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ANIMAL BEHAVIOUR, 2005, 69, 1093–1100doi:10.1016/j.anbehav.2004.09.009

Experience-induced modulation of the use of spatial

information in the domestic chick

RAFAEL FREIRE & LESLEY J. ROGERS

Centre for Neuroscience and Animal Behaviour, University of New England

(Received 10 May 2004; initial acceptance 3 July 2004;

final acceptance 18 September 2004; published online 17 February 2005; MS. number: 8121)

We examined whether experience of opaque or transparent screens modulates the use of spatial cues bydomestic chicks. From 10–12 days of age, chicks could lose sight of an imprinting stimulus or theircagemate in cages with opaque screens, but were largely unable to do so in cages with transparent screens.Chicks were then trained to find the imprinting stimulus behind one of two screens. Unrewarded probetests were presented to chicks with the apparatus rotated by 180 � so that proximal (intra-apparatus) anddistal (extra-apparatus) cues indicated opposite sides of the apparatus. In tests with two distinctly differentscreens, chicks using the left eye (LES) chose the distal side more than the proximal side, whereas chicksusing the right eye (RES) chose the proximal side more than the distal side. When using both eyes (BIN) intests with different screens, chicks reared in cages with opaque screens were quicker in making a choice,and tended to choose the screen on the distal side more often than chicks reared in cages with transparentscreens. Chicks reared in cages with opaque screens failed to make a choice less often than chicks reared incages with transparent screens in BIN tests with different screens and in monocular tests with identicalscreens. The results suggest that chicks using the LES are less easily disrupted by conflicting spatial cuesthan chicks using the RES, and that experience of visual barriers from 10–12 days of age improves thechick’s ability to attend to distal cues.

� 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Changes to the environment early in life, such as theaddition of nesting material and other objects to cages,have been found to improve spatial memory in rats, Rattusnorvegicus (Pham et al. 1999) and mice, Mus musculus(Williams et al. 2001). Providing domestic chicks, Gallusgallus domesticus, with the opportunity to move aroundvisual barriers between days 8 and 12 also improves spatialmemory, as assessed by visual displacement and detourtests, compared with chicks provided with similar-sizedtransparent barriers (Freire et al. 2004). Visual barriersallow chickens to move out of sight of conspecifics, anaction that shows a peak at around 11 days of age inseminatural environments (Vallortigara et al. 1997) andappears to be one of the factors that contributes to theapparent sensitive period for effective spatial learningin chickens around this age (Rashid & Andrew 1989;Andrew 1991; Dharmaretnam & Andrew 1994). Presently,it therefore seems likely that environment-induced

Correspondence: R. Freire, Centre for Neuroscience and AnimalBehaviour, School of Biological, Biomedical and Molecular Sciences,University of New England, Armidale, NSW 2351, Australia (email:rfreire2@une.edu.au).

103–3472/04/$30.00/0 � 2004 The Association for the S

modulation of spatial cognition could be a reasonablycommon phenomenon in vertebrates.As in mammals, the right hemisphere in birds appears

to play a key role in the processing of spatial information(Vallortigara & Regolin 2002). In birds, hemisphericspecialization can be shown by monocular testing, sincethe complete decussation at the optic chiasma allowsinformation from each eye to be processed largely by thecontralateral hemisphere (Rogers 1995). Rashid & Andrew(1989) trained domestic chicks to find food in one cornerof a square tray with each wall painted differently andthen examined the search patterns when the chicks werepresented with the tray rotated by 180 �, so that proximal(intratray) cues indicated one corner and distal (extratray)cues indicated the opposite corner. Using monoculartesting, they showed that at certain ages chicks usingthe left-eye system (LES, and so the right hemisphere)made more use of distal (extra-apparatus) cues than chicksusing the right-eye system (RES). With domestic chickstrained to find food in the centre of an arena, Tommasi &Vallortigara (2001) found that, when they were tested ina larger arena, chicks using the LES searched in the centre,whereas chicks using the RES searched on the basis ofabsolute distance from the walls. Similarly, marsh tits,

