Valerio etal 2010

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Valerio etal 2010
  Directional learning, but no spatial mapping by rats performing a navigationaltask in an inverted orientation Stephane Valerio 1 , Benjamin J. Clark 1 , Jeremy H.M. Chan, Carlton P. Frost, Mark J. Harris, Jeffrey S. Taube * Department of Psychological and Brain Sciences, Dartmouth College, 6207 Moore Hall, Hanover, NH 03755, USA a r t i c l e i n f o  Article history: Received 14 October 2009Revised 1 January 2010Accepted 20 January 2010Available online 28 January 2010 Keywords: Spatial navigationVestibular systemHead direction cellsSpatial task a b s t r a c t Previous studies have identified neurons throughout the rat limbic system that fire as a function of theanimal’s head direction (HD). This HD signal is particularly robust when rats locomote in the horizontaland vertical planes, but is severely attenuated when locomoting upside-down (Calton & Taube, 2005).GiventhehypothesisthattheHDsignalrepresentsananimal’ssenseofdirectionalheading,weevaluatedwhether rats could accurately navigate in an inverted (upside-down) orientation. The task required theanimals tofind anescapehole whilelocomoting inverted onacircular platformsuspended fromthe ceil-ing. In Experiment 1, Long-Evans rats were trained to navigate to the escape hole by locomoting fromeither one or four start points. Interestingly, no animals from the 4-start point group reached criterion,evenafter29daysoftraining.Animalsinthe1-startpointgroupreachedcriterionafteraboutsixtrainingsessions. In Experiment 2, probe tests revealed that animals navigating from either 1- or 2-start pointsutilized distal visual landmarks for accurate orientation. However, subsequent probe tests revealed thattheir performance was markedly attenuated when navigating tothe escape hole froma novel start point.This absence of flexibilitywhile navigating upside-downwas confirmed in Experiment 3 where we showthat the rats do not learn to reach a  place , but instead learn separate  trajectories  to the target hole(s).Based on these results we argue that inverted navigation primarily involves a simple directional strategybased on visual landmarks.   2010 Elsevier Inc. All rights reserved. 1. Introduction Thedifferentstrategiesusedbyhumansandanimalstonavigateaccurately in their environment have been studied extensively byneuroscientists(Aguirre,Zarahn,&D’Esposito,1998;Hartley,Magu-ire, Spiers, & Burgess, 2003; Maguire et al., 1998; O’Keefe & Nadel,1978). Thetheoriespostulatedtoaccount forsuccessful navigationhavebeenshaped,inpart,bythediscoveryofdifferenttypesofspa-tially tuned neurons that code for location (place cells), directionalorientation(headdirectioncells),andgridoftheenvironment(gridcells) using an allocentric frame of reference (Ekstrom et al., 2003;Hafting,Fyhn,Molden,Moser,&Moser,2005;O’Keefe&Dostrovsky,1971; Taube, Muller, & Ranck, 1990). Indeed, these findings, com-bined with animal lesion studies showing that disruption of brainareascontainingtheseneuronspreventsanimalsfromsolvingallo-centric spatial tasks (Morris, Garrud, Rawlins, &O’Keefe, 1982), ledtotheideathattheseneurons,primarilyfoundinthelimbicsystem,weretheneuralsubstrateofaflexiblerepresentationofspaceor cog-nitivemap (O’Keefe&Nadel,1978).Ontheotherhand,whenfacingaspatial problem, rats do not always rely on a cognitive map-basedstrategy,butmayalso:(1)useaspecificlandmarksituatedclosetothe goal (beacon navigation), (2) use a small number of distal cuesthatprovidesthemthedirectiontofindthegoal(directionstrategy),or(3)simplylearnasequenceofturns(e.g.,left–right–left)andthususeabody-centered(egocentric)frameofreference(responsestrat-egy)(McDonald&White,1993;Packard&McGaugh,1996;Skinneretal.,2003).Most studies on mammalian navigation have involved two-dimensional terrestrial environments. However, many species liveand move about in a three-dimensional (3-D) environment – themost obvious species include marine and arboreal mammals, aswell as rodents that live in subterranean environments. But hu-mans also frequently navigate in 3-D environments, particularlyair pilots and astronauts, many of whom frequently encounterperiods of disorientation that often lead to motion sickness, whichcan have serious consequences during flight (Oman, 2007). Thus,determiningwhichstrategiesandreferenceframesareusedduringnavigation in a 3-D environment is crucial for understanding howanimals live and orient in a 3-D world.Previous experiments have studied how head direction (HD)cells respond when a rat locomotes in a vertical plane or upside-down in 1-g (Calton & Taube, 2005; Stackman, Tullman, & Taube,2000)andin0-g(Taube,Stackman,Calton,&Oman,2004).Inthese 1074-7427/$ - see front matter    2010 Elsevier Inc. All rights reserved.doi:10.1016/j.nlm.2010.01.007 *  Corresponding author. Fax: +1 603 646 1419. E-mail address: (J.S. Taube). 