Learning and Motivation 43 (2012) 66– 77

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Learning and Motivation

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The effect of asymmetrical sample training on retention functions forhedonic samples in rats

Sabrina Simmons, Angelo Santi ∗

Wilfrid Laurier University, Canada

a r t i c l e i n f o

Article history:Received 20 September 2011Received in revised form 28 February 2012Available online 7 April 2012

Keywords:Asymmetrical retention functionsSignal detectionSingle code/default strategyAsymmetrical sample trainingInstructional ambiguityDelayed matching-to-sampleRats

a b s t r a c t

Rats were trained in a symbolic delayed matching-to-sample task to discriminate sam-ple stimuli that consisted of the presence of food or the absence of food. Asymmetricalsample training was provided in which one group was initially trained with only the foodsample and the other group was initially trained with only the no-food sample. In addi-tion, within each group half of the rats were trained with an illuminated intertrial interval(ITI) and the remaining rats with a dark ITI. While the retention functions did not differas a function of which sample was trained first, they did differ as a function of the sim-ilarity in the illumination conditions during the ITI and the delay interval. Symmetricalretention functions were obtained when the lighting conditions were similar and slightlyasymmetrical retention functions were obtained when the lighting conditions were dissim-ilar. Probe tests confirmed that features of the no-food sample were attended to and usedto generate a memory representation for the no-food sample. The results are not consistentwith the hypothesis that asymmetrical sample training encourages coding of the sampleintroduced initially and default responding to the subsequently introduced sample. Ratsgenerate memory representations for both samples when asymmetrical sample training isgiven with hedonic samples.

© 2012 Elsevier Inc. All rights reserved.

Delayed matching to sample (DMTS) is a procedure commonly used to study memory in animals. In this procedure, ananimal learns to respond to one comparison stimulus after being presented with one sample, and to respond to a differentcomparison stimulus after being presented with a different sample. A number of studies conducted with pigeons haveused presence/absence samples. A presence sample involves the presentation of a stimulus such as food or a red hue asthe sample stimulus whereas an absence sample involves the absence of that stimulus (i.e., no-food or no red hue). Whenpresence/absence samples are used, asymmetrical retention functions are obtained where accuracy for the absence sampleremains high while accuracy for the presence sample declines sharply with increasing retention intervals (Colwill, 1984;Dougherty & Wixted, 1996; Grant, 1991; Wilson & Boakes, 1985; Wixted, 1993). A single code/default strategy was suggestedas an explanation of these asymmetrical retention functions. An animal using such a strategy is said to code only the presencesample and respond by default to the absence sample whenever there is no memory of the presence sample. A number ofalternative hypotheses have also been proposed including confusing the delay interval with the absence sample (Sherburne& Zentall, 1993a); differential pecking behavior during presence and absence samples (Sherburne & Zentall, 1993b; Weaver,Dorrance, & Zentall, 1999; Zentall, Kaiser, Clement, Weaver, & Campbell, 2000); and a signal detection account of performance

in the presence/absence task which, unlike the default response strategy, assumes that performance on absence-sample trialsinvolves an actual memory for non-occurrence (Dougherty & Wixted, 1996; White & Wixted, 2010; Wixted, 1993; Wixted& Dougherty, 1996; Wixted & Gaitan, 2004).

∗ Corresponding author at: Department of Psychology, Wilfrid Laurier University, Waterloo, Ontario, Canada N2L 3C5.E-mail address: asanti@wlu.ca (A. Santi).

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S. Simmons, A. Santi / Learning and Motivation 43 (2012) 66– 77 67

With the exception of the study by Wilson and Boakes (1985) in which jackdaws were also studied, all of the previouslyublished experiments on presence/absence retention functions have been conducted in pigeons. Santi, Simmons, andischler (2011) recently trained rats to discriminate samples consisting of the presentation of a food pellet (food sample)

r the absence of a food pellet (no-food sample) by responding to a stationary or moving lever. They obtained retentionunctions for the food and no-food samples and found that rats, like pigeons and jackdaws, exhibit more rapid forgettingn food-sample trials than on no-food-sample trials. In addition, they showed that these asymmetrical functions could note explained in terms of mediation of choice responding by magazine head-entry behavior during the retention interval.ore importantly, the asymmetrical retention functions were obtained when retention intervals were manipulated within

essions, but not when the retention intervals were manipulated between sessions. The differential effect of within versusetween session manipulations of retention intervals is consistent with previous findings reported by Wixted (1993) for theresence and absence visual samples in pigeons. Wixted argued that these results were consistent with a signal-detectionccount of performance on the presence–absence task. The signal-detection model maintains that one response (i.e., theesponse correct for the presence sample) is made when the subjective “strength of evidence” that a presence sample wasresented falls above a decision criterion and the alternative response is selected (i.e., the response correct for the absenceample) when the “strength of evidence” falls below the criterion. When retention intervals are manipulated within sessions,he detection model predicts asymmetrical retention functions for presence and absence samples. However, when delays manipulated between sessions, the detection model predicts that the retention functions for the presence and absenceamples will decline symmetrically, because the decision criterion can be altered (i.e., shifted leftward) when the sameetention interval occurs on all trials in a session, but not when the retention interval varies randomly from trial to trial in

session. Because a given delay interval is in effect for an extended number of sessions in the between manipulation, theecision criterion can be adjusted to maintain a more favorable reinforcement probability (averaged across trials).