93tudy of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

ANIMAL BEHAVIOUR, 69, 51094

Parus palustris, using monocular retrieval of cached fooditems use relative spatial position when using the LES butonly local cues when using the RES (Clayton & Krebs1994). Hence, monocular testing of birds in spatial taskshas proved to be a useful technique for identifying the roleof each hemisphere in spatial processing.In addition, lesioning of the hippocampus in birds

impairs spatial memory but has little effect on othercognitive tasks (Hampton & Shettleworth 1996; Fremouwet al. 1997), and spatial memory in lesioned zebra finches,Taeniopygia guttata, can be improved by transplantation ofembryonic tissue into the hippocampus (Patel et al. 1997).In domestic chicks, dendrite length and linear spinedensity are greater in the right than in the left hippocam-pus (Freire & Cheng 2004), and lesioning of the righthippocampus, but not the left hippocampus or a shamoperation, impairs relocation of hidden food using relativepositional information (Tommasi et al. 2003). Thus, thechicken brain is lateralized in its processing of spatialinformation: the right hippocampus processes distal andgeometric cues but the left hippocampus does not.Evidence that experience during development may

modulate the above lateralization in spatial cognitionand hippocampal function has, however, proved to beelusive. Freire & Cheng (2004) found no evidence thatrearing with visual barriers, which are known to improvespatial memory, influenced asymmetry in the length ofdendrites and linear spine density in the multipolarprojection neurons of the hippocampus of domesticchicks. In contrast, exposing rats to a novel environmentearly in life (which is known to improve spatial memory;Tang 2001) produced a long-lasting right-shift in hippo-campal volume (Verstynen et al. 2001).Lateralization in brain structure and certain visual

functions in the chicken are susceptible to environmentalmodulation at certain ages, and even a brief exposure tolight during the later part of incubation leads to asymme-try in the visual projections (Koshiba et al. 2003). Aschickens have several shifts in hemispheric dominance inthe first few weeks of life (summarized in Rogers 1995), itmay be that different types of experiences influence thedevelopment of brain and behaviour during periods whena particular hemisphere is most active. If so, for thereasons presented above, we would predict that experi-ence with spatial cues at around days 10 and 11 of agemight have an effect on the right hemisphere and hencespatial ability using the LES later in life.We used a variation of the rotated floor test (RFT, Rashid

& Andrew 1989) to test the hypothesis that experience ofvisual barriers from days 10–12 modulates the chick’s useof distal and proximal cues. One modification requiredthe chicks to locate an imprinting stimulus hidden in oneof two locations at opposite ends of a rectangular appa-ratus. Testing involved rotating the apparatus by 180 � sothat proximal (intra-apparatus) and distal (extra-appara-tus) cues now indicated opposite sides, and thus provideda means of assessing the choice made by the chick. Wemanipulated experience before testing by giving thechicks experience of visual barriers or transparent barriersbetween 10 and 12 days of age. By using monoculartesting in addition to binocular, we examined whether

this manipulation of experience could generate theknown asymmetry in performance in the RFT (Rashid &Andrew 1989). We predicted that chicks reared withvisual barriers would pay more attention to distal cuesthan proximal cues in the RFT, when tested using the lefteye.

METHODS

We used 64 broiler chicks (Kootingal Hatchery, Kootingal,NSW, Australia) obtained as fertile eggs at 18 days ofincubation and hatched in two replicates of 32. Chickswere reared in isolation from about 2 h after hatching to 7days of age in grey sheet metal cages (25 ! 25 cm and30 cm high). A yellow tennis ball was suspended by string10 cm above the floor in the centre of the cage to providean imprinting stimulus (broiler chicks are known toimprint on tennis balls, Freire & Nicol 1999). Temperaturewas maintained at 35 �C for the first week after hatchingand lighting from halogen strips was provided on 12:12 hlight:dark cycle. The floor of the cage was lined with whitepaper and sprinkled with chick starter crumbs that wereperiodically tapped with a round dowel (as recommendedin Hawkins et al. 2001) to encourage pecking and eatingwhich can be a concern in isolation-housed chicks. Nochicks died in the first week and they showed no signs ofdistress. Water was available ad libitum from a clearPerspex petri dish for the first 3 days and for the remainderof the time from a bird drinker placed outside the cagewith the cup on the floor. In the first few days chicks weresexed by inspection of the wing feathers, as advised by thehatchery.

