the stimula-tion condition. Circles represent data from monkey W (n = 34); squares represent data from monkey R (n = 22). Dashed line is the unity line (slope = 1). Ipsi-lateral and contralateral refers to the direction of the saccade. a Prosaccades; b antisaccades

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sites. The Wrst eVect is generally considered as the mecha-nism by which microstimulation evokes motor responses.For example, suprathreshold stimulation of the frontal eyeWelds has been attributed to the excitation of corticotectaland corticopontine axons (Bruce et al. 2004).

An example in which microstimulation might have dis-rupted normal processing in a cortical area is a recent studyby Churchland and Shenoy (2007). The authors used sub-threshold microstimulation in the premotor cortex whilemonkeys performed a delayed-reach task. They found anincrease in reaction times when microstimulation wasapplied around the time of the go signal. The authors inter-preted this eVect as a disruption of normal preparatoryactivity in premotor cortex. A disruptive microstimulationeVect of this nature is reminiscent of the inhibitory eVectsof single pulse transcranial magnetic stimulation onDLPFC function seen in human studies (Muri et al. 1996;Jahanshahi and Dirnberger 1999; Mull and Seyal 2001).

Two alternative explanations can therefore be proposedto account for our Wndings. The Wrst is that microstimula-tion disrupted neural processes in the DLPFC, and thus anyputative suppression signal that would be sent to saccade-related brain areas. The second is that microstimulationexerted a predominately excitatory eVect and evoked actionpotentials in pyramidal axons exiting the DLPFC. In thiscase, our data would be inconsistent with the hypothesis

that DLPFC neurons provide saccade suppression for con-tralateral saccades. Instead, the data could be interpreted inthe context of a more general role of the DLPFC in atten-tional selection as supported by single neuron recordings inmonkeys (Rainer et al. 1998; Hasegawa et al. 2000; Everlinget al. 2002; Desouza and Everling 2004; Lebedev et al.2004; Everling et al. 2006) and functional imaging studiesin humans performing spatial working memory or spatialattention tasks (Owen et al. 1996; Vandenberghe et al.1997; Rowe and Passingham 2001; Abe et al. 2007; Lukset al. 2007).

The idea that the DLPFC participates in the suppressionof the automatic prosaccade during the antisaccade taskwas initially proposed to account for the results of lesionstudies in human subjects. Although Guitton et al. (1985)hypothesized in their seminal investigation of antisaccadeperformance in patients with frontal lobe damage thatlesions of the frontal eye Welds are responsible for theincreased error rates, subsequent studies have pointedtowards a more circumscribed role of the DLPFC inresponse suppression in the antisaccade task (Pierrot-Deseilligny et al. 1991; Walker et al. 1998; Gaymard et al.2003; Ploner et al. 2005). The majority of studies havereported a bilateral increase in error rates in patients withDLPFC lesions with little impairment of reaction times. Incontrast, an increase in antisaccade reaction times has beenreported after isolated FEF lesions (Rivaud et al. 1994;Gaymard et al. 1999). In addition to these DLPFC lesionstudies, position emission tomography (Sweeney et al.1996; Doricchi et al. 1997) and fMRI studies (McDowellet al. 2002; Curtis and D’Esposito 2003; Desouza et al.2003; Ford et al. 2005; Brown et al. 2007) have also foundhigher DLPFC activations for antisaccade compared withprosaccade trials. Ford et al. (2005) observed a higher levelof preparatory activation before correct, compared withincorrect, antisaccade responses. This Wnding supports theidea that a certain activation level in the DLPFC is neces-sary for saccade suppression.

A possible role for the DLPFC in response suppressionhas also been suggested by single neuron recordings inmonkeys performing pro- and anti-saccades (Funahashiet al. 1993; Everling and Desouza 2005; Johnston andEverling 2006b; Johnston and Everling 2006a). Recently,we recorded from DLPFC neurons that project directly tothe SC (Johnston and Everling 2006a). We found that thesecorticotectal neurons had increased pre-stimulus activityand higher contralateral stimulus-related responses on anti-saccade trials compared with prosaccade trials. We hypoth-esized that the DLPFC excites either Wxation or inhibitoryinterneurons in the ipsilateral SC, which then suppress theactivity of saccade-related neurons.

The other explanation of our results is that microstimula-tion evoked an attentional or preparatory bias towards the

Fig. 6 Relationship between changes in saccadic reaction times(SRT) and error rates. The percent change in SRT is plotted against thecorresponding diVerence in error rate for each stimulation site (n = 56).Circles represent data from monkey W (n = 34); squares represent datafrom monkey R (n = 22). Filled symbols indicate signiWcant sites. Sol-id and dashed lines are best-Wt lines through the signiWcant site data formonkey W and monkey R, respectively

