Behavioural Elsevier
Processes,
17 (1988) 229-238
229
EFFECTS OF INTERTRIAL REINFORCERS ON RATS' TIMING BEHAVIOR DONALD M. WILKIE, LAWRENCE A. SYMONS and RICHARD C. TEES Department of Psychology, Canada V6T lY7 (Accepted 28 March
University of British Columbia, Vancouver, B.C.
19Q,
ABSTRACT Wilkie, D.M., Symons, L.A. and Tees, R.C. 1988. Effects of intertrial reinforcers on rats' timing behavior. Behav. Processes 17 : 229-238 Two experiments were conducted to assess the effects of non-contingent intertrial interval (ITI) reinforcers on rats' discriminationof duration. In the first experiment, rats' discriminationof a 2 vs. 8 s of light was significantly disrupted when reinforcerswere presented in the ITI. Disruption was not different on short (2 s) and long (8 s) trials. The second experiment showed that this disruptive effect was not specific to trials preceded by IT1 reinforcers; responding on empty IT1 trials run in the same session as ITI-reinforcer trials was also disrupted. This disruption however was not as great as on the ITI-reinforcertrials. The results of these experiments show that ITI reinforcers affect timing discriminationsin much the same way they affect classical conditioningand delayed matching to sample. However, detailed examination of the results suggests that the deleterious effects of ITI reinforcers in these different paradigms might be produced by different rather than the same mechanism. The results also support the conclusion that pre-trial reinforcement "priming" produces disruption rather than facilitation in complex tasks.
INTRODUCTION The purpose of this research was to collect data relevant to two issues. The
first
concerns "priming", the
effect of pre-trial reinforcementon
subsequent performance. Although Gallistel (19731 suggested that priming effects were specific to use of electrical stimulation of the brain as reinforcement, subsequent research has demonstrated the effect with such natural reinforcers as food and water (van der Kooy and Hogan, 1978) and aggressive displaying (Bols, 19761. Priming may have facilitatingor interfering effects on subsequent responding depending upon whether the response is already learned (where facilitationis often observed) or is being acquired (cf. Terry, 19871. Although many studies of priming have used simple runway or bar press procedures, there have been a few recent studies involving more complex tasks
0376.6357/88/$03.50
0
1988 Elsevier
Science
Publishers
B.V. (Biomedical
Division)
230
such as delayed matching (1985), Spetch reinforcers
to sample and its variants.
(1985) and Wilkie
had deleterious
subsequent
trials.
effect
Although
retention
these studies
more complex
tasks might be harmed
reinforcers,
recent results reported
generalization alternation.
reinforcers
when pre-trial
feeding
disrupted
(1987) suggest that this
reinforcers
was to examine priming
arm was rewarded.
correct
spatial
effects
chose
Terry
arm choice, but only when
effect.
Thus it is not as yet clear
will have a deleterious Accordingly,
effect on performance
one rationale
in another variant
for the present
in
studies
of delayed matching,
event
discrimination.
The second issue concerns of event duration,
the relationship
and classical
have been viewed as mostly unrelated evidence,
however,
classical
conditioning.
facilitate
of striking
both classically
Locurto,
matching
to sample paradigm
phenomena.
conditioned
and matching
Autoshaped
responding
autoshaped
1975).
related
delay between
sample offset and the presentation
to depend upon the mean IT1 duration (Gibbon et al., 1977; Roberts and matching
of the duration reinforcer
in autoshaping
sample and choice stimuli 1984; Wilkie,
in a delayed
variable.
accuracy decreases
in both paradigms
rather than local variations 19821.
as the
appears
in duration
Fourth, both autoshaped
are linear functions
of the log of the ratio CS onset to
(Gibbon et al.. 1977) and the delay between
in delayed
1984).
(e.g., Gibbon,
accuracy
of the choice stimuli
responding
and Kraemer,
accuracy
(ITI)
Second, both autoshaped
of IT1 to the trial length variable--the
interval
and
interval
responding
et al., 1977); matching
(e.g., Gibbon
Third,
these
to the time between CS onset and
reinforcement
responding
delayed matching
in the intertrial
are affected by the trial duration
is inversely
increases (e.g., Blough, 1959).