When the chicks were 8 days old, we paired them (onemale and one female as far as possible), marked the backfeathers of one chick with a black marker pen and placedeach pair into a cardboard box (50 ! 50 cm and 60 cmhigh). Pairing at this age was undertaken to uphold thewelfare of the chicks by allowing them to express socialbehaviour, as well as allowing chicks to observe a socialcompanion disappear from sight. Chicks were observedregularly during this time to ensure that fighting did notbreak out and that welfare was not compromised: fightingwas not observed nor were there any other signs thatpairing at this age caused distress. There was no indicationthat the marking placed on the back of one of the chicksattracted pecking or otherwise compromised welfare inany way. A yellow tennis ball was suspended by a string inthe centre of the pen, 10 cm above the floor. The floor wascovered with wood shavings and food (starter crumbs) andwater (from an externally placed drinker) were available adlibitum. From 8 days onwards, the temperature was 30 �Cand the lighting schedule remained unchanged. At lights-on on day 10, we added two screens (20 ! 20 cm and30 cm high) to each box centrally 10 cm either side of theimprinting stimulus. Opaque screens made of wood(0.5 cm thick) and painted grey were added to eight boxes(Treatment O). Transparent screens made from 0.3-cm-thick acrylic sheet were added to the other eight boxes(Treatment T). All the screens were removed just beforelights-off on day 12 after hatching.

FREIRE & ROGERS: SPATIAL MEMORY IN CHICKENS 1095

At 13 and 14 days of age the chicks were trained andtested in a rotated floor test. The apparatus (Fig. 1)consisted of a rectangular pen (160 ! 65 cm and 60 cmhigh) made from cardboard sheeting and with woodshavings on the floor. Parallel to, and 30 cm from, theshorter sides were two screens (20 ! 20 cm and 30 cmhigh) made from wood. Half the chicks were trained andtested with both screens covered in white plastic paper,making them visually similar (below we refer to them asidentical). We made the apparatus as uniform as possible,and hence in tests with identical screens there may nothave been any obvious proximal cues discernible to thechicks. The remainder of the chicks were trained andtested with one screen covered in white plastic paper andthe other in a floral plastic wallpaper (mainly yellow, redand green on a white background), making them visuallydissimilar (different).All treatments and procedures were approved by the

University of New England’s Animal Ethics Committee.After the tests were completed, the chicks were given awayto a local grower of broiler chickens where they were keptuntil they reached slaughter weight.

Training Phase

Each chick was trained to locate the imprinting stimulus(yellow ball) behind one of the screens in the testapparatus. The type of screen and its relative position tothe chick (i.e. on the left or right) was balanced (as far aspossible) for rearing treatment and sex. Training involvedplacing a chick in the centre of the apparatus next to theball. We moved the ball slowly behind one screen and thechick usually followed it. If a chick did not follow the ball,

Screens

Start cage

(a) Start

(c) Start

Ball

(b) Release

(d) Release

Figure 1. Rotated floor test apparatus showing the two screens and

the arrangement at the start and at the point of release of the chick

in relocation trials (a and b, respectively) and probe tests (c and d,

respectively). In probe tests, the apparatus was rotated by 180 �. Thefigure shows training in one direction with different screens

(distinguished by solid and dotted lines) for ease of explanation,

although the direction of displacement of the ball and the screenbehind which the ball was hidden were, as far as possible, balanced

for rearing treatment and sex.