1 These authors contributed equally to this study. Neurobiology of Learning and Memory 93 (2010) 495–505 Contents lists available at ScienceDirect Neurobiology of Learning and Memory journal homepage:  experiments, it was observedthat the vertical planewas treatedasan extension of the floor and that HD cells maintained their direc-tion-specific firing when the rat locomoted on the wall. However,when the rats locomoted upside-down on the ceiling, HD cellsshowed a dramatic change in their activity, including a completeloss of directional-specific firing in two-thirds of the cells, and amarkedreductionindirectionalactivityintheremainingcells(Cal-ton & Taube, 2005). Furthermore, in cells that maintained somesemblanceofdirectionalfiring,therewasfrequentlyashiftintheirpreferred firing direction. However, even with this distorted direc-tionalsignal,ratsperformedquitewellinthetask–climbingaver-tical wall to a ceiling, then traversing the ceiling inverted, andfinallydownanotherwallinordertoreacharewardcompartment.Observationsoftheratsappearedtoindicatethattheyknewwhichdirection to go in order to obtain the reward. This accuracy in thetask was attributed to the low navigational demand and may nothave required a functional HD system. To date, there are no exper-iments that have tested the extent to which rats are capable of accuratenavigationinaninvertedorientation. Inthepresentstudywe devised a more demanding spatial paradigm in order to exam-ine whether rats are capable of accurate navigation in an invertedorientation, and if so, what spatial strategies they use to performthe task. 2. General methods  2.1. Subjects Experimentally naive female Long-evans rats served as subjectsfor all three experiments. The animals were housed singly in clearplastic cages in a room maintained on 12h light/dark cycle. Waterandfoodwereavailable ad libitum inthehomecage.Allprocedureswereconductedunder aninstitutionally approvedanimal careanduse protocol.  2.2. Apparatus The ‘‘inverted maze” apparatus consisted of a 190-cm diametercircular wire mesh that was suspended   50cm from the ceiling,withfour evenlyspaced holes that formedfour possibleescape ex-its from the maze (Fig. 1). Each one of these 15-cm holes could beclosed with a piece of wood. The wire mesh was composed of me-tal screening with square size of 0.5  0.5cm. The maze was in aroom containing many salient visual cues including posters, a TV,adoor, ablackboard, twowindowsalongonewall, andadeskwitha computer on it.  2.3. Behavioral procedure Prior to the commencement of training all rats were handled3min per day for 3days. The basic task involved the rat beingplaced in an inverted position, either on the periphery of the mazeor at the maze center (see below), and because rats contain a nat-ural proclivity to remain in an upright position they were highlymotivatedtofindaholetoescapethroughinordertoenablethem-selves to return to an upright position on top of the circular board.For all experiments three of the four holes were blocked with apiece of wood – thus, only one hole was open to escape through.Although the rat could see the small depression made by the fourholes, it was not possible for it to determine which hole was thecorrect one to escape through without being close to it becauseofthesmallanglebetweenitsheadandthewiremeshsurface.Thispoint is firmlydemonstrated by noting the great difficulty rats hadin finding the correct hole in the four start position condition (seeresults below). Throughout all the experiments there was no at-tempt to disorient the rat when bringing it into the room or whenplacing it on the inverted maze.  2.3.1. Pretraining  Before being trained to solve the spatial task, the rats weretrained to climb short distances in an inverted position on the in-verted maze. More specifically, pretraining consisted of eight trialsper day where the rats were released on the maze (upside-down)in close proximity to one of the four escape holes. This procedureenabled them to learn how to locomote inverted on the mazeand to understand how to exit from the maze by passing throughthe hole and returning to a normal upright position on top of themaze. The target hole and the release direction of the rats werevaried each trial. Depending on the improvement of the rats, theexperimenter progressivelyincreasedthedistancebetweenthere-lease point and the hole until the animals were able to reach thetargetholewhenreleased20cmawayfromit. Whenanimalswereabletoclimbtotheopenholefromthatdistance, theanimalswereassignedto a testing groupand the training phase started. The ratsthat could not complete this pretraining phase after 8days wereconsidered ‘‘bad climbers” and excluded from the experiments.  