Traditionally,
1977) and matching
(e.g., Grant,
accuracy
delayed matching,
There is some recent
between
increases
Gold, and Terrace,
responding
between
conditioning.
parallels
First,
Baldock,
Santi,
rats on delayed
the initial arm entry was not rewarded
had no deleterious
these more complex tasks.
timing
on these
of pre-trial
On the final part of the trial the subject
Entry into the non-forced
that pre-trial
duration
by Terry
on
that performance
Terry trained
the forced choice was rewarded--when pre-trial
of sample information
suggest
On the first part of the trial the subject was forced to one arm
both arms.
reported
that pre-trial
by the presentation
might be premature.
of a T-maze when it was fed. between
Santi and Roberts
(1984) each reported
matching
(Roberts and Kraemer,
1982;
231
There is also evidence of parallels between classical conditioning and event duration discrimination. In several experiments Holder and Roberts (e.g. 1985) have shown that classical conditioningtreatments that change the signal value of a the sample stimulus (the CS) also change the effective duration of that stimulus. In one set of their experiments they used a cross-modal transfer procedure in which rats were first trained to time light duration using a peak procedure; food reward for lever pressing was available 40 s after light onset. This training produced a peak in response rate about 40 s after light onset. On test trials the light was immediately preceded by a short or long interval of sound. Holder and Roberts reasoned that if sound was timed, the peak rate during light should differ after short and long sounds. However, if sound was not timed, the peak rate should not differ after short and long sound. The sound stimulus was used as a CS in various classical conditioningprocedures. In different phases the sound was presented alone (CS only), followed by food (forward CS-US pairing), preceded by food (backward US-CS pairing), and extinguished. Sound affected the light peak rate on test trials only when the sound was an effective CS, as indicated by the occurrence of CRs during sound presentation. The sound was timed after forward pairing but not after CS alone, backward pairing or during extinction. Although it is clear that there are parallels between delayed matching, timing and classical conditioning, it is not yet established to what extent there is overlap. It is still unclear if all, most, or only certain types of variables affect responding in these different procedures. It is also possible that the same variable might have a facilitative or deleterious effect on responding in different procedures, but produce these effects in quite different ways. Accordingly, further studies of the relationship between these procedures are clearly warranted. Another manipulation that affects both delayed matching and classical conditioning in similar ways is presenting reinforcers during intertrial interval. Gamzu and Williams (1971) and Gibbon, Locurto, and Terrace (1975) have found that the non-contingent presentation of reinforcers during the IT1 disrupts autoshaped responding. Wilkie (1984) showed delayed matching to sample was also disrupted by IT1 reinforcers. A subsequent more detailed study by Spetch (1985) however suggested that this variable might operate differently in the two cases. Spetch varied both the number and temporal location of the reinforcers presented in the IT1 (either early, middle, or
232
late in the ITI). Her pigeons' delayed matching to sample accuracy was most affected by temporal location--a single reinforcer presented late in the ITI produced maximum disruption. This result stands in contrast to that reported for IT1 reinforcement in the autoshapingparadigm in which frequency rather than location of reinforcers is important (e.g., Jenkins, Barnes, and Barrera, 1981). The purpose of the present research was to further examine the effects of non-contingent IT1 reinforcers. In particular we studied the effects of IT1 reinforcers in an event discriminationparadigm. Although IT1 reinforcers have been found to disrupt a variety of complex tasks, Terry's (1987) failure to find disruption in a spatial memory task argues that additional study is warranted. Holder and Roberts' analysis of the relationshipbetween timing and classical conditioning suggests that IT1 reinforcers should disrupt timing.