The different results obtained as a function of whether the delay interval is manipulated within or between sessions areore difficult to explain in terms of a single code/default strategy because this hypothesis predicts asymmetric functions for

ood and no-food samples regardless of the way in which the delay interval is manipulated. Wixted and colleagues (Dougherty Wixted, 1996; Wixted & Dougherty, 1996; Wixted & Gaitan, 2004) have argued that the signal detection model predictssymmetric functions whenever memory for events which differ in salience are tested using a within-session manipulationf retention intervals (e.g., presence versus absence of a visual sample or a food sample, or memory for event duration).n these cases, very little forgetting is observed for the less salient sample (e.g., absent visual sample, no food, or a shorturation sample); however, rapid forgetting is observed for the more salient sample (e.g., presentation of a visual sample or

food sample, or a long duration sample). In general, Wixted and Gaitan (2004) suggested that when sample stimuli differn salience, animals transform the nominal discrimination task into a detection task in which they respond on the basis ofhe presence or absence of the more salient sample.

One set of findings which appears to be more easily explained in terms of single-code/default strategy rather than theetection model is the effect of asymmetrical sample training on retention functions for presence versus absence samples.rant and Blatz (2004) used a visual presence/absence sample in which the presence sample was a vertical line on a greenackground, and the absence sample was a vertical line on a black background. One group of pigeons was trained initiallyith only the presence-sample being presented on each trial, and a second group was trained with only the absence-

ample on each trial. Subsequently both groups of pigeons were trained with both samples and retention functions werebtained. Grant and Blatz found that in both groups the retention function for the initially trained sample declined rapidlyo below chance levels while the retention function for the sample trained second remained above chance. Consequently,hey argued that both groups of pigeons only coded the sample with which they were initially trained and responded byefault to the comparison stimulus that was designated correct for the sample introduced subsequently. Grant and Blatz alsoonducted transfer tests in which features of the presence and absence samples were modified. Consistent with their single-ode/default strategy explanation, they found that changing the features of the initially trained sample reduced accuracyore than changing the attributes of the sample introduced second for both groups of pigeons.Grant and Blatz (2004) speculated that asymmetrical training per se regardless of the nature of the samples employed

ill produce asymmetrical sample coding. Grant (2006) produced some evidence consistent with this suggestion in a studyhich employed color (i.e., red and green) and line orientation (i.e., vertical and horizontal) stimuli. Retention asymmetriesere obtained for both the color and line orientation stimuli with accuracy for the first-trained sample declining more

apidly than accuracy for the second-trained sample as the retention interval was increased.While the two stimuli used within each dimension in Grant (2006) were equally salient, the presence sample in the

rant and Blatz (2004) study was presumably more salient than the absence sample. From a detection model and stimulusalience perspective, it is difficult to account for the effect of asymmetrical sample training on the retention functions forhese nonhedonic samples. Given the support obtained for the detection model in our recent rat experiments (Santi et al.,011), we thought it would be informative to examine the effect of asymmetrical sample training with hedonic samples onubsequent retention functions in rats.

xperiment 1

The first experiment had two objectives: (1) to determine the effects of an asymmetrical training procedure on theetention functions for hedonic samples in rats; and (2) to assess whether ITI and delay interval illumination affect these

68 S. Simmons, A. Santi / Learning and Motivation 43 (2012) 66– 77

retention functions. One group of rats was initially trained with only the food sample being presented on each trial, anda second group was initially trained with only the no-food sample presented on each trial. Following training with bothsamples, retention functions were obtained. Based on the findings of Grant and Blatz (2004) and Grant (2006), it mightbe predicted that the rats would code the sample introduced initially and respond by default to the sample introducedsubsequently. If this were the case, then we would expect to observe the usual food/no-food retention asymmetry for ratstrained initially with the food sample, but a reversal of this asymmetry for rats trained initially with the no-food sample.That is, rats trained initially with the no-food sample might code features of the no-food sample and respond by default ontrials initiated by the food sample. Consequently, for this group, the retention function for the no-food sample would declinerapidly to below chance levels while the retention function for the food sample trained second should remain above chance.

On the other hand, asymmetrical sample training could have different effects for hedonic samples which differ substan-tially in salience than for visual samples which are either equally salient (Grant, 2006) or differ moderately in salience (Grant& Blatz, 2004). Because of the high salience of food samples, regardless of whether they are introduced first or second, ratsmight acquire the discrimination as a detection task and respond on the basis of the presence or absence of the highly salientfood sample. If this were to occur, then we would expect to observe the retention function for the food sample to declinemore rapidly than the retention function for the no-food sample regardless of which sample was trained first.

Finally unlike the typical procedure in which both food and no-food samples are trained concurrently, it may be that anasymmetrical training procedure with hedonic samples discourages rats from using a detection strategy based on stimulussalience and encourages them to generate memory codes for both the food and the no-food sample. If this were the case, theninstead of asymmetrical retention functions, we would expect to observe symmetrical retention functions with accuracy forboth samples dropping to chance levels.

The second objective of this experiment was to determine whether illumination of the ITI and the delay interval affects thenature of the retention functions for hedonic samples. As noted earlier, there have been a number of alternatives proposedto the single-code/default strategy explanation of asymmetrical food/no food retention functions. One alternative, whichhas been ruled out in pigeons, is ambiguity between the delay interval and the ITI (Zentall, 1997, 2005). According to thishypothesis, when stimulus conditions are the same during the ITI and the delay interval, a pigeon presented with a noveldelay interval may perceive it to be an ITI and expect to receive a sample at the end of the interval. However, because nosample is presented following a delay interval, the pigeon responds to the comparison associated with the no-food samplebecause no sample is more similar to a no-food sample than to a food sample. This explanation was ruled out in pigeons bystudies which reported asymmetrical functions even when testing was conducted with an illuminated ITI and a dark delayinterval (Grant, 2009; Sherburne & Zentall, 1993b). In order to assess whether ITI/delay interval ambiguity has any effect onthe retention functions for hedonic samples following asymmetrical sample training, half of the rats were trained with anilluminated ITI and the remaining rats with a dark ITI. During subsequent delay tests, the illumination condition during thedelay interval was randomly varied within session for all rats.