we gently pushed it with a board until it was next to theball. After the chick had remained with the ball for 15 s,we placed the chick in a circular start cage (20 cmdiameter, 30 cm high, made from wire) in the centre ofthe apparatus. We placed the ball next to the start cage,and when the chick was oriented towards it, moved theball away from the chick until it was behind a screen. Thechick was then released and allowed to search for the ball.After the chick approached to within 5 cm of the ball for15 s, or after 2 min, we picked up the chick (this wastermed a displacement trial). The chick was then replacedin the start cage and received another displacement trial.Once the chick had completed three displacement trials,we put it back in the home pen for 5–15 min.The chick was then placed into the apparatus again and

received two consecutive displacement trials. If a chickfailed to approach to within 5 cm of the ball within 2 minof release in both trials, it was deemed not to beimprinted, and was removed from the experiment (eightchicks were not imprinted). After the second displacementtrial, we placed the chick in the start cage again for 15 s.With the ball already behind the screen, the chick wasreleased and allowed to search for the ball (this was termeda relocation trial; Fig. 1a, b). After the chick had spent 15 swithin 5 cm of the ball, or after 1 min, it was returned tothe home pen for 5–15 min. Chicks that did not approachthe imprinting stimulus in the required time receiveda latency score of 1 min.The chick then received another relocation trial. If

a chick failed to approach the ball within 1 min, it wasplaced in the start cage and received a displacement trial,after which we returned it to the home pen for 5–15 min.The sequence of relocation trial followed by displacementtrial continued until the chick reached the criterion ofapproaching to within 5 cm of the ball within 15 s ofrelease. Once this criterion was reached we returned thechick to the home pen and began the probe testing phase.

Probe Testing Phase

The testing phase consisted of unrewarded probe tests(i.e. no imprinting stimuli were placed in the apparatus)with the apparatus rotated by 180 � (Fig. 1c, d) interspersedwith rewarded relocation trials and, if necessary, displace-ment trials, as described above. Each chick received threeprobe tests, one binocular (BIN), one using the left eye(LES) and one using the right eye (RES), determinedaccording to a semirandom sequence. We carried outmonocular testing by temporarily placing a conical pieceof tape (2 ! 2.5 cm) over one eye 10–15 min beforetesting. The cone was attached to the feathers aroundthe eye and did not press on the eye. Chicks initiallyattempted to remove the cone, but after about 7 min suchactivities were not observed and chicks showed normalbehaviour in the home pens. We carefully removed thepatches immediately after testing. The probe tests endedwhen a chick moved behind one screen, or after 3 min.Chicks that did not move behind a screen in the requiredtime received a latency score of 3 min.

ANIMAL BEHAVIOUR, 69, 51096

After a probe test, the chick was returned to the homepen for about 10 min. It was then returned to the startcage and presented with a relocation trial as describedabove (i.e. the apparatus and the ball were as in thetraining phase). When a chick approached to within 5 cmof the ball within 15 s of release, we returned it to thehome pen. If it failed to approach the ball within 1 min itwas placed in the start cage again and presented witha displacement trial, and then returned to the home pen.We repeated this procedure until the chick approached towithin 5 cm of the ball within 15 s of release (criterion)and could be presented with another probe test.Behaviour in the relocation trials and probe tests was

recorded by an overhead video camera (Kobi DSP colourCCD) and VCR (JVC, J695). We used a blind procedure torecord the side of the first screen that the chick walkedbehind and the latency to walk behind a screen byallocating each chick that was displayed on the videorecordings a random number from 1 to 64. Correspondingrearing treatment and sex were not disclosed until com-pletion of the behavioural observations, although the typeof screens used could be seen on the recordings.