2.3.2. Training  Atthestartoftrainingtherats’cageswerebroughtintothemazeroom on a cart and placed 2m away from the maze on one side of theroom. Atypicaltrainingtrial consistedofreleasingtheratfromone of the four start points (north, south, west and east) along theperimeter of the maze and allowing it to locomote inverted to thecorrect uncovered escape hole. In cases when the rats could not Target hole 1  9  0  c m NSEW Fig. 1.  The test roomand the inverted maze apparatus. The apparatus consisted of a 190-cm diameter circular wire mesh that was suspended  50cmfromthe ceiling, withfour evenly spaced holes that formed four possible escape exits from the maze. Each one of these 15-cm holes could be blocked with a piece of wood. The wire mesh wascomposed of metal screening with square size of 0.5  0.5cm.496  S. Valerio et al./Neurobiology of Learning and Memory 93 (2010) 495–505  reachtheholeafter30s,theywereledtoitbytheexperimenterandgiven a score of 30s. We could not allow for more search time be-causetheratsfrequentlybecametiredwhilesuspendedandwouldinevitably fall off the maze after a minute. Moreover, we observedthat well-trained rats were proficient at navigating to the escapehole in less than 30s, regardless of whether the W, E, S, or N startpoint was used (see Fig. 1 right panel). At the end of each trial therat was returned to its home cage while the other rats were givenatrainingtrial.Eachtrainingsessionconsistedoffourtrialsandani-malsweregivenonetrainingsessionperday.Thelatencytofindthetarget hole was timed by the experimenter standing at differentlocationsaroundtheedgeofthecircularplatform.Alltrainingtrialswere videotaped via a video camera placed on the floor below themaze and aimed upward at the inverted maze.  2.3.3. Criterion In all three experiments, the animals were considered to havelearned the task when they accurately reached the target hole(in<30s) on three out of four trials for two consecutive days.  2.4. Statistical analysis InExperiment 1, anANOVAwithrepeatedmeasuresforthefac-tor  session  was performed to analyze the performance of animalstrainedfromfourstartpoints(4SP).Inallothercaseslearningcom-parisons were performed on the mean group value (±SEM) for thenumber of training days necessary to reach criterion, which wereanalysed using Student’s  t  -test. All tests comparing the perfor-mance of animals in a specific testing situation with their perfor-mance in their last regular training session were analysed usinga two-way ANOVA ( session   group ), with repeated measures forthe factor  session . 3. Experiment 1 The aim of the first experiment was to test whether, given themarked disruption of the head direction signal during invertedlocomotion, the rats were able to learn a spatial task that requiredthemto navigate accurately upside-down. This task resembled theclassic Morris water maze task (Morris et al., 1982) in that the ratshad to locate a spatial location relative to room cues, either fromone or four different entry points, in order to avoid an aversivestimulus, which in this case was the continuous state of being inan inverted position.  3.1. Method Twogroupsof animalsweretrainedtotwodifferent versionsof the task: (1) 11 animals were trained to find the target hole fromfourstartpoints(4SP,Fig.2E)and(2)10animalsweretrainedfromone start point (1SP, Fig. 2A). For each animal, the target hole waskept constant during the entire training procedure.Following training on the inverted hole board escape task, the4SP group was trained in a Morris water maze task. The pool(180cm diameter, 50cm high) was positioned 14cm above thefloor. The water, which was   23cm in depth, was made opaquewith white powder and a 13cm diameter plastic platform was lo-catedin one quadrant of the pool, suchthat its topwas submerged  1cmbeneaththesurface.Theplatform’slocationwasfixedforalltrials.Eachtrialwasvideotapedfromanoverheadcamera.Foreachtrial we measured the time it took for themto find and climb ontothe platform (escape latency). Animals received four trials per dayuntil they reached the learning criterion, which was defined asfinding the platform in less than 10s in three out of four trialson two consecutive days.  3.2. Results and discussion The rats trained from one start point reached criterion after amean of 5.9±0.57 (SEM) training sessions. The mean latency toreach the escape hole on their last training day was 16.2±1.41s,range 9–22s. In contrast, none of the 11 rats trained in the 4SPcondition could reach criterion even after 29 training sessions,although there was a small improvement over time. As shown inFig. 