Method

SubjectsSixteen experimentally naïve male hooded Long-Evans rats were used as subjects. They were obtained from Charles River

Canada (St. Constant, Quebec) at 43–46 days old. They were individually housed in clear Plexiglas cages in a temperature-and humidity-controlled holding room under an alternating 12 h light-dark cycle. Experimental sessions were given duringthe light phase. The rats had free access to water but their diet was restricted to maintain bodyweight at approximately 85%of their free-feeding weights.

ApparatusFour Coulbourn modular operant test cages (Model #E10-10), housed individually in isolation cubicles (Model #E10-20)

and located in the same room, were used. The cubicles were equipped with a ventilation fan and baffled air intake exhaustsystem. Each test cage was equipped with a 45-mg pellet dispenser (Model #E14-06), which was mounted in the center ofthe front wall of each test cage just above the steel-grid flooring. Two retractable levers (Model #E23-07) were mounted2.5 cm above the floor on either side of the pellet feeder, and a cue-light was positioned above each lever. A single Sonalerttone module (Model # E12-02) was mounted 2.5 cm above each cue-light: one tone module was 2.9 kHz, the other was4.5 kHz. The spatial location of the tone modules (left and right) was counterbalanced across boxes. A houselight was alsoinstalled 6.5 cm above the pellet feeder and positioned so that the light was facing upward and reflected off of the ceiling ofthe test cage (Coulbourn Model #E11-01 with bulb #SL1819X). All of the experimental events and responses were controlledby a microcomputer located in the same room as the four test cages.

Procedure

Preliminary trainingEach rat received several sessions of combined magazine and lever training. The rats were placed in the operant chamber

with both the right and left levers retracted. Each trial began with the entry of the left or right lever into the chamber. Thelever remained extended until it was pressed or 60 s had elapsed, whichever occurred first. Either event resulted in delivery

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S. Simmons, A. Santi / Learning and Motivation 43 (2012) 66– 77 69

f a 45-mg food pellet and retraction of the lever. Pellet delivery produced an audible “click”, and the light in the magazineas illuminated for 0.5 s. The houselight was illuminated during all of the preliminary training sessions. The sessions were

erminated when the rat pressed the lever 60 times or 60 min had passed. After rats had acquired the lever press to atationary lever, they were trained to respond to both a stationary lever and a moving lever on both the right and the leftide of the test chamber. A stationary lever was a lever which entered the chamber and remained extended until a leverress occurred or 6 s elapsed without a lever press, whichever occurred first. If no response occurred, the stationary leveras retracted and it was presented again after 5 s had elapsed. The moving lever was inserted and retracted at 0.6 s intervals.

hese 1.2-s cycles continued for a maximum of 5 repetitions (i.e., 6 s). If no response occurred within 6 s, the moving leveras retracted and it was presented again for another set of 5 cycles of 1.2 s after 5 s had elapsed. Responses to either the

tationary or the moving lever within the 6-s presentation interval resulted in immediate retraction of the lever and deliveryf a food pellet. Preliminary training continued until all of the rats were reliably pressing both the stationary and movingever.

ingle-sample trainingEach trial began with the presentation of a sample stimulus for 4 s. For the eight rats randomly assigned to group food-

rst, the sample consisted of a food pellet and the simultaneous presentation of tone and magazine light illumination for s. The food pellet was always presented at the beginning of the 4-s sample. For the remaining eight rats assigned to groupo-food-first, the sample consisted of the simultaneous presentation of tone and magazine light illumination for 4 s but no

ood-pellet delivery. For rats in group food-first, only the food sample was presented on every trial. For the rats in groupo-food-first, only the no-food sample was presented on every trial. Regardless of whether the sample was food or no food,

t was always accompanied by the presentation of a tone from the module on the right side of the chamber. For half of theats in each group, this was a high frequency (4.5 Hz) tone and for the remaining rats in each group, it was a low frequency2.9 Hz) tone. Tone frequency was introduced and counterbalanced between rats so that it provided a stimulus feature whichould be altered in Experiment 2 in order to determine which features of a sample had been attended to and coded intoemory. Throughout Experiment 1, each rat heard the same tone during both the initially and the subsequently trained

ample.Each session consisted of 64 trials separated by an ITI of 8, 16, 32 or 64 s, randomly selected without replacement. The ITI

ighting conditions were counterbalanced such that eight rats (four from group food-first and four from group no-food-first)ad the houselight illuminated during the ITI (group ITI light), while the remaining eight rats (four from group food-firstnd four from group no-food-first) experienced a dark ITI (group ITI dark). Each trial began with the presentation of theample stimulus. Following presentation of the sample, the rats were presented with either the stationary lever on the leftnd the moving lever on the right, or the moving lever on the left and the stationary lever on the right. The moving leveras retracted and inserted at 0.6-s intervals, and these repeating 1.2-s cycles continued until the first lever press after a

.4-s delay. The 2.4-s delay between the insertion of the levers and the recording of the choice response was used so thathe animal had enough time to determine which lever was moving before making its response. This will be referred to as a-s delay condition. For four rats in the food-first group, a press to the moving lever following the food sample was correct,hile for the remaining four rats a press to the stationary lever following the food sample was correct. For four rats in theo-food-first group, a press to the moving lever following the no-food sample was correct, while for the remaining four rats

press to the stationary lever following the no-food sample was correct. For all rats, a response to either lever retracted bothevers. A correct response resulted in delivery of a food pellet and illumination of the magazine light for 0.5 s followed byhe ITI. An incorrect response was only followed by an ITI. Each combination of sample and comparison lever test conditionccurred twice in each block of four trials. The order of trial presentation was randomized individually for each rat in eachession. All rats received 15 sessions of single-sample training.