Statistical Analysis

Four chicks from Treatment O (three males and onefemale) and four chicks from Treatment T (two males andtwo females) were deemed not to be imprinted on thetennis ball and were removed from the experiment. Thus,28 chicks from each rearing treatment were tested witheither identical (NZ 14) or different (NZ 14) screens.There were three possible outcomes to the probe tests:

(1) the chick walked behind the screen on the same side asthe imprinting stimulus was located during relocationtrials (indicating a preference to use distal, extra-apparatuscues), (2) the chick walked behind the opposite side(indicating a preference to use proximal, intra-apparatus)cues or (3) the chick failed to walk behind a screen within3 min (fail). In some cases, tabulation of the outcomeresulted in many cells with expected counts of less than 5,so we grouped the data in two meaningful ways foranalysis using a chi-square test (Sokal & Rohlf 1981). First,we examined the effect of screen type on the outcome ofthe probe tests by combining the two rearing treatments.Second, the effect of rearing treatment on outcome wasexamined by combining screen type. In addition, wecombined the number of chicks choosing the screenindicated by the proximal cues or failing to choose a screenif required.As the sex of the chicks had no effect on the number of

trials required to reach criterion during training (ANOVA:F1,48 Z 0, 0.98), the mean latency of training trials(F1,48 Z 0.1, 0.75) or the outcome or latency to choosea screen in the probe tests (F1,48 Z 0.35, 0.57), sex wasremoved from the analysis. Hence, we analysed responselatencies in the probe tests using an ANOVA, with rearingtreatment (Treatment O or T) and screen type (identical ordifferent) as the between-subject factors, and eye system(BIN, RES or LES) as the within-subject factor, followed byF tests when appropriate.

RESULTS

Relocation Trials

Chicks required one to four relocation trials to reachcriterion during the training phase. After the first probetrial, chicks again took one to four relocation trials toreach criterion. After the second probe trial, they took oneto three relocation trials to reach criterion. The number oftrials decreased as the experiment progressed, so thatfewer trials were required to reach criterion at the end oftesting than during the training phase (ANOVA:F2,104 Z 10.2, P ! 0.0001; Fig. 2), with no significantinteractions (phase)treatment interaction: F2,104 Z 1.6,0.29; phase)screen interaction: F2,104 Z 0.8, 0.51). Thenumber of trials required to reach criterion was notsignificantly influenced by the type of screen used (iden-tical or different screens: F1,52 Z 0.05, 0.079), rearingtreatment (F1,52 Z 0.15, 0.65) or their interaction (rear-ing)screen interaction: F1,52 Z 1.3, 0.25). Similarly, themean latency to approach the imprinting stimulus duringthe relocation trials decreased as the experiment pro-gressed (F2,104 Z 5.9, P ! 0.01; Fig. 3), with no significantinteractions (phase)treatment interaction: F2,104 Z 1.0,0.38; phase)screen interaction: F2,104 Z 1.9, 0.20). Again,the mean latency to approach the imprinting stimuluswas not significantly influenced by the type of screen used(F1,52 Z 0, 0.94), rearing treatment (F1,52 Z 0.04, 0.85) ortheir interaction (rearing)screen interaction: F1,52 Z 0.8,0.37).

Probe Tests

In general, chicks oriented on the basis of distal cuesmore and failed to make a choice less in probe tests withidentical screens than in probe tests with different screensin BIN (chi-square test: c1

2 Z 5.8, P! 0.01; Fig. 4), LES(c1

2 Z 3.6, P Z 0.057; Fig. 5) and RES tests (c22 Z 10.5,

P ! 0.01; Fig. 5). The latency to make a choice was shorter

11.21.41.61.8

22.22.42.62.8

3

Nu

mbe

r of

tri

als

1 2 3 1 2 3 1 2 3 1 2 3

Treatment O/ identical

screens

Treatment O/ different

screens

Treatment T/ identical

screens

Treatment T/ different

screens

Phase

Figure 2. Mean number of relocation trials required to reach

criterion during the training phase (phase 1), after the first probe test

(phase 2) and after the second probe test (phase 3) using identicaland different screens. Treatment O: opaque screens; Treatment T:

transparent screens.

