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A cautionary note regarding drug and brain lesion studies that use swimming pool tasks: partial reinforcement impairs acquisition of place learning in a swimming pool but not on dry land

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A cautionary note regarding drug and brain lesion studies that use swimming pool tasks: partial reinforcement impairs acquisition of place learning in a swimming pool but not on dry land
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  Behavioural Brain Research 112 (2000) 43–52 Research report A cautionary note regarding drug and brain lesion studies that useswimming pool tasks: partial reinforcement impairs acquisition of place learning in a swimming pool but not on dry land Claudia L.R. Gonzalez *, Bryan Kolb, Ian Q. Whishaw Department of Psychology and Neuroscience ,  Uni   ersity of Lethbridge ,  Lethbridge ,  Alberta ,  Canada T  1 K   3  M  4  Received 15 November 1999; received in revised form 25 January 2000; accepted 30 January 2000 Abstract Spatial tasks are used widely in neurobiological studies because it is thought that they provide an unbiased assessment of theintegrity of neural structures that mediate spatial learning. For example, in the Morris swimming pool place task, animals arerequired to locate a hidden platform in a swimming pool in relation to environmental cues. Treatments that result in an animal’sfailure to find the platform are assumed to reflect defects in the function of neural systems involved in spatial learning. The presentstudy demonstrates, however, that an animal’s reinforcement history can contribute to its spatial performance. Animals weretrained in the Morris place task with the platform present on 100, 75 or 50% of trials. Relative to the 100% group, the 75% groupwas impaired in place acquisition, and the 50% group failed to learn. Even placing the 50% group animals onto the platform atthe completion of an unsuccessful trial failed to improve acquisition. Animals trained to search for food on an identical dry mazeproblem were not affected by similar reinforcement schedules. The present findings demonstrate that the Morris swimming poolplace task does not provide an unbiased assessment of spatial learning: A treatment effect may be confounded with reinforcementhistory. The results are discussed in relation to widespread applications of the Morris place task to neurobiological problems.© 2000 Elsevier Science B.V. All rights reserved. Keywords :   Learning and memory; Partial reinforcement; Spatial learning; Water mazewww.elsevier.com / locate / bbr 1. Introduction The proposal that the Morris swimming pool tasks‘lend themselves to a variety of behavioral investiga-tions, including pharmacological work and studies of cerebral function’ [26] has been fully confirmed by anow enormous literature. In studies using these tasks itis assumed that the performance of the animal providesan unbiased assessment of treatment or lesion effects.That is, behavioral deficits obtained with pharmacolog-ical treatments [8,10,11,16,21,24,38,41,43] or brain le-sions [5,9,14,16–18,27–29,32,35–37] are interpreted asbeing due to interruptions of process that affect learn-ing and memory or spatial guidance. For example, theMorris place task requires a rat to escape to a hiddenplatform located just beneath the water surface andtherefore offers no local cues to guide escape behavior.Normal rats very quickly learn to swim directly to-wards the platform from any starting position. Animalssubjected to drug treatments or brain lesions may failto find the platform on many trials in the allotted timeand may never become proficient in performing thetask over the testing period. Thus, their behavioraldeficit suggests that some aspects of spatial learninghave been disrupted by the treatment.Although most studies using the Morris task includeprocedures to ensure that behavioral deficits are notdue to ‘side effects’, such as motor incapacity, inade-quate motivation, or sensory impairment, studies of conditioning suggest another problem that can con- * Corresponding author. Tel.:  + 1-403-3292440; fax:  + 1-403-3292555. E  - mail address :   claudia.gonzalez@uleth.ca (C.L.R. Gonzalez)0166-4328 / 00 / $ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S0166-4328(00)00162-5  C  . L . R .  Gonzalez et al  .  /   Beha  ioural Brain Research  112 (2000) 43–52  44 found the interpretation of spatial studies. Incrementalmodels of learning [31]; see also [1,19,20] demonstratethat although rapid learning occurs if a stimulus ispaired with reinforcement on every trial, acquisitionrate is reduced if the incidence of pairing is reduced. Inprinciple, the finding that acquisition is related incre-mentally to reinforcement frequency may be relevant tostudies of place learning in the swimming pool. After aparticular treatment an animal may fail to find theplatform on some or many trials. Thus, the animal isinadvertently subjected to a partial reinforcementschedule. Stated differently, if an animal’s reinforce-ment history during learning contributes to task acqui-sition, it will be difficult to dissociate behavioral deficitsfrom reinforcement effects.The purpose of the present study was to determinewhether reinforcement contingencies influence learningin the Morris place task. Rats were trained on sched-ules on which the platform was present on all trials, on75% of trials, or on only 50% of trials. In the firstexperiment, animals were started from a single locationon the periphery of the pool. In addition, a group of rats was included that received a 50% schedule, butwere additionally placed on the platform that wasintroduced into the pool at the completion of a 60 secnon-reinforced trial. In a second experiment, ratsbegan their swims from four different positions. In athird experiment very similar training procedures wereused, except that food deprived rats searched for a foodpellet that was present on every trial or on only 50% of trials. 2. Materials and methods: experiment 1 2  . 1 .  Animals Forty-two adult, male, Long–Evans rats (Universityof Lethbridge vivarium) weighing 350–450 g, werehoused in groups of four to six individuals in wire meshcages in a laboratory with room temperature main-tained at 20°C and lighted on a 12 / 12 h light / dark cycle(08:00–20:00 h). 2  . 2  .  Apparatus The water maze consisted of a circular pool (diame-ter 1.55 m, height 46 cm), the inside of which waspainted white, filled to a height of 25 cm with   22°Cwater in which 500 g of instant powdered milk wasdissolved. A clear Plexiglas platform (11 × 12 cm 2 ) waspresent inside the pool; its top surface was 1 cm belowthe surface of the water, and thus the platform wasinvisible to a viewer inside the pool [42]. 2  . 3  .  Swimming analysis The latency to find the platform was timed by anexperimenter standing by the pool’s edge. In addition,its swimming paths were recorded via a video cameraand Poly-Track video tracking system (San Diego In-struments). The tracking system also provided a recordof swim latency and scores of swim distance and head-ing direction [26]. 2  . 4  .  Training  Swim training was conducted on 5 consecutive days,with each rat receiving eight trials on each day, accord-ing to previous procedures [27,35]. For each trial, a ratwas placed into the water facing the wall of the pool. If a rat did not find the platform or if the platform wasabsent, it was removed after 60 s from the pool andreplaced in its holding cage. If on a particular trial a ratfound the platform, it was permitted to remain therefor 10 s before being returned to its holding cage. Trialswere separated by 5 min, during which other membersof the group received training. 2  . 5  .  Probe trial  At the end of 40 training trials, each rat received aprobe trial. For this trial, the platform was removedfrom the pool and the rat was allowed to swim for 60s, before being removed from the pool.Probe trials were analyzed using a preference analysis[3]. The movement tracking system collected swim du-ration for each pool quadrant. The quadrant in whichthe platform had been located previously was desig-nated as the target quadrant (T). The swim times in theremaining three quadrants (A, B, C) were then sub-tracted from the swim time in the target quadrant andthe resultant scores were added and their averagederived to produce a preference score according to thefollowing formula:(T − A) + (T − B) + (T − C)3  = Preference score 2  . 6  .  Statistical analysis Swimming performance was analyzed using analysisof variance (ANOVA) and follow-up Fisher’s tests [47].Two analyses were done. (a) Trials: an analysis wasdone on trials in which the platform was present in thepool; e.g. trials one, three, five and seven on each dayfor animals given 50% reinforcement. (b) Reinforce-ments: an analysis was done comparing the first 20trials of group 100%, the 20 first trials with the plat-form present for the 75% group, and the 20 trials withplatform for the 50%. Two analyses were used because,  C  . L . R .  Gonzalez et al  .  /   Beha  ioural Brain Research  112 (2000) 43–52   45 according an incremental model of learning, learning isproportional to the percentage of reinforced trials,whereas according to an invariance model of learning,rate of acquisition is related to the number of rein-forced trials [46]. 3. Materials and methods: experiment 2 3  . 1 .  Animals Fourteen adult, male, Long–Evans rats (Universityof Lethbridge vivarium) were used in this study. 3  . 2  .  Feeding  Feeding was restricted by giving the rats weigheddaily quantities of food to maintain the rats at 90% of their expected body weights. Medium (500 mg) rodentpellets (Bio-Serv Inc., Frenchtown, NJ), were used asreward during behavioral testing. The time taken to eata 500-mg food pellet approximated 10 s, the time ratswere left on the platform in experiment 1. Thus, whileeating the food pellet, the rat remained at the foodlocation for   10 s [45]. After testing each day, the ratswere supplementary fed with LabDiet laboratory ro-dent pellets in their home cages. 