ingle-sample delay testingIt is somewhat unclear why single-sample delay testing was conducted by Grant and Blatz (2004). However, in order to

ollow their procedures as closely as possible, similar delay testing was conducted in the present study. All parameters androcedures for test sessions were identical to those of training except that the delay between end of the sample sequencend the entry of the comparison levers into the chamber was 0, 5, 10, and 15 s. Within each block of 32 trials, 20 trialsccurred at the 0-s baseline delay, and 4 trials occurred at each of the other delays (5, 10, and 15 s). For one half of the trialsnvolving the 5, 10, and 15-s delays, the entire delay was spent in darkness. On the other half of the trials, the houselight

as illuminated for the entire delay. Each of the eight test sessions consisted of 64 trials.

wo-sample trainingIn this phase, both groups of rats were exposed to the same procedure. Half of the trials in each session were the same

s those during single-sample training. The remaining trials involved the sample that was not presented during single-ample training. On these trials, the no-food sample was presented to rats in the food-first condition and the food sample

as presented to rats in the no-food-first condition. For all rats, the comparison lever that was incorrect on trials with the

nitially trained sample was correct on trials with the newly introduced sample. Each session consisted of 128 trials. Eachombination of sample type and comparison test condition occurred once in each block of four trials. The order of trialresentation was randomized individually for each rat in each session. All rats received either 20 or 26 sessions of training.

70 S. Simmons, A. Santi / Learning and Motivation 43 (2012) 66– 77

The study was run in two replications with 8 rats each. Although all rats acquired the task and were performing well after20 sessions of training, one replication group could not be moved to delay testing because of a two week shutdown of thelab. Consequently, this group received four training sessions just prior to the shutdown and two training sessions after theshutdown.

Two-sample delay testingAll parameters and procedures for test sessions were identical to those of training except that the delay between end of

the sample sequence and the entry of the comparison levers into the chamber was 0, 5, 10, and 15 s. Within each session,40 trials for each sample (food, no food) occurred at the 0-s baseline delay, and 8 trials for each sample occurred at each ofthe other delays (5, 10, and 15 s). For one half of the trials involving the 5, 10, and 15-s delays, the entire delay was spent indarkness. On the other half of the trials, the houselight was illuminated for the entire delay. All rats received 15 sessions ofdelay testing.

For all of the statistical tests reported in this article, the rejection region was p < 0.05.

Results and discussion

Single-sample trainingAll rats in both the food-first and the no-food-first condition rapidly acquired the single-sample discrimination to 90%

accuracy. An ANOVA was conducted on the training data with training condition (food-first, no-food-first) and ITI illumination(light and dark) as between-subjects factors and session (the first 15 sessions which were common to all rats) as a within-subjects factor. There was a significant main effect of session, F(14,168) = 36.19, and an ITI illumination × session interaction,F(14,168) = 1.83. This interaction occurred because during the third and fourth training sessions, rats in the ITI dark groupperformed more accurately (87.5%) than rats in the ITI light group (73.5%). However, by the fifth session of training, accuracyaveraged above 90% in both groups and it remained at that level throughout the remaining sessions.

Single-sample delay testingAccuracy was very high across all conditions except for a slight decrease across delays. Averaged across all rats, accuracy

at the four delays was 96.0%, 94.1%, 94.1% and 89.5%. An ANOVA was conducted on the test data with training condition (food-first, no-food-first) and ITI illumination (light and dark) as between-subjects factors and delay interval illumination (light,dark), and delay interval length (0, 5, 10, and 15 s) as within-subjects factors. For purposes of analysis, one half of all 0-s delayswere randomly designated as dark delays, and the other half were randomly designated as illuminated delays. There wasa significant main effect of delay interval length, F(3,26) = 6.72, but no other significant effects were found. Grant and Blatz(2004) also reported a slight but statistically significant decrease in accuracy across delays in pigeons when single-sampletraining was given with visual presence/absence stimuli.

Two-sample trainingFig. 1 presents accuracy data during two-sample training for group ITI light in the top panel and group ITI dark in the lower

panel. During the first block of training, accuracy for the sample that the rats had been initially trained with remained high(i.e., the food sample for the group trained initially with the food sample, and the no-food sample for the group trained initiallywith the no-food sample), while accuracy for the introduced sample was quite low (i.e., the no-food sample for the food-first group, and the food sample for the no-food-first group). Accuracy for the introduced sample increased rapidly duringthe second block of training sessions. An ANOVA was conducted on accuracy data for the first 20 sessions of training withsample (food, no-food) and block (1, 2, 3, 4) as within-subject factors and ITI illumination (light, dark) and training condition(food-first, no-food-first) as between-subjects factors. There were significant main effects of sample, F(1,12) = 4.84, and ofblock, F(3,36) = 121.99, as well as significant interactions of sample × block, F(3,36) = 2.80, sample × block × ITI illumination,F(3,36) = 3.33, and sample × block × training condition, F(3,36) = 71.54. No other main effects or interactions were statisticallysignificant. The sample × block × ITI illumination interaction occurred because, for group ITI dark, accuracy on no-food-sample trials (collapsed across food-first and no-food-first conditions) was significantly higher than accuracy for food-sampletrials in block 1, F(1,12) = 10.70, but not blocks 2, 3 and 4, F’s ≤ 1.54. In group ITI light, there was no difference in accuracy asa function of sample type after collapsing across the training conditions. The sample × block × training condition interactionoccurred as a result of a significant sample × training condition interaction during block 1 of acquisition, F(1,12) = 206.36.No statistically significant differences were obtained at blocks 2–4. This indicates that during the initial block of training,accuracy for the sample that the rats were initially trained with remained quite high, while accuracy for the introducedsample was low. Across session blocks, accuracy for the introduced sample increased to levels equivalent to that of theinitially trained sample.