097

in probe tests with identical screens than in probe testswith different screens (pooled meansG SEZ 74.9G 7.4 sand 105.6G 7.7 s, respectively; ANOVA: F1,52 Z 11.08,P! 0.01), suggesting that the use of different screens inthe probe tests impaired performance. No significantinteractions were found between the type of screen usedin probe tests and other factors (screen)eye system:F2,104 Z 0.48, 0.62; screen)treatment: F1,52 Z 1.59, 0.21;screen)eye system)treatment: F2,104 Z 0.54, 0.35). Thedata were further analysed to determine the effect ofrearing treatment on outcome and latency to makea choice in BIN, LES and RES probe tests.

Binocular probe testsIn probe tests with identical screens, Treatment O and T

chicks showed a preference to respond to distal cues(binomial test: P ! 0.0001 and P! 0.05, respectively;Fig. 4). In probe tests with two different screens, however,

5

10

15

20

25

30

35

1 2 3 1 2 3 1 2 3 1 2 3

Treatment O/ identical

screens

Treatment O/ different

screens

Treatment T/ identical

screens

Treatment T/ different

screens

Phase

Late

ncy

(s)

Figure 3. Mean latency to approach the imprinting stimulus in

relocation trials during the training phase (phase 1), after the firstprobe trial (phase 2) and after the second probe trial (phase 3) using

identical and different screens. Treatment O: opaque screens;

Treatment T: transparent screens.

0

2

4

6

8

10

12

14

16

Identicalscreens

Differentscreens

Identicalscreens

Differentscreens

Treatment O Treatment T

Rearing treatment

Nu

mbe

r of

ch

icks

Figure 4. Number of chicks choosing the screen indicated by the

distal (-) or proximal (,) cues, or failing to make a choice (G) inbinocular probe tests. Treatment O: opaque screens; Treatment T:

transparent screens. NZ 14 per group.

Treatment O chicks chose the screens indicated by thedistal cues more than Treatment T chicks which tended tochoose at random or failed to choose a screen (chi-squaretest: c1

2 Z 3.6, P Z 0.058; Fig. 4). In addition, there wasa significant interaction between eye system and rearingtreatment in response latency (ANOVA: F2,104 Z 6.1,P! 0.01; Fig. 6), indicating that Treatment O chicks werefaster at making a choice than Treatment T chicks (F test:F1,149 Z 14.3, P ! 0.001; Fig. 6).

Monocular probe testsIn general, chicks failed to choose a screen more often

in monocular probe tests (50) than when tested binocu-larly (3). Treatment O chicks failed 22 times and Treat-ment T chicks failed 28 times in monocular tests. Overall,chicks chose the side indicated by distal cues in monoc-ular probe tests with identical screens (28/10; binomialtest: P ! 0.01; Fig. 5a). In probe tests with differentscreens, however, LES chicks still chose the distal sidemore often than the proximal side, whereas RES chickschose the proximal side more often than the distal side(chi-square test: c1

2 Z 4.2, P! 0.05; Fig. 5b).Generally, rearing treatment had no effect on the

number of chicks that chose the distal side when testedmonocularly. In probe tests with identical screens, how-ever, Treatment O chicks failed to choose a screen lessoften than Treatment T chicks (chi-square test: c2

2 Z 7.3,P! 0.05; Fig. 5a). Rearing treatment did not influence

0123456789

LES RES LES RES

Treatment O Treatment T

(a)

(b)

LES RES LES RES

Treatment O Treatment T

Rearing treatment

0

2

4

6

8

10

12

Nu

mbe

r of

ch

icks

Figure 5. Number of chicks choosing the screen indicated by thedistal (-) or proximal (,) cues, or failing to make a choice (G)

when using the left eye (LES) or right eye (RES) in probe tests with (a)

identical and (b) different screens. Treatment O: opaque screens;Treatment T: transparent screens.