3A, these rats improved their mean latency to the target holewith training days, and the ANOVA confirms this slight improve-ment over training sessions;  F  (28,280)=3.864,  p  <0.0001, butnone of the rats met criterion. Although statistically there was animprovement over time, the graph in Fig. 3 clearly indicates thatthis improvement was minimal, particularly after training Day 6where subsequent learning is virtually absent.Part of the overall small improvement is due to correct re-sponses on some trials when the start point was at one of thetwolocationsadjacenttothetargethole. Onthelast3daysof test-ing, the 4SP rats needed an average of 20.9±1.6s to reach the tar-getholewhenreleasedfromthenearerstartpoints. However, theyapparently could  not   use this knowledge to improve their perfor-mance from the farther start points, as the average time to reachthe escape hole was 29.7±0.3s, which is almost the entire 30slength of the trial. This latency difference could be explained bythe longer distances the rats had to walk when the escape holewas on the opposite side of the platform from the release point.Furthermore, because it was difficult to match the distances therats had to locomote in the two versions of the task (1SP vs.4SP), we cannot exclude the possibility that this distance factorcontributed to the observed learning deficit (i.e., longer escapelatencies)inthe4SPversionofthetask.However,analysisofhead- 2SPSameplace –Opposite turn2SPDifferentplace –Opposite turn2SPDifferentplace –Sameturn B C DA E 1SP4SP Fig. 2.  Experimental protocol for the one (1SP), two (2SP), and four (4SP) start points training conditions. (A) 1SP condition, where the rat is always released from the sameentry point. (B) 2SP-Same place-Opposite turn: the rats had to learn to reach a single target hole from two different start points (e.g., south and east) that required twooppositeturns(rightfromsouth,andleftfromeast).(C)2SP-Differentplace-Oppositeturn:theratswerealsoreleasedinthemazefromtwostartpoints,butinthisconditionthey hadtoreachone of two distinct target holes (different place) depending ontheir releasepoint in the maze. Thestart points were selectedsuch that the ratshadtolearntwooppositeturns,similartotheconditionsinB.(D)2SP-Differentplace-Sameturn:theratsweretrainedtoreachtwodifferentescapeholes(differentplace)fromtwostartpoints,butinthiscase,bothescapeholeswereinthesamerelativepositionfromthestartpoint.Inthiscondition,learningaparticularegocentricturn(e.g.,alwaysturnright)could be a successful strategy for the rats. (E) 4SP condition, where the rat is released from one of four different entry points. S. Valerio et al./Neurobiology of Learning and Memory 93 (2010) 495–505  497  ing trajectory (measured after the rat had walked a body lengthfrom their release point), revealed that the poor performance therats showed from the farther start points was more likely due toan initial poor directional heading. Indeed, when released fromthe two closer start points, the rats directional heading only devi-ated 12.9±3.7   away from the correct direction to the target hole,but deviated by 53.1±4.9   when released from the two fartherstart points. Thus, 4SP animals were modestly able to learn howto reach the escape hole from the two close start points, but evenafter 29 trainingsessions, this knowledge did not appear to be suf-ficiently flexible to transfer to the two other more difficult startpoints that were distant to the escape hole.In order to verify that these animals were able to learn a morenormal spatial task under similar room conditions, we trainedthem in a comparable version of the standard water maze task.The results indicate that these animals display normal perfor-mance in this task and readily acquire the task. On average, theanimalsonlyrequired3.9±0.5sessionstoreachthelearningcrite-rion in finding and escaping onto the submerged platform. Thisfinding indicates that their poor performance in the 4SP invertedmaze cannot be attributed to: (1) a general learning deficit, (2)an inability to see the surrounding visual landmark cues, or (3)the stressful characteristic of the task, because they showed nor-mal performances in the water maze task. Moreover, the fact thatthe rats were able to learn the inverted escape task in the 1SP ver-sion of the task also supports this view.In summary, although we cannot exclude the possibility thatdistance played a role in the difficulty the animals had in learningthe4SPcondition, it isnoteworthythat the spatial knowledgetheyhad was not sufficiently flexible to enable them to reach criterion.Thus, our hypothesis is that the learning deficit is due to an inabil-ity to form a flexible, map-like representation of the environmentwhen in an inverted position. To test this hypothesis, in Experi-ments 2 and 3 we tested whether the deficit was due to the in-creased spatial demand of the 4SP condition. 