Two-sample delay testing

A preliminary analysis of the retention test data indicated that there was no significant main effect or interactions asso-

ciated with the training condition variable. In order to simplify the presentation and analysis of data, this variable was notincluded in the subsequent analyses. Fig. 2 shows the data for group ITI light in the top panel and group ITI dark in the lowerpanel. When the illumination condition during the delay interval was the same as during the ITI, both groups exhibited

S. Simmons, A. Santi / Learning and Motivation 43 (2012) 66– 77 71

Group ITI Light

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ig. 1. Accuracy during two-sample training as a function of sample type (food and no food) and training condition (food first, no food first). The data forroup ITI light are shown in the top panel and the data for group ITI dark are shown in the lower panel. Error bars represent the standard error of the mean.

arallel retention functions for the food and no-food samples. However, when the illumination conditions differed betweenhe delay interval and the ITI, both groups exhibited a moderate asymmetry with a greater decline in accuracy for the foodample than for the no-food sample. An ANOVA was conducted on the retention testing data with sample (food, no food),elay interval illumination (light, dark) and delay interval length (0, 5, 10, 15 s) as within-subjects factors and ITI illuminationlight, dark) as a between-subjects factor. There was a significant main effect of delay interval length, F(3,42) = 249.43, as wells significant interactions of sample × delay interval length, F(3,42) = 3.72, sample × delay interval illumination × ITI illumi-ation, F(1,12) = 19.61, and sample × delay interval length × delay interval illumination × ITI illumination, F(3,42) = 8.86. The

nteraction of sample × delay interval length × delay interval illumination was statistically significant both for group ITI light,(3,42) = 6.48, and group ITI dark, F(3,42) = 4.26. For group ITI light, there was a significant sample × delay interval lengthnteraction when the delay interval was dark, F(3,42) = 2.82, but not when it was light, F(3,42) = 2.01. When the delay interval

as dark, accuracy for the no-food sample was higher than for the food sample at the longer delays, but the difference wastatistically significant only at the 10-s delay, F(1,14) = 5.04. For group ITI dark, the sample × delay interval length interactionas significant when the delay interval was light, F(3,42) = 6.01, but not when it was dark, F < 1. When the delay interval was

ight, accuracy for the no-food sample was higher than for the food sample at both the 10-s and 15-s delays, F(1,14) = 5.23nd 6.52, respectively.

The present results indicate that asymmetrical sample training with hedonic samples in rats produces different resultsrom those obtained with nonhedonic samples in pigeons (Grant, 2006; Grant & Blatz, 2004). Asymmetrical training withonhedonic samples in pigeons appears to encourage coding of the sample introduced initially and default responding tohe subsequently introduced sample. This clearly does not occur when rats are given asymmetrical training with hedonicamples. Regardless of whether the food sample was introduced first or second in training, symmetric retention functionsor food and no-food samples were obtained when the illumination conditions during the ITI and the delay interval were theame. However, when the illumination conditions during the ITI and delay intervals differed, asymmetric retention functionsor food and no-food samples were obtained.

While the retention functions did not differ as a function of whether the food sample or the no-food sample was trainedrst, they are different from those previously reported for rats (Santi et al., 2011). In the Santi et al. study, rats were trainedoncurrently with food and no-food samples and both the ITI and delay interval were dark. When the delay interval was

72 S. Simmons, A. Santi / Learning and Motivation 43 (2012) 66– 77

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Fig. 2. The mean percentage of correct responses for the food sample and the no-food sample as a function of delay interval illumination and delay intervallength. The data for group ITI light are shown in the top panel and the data for group ITI dark are shown in the lower panel. Error bars represent the standarderror of the mean.

varied within sessions, the retention function for food samples decreased markedly, but the retention function for no-foodsamples was unaffected by delay interval duration. Thus, unlike Santi et al. (2011), in the present study accuracy decreasedsignificantly for both the food and the no-food samples when the delay interval was increased. The size of the asymmetrywas either smaller or entirely absent depending on the similarity of the ITI and delay interval illumination condition.

These results are difficult to explain on the basis of previous hypotheses developed to explain differences in retentionfunctions for asymmetrical events. First, they are difficult to explain in terms of the operation of a single code/default responsehypothesis. Although, it could be claimed that rats coded the food sample and responded by default on trials initiated bythe no-food sample regardless of whether the food sample was trained initially or introduced subsequently, this wouldnot explain why the resulting asymmetry is attenuated relative to previous findings nor why it was only present when thelighting conditions during the ITI and the delay interval were different. The results are also inconsistent with the instructionalconfusion hypothesis (Zentall, 1997, 2005). According to this hypothesis, animals may confuse a novel delay interval withthe ITI and make the comparison response that is consistent with not having been presented with a sample (i.e., makingthe response associated with the no-food sample). But clearly the present results are opposite to what would be expectedaccording to this explanation. Finally, the detection model based on stimulus salience has difficulty with these findings. Thatis, if it is assumed that stimulus salience is not altered by asymmetrical sample training, then the food sample should bemore salient than the no-food sample, regardless of whether the food sample was trained first or second. Consequently,the rats should have responded based on the detection of the more salient sample and exhibited faster forgetting for thefood sample. It is unlikely that extended experience with food and no-food samples alters the salience of hedonic samplesbecause following the extensive between-session retention testing reported in Experiment 2 of Santi et al. (2011), a within-session test of retention once again produced asymmetrical retention functions which were consistent with the criterionshift predictions of the detection model.