FREIRE & ROGERS: SPATIAL MEMORY IN CHICKENS 1

ANIMAL BEHAVIOUR, 69, 51098

response latency in probe tests using the LES (F test:F1,149 Z 2.4, 0.13) or RES (F1,149 Z 0.3, 0.86). However,Treatment O chicks responded faster when using the LESthan the RES (F test: F1,104 Z 5.0, P! 0.05; Fig. 6) whereasTreatment T chicks showed no asymmetry in responselatency (F1,104 Z 0.02, 0.88).

DISCUSSION

In summary, chicks chose the screen on the distal side (i.e.the same side as during training) more often and witha shorter latency in probe tests with identical screens thanin probe tests with different screens. In probe tests withdifferent screens, however, LES chicks chose the distal sidemore often than the proximal side, whereas RES chickschose the proximal side more often than the distal side. InBIN probe tests with different screens, Treatment O chickswere quicker in making a choice than Treatment T chicksand they tended to choose the screen on the distal sidemore often than Treatment T chicks. Treatment O chicksalso failed to make a choice less often than Treatment Tchicks in monocular probe tests with identical screens.We found no gender effects on performance in the

rotated probe tests, suggesting similar spatial cognitiveabilities in male and female chicks. Similarly, Tommasi &Vallortigara (2004) found no difference in male andfemale chicks’ use of proximal cues for locating hiddenfood, although males responded more strongly to non-spatial changes to the proximal cues (i.e. their removal orreplacement). In addition, no evidence was found that therearing treatment influenced the ability to walk arounda screen in the probe tests. First, Treatment O and T chicksdid not differ in either the number of training trials or inthe latency to move around a screen during training.Second, no evidence was found that rearing treatmentinfluenced performance in BIN probe tests with identicalscreens, or in monocular probe tests with differentscreens. Thus, the results suggest that Treatment O andT chicks were equally motivated to search for the imprint-ing stimulus.

0

20

40

60

80

100

120

140

160

Treatment O Treatment T

Rearing treatment

Late

ncy

(s)

* ***

Figure 6. Mean latency to chose a screen in probe tests when thechicks were tested binocularly (-), and using the left (G) and right

(,) eyes for chicks in Treatment O (opaque screens) and Treatment

T (transparent screens). Data for identical and different screens arepooled because they were not significantly different. *P! 0.05;

***P! 0.001.

Our findings that chicks generally chose the screen onthe distal side in probe tests with two identical screenssupport previous work indicating that chicks readily usedistal cues in the RFT (Rashid & Andrew 1989; Regolinet al. 2005). Generally, fewer chicks showed a preferencefor the distal side in probe tests with two different screens,suggesting that the relatively obvious proximal cuespresented a conflict situation. Indeed, the longer latencyto choose a side and the greater number of failures inprobe tests with two different screens than with twoidentical screens supports this idea. Our findings extendearlier work indicating that chicks are able to orientaccurately and quickly using distal cues by showing thatthe presence of obvious proximal cues that presentconflicting information may interfere with orientationbased on distal cues.

Furthermore, although chicks responded strongly todistal cues in BIN probe tests with identical screens, therewas a tendency for Treatment O chicks to use distal cuesmore often than Treatment T chicks in BIN probe testswith different screens (P Z 0.058), suggesting that ina conflict situation Treatment O chicks were still able toattend to distal cues but that Treatment T chicks weremore likely to switch to proximal cues or fail to choose.Indeed, the finding that Treatment O chicks were faster atchoosing a screen in BIN probe tests than were TreatmentT chicks supports the suggestion that Treatment T chickswere in greater conflict than Treatment O chicks. Gener-ally, chicks were more likely to fail in monocular than inbinocular probe tests indicating that covering one eyesignificantly impaired performance in probe tests. Treat-ment O chicks failed to make a choice less often thanTreatment T chicks in monocular probe tests with identi-cal screens. It is unlikely that the latter finding indicatesa motivational difference in search behaviour betweenTreatment O and T chicks, since no other evidence forsuch differences either in training or in other probe testswas found. Instead, it may be that Treatment O chickswere better able to detect and use subtle proximal cuesthan Treatment T chicks when orientation was impairedby covering one eye, although this remains to be de-termined. Combined, the above findings indicate thatrearing with opaque screens improved the chick’s re-sponse to distal cues by apparently reducing the effect ofinterference from conflicting proximal cues.