4. Experiment 2 The aim of the second experiment was to test whether the ratshad a flexible representation of the environment when navigatingin the inverted maze. Experiment 1 showed that they could solvethetaskwhentheywerereleasedfromaunique(single)startpoint.Whattype of strategydidtheyuseinthis situation? Didtheylearnthe location using a simple response strategy (i.e., turn right), ordid they learn the location using a more cognitive map-like repre-sentation? We also wanted to determine whether the rats couldlearnamoredemandingversionofthetaskbyusingtwostartpoints.Using this procedure and several probe trials enabled us to deter-minewhatstrategiesandcuestheratswereusingtosolvethetask. 4.1. Method4.1.1. Training  Twonewgroupsofratsweretrainedtoreachtheescapeholeinthe inverted maze, either from one start point (1SP,  n  =7), or fromtwostart points(2SP,  n  =11). To directlycomparethesetwolearn-ing conditions, and to reduce the non-specific difficulties of thetask, we trained the 2SP rats from the two closest start points of their assigned hole (e.g., Fig. 2B). After reaching criterion the ratswere submitted to a series of probe tests. In between each of theseprobe tests the rats were trained again to criterion. Analysis of thepercentage of correct trial revealed no differences between groupsfor any of the criterion performance scores on days precedingprobes (  p  >0.05). 4.1.2. Probe tests Asinthetrainingsessions, theratshad30stofindthehole, butwhen they failed to find it, they were removed from the maze, re-turned to their home cages (no correction procedure), and the trialwas scored 30s. Center probe.  The rats were released from the center of themaze facing one of the four cardinal points (four trials). The targethole was open and the rats had 30s to reach it. The experimenterscored the latency they needed to reach the hole when releasedfrom this novel start point. This test enabled us to determinewhether the rats had learned the task using a cognitive map-likestrategy - if so, then their performance should be good when re-leased from the novel center point. In contrast, if they had learneda more rigid strategy ( response  or  directional  strategy), then theyshould be impaired on this probe. Olfactory probe.  In order to test whether rats used olfactorycues to findthe target hole, theywere submitted toa four trial testafter the maze had been rotated 90   and cleaned with alcohol. If they were using olfactory cues to locate the correct target hole,then their behavior should be altered by the suppression of thesecues during the maze rotation trial. Reversal probe.  To makesure that the rats couldnot see thecorrect escape hole from the start point, they were submitted to atraining session (four trials) with a new target hole located in theopposite quadrant of the maze and released from new startingpoints that remained in the same relative position to the escapehole for both the 1SP and 2SP conditions. Good performance onthe reversal probe would suggest that they could see the correcttarget hole fromtheir start positionand/or were usinga simple re-sponse strategy. Training day 0510152025305101525 A    L  a   t  e  n  c  y   (  s  e  c   ) Training day 051015202530 B    L  a   t  e  n  c  y   (  s  e  c   ) 051020 Fig. 3.  Escape latencies for the 4SPand 1SPconditions in Experiment 1. (A) Meanescape latencies for the 29 trainingsessions (four trialsper session) for the animals trainedto reach the escape hole from4-start points. (B) Mean escape latencies for the animals trained to reach the escape hole froma single start point (1SP condition). Because thetwo groups did not reach criterion in the same amount of time, the graph plots the individual learning curve for each animal.498  S. Valerio et al./Neurobiology of Learning and Memory 93 (2010) 495–505 Blind start probe.  Inthis four trial test, the rats weregivenaregular training session (same hole and start point), except thatduring the last two trials the experimenter used his lab coat toshieldtheratsfromviewingvisualcues withintheroomuntil theywere released on the maze. This probe tested the extent to whichrats used spatial information they derived from room cues beforebeing placed upside-down on the maze periphery. Curtain probe.  Finally, in order to verify that distal visualcues were essential for performing the task accurately, we sup-pressed these cues by hanging a black floor-to-ceiling curtainaroundthemaze.Tocompletelyeliminateanyvisualcues,thistestwas performed under infrared light. The rats were given four reg-ular training trials in these conditions. 