Given the symmetrical retention functions obtained when the ITI and delay illumination conditions are similar and the

modest asymmetry observed when the ITI and delay illumination conditions differ, it seems that compared to previousstudies, in which food and no-food samples were trained concurrently, single-sample training with hedonic samples resultsin the generation of a memory representation for both the food and the no-food sample which does not have the “strength

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f evidence” difference that allows a detection strategy to be applied. If this assumption is correct, there should be somevidence that features of the no-food sample are attended to and used to generate a memory representation for the no-foodample. Experiment 2 was conducted to determine which features of the food and no-food samples, in addition to food itself,ay have been used to generate these memory representations.

xperiment 2

Experiment 2 was based on the assumption that changes to features of a sample that are attended to and used to generate memory code would be more disruptive than changes to features of a sample that are attended to but not used to generate memory code. On intermittent probe trials, various features of the sample were changed or eliminated. During the first setf test sessions, the frequency and location of the tone present during sample presentation was modified, or the tone was notresented. During the second set of test sessions, the magazine light was not presented during sample presentation, or bothhe magazine light and the tone were not presented. Assuming that rats only used the occurrence of a food pellet to generate

memory representation for the food-sample there should be no effect of changing the tone location and frequency, or ofot presenting the tone and/or the magazine light on food-sample trials. However, if features of the no-food sample (i.e.,he tone location and frequency, or the tone and/or the magazine light) are used to generate a memory representation forhe no-food sample, then the absence of one or more of these features would result in a decrease in accuracy. The memoryepresentations generated for the food and the no-food samples could be retrospective codes of one or more sample featuresr they could be prospective codes regarding which lever to press. While the present study cannot provide any informationoncerning the content of the memory code, it should provide evidence regarding the features of the no-food samples whichre necessary for generating an appropriate memory representation for these samples.

ethod

ubjects and apparatusThe sixteen rats tested in Experiment 1 were used in this study. The apparatus was the same as in Experiment 1.

rocedureAll rats were returned to baseline training for 2 sessions of 128 trials each before receiving probe testing sessions.

rocedures during baseline training were identical to those of two-sample training.The first eight sessions of probe testing consisted of 64 trials. On 48 trials of each session, the food and no-food samples

hat the rats had been previously trained on were presented. The remaining 16 trials were probe trials. Eight were trials inhich a different tone (altered location and frequency) was presented during the sample (4 food and 4 no-food samples), and

ight were trials in which the tone was absent during sample presentation (4 food and 4 no-food samples). In each block of trials, there were 4 regular trials and 2 probe trials. On probe trials, the rats were reinforced according to the contingenciesxperienced during two-sample training. On probe trials in which the tone location and frequency was changed, the toneas presented from the left side of the chamber for all of the rats and the frequency experienced was higher (4.5 kHz) than

he baseline tone for half of the rats and lower (2.9 kHz) for the remaining rats.The second eight sessions of probe testing were the same as those described above except that the 16 probe trials per

ession were different. Eight were trials in which the magazine light was not presented during sample presentation (4 foodnd 4 no-food samples), and eight were trials in which both the magazine light and the tone were absent during sampleresentation (4 food and 4 no-food samples). All other aspects of the procedure were the same as previously described.

esults and discussion

Fig. 3 presents accuracy on baseline trials, different-tone trials, and no-tone trials for both the food and no-food sampleveraged over ITI illumination and training conditions. Trials for which the location and frequency of the tone was changedere responded to very accurately. The only reduction in accuracy occurred when the tone was absent during the presen-

ation of the no-food sample. An ANOVA was conducted on the data with sample (food, no-food) and trial type (baseline,one2, no tone) as within-subjects factors and ITI illumination (light, dark) and training condition (food-first, no-food-first)s between-subjects factors. The only statistically significant effects were a main effect of trial type, F(2,24) = 22.45, and

trial type × sample interaction, F(2,24) = 9.65. The interaction was due to a significant effect of trial type for the no-foodample, F(2,24) = 16.96, and no significant effect of trial type for the food samples, F(2,24) = 2.49. For the no-food samples,n LSD test indicated that there was no difference in accuracy between the baseline and the probe trials in which the toneas altered in frequency and location, however accuracy was significantly lower when the tone was absent relative to both

f the other conditions. These results indicate that the rats were attending to the tone and using it in conjunction with thebsence of a pellet to generate a memory representation when the sample was no-food, but that the specific location and

requency of the tone was not relevant. The data are inconsistent with the hypothesis that rats were responding by defaulto the comparison lever correct for the no-food sample on trials in which the food sample is not presented.

Fig. 4 presents accuracy on baseline trials, no-magazine-light trials and no-tone-no-magazine-light trials for both theood and no-food samples. The data for group ITI light are shown in the top panel and for group ITI dark in the lower panel.

74 S. Simmons, A. Santi / Learning and Motivation 43 (2012) 66– 77

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Fig. 3. The mean percentage of correct responses for the food sample and the no-food sample on baseline trials, different-tone probe trials, and no-toneprobe trials. Error bars represent the standard error of the mean.

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Fig. 4. The mean percentage of correct responses for the food sample and the no-food sample on baseline trials, no-light probe trials, and no-tone/no-lightprobe trials. The data for group ITI light are shown in the top panel and the data for group ITI dark are shown in the lower panel. Error bars represent thestandard error of the mean.