3  . 3  .  Apparatus In order to maintain the same conditions of experi-ment 1, the dry maze was the same pool (in the sameroom). A circular wooden table 25 cm in height wasplaced in the pool (water absent). Thus, an animal onthe table would view the same room cues as an animalswimming in experiment 1. Tanslucent white food cups(4.5 cm diameter) were attached to the floor in thecenter of each the four quadrants (NE, NW, SE, SW).A food pellet was hidden in the NW food cup (samelocation were the platform was placed in the swimmingtask) on reinforced trials. 3  . 4  .  Tracking analysis The latency to find the food was timed by an experi-menter standing by the pool’s edge. In addition, searchpath, time, and distance were recorded via the sametracking system used in experiment 1. 3  . 5  .  Training trials As in experiment 1, training was conducted on 5consecutive days, with each rat receiving eight trials oneach day. For each trial, a rat was placed by hand onthe surface of the table facing the wall. If a rat did notfind the food within 60 s, it was removed and replacedin its holding cage. One pellet was used as reward.Trials were separated by 5 min, during which othermembers of the group received training. 3  . 6  .  Probe trial  At the end of 40 training trials, each rat received aprobe trial. For this trial, the food was removed fromthe food cup and the rat was allowed to search for 60s, before being removed. Probe trials were analyzedusing the same protocol than experiment 1. 4. Procedure: experiment 1 Two experiments were performed. In the first experi-ment rats were started from the south (S) location onevery trial and the platform was located in the center of the northwest (NW) quadrant of the swimming pool. Inthe second experiment, four starting locations wereused (N, S, E, W) and the platform was also located inthe NW quadrant of the swimming pool. 4  . 1 .  One starting location Twenty-four rats were divided into four groups of sixrats each, which received one of the followingtreatments.1. 100% reinforcement: the platform remained in thepool for all training trials.2. 75% reinforcement: the platform was removed fromthe pool on 75% of trials (trials four and eight eachday) so that the rats were required to search for 60s.3. 50% reinforcement: the platform was removed fromthe pool for 50% of training trials (trials two, four,six and eight).4. 50% reinforcement-placed: the platform was re-moved from the pool for 50% of training trials(trials two, four, six and eight). On trials on whichthe platform was removed for training, it wasquickly replaced at the end of the 60 s swim, and therat was placed upon it for 10 s. 4  . 2  .  Four starting locations Twelve rats were divided into two groups of six ratseach, which received one of the following treatments:1. 100% reinforcement: the platform remained in thepool for all training trials and animals were releasedfrom four different positions (N, S, E and W).2. 50% reinforcement: the platform was removed fromthe pool for 50% of training trials (trials two, four,six and eight), and the animals were released fromfour different starting positions (N, S, E and W).  C  . L . R .  Gonzalez et al  .  /   Beha  ioural Brain Research  112 (2000) 43–52  46 5. Procedure: experiment 2 Rats were started from the south (S) location onevery trial and the food was located in the center of thenorthwest (NW) quadrant of the pool.Fourteen rats were divided in two groups of seveneach, and each group received one of the followingtreatments:1. 100% reinforcement: one pellet was present in thefood cup on every training trial.2. 50% reinforcement: the food was present on only50% of the training trials (trials two, four, six andeight), so that the rats were required to search for 60s. 6. Results: experiment 1 The main finding of the present study was that theperformance of the animals that were continuouslyreinforced for finding the platform was superior on allmeasures in comparison to animals for which the plat-form was not present on a subset of trials. Indeed,animals for which the platform was present on only50% of trials displayed very little learning of the plat-form location. The typical swim paths in Fig. 1 for a100% reinforced animal and a 50% reinforced animalillustrate this result. 6  . 1 .  One starting location 6  . 1 . 1 .  Latency Latency measures are summarized in Fig. 2, left top.When analyzed by trials an ANOVA on latency indi-cated that the reinforcement history of the animals hada significant effect on acquisition, group  F   (3, 20) = 18.93  P  0.001. There was an effect of trial blocks,  F  (4, 80) = 10.81,  P  0.001, but no group by trial blockinteraction. Follow-up tests on the main effect of groupshowed the following significant differences in latenciesbetween the groups: 100  75  50% groups (Fisher’sLSD tests,  P s  0.02). When analyzed by reinforce-ments (Fig. 2, top right) similar results were observed,group  F   (3, 20) = 11.16  P  0.001. There was an effectof trial blocks,  F   (4, 80) = 20.79,  P  0.001, and groupby trial blocks interaction  F   (12, 80) = 2.54,  P  0.01. 