GENERAL METHODS Subjects and Apparatus The subjects were six male Long-Evans Hooded rats born and raised in our colonies. All rats had previously been used in an experiment involving similar timing discriminations (Tees and Symons, in press). Four of the rats were housed individually in a colony with a 12-hour light/darkcycle (lights on from 06:OO to 18:OO). The two remaining animals were housed individually in a permanently dark colony. All subjects were maintained at 85-90X of their free-feeding weight and had ad lib access to water. The testing took place in two identical lever boxes (27 x 24 x 25 cm). The lid, back and side walls were Plexiglas and the front wall was stainless steel. The floor consisted of brass rods. Noyes precision pellets (45 mg) were delivered to a food cup mounted 8 cm above the floor. Two retractable levers (Colbourn E23-05) were mounted on either side of the food cup. A 6-w lamp was located 10 cm above the food cup. Each box was housed in a large insulated, light-proof chamber. Experimental control and data collection were carried out by a minicomputer running KANE state notation software (Gilbert and Rice, 1979). Procedure The rats were tested hours.
5 days per week between
approximately
14:00 to 16:00
A trial began with an intertrial interval (IT11 averaging 30 s in
duration with a minimum duration of 15 s. No light was on during the ITI.
233
Following the ITI, the lamp was illuminated for either 2 or 8 s (each stimulus duration was equally likely on each trial). The two levers were then extended. The rat was reinforced for a correct left lever response after the 2-s stimulus and a correct right lever response after the 8-s stimulus. The levers were withdrawn after a response was made. Each session consisted of 60 trials. The frequency of correct responses to short (2 s) and long (8 s) stimuli were recorded.
EXPERIKENT 1 Because the rats had been previously trained to discriminate 2 and 8 s of light, no preliminary training was needed. The experiment was divided into 4 blocks of 3 sessions each. The first block represented a baseline phase and consisted of the procedure described above. The second block of 3 sessions comprised test sessions. During the second and subsequent ITIs of these sessions, there was a 0.5 probability of the subject receiving a pellet every 5 s.
The number of IT1 reinforcers delivered averaged 142 per session. The
final 2 blocks of 3 sessions were baseline phases identical to the first baseline phase. Thus a 2 x 4 (stimulus duration x blocks of sessions) repeated measures design was used. Results Figure 1 shows the percent correct responses to the short and long stimuli over the different blocks of sessions. The non-contingent IT1 food presentations clearly disrupted the rats' timing accuracy. Although there was a tendency for the rats to choose the short alternativemore often during the ITI-reinforcer sessions, this effect was not significant in the repeated measures analysis of variance. In this analysis there was a significant effect of blocks, F(3.5) = 11.6, R = 0.0004, but no short/long difference, F(1,5) < 1, R > 0.05, and no interaction, F(3.151 < 1, R > 0.05). Post-hoc testing (Newman-Keuls)showed the ITI-food presentation blocks differed from all the non-food blocks. None of the three no-food blocks differed significantly. Discussion Intertrial interval reinforcers have a deleterious effect on timing accuracy. This finding represents another instance in which "priming" produces a deleterious rather than a facilitative effect. It also is another case of the same manipulation affecting timing in the same way as classical conditioning. It remains to be established however if the mechanism(s) of
234
Fig. 1. Percent correct on short- and long-sampletrials during blocks of sessions in which food reinforcers were, or were not, presented during the ITI.
action of the IT1 reinforcers is the same in the two situations. One piece of evidence suggesting that the mechanisms might differ is the failure to observe a significant selective effect of IT1 reinforcers on accuracy on long-sample trials. When Holder and Roberts found that classical conditioning manipulations disrupted the timing of a stimulus, these had a specific effect, namely, subjects tended to treat a non-timed stimulus as being short.
EXPERIUENT 2 This experiment was a systematic replication of Experiment 1.
Some trials
were preceded by IT1 reinforcers, others were not. We were interested in determining if the disruption of timing produced by IT1 reinforcers was specific to those trials preceded by reinforcementor whether there was a general disruptive effect. We were also interested in further attempting to see if long-sampletrials were especially disrupted.
235
Subiects and Apparatus Due to illness only five of the six rats used in Experiment 1 served in this study. The apparatus was identical to Experiment 1. Procedure The experiment consisted of 12 sessions. Baseline sessions alternated with test sessions; 6 baseline sessions and 6 test sessions were run. Each session comprised 60 trials. During the test sessions, each trial had a 0.5 probability of two pellets being delivered one right after the other, 5 s before light onset. Otherwise the trial was identical to a baseline trial. The design, then, was a 2 x 3 (stimulus duration x condition) repeated measures factorial. Results Figure 2 shows the percent correct responses in each of the three conditions: baseline sessions in which no food was presented in ITI, and the two types of trials during test sessions in which food was presented in some of the ITIs. The three conditions differed from each other. Again, there was no difference on short and long trials.