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contralateral visual Weld. A role of the DLPFC in atten-tional selection has been suggested by many neuropsycho-logical and functional imaging studies that employedspatial working memory or spatial attention tasks (Owenet al. 1996; Vandenberghe et al. 1997; Rowe and Passing-ham 2001; Peers et al. 2005; Abe et al. 2007; Luks et al.2007). Furthermore, single neuron recordings in monkeysfound that DLPFC neurons have increased responses forattended stimuli (Rainer et al. 1998; Everling et al. 2002;Everling et al. 2006), with a bias towards the contralateralWeld. In fact, Lebedev et al. (2004) demonstrated that sig-nals related to attentional selection are more prevalent inDLPFC neurons than those related to working memory,

long considered a key function of this area. While neuronsin the DLPFC appear to reXect attentional selection, themain frontal area that has been implicated in directingattention has been the frontal eye Weld (FEF) (Corbettaet al. 1993; Corbetta et al. 1998). Previously, Moore andArmstrong (2003) showed that microstimulation in the FEFincreases visual responses in neurons with correspondingresponse Welds in V4. Therefore, our observed eVects onSRTs and error rates may be mediated by projections fromthe DLPFC to the FEF (Pandya and Kuypers 1969), or byprojections from the DLPFC to posterior cortical areas(Goldman-Rakic 1988). Alternatively, projections from theDLPFC to the ipsilateral SC (Goldman and Nauta 1976;

Table 2 A summary of saccade metrics (duration, peak velocity, and horizontal amplitude for signiWcant stimulation sites (n = 32))

Control Microstimulation P value (Wilcoxon)

Monkey W (n = 18)


Duration (ms) 38.5 § 0.3 38.5 § 0.2 1.000

Peak velocity (°/s) 332.5 § 2.5 336.7 § 2.4 0.031*

Horizontal amplitude (°) 7.85 § 0.02 7.90 § 0.04 0.115


Duration (ms) 37.4 § 0.2 37.4 § 0.2 1.000

Peak velocity (°/s) 373.5 § 2.9 375.7 § 2.3 0.493



Duration (ms) 48.1 § 0.8 46.6 § 0.4 0.018*

Peak velocity (°/s) 290.1 § 3.3 294.6 § 3.4 0.142



Duration (ms) 40.9 § 0.5 39.5 § 0.5 0.009*

Peak velocity (°/s) 330.8 § 3.8 327.7 § 3.8 0.356

Horizontal amplitude (°) 8.28 § 0.07 8.02 § 0.09 0.007*

Monkey R (n = 14)


Duration (ms) 38.1 § 0.4 38.1 § 0.4 1.000

Peak velocity (°/s) 380.2 § 2.52 383.3 § 5.3 0.557



Duration (ms) 39.8 § 0.3 39.5 § 0.2 0.219

Peak velocity (°/s) 348.5 § 1.9 345.6 § 2.3 0.175



Duration (ms) 49.0 § 1.4 49.5 § 1.2 0.258

Peak velocity (°/s) 333.7 § 5.4 338.4 § 7.1 0.175



Duration (ms) 36.5 § 0.6 36.1 § 0.8 0.375

Peak velocity (°/s) 208.1 § 8.2 197.4 § 9.0 0.007*

Horizontal amplitude (°) 4.79 § 0.24 4.53 § 0.27 0.004** P < 0.05

123

472 Exp Brain Res (2008) 190:463–473

Leichnetz et al. 1981; Johnston and Everling 2006a) mightbias preparatory activity for contralateral saccades. Aninduced attentional bias by DLPFC microstimulationtowards the contralateral Weld could explain the delay ofipsilateral prosaccades and antisaccades. In addition, anincrease in contralateral attention could also account for theincrease in direction errors toward the contralateral side,especially if we assume that microstimulation increased theactivity of saccade neurons in the ipsilateral SC. Anincreased activity of these neurons at the time of the arrivalof the visual signal in the SC (»50–70 ms following stimu-lus onset) would have made it more likely that the stimulus-related burst reached the saccade threshold and triggered ashort-latency prosaccade. However, this model would alsopredict faster contralateral saccades. Although we foundsigniWcantly shorter SRTs for contralateral prosaccades inmonkey W, monkey R did not show any diVerences. Itshould be noted here that monkey R already had very shortSRTs for contralateral (leftward) prosaccades in the controlcondition so that microstimulation may not have been ableto reduce SRTs any further. For contralateral antisaccades,we did not Wnd signiWcant reductions in SRTs in eithermonkey. A possible explanation is that the beneWt of a con-tralateral attentional bias was ineVective in this conditionbecause antisaccades may initially require a shift of atten-tion to the stimulus, i.e., in this condition toward the ipsilat-eral side.

In contrast to eVects on SRTs, we found only small, andmostly inconsistent diVerences for saccade duration, veloc-ity, and amplitude between microstimulation and controltrials for prosaccades. This is consistent with inactivationstudies of the lateral prefrontal cortex, which also did notobserve eVects on saccade velocities and saccade ampli-tudes (Sawaguchi and Iba 2001; Kuwajima and Sawaguchi2007). However, we found a consistent and signiWcantreduction in the amplitudes of ipsilateral antisaccades onmicrostimulation trials. We hypothesize that the reductionin antisaccade amplitude was a result of a residual stimulusactivity in the ipsilateral SC, which entered into the compu-tation of the saccade amplitude. Such a residual activitycould be the result either of an insuYcient suppression oran increased activation.

Although we cannot be certain about the neural mecha-nisms that underlie our observed Wndings, they show thatmicrostimulation of the right DLPFC in nonhuman primatesprolongs ipsilateral prosaccades and antisaccades and leadsto more direction errors on antisaccade trials in this condi-tion. These results provide causal evidence that the DLPFCinXuences saccadic eye movements in non-human primates.

Acknowledgments This research was supported by grants from theCanadian Institutes of Health Research (CIHR) and the Natural Sci-ences and Engineering Research Council of Canada (NSERC).

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