4.2. Results and discussion4.2.1. Training  Both 1SP and 2SP groups reached criterion. As shown in Fig. 4Athe rats trained from one start point were much faster to learn thetask than those trained from two start points with the mean num-ber of training sessions to reach criterion: 1SP=5±0.7 sessions;2SP=13.3±1.6 sessions. Statistical analysis confirms that the1SP group needed significantly fewer training sessions to learnthetask[ t  (16)=  3.899,  p  <0.01].Itisnoteworthythatthe2SPratsrequiredmorethantwiceas manytrainingsessionsas the 1SPratsto reach criterion. This difference suggests that when learning toreach the target hole from a second start point, the animals donot benefit from knowing how to reach it from one start point.One possible explanation for this deficiency is that the 2SP ratswere required to build a more complex representation of the envi-ronment to encode the target hole location compared to the 1SPrats, which could have used a simple response strategy. Thus, sev-eral probe tests were conducted to determine what kind of spatialstrategy each group used to solve the task. 4.2.2. Center probe This probe determined whether the animals used a flexible,map-like representation of the environment that permits them toreach the target hole from a new start point – in this case the cen-ter of the maze. Fig. 4Bshows that the performance of bothgroupswas severely altered in this testing condition. Indeed, there is aclear increase in the mean latency to reach the target hole duringtheprobesession(mean=28.38±0.43s) comparedwiththemeanlatency observed on the last regular training session (mean=15.60±0.58s). A two-way ANOVA with repeated measures con-firms this strong  session  effect;  F  (1, 16)=85.342,  p  <0.0001. Thisanalysis also reveals a global  group  effect [ F  (1,16)=5.088,  p  <0.05], reflecting the fact that the 1SP group is on average fasterto reachthe holethanthe 2SP group. But more importantly for ourquestion of whether the animals used a map-like representation,there is no  session   group  interaction (  p  >0.05). This result indi-cates that both groups were similarly affected by this testing situ-ation, where both groups displayed poor performances whenreleased from a newstart point. This absence of flexibility may ac-count for the poor performance we observed in the 4SP conditionin Experiment 1. However, the question now arises as to whatstrategy they used when performing the 1SP and 2SP versions of the task. 1SP 2SP    N  u  m   b  e  r  o   f   S  e  s  s   i  o  n  s   t  o   C  r   i   t  e  r   i  o  n 0246810121416 B A Criterion Center Probe    L  a   t  e  n  c  y   (  s  e  c    ) 051015202530 2SP1SP C Criterion OlfactoryProbe    L  a   t  e  n  c  y   (  s  e  c   ) 051015202530 D Criterion ReversalProbe    L  a   t  e  n  c  y   (  s  e  c   ) 051015202530 Criterion Curtain Probe G    L  a   t  e  n  c  y   (  s  e  c   ) 051015202530 E 1 Trial 0510152025302 3 4    L  a   t  e  n  c  y   (  s  e  c   ) RegulartrialsBlindfolded trials    L  a   t  e  n  c  y   (  s  e  c   ) 051015202530 F Fig. 4.  Results from Experiment 2. (A) Mean number of sessions to criterion for the animals trained to reach the escape hole from 1-start point (1SP) and 2-start points(2SP). (B) Mean escape latency that 1SP and 2SP animals needed to reach the escape hole when released on the maze from a novel starting location (center of the maze),compared with the mean escape latency on their last regular training session (criterion). (C) Mean escape latency that 1SP and 2SP animals needed to reach the hole afterthe maze was cleaned with alcohol and rotated 90   (olfactory probe), compared with the mean escape latency on their last regular training session (criterion). (D) Meanescape latency that 1SP and 2SP animals needed to reach a new target hole (reversal probe), compared with the mean escape latency on their last regular training session(criterion). (E) Mean escape latency that 1SP and 2SP needed to reach the new target hole over the course of the reversal session (four probe trials). (F) Mean escapelatency that 1SP and 2SP animals needed to reach the escape hole when the rats were shielded from viewing visual cues within the room before they were released on themaze (blindfolded trials), compared with the mean escape latency on the first two regular trials of the same session (regular trials). (G) Mean escape latency that 1SP and2SP animals needed to reach the escape hole when deprived of distal visual cues (curtain probe), compared with the mean escape latency on their last regular trainingsession (criterion). S. Valerio et al./Neurobiology of Learning and Memory 93 (2010) 495–505  499
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