6  . 1 . 2  .  Distance Distance measures are summarized in Fig. 2, leftbottom. An ANOVA on latency indicated that thereinforcement history of the animals had a significanteffect on acquisition, group,  F   (3, 20) = 12.27.  P  0.001. There was an effect of trial blocks,  F   (4, 80) = 24.25,  P  0.001, but no group by trial blockinteraction. Follow-up tests on the main effect of thegroup analysis showed the following significant differ-ences in latencies between the groups: 100 = 75  50%groups (Fisher’s LSD tests,  P s  0.02). The analysis byreinforcements (Fig. 2, right bottom) gave similar re-sults, group  F   (3, 20) = 7.64,  P  0.002. There was aneffect of trial blocks,  F   (4, 80) = 21.45,  P  0.001, butno group by trial block interaction. 6  . 2  .  Four starting locations 6  . 2  . 1 .  Latency Latency measures are summarized in Fig. 3, left top.An ANOVA on the latency on those trials on which theplatform was present indicated that the reinforcementhistory of the animals had a significant effect on acqui-sition, group  F   (1, 10) = 40.2,  P  0.001. There was aneffect of trial blocks,  F   (4, 40) = 10.62,  P  0.001), anda group by trial block interaction,  F   (4, 40) = 3.56, P  0.02. The analysis on reinforcements (Fig. 3, righttop) gave the same pattern of results, a significant effectof group  F   (1, 10) = 16.73,  P  0.003), an effect of trialblocks,  F   (4, 40) = 12.69,  P  0.001), and the interac-tion  F   (4, 40) = 4.59,  P  0.01). Fig. 1. Swim paths for two rats (median rats of the groups) on thefirst swim of each day’s training. The 100% rat had a platformlocated in the pool on every trial whereas the 50% rat had a platformlocated in the pool on every second trial (on the trials illustrated theplatform was present). Note the failure to learn the platform locationby the 50% rat.  C  . L . R .  Gonzalez et al  .  /   Beha  ioural Brain Research  112 (2000) 43–52   47Fig. 2. Place task acquisition (mean and standard error) as a function of reinforcement. Left panel shows data analyzed by trials and right paneldata analyzed by reinforcers. The top panel shows latency and the bottom panel shows distance (100, 75, 50% indicate percentage of swims inwhich the platform was present; 50% + P indicates platform was present on 50% of trials and on non-reinforced trial the rat was placed on theplatform at the completion of the swim). For every trial the platform remained in the same location and animals were always released from thesame starting point. 6  . 2  . 2  .  Distance Distance measures are summarized in Fig. 3, leftbottom. An ANOVA on latency indicated that thereinforcement history of the animals had a significanteffect on acquisition, group  F   (1, 10) = 35.31  P  0.001.There was an effect of trial blocks,  F   (4, 40) = 10.22, P  0.001, but no group by trial block interaction. Forreinforcements (Fig. 3, right bottom) there was a sig-nificant effect of group  F   (1, 10) = 11.67,  P  0.01, aneffect of trial blocks,  F   (4, 40) = 13.96,  P  0.001 andthe group by trial blocks interaction was also signifi-cant,  F   (4, 40) = 4.19,  P  0.01. 6  . 3  .  Probe trials Performance on the probe trials for both trainingprocedures described above is summarized in Fig. 4. AnANOVA on the probe trial of swims beginning at asingle starting point indicated that the reinforcementhistory of the animals had a significant effect on probetrial performance, group  F   (3, 20) = 13.85,  P  0.001.Follow-up tests on the main effect of group showed thefollowing significant differences in probe trial perfor-mance between the groups: 100  75  50% reinforcedgroup, Fisher’s LSD  P s  0.001. Performance on theprobe trial for the experiment in which swims startedfrom four locations is summarized in Fig. 4 (rightpanel). An ANOVA on latency indicated that the rein-forcement history of the animals had a significant effecton probe trial performance, group  F   (1, 10) = 47.72, P  0.001. Follow-up tests on the main effect of groupshowed the following significant differences in probetrial performance between the groups: 100  50% rein-forced group, Fisher’s LSD  P s  0.001. 6  . 4  .  Results :   experiment  2  When partial reinforcement occurred for the dry landspatial task, performance was indistinguishable fromanimals exposed to a continuous schedule. Further-more, when the number of reinforcements as opposedto the number of trials was analyzed, animals on a 50%schedule performed generally better than animals onthe continuous schedule. 6  . 4  . 1 .  Latency Latency measures are summarized in Fig. 5, left top.When analyzed by trials, no differences among the
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