FOOb ITI BASELINE
TEST
EMPiY
ITI
236
A repeated measures analysis of variance showed a main effect for condition, F(2, 8) = 13.4, R = 0.003, but no effect for stimulus duration nor any interaction effect [E(l, 4) < 1, R > 0.05 and F(2,8) < 1, R > 0.05, respectively]. Newman-Keulspost-hoc tests revealed that all three conditions differed from each other. Discussion As in Experiment 1 non-contingent ITI reinforcershad a deleterious effect on event duration discriminationaccuracy. Unlike Experiment 1 however there was no suggestion of a selective effect of IT1 reinforcementon long-sample trials. During ITI-reinforcementsessions performancewas more impaired on those trials following IT1 reinforcers, although performance on trials following empty ITIs was still impaired relative to baseline sessions. Thus it seems that non-contingentIT1 reinforcementproduces both a specific and a general disruptive effect.
GENERAL DISCUSSION Presentation of reinforcers in the IT1 disrupts classical conditioning (e.g., Gamzu and Williams, 1971), and delayed matching of both spatial (Wilkie, 1984) and color (Spetch, 19851 cues. The present experiments show that event duration discrimination is also disrupted. On the surface these findings support a hypothesis that timing, delayed matching and classical conditioning are related. However, careful examinationof the effect of IT1 reinforcement on classical conditioning and delayed matching reveals differences suggesting that conditioning and matching are affected in different ways (Spetch, 1985). The present results suggest that there may be differences between classical conditioningand timing. Holder and Roberts (e.g., 1985) have clearly demonstrated that manipulationsthat affect classical conditioningalso affect event duration discriminationand in a very specific way, namely by decreasing accuracy on long-sample trials. In our studies we found little evidence that IT1 reinforcementselectively affected long-sample trials; rather, long- and short-sampletrials seemed to be equally affected by non-contingentIT1 reinforcers. It should be noted that our procedures and those used by Holder and Roberts are somewhat different and these differencesmay be important in our failure to find a specific type of disruption. However, other aspects of our data also suggest different modes of action of ITI reinforcers in the two paradigms. Intertrial interval
231
reinforcers affect classical conditioning at a global rather than a local level. That is, the deleterious effect of IT1 reinforcers is not due to local interference by events immediately surrounding a given trial (cf. Jenkins, Barnes, and Barrera, 1981). Rather it is the overall rates of reinforcement associated with the CS and the background or context that seems to be important. Our results, in contrast, show local effects. Although timing accuracy was reduced on trials preceded by empty ITIs in sessions in which IT1 reinforcers occur, performance was much more disrupted on trials preceded by ITI reinforcers. Local effects of ITI reinforcementon delayed matching were also seen by Spetch (1985). Our results also clearly support the emerging conclusion in the reinforcement priming literature that pre-trial reinforcement interfereswith performance on more complex
tasks.
ACKNOWLEGEUENT This research was supported by the Natural Sciences and Engineering Research Council of Canada. M. Hana and C.K. Kim assisted with the experiments.