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S. Simmons, A. Santi / Learning and Motivation 43 (2012) 66– 77 75

he absence of only the magazine light during sample presentation did not reduce accuracy on either food or no-food samplerials. However the absence of both the tone and the magazine light during sample presentation resulted in a large reductionn accuracy but only for the no-food-sample trials. An ANOVA was conducted on the data with sample (food, no-food)nd trial type (baseline, no light, no tone-no light) as within-subjects factors and ITI illumination (light, dark) and trainingondition (food-first, no-food-first) as between-subjects factors. There were significant main effects of ITI illumination,(1,12) = 9.76, of sample, F(1,12) = 40.05, and of trial type, F(2,24) = 131.41. There were also significant two-way interactionffects of sample × ITI illumination, F(1,12) = 5.80, trial type × ITI illumination, F(2,24) = 15.87, trial type × training condition,(2,24) = 3.63, and sample × trial type, F(2,24) = 42.64. The three-way interaction of sample × trial type × ITI illumination waslso significant, F(2,24) = 9.39. A simple main effect analysis of the three-way interaction showed that the sample × trial typenteraction was significant for group ITI light, F(2,24) = 6.09, as well as group ITI dark, F(2,24) = 45.94. When the sample

as food, there was no statistically significant effect of trial type for either group ITI light, F(2,24) = 3.12 or group ITI dark,(2,24) = 3.00. However, when the sample was no food, there was a significant effect of trial type for both group ITI light,(2,24) = 14.61, and group ITI dark F(2,24) = 79.86, but the effect was greater for group ITI dark. For group ITI light, an LSDest indicated that accuracy for the no-food sample was significantly lower on trials when neither the tone nor the lightas presented compared to baseline trials and compared to trials in which only the tone was presented. There was no

ignificant difference between the baseline trials and trials in which only the tone was presented. For group ITI dark, an LSDest indicated that accuracy for the no-food sample was significantly lower on trials when neither the tone nor the lightere presented compared to baseline trials and compared to trials in which only the tone was presented. There was also

ignificantly lower accuracy on trials in which only the tone was presented compared to baseline trials. In summary, both ITIroups showed the greatest drop in accuracy for no-food samples when neither the light nor the tone were presented, andhis drop was greater for group ITI dark than group ITI light, F(1,12) = 16.74. On these test trials, accuracy for group ITI lightemained significantly above chance, t(7) = 6.12, while accuracy for group ITI dark did not differ significantly from chance,

< 1.A simple main effects analysis of the trial type × training condition interaction showed that there was a significant effect

f trial type for both the group trained with food first, F(2,24) = 40.35, and the group trained with no-food first, F(2,24) = 85.69.he interaction occurred because there was no significant difference in accuracy between the two training conditions onither baseline trials or test trials in which neither the tone nor the light was presented, F(1,12) = 2.55, and F < 1, respectively;owever, on test trials in which no light was presented, accuracy was somewhat lower for group food-first (M = 91.0%), than

or group no-food-first (M = 96.9%).Overall, the most important finding from transfer testing was the sample × trial type interaction. Accuracy on trials

nvolving the food sample was not affected by the absence of the tone and the magazine light during sample presentation.his suggests that the rats were primarily coding the occurrence of the food pellet on food-sample trials and that the featuresf tone and magazine light were not required for this coding. It is important to note that the delivery of a food pellet at thetart of a food sample (as well as during the reinforcement for a correct response) produced a very audible click from theotary solenoid and that this click strongly controlled retrieval of the pellet from the magazine regardless of whether theone and/or magazine light were presented. Unlike the trials involving food samples, accuracy on trials involving the no-foodample was significantly reduced when the tone and the magazine light were both absent. This reduction in accuracy wasreater for group ITI dark than for group ITI light. For group ITI dark, the test trials in which neither the tone nor the magazineight were presented on no-food sample trials were essentially extensions of the ITI because no stimulus changes markedhe transition from an ITI to the presentation of the no-food sample on these test trials. In the absence of any information onhese trials, the rats responded at chance levels. This finding supports the claim that responding on no-food-sample trialsas based on a memory representation for the no-food sample, and not on a default response strategy. For group ITI light,

he houselight was terminated at the end of the ITI, so that no-food-sample trials in which neither the tone nor the magazineight was presented would not be experienced as extensions of the ITI. It seems that multiple stimulus features (i.e., offsetf the houselight, presentation of the tone, and the magazine light) were used to generate a memory representation for theo-food sample in group ITI light. In the absence of the tone and the magazine light, the offset of the houselight by itselfith no food pellet delivery was partially successful in generating an appropriate memory code for the no-food sample. As

result, when presented with the comparison alternatives on these test trials, the rats in group ITI light responded to theo-food associated comparison at levels significantly above chance but the rats in group ITI dark did not.

eneral discussion

Asymmetrical training with hedonic samples in rats produces different results from those obtained when symmetricalraining with hedonic samples is given to rats (Santi et al., 2011) or when asymmetrical training with nonhedonic samples isiven to pigeons (Grant, 2006; Grant & Blatz, 2004). In pigeons, asymmetrical training with nonhedonic samples encouragesoding of the sample introduced initially and default responding to the subsequently introduced sample. This does not occurhen rats are given asymmetrical training with hedonic samples. Regardless of whether the food sample was trained first or

econd, symmetric retention functions were obtained when the ITI and delay interval illumination conditions were the same,nd moderately asymmetric retention functions were obtained when the ITI and delay interval illumination conditions wereifferent. It is important to note that even in the conditions which produced asymmetric retention functions, accuracy foroth the food and the no-food samples declined as the delay was increased. The retention functions obtained in Experiment