Providing chickens with experience of opaque screensfor 5 days, at 8–12 days of age, improves search in a visualdisplacement test and a detour test, suggesting that thereis an effect of experience on the chick’s ability to orient(Freire et al. 2004). It appears that day 11 may be a crucialperiod since chickens in the laboratory spend more timeout of sight of an imprinting stimulus on this day than inthe days before or after (Freire et al. 2004) and similarlychickens in semiextensive housing systems actively moveout of sight of the mother at 11 days of age (Vallortigaraet al. 1997), a time when the right hemisphere assumesdominant control of visual behaviour (Rogers & Ehrlich1983).

Although it appears that moving out of sight shapes thedevelopment of spatial memory, the precise experiencesthat shape this remain to be determined. It does not

FREIRE & ROGERS: SPATIAL MEMORY IN CHICKENS 1099

appear that the locomotory changes associated withmoving around objects are important, since the trans-parent screens used in the present study and by Freireet al. (2004) would be expected to allow similar locomo-tory patterns. Chicks reared in pens without screens andperiodically placed out of view of an imprinting stimulusby the experimenter showed no improvement in spatialmemory relative to chicks similarly handled but withoutbeing placed out of sight of the imprinting stimulus,suggesting that the chick’s active behaviour is central tothe developmental process (Freire et al. 2004). Onepossibility is that, by moving out of sight of prominentvisual cues such as the imprinting stimulus or the otherchick, the chicks are actively facilitating their ability tolearn to use distal spatial cues.Chicks using the LES based their orientation more on

distal cues than proximal cues, whereas chicks using theRES based their orientation more on proximal cues thanon distal cues in probe tests with different screens,supporting previous research showing that the processingof distal information involves the LES (Rashid & Andrew1989; Tommasi & Vallortigara 2001; Regolin et al. 2005)and presumably the right hippocampus (Tommasi et al.2003). Perhaps surprisingly though, we found that chicksusing the RES and LES responded equally well to distalcues in probe tests with identical screens, indicating thatthe RES is able to attend to distal cues if there are noobvious proximal cues. Similarly, Vallortigara et al. (2004)found that chicks using the RES could orient using distalcues, although not as well as chicks using the LES. It isunlikely that the left hemisphere has some rudimentaryability to process distal information since lesions to theright hippocampus, but not the left hippocampus, disruptthe response to geometric information (Tommasi et al.2003). Instead, it may be that the left hemisphere is ableto attend to distal cues in the absence of obvious proximalcues by engaging the right hemisphere through ipsilateralconnections, or perhaps through connections to the righthemisphere such as the thalamofugal visual projectionsthrough the supraoptic decussation (Rogers 1995). It maybe that, in normal functioning, the pathways allowing theleft hemisphere to process distal information are largelyredundant, but when there are no obvious proximal cues,the left hemisphere may be able to attend to distal cues.Although the above is speculative, it would suggest thatthe left hemisphere has not lost its ability to respond todistal cues, but that instead its primary role is to respondto proximal cues.In conclusion, response to distal cues in a rotated floor

test was impaired by obvious proximal cues, indicatingthat relatively obvious proximal cues are necessary forconflict to arise. Previously reported advantages in the useof distal cues by the LES were confirmed, and indicatedthat chicks using the LES were less disrupted by conflict-ing proximal cues than chicks using the RES. Rearing withopaque screens improved the chicks’ ability to use distalcues and ignore conflicting proximal cues, suggesting thatthe previously reported experience-induced improvementin spatial memory (Freire et al. 2004) may arise becauseexperience of visual barriers biases the chick’s orientationmechanism towards using distal cues.

Acknowledgments

We are grateful to funding from a VC post-doctoral fellowship and the Faculty of Sciences. L.J.R.is grateful for funding from the Australian ResearchCouncil.

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