REFERENCES Blough, D. S. (1959). Delayed matching in the pigeon. Journal of the Experimental Analysis of Behavior, 2, 151-160. Bols, R. J. (1976). Factors influencingthe reinforcing value of aggressive display in the Siamese fighting fish (Betta splendensl. Unpublished doctoral dissertation, University of Toronto. Gallistel, C. R. (1973). Self-stimulation:The neurophysiology of reward and motivation. In J. A. Deutsch (Ed.), The physiological basis of memory (pp. 175-267). New York: Academic Press. Gamsu, E., and Williams, D. R. (1971). Classical conditioning of a complex skeletal response. Science, 171, 923-925. Gibbon, J., Baldock, Fl.D., Locurto, C. Fl.,Gold, L., and Terrace, H. S. (1977). Trial and intertrial durations in autoshaping. Journal of Experimental Psychology: Animal Behavior Processes, 2, 264-284. Gibbon, J., Locurto. C. E., and Terrace, H. S. (1975). Signal-food contingency and signal frequency in a continuous trials auto-shaping paradigm. Animal Learning h Behavior, 3, 317-324. Gilbert, S. G.. and Rice, D. C. (1979). NOVA SKED II: A behavioral notation language utilizing the Data General Corporation real-time disk-operating system. Behavior Research Methods 6 Instrumentation,ll. 71-73. Grant, D. S. (1975). Proactive interference in pigeon short-term memory. Journal of Experimental Psychology: Animal Behavior Processes, 1, 217-227. Jenkins, H. H., Barnes, R. A., and Barrera, F. J. (1981). Why autoshaping depends on trial spacing. In L. C. Locurto, H. S. Terrace, and J. Gibbon (Eds.), Autoshaping and conditioning theory (pp. 255-284). New York: Academic Press.
238
Holder, M. D., and Roberts, S. (1985). Comparison of timing and classical conditioning. Journal of ExperimentalPsychology: Animal Behavior Processes, ll, 172-193. Roberts, W. A., and Kraemer, P. J. (1982). Some observationsof the effects of intertrial interval and delay on delayed matching to sample in pigeons. Journal of ExperimentalPsychology: Animal Behavior Processes, 8, 342-353. Santi, A. (1984). The trial spacing effect in delayed matching-to-sampleby pigeons is dependent upon the illumination condition during the intertrial interval. Canadian Journal of Psychology, 38, 154-165. Santi, A., and Roberts, W. A. (1985). Reinforcement expectancy and trial spacing effects in delayed matching-to-sampleby pigeons. Animal Learning h Behavior, 13, 274-284. Spetch, M. L. (1985). The effect of intertrial interval food presentations on pigeons' delayed matching to sample accuracy. Behavioural Processes, l-l, 309-315. Tees, R. C., and Symons, L. A. (in press). Intersensory coordination and the effects of early sensory deprivation. Developmental Psychobiology. Terry, W. S. (1987). Food priming and short-term retention in the delayed alternation task. Learning and Motivation, l8, 261-273. van der Kooy, D., and Hogan, J. A. (1978). Priming effects with food and water reinforcers in hamsters. Learnins and Motivation, 3, 332-346. Wilkie, D. M. (1984). Pigeons' spatial memory IV: Effects of intertrial interval manipulations on delayed matching of key locations. Canadian Journal of Psychology, 38, 178-195.
Processes,
17 (1988) 229-238
229
EFFECTS OF INTERTRIAL REINFORCERS ON RATS' TIMING BEHAVIOR DONALD M. WILKIE, LAWRENCE A. SYMONS and RICHARD C. TEES Department of Psychology, Canada V6T lY7 (Accepted 28 March
University of British Columbia, Vancouver, B.C.
19Q,
ABSTRACT Wilkie, D.M., Symons, L.A. and Tees, R.C. 1988. Effects of intertrial reinforcers on rats' timing behavior. Behav. Processes 17 : 229-238 Two experiments were conducted to assess the effects of non-contingent intertrial interval (ITI) reinforcers on rats' discriminationof duration. In the first experiment, rats' discriminationof a 2 vs. 8 s of light was significantly disrupted when reinforcerswere presented in the ITI. Disruption was not different on short (2 s) and long (8 s) trials. The second experiment showed that this disruptive effect was not specific to trials preceded by IT1 reinforcers; responding on empty IT1 trials run in the same session as ITI-reinforcer trials was also disrupted. This disruption however was not as great as on the ITI-reinforcertrials. The results of these experiments show that ITI reinforcers affect timing discriminationsin much the same way they affect classical conditioningand delayed matching to sample. However, detailed examination of the results suggests that the deleterious effects of ITI reinforcers in these different paradigms might be produced by different rather than the same mechanism. The results also support the conclusion that pre-trial reinforcement "priming" produces disruption rather than facilitation in complex tasks.
INTRODUCTION The purpose of this research was to collect data relevant to two issues. The
first
concerns "priming", the
effect of pre-trial reinforcementon
subsequent performance. Although Gallistel (19731 suggested that priming effects were specific to use of electrical stimulation of the brain as reinforcement, subsequent research has demonstrated the effect with such natural reinforcers as food and water (van der Kooy and Hogan, 1978) and aggressive displaying (Bols, 19761. Priming may have facilitatingor interfering effects on subsequent responding depending upon whether the response is already learned (where facilitationis often observed) or is being acquired (cf. Terry, 19871. Although many studies of priming have used simple runway or bar press procedures, there have been a few recent studies involving more complex tasks
0376.6357/88/$03.50
0
1988 Elsevier
Science
Publishers
B.V. (Biomedical
Division)
230
such as delayed matching (1985), Spetch reinforcers
to sample and its variants.
(1985) and Wilkie
had deleterious
subsequent
trials.
effect
Although
retention
these studies
more complex
tasks might be harmed
reinforcers,
recent results reported
generalization alternation.
reinforcers
when pre-trial
feeding
disrupted
(1987) suggest that this
reinforcers
was to examine priming
arm was rewarded.
correct
spatial
effects
chose
Terry
arm choice, but only when
effect.
Thus it is not as yet clear
will have a deleterious Accordingly,
effect on performance
one rationale
in another variant
for the present
in
studies
of delayed matching,
event
discrimination.
The second issue concerns of event duration,
the relationship
and classical
have been viewed as mostly unrelated evidence,
however,
classical
conditioning.
facilitate
of striking
both classically
Locurto,
matching
to sample paradigm
phenomena.
conditioned
and matching
Autoshaped
responding
autoshaped
1975).
related
delay between
sample offset and the presentation
to depend upon the mean IT1 duration (Gibbon et al., 1977; Roberts and matching
of the duration reinforcer
in autoshaping
sample and choice stimuli 1984; Wilkie,
in a delayed
variable.
accuracy decreases
in both paradigms
rather than local variations 19821.
as the
appears
in duration
Fourth, both autoshaped
are linear functions
of the log of the ratio CS onset to
(Gibbon et al.. 1977) and the delay between
in delayed
1984).
(e.g., Gibbon,
accuracy
of the choice stimuli
responding
and Kraemer,
accuracy
(ITI)
Second, both autoshaped
of IT1 to the trial length variable--the
interval
and
interval
responding
et al., 1977); matching
(e.g., Gibbon
Third,
these
to the time between CS onset and
reinforcement
responding
delayed matching
in the intertrial
are affected by the trial duration
is inversely
increases (e.g., Blough, 1959).
Traditionally,
1977) and matching
(e.g., Grant,
accuracy
delayed matching,
There is some recent
between
increases
Gold, and Terrace,
responding
between
conditioning.
parallels
First,
Baldock,
Santi,
rats on delayed
the initial arm entry was not rewarded
had no deleterious
these more complex tasks.
timing
on these
of pre-trial
On the final part of the trial the subject
Entry into the non-forced
that pre-trial
duration
by Terry
on
that performance
Terry trained
the forced choice was rewarded--when pre-trial
of sample information
suggest
On the first part of the trial the subject was forced to one arm
both arms.
reported
that pre-trial
by the presentation
might be premature.
of a T-maze when it was fed. between
Santi and Roberts
(1984) each reported
matching
(Roberts and Kraemer,
1982;
231
There is also evidence of parallels between classical conditioning and event duration discrimination. In several experiments Holder and Roberts (e.g. 1985) have shown that classical conditioningtreatments that change the signal value of a the sample stimulus (the CS) also change the effective duration of that stimulus. In one set of their experiments they used a cross-modal transfer procedure in which rats were first trained to time light duration using a peak procedure; food reward for lever pressing was available 40 s after light onset. This training produced a peak in response rate about 40 s after light onset. On test trials the light was immediately preceded by a short or long interval of sound. Holder and Roberts reasoned that if sound was timed, the peak rate during light should differ after short and long sounds. However, if sound was not timed, the peak rate should not differ after short and long sound. The sound stimulus was used as a CS in various classical conditioningprocedures. In different phases the sound was presented alone (CS only), followed by food (forward CS-US pairing), preceded by food (backward US-CS pairing), and extinguished. Sound affected the light peak rate on test trials only when the sound was an effective CS, as indicated by the occurrence of CRs during sound presentation. The sound was timed after forward pairing but not after CS alone, backward pairing or during extinction. Although it is clear that there are parallels between delayed matching, timing and classical conditioning, it is not yet established to what extent there is overlap. It is still unclear if all, most, or only certain types of variables affect responding in these different procedures. It is also possible that the same variable might have a facilitative or deleterious effect on responding in different procedures, but produce these effects in quite different ways. Accordingly, further studies of the relationship between these procedures are clearly warranted. Another manipulation that affects both delayed matching and classical conditioning in similar ways is presenting reinforcers during intertrial interval. Gamzu and Williams (1971) and Gibbon, Locurto, and Terrace (1975) have found that the non-contingent presentation of reinforcers during the IT1 disrupts autoshaped responding. Wilkie (1984) showed delayed matching to sample was also disrupted by IT1 reinforcers. A subsequent more detailed study by Spetch (1985) however suggested that this variable might operate differently in the two cases. Spetch varied both the number and temporal location of the reinforcers presented in the IT1 (either early, middle, or
232
late in the ITI). Her pigeons' delayed matching to sample accuracy was most affected by temporal location--a single reinforcer presented late in the ITI produced maximum disruption. This result stands in contrast to that reported for IT1 reinforcement in the autoshapingparadigm in which frequency rather than location of reinforcers is important (e.g., Jenkins, Barnes, and Barrera, 1981). The purpose of the present research was to further examine the effects of non-contingent IT1 reinforcers. In particular we studied the effects of IT1 reinforcers in an event discriminationparadigm. Although IT1 reinforcers have been found to disrupt a variety of complex tasks, Terry's (1987) failure to find disruption in a spatial memory task argues that additional study is warranted. Holder and Roberts' analysis of the relationshipbetween timing and classical conditioning suggests that IT1 reinforcers should disrupt timing.
GENERAL METHODS Subjects and Apparatus The subjects were six male Long-Evans Hooded rats born and raised in our colonies. All rats had previously been used in an experiment involving similar timing discriminations (Tees and Symons, in press). Four of the rats were housed individually in a colony with a 12-hour light/darkcycle (lights on from 06:OO to 18:OO). The two remaining animals were housed individually in a permanently dark colony. All subjects were maintained at 85-90X of their free-feeding weight and had ad lib access to water. The testing took place in two identical lever boxes (27 x 24 x 25 cm). The lid, back and side walls were Plexiglas and the front wall was stainless steel. The floor consisted of brass rods. Noyes precision pellets (45 mg) were delivered to a food cup mounted 8 cm above the floor. Two retractable levers (Colbourn E23-05) were mounted on either side of the food cup. A 6-w lamp was located 10 cm above the food cup. Each box was housed in a large insulated, light-proof chamber. Experimental control and data collection were carried out by a minicomputer running KANE state notation software (Gilbert and Rice, 1979). Procedure The rats were tested hours.
5 days per week between
approximately
14:00 to 16:00
A trial began with an intertrial interval (IT11 averaging 30 s in
duration with a minimum duration of 15 s. No light was on during the ITI.
233
Following the ITI, the lamp was illuminated for either 2 or 8 s (each stimulus duration was equally likely on each trial). The two levers were then extended. The rat was reinforced for a correct left lever response after the 2-s stimulus and a correct right lever response after the 8-s stimulus. The levers were withdrawn after a response was made. Each session consisted of 60 trials. The frequency of correct responses to short (2 s) and long (8 s) stimuli were recorded.
EXPERIKENT 1 Because the rats had been previously trained to discriminate 2 and 8 s of light, no preliminary training was needed. The experiment was divided into 4 blocks of 3 sessions each. The first block represented a baseline phase and consisted of the procedure described above. The second block of 3 sessions comprised test sessions. During the second and subsequent ITIs of these sessions, there was a 0.5 probability of the subject receiving a pellet every 5 s.