76 S. Simmons, A. Santi / Learning and Motivation 43 (2012) 66– 77

1 were also different from those reported when rats are concurrently trained with food and no-food samples prior todelay testing (Santi et al., 2011). When symmetrical sample training is given and delay intervals are varied within sessions,retention functions for the food samples show a steep decline, while the functions for the no-food samples are relatively flat(Santi et al., 2011). Overall, the results of Experiment 1 indicate that rats do not respond on the basis of a single code/defaultstrategy, nor do they respond in terms a detection strategy based on stimulus salience (i.e., strength of evidence along acontinuous dimension), when asymmetrical sample training is given with hedonic samples. That is, rats appear to generatea memory representation for both the food and the no-food sample. Evidence for this was provided in Experiment 2. Alteringfeatures of the samples, such as magazine light illumination and tone had no effect on accuracy for the food-sample trials. Thissuggests that on food-sample trials the rats were primarily using the delivery of a food pellet to code either the occurrenceof food in memory (i.e., a retrospective sample code) or an instruction regarding which lever to press (i.e., a prospectiveresponse code). Unlike food-samples, accuracy for the no-food samples was moderately reduced by the absence of tone andmore strongly reduced by the absence of both the tone and the magazine light, particularly in group ITI dark. This indicatesthat responding on no-food-sample trials was not based on a default response strategy. It appears that the compound ofthe tone and magazine light on no-food-sample trials (along with the offset of the houselight during sample presentationfor group ITI light) was used to generate either a prospective memory code or a retrospective memory code for the no-foodsample.

The evidence presented in Experiment 2 is consistent with the claim that rats were generating memory codes for boththe food and no-food samples that differed qualitatively but not quantitatively on a “strength of evidence” continuum.The absence of an asymmetry in the retention functions for food and no-food samples in Experiment 1 when the ITI anddelay interval illumination were similar is consistent with the assumption that memory codes were generated for bothsamples. Even in the case of the modest asymmetry which occurred when the ITI and delay interval illumination differed,it is important to note that accuracy declined as a function of delay interval for both samples. This reinforces the claim thatresponding to the comparison alternative correct for the no-food sample did not occur by default.

The differential effects of ITI and delay interval illumination on the retention functions were surprising and appear to bethe result of somewhat different processes. The instructional ambiguity hypothesis (Zentall, 1997, 2005) may be applicableto these results, although from a different perspective than that outlined in the introduction to this paper. Kaiser, Zentall,and Neiman (2002) considered the effects of similarity between the ITI and a gap in a signal on pigeons’ timing of that signalin the peak procedure. They noted that gaps which are identical to an ITI could encourage pigeons to treat a gap as if it werean ITI. They also assumed that pigeons might use the ITI to clear out the memory trace from the preceding trial so that itdoes not affect performance on the following trial. As a result, they expected that pigeons might reset and start timing anew interval when the gap ended. They suggested that resetting memory during the gap could be thought of as instructionalambiguity (Zentall, 1997, 2005) due to the similarity of the ITI and gap. The assumption that pigeons might use the ITI toclear out a memory trace from the preceding trial may be relevant to the present results. That is, when the delay intervalillumination is the same as illumination during the ITI, there may be a rapid resetting of the memory code for the food or theno-food sample generated on that trial. This might be expected to produce equivalent declines in retention for both samples.On the other hand when the delay interval illumination differs from that which occurs during the ITI, the memory codesmay not be subject to rapid resetting. In the case of group ITI light, recall that the testing in Experiment 2 showed that offsetof the houselight by itself in the absence of the tone, the magazine light, or delivery of a pellet was sufficient to generateresponding to the comparison alternative correct for the no-food sample at a level significantly above chance. It may be thatan extended delay interval with no pellet delivery and the houselight not illuminated was treated as being more similar toa no-food sample than a food-sample and resulted in a detectable bias to respond to the comparison correct for the no-foodsample. This is also a type of instructional ambiguity, called sample-delay ambiguity, which has been documented in studiesof memory for number in both pigeons and rats (Keough, Santi, & Van Rooyen, 2007; Santi, Lellwitz & Gagne, 2006; Santi& Van Rooyen, 2007; Van Rooyen & Santi, 2008; Zentall, 2007). Finally, in the case of group ITI dark, illumination of thehouselight during the DI was a novel event but more similar to trial events like illumination of the magazine light during asample than to a dark ITI. Thus, an extended delay interval with no pellet delivery and the houselight illuminated may havebeen treated as being more similar to a no-food sample than a food-sample and resulted in a detectable bias to respond tothe comparison correct for the no-food sample.

In summary, the present results demonstrate that when asymmetrical sample training is provided with hedonic samples,rats appear to generate memory representations for both the food sample and the no-food sample. The memory representa-tions appear to differ qualitatively but not quantitatively because the retention functions are inconsistent with the hypothesisthat rats are employing a detection strategy based on a “strength of evidence” continuum. Unlike the results of asymmetricaltraining with nonhedonic samples in pigeons, asymmetrical training with hedonic samples does not result in the rats codingthe sample introduced initially and then responding by default to the sample introduced subsequently. This discrepancyencourages additional research in which pigeons are asymmetrically trained with hedonic samples, and rats are trainedasymmetrically with nonhedonic samples. It may be that in both rats and pigeons, asymmetrical training results in defaultresponding to the sample introduced subsequently only for nonhedonic samples.

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cknowledgments

This research was supported by Grant OGPOOD6378 from the Natural Sciences and Engineering Research Council ofanada to A. S. The authors thank Kelly Putzu and Tammy Buitenhuis for their assistance in caring for the animals.

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