The number of IT1 reinforcers delivered averaged 142 per session. The
final 2 blocks of 3 sessions were baseline phases identical to the first baseline phase. Thus a 2 x 4 (stimulus duration x blocks of sessions) repeated measures design was used. Results Figure 1 shows the percent correct responses to the short and long stimuli over the different blocks of sessions. The non-contingent IT1 food presentations clearly disrupted the rats' timing accuracy. Although there was a tendency for the rats to choose the short alternativemore often during the ITI-reinforcer sessions, this effect was not significant in the repeated measures analysis of variance. In this analysis there was a significant effect of blocks, F(3.5) = 11.6, R = 0.0004, but no short/long difference, F(1,5) < 1, R > 0.05, and no interaction, F(3.151 < 1, R > 0.05). Post-hoc testing (Newman-Keuls)showed the ITI-food presentation blocks differed from all the non-food blocks. None of the three no-food blocks differed significantly. Discussion Intertrial interval reinforcers have a deleterious effect on timing accuracy. This finding represents another instance in which "priming" produces a deleterious rather than a facilitative effect. It also is another case of the same manipulation affecting timing in the same way as classical conditioning. It remains to be established however if the mechanism(s) of
234
Fig. 1. Percent correct on short- and long-sampletrials during blocks of sessions in which food reinforcers were, or were not, presented during the ITI.
action of the IT1 reinforcers is the same in the two situations. One piece of evidence suggesting that the mechanisms might differ is the failure to observe a significant selective effect of IT1 reinforcers on accuracy on long-sample trials. When Holder and Roberts found that classical conditioning manipulations disrupted the timing of a stimulus, these had a specific effect, namely, subjects tended to treat a non-timed stimulus as being short.
EXPERIUENT 2 This experiment was a systematic replication of Experiment 1.
Some trials
were preceded by IT1 reinforcers, others were not. We were interested in determining if the disruption of timing produced by IT1 reinforcers was specific to those trials preceded by reinforcementor whether there was a general disruptive effect. We were also interested in further attempting to see if long-sampletrials were especially disrupted.
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Subiects and Apparatus Due to illness only five of the six rats used in Experiment 1 served in this study. The apparatus was identical to Experiment 1. Procedure The experiment consisted of 12 sessions. Baseline sessions alternated with test sessions; 6 baseline sessions and 6 test sessions were run. Each session comprised 60 trials. During the test sessions, each trial had a 0.5 probability of two pellets being delivered one right after the other, 5 s before light onset. Otherwise the trial was identical to a baseline trial. The design, then, was a 2 x 3 (stimulus duration x condition) repeated measures factorial. Results Figure 2 shows the percent correct responses in each of the three conditions: baseline sessions in which no food was presented in ITI, and the two types of trials during test sessions in which food was presented in some of the ITIs. The three conditions differed from each other. Again, there was no difference on short and long trials.
FOOb ITI BASELINE
TEST
EMPiY
ITI
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A repeated measures analysis of variance showed a main effect for condition, F(2, 8) = 13.4, R = 0.003, but no effect for stimulus duration nor any interaction effect [E(l, 4) < 1, R > 0.05 and F(2,8) < 1, R > 0.05, respectively]. Newman-Keulspost-hoc tests revealed that all three conditions differed from each other. Discussion As in Experiment 1 non-contingent ITI reinforcershad a deleterious effect on event duration discriminationaccuracy. Unlike Experiment 1 however there was no suggestion of a selective effect of IT1 reinforcementon long-sample trials. During ITI-reinforcementsessions performancewas more impaired on those trials following IT1 reinforcers, although performance on trials following empty ITIs was still impaired relative to baseline sessions. Thus it seems that non-contingentIT1 reinforcementproduces both a specific and a general disruptive effect.
GENERAL DISCUSSION Presentation of reinforcers in the IT1 disrupts classical conditioning (e.g., Gamzu and Williams, 1971), and delayed matching of both spatial (Wilkie, 1984) and color (Spetch, 19851 cues. The present experiments show that event duration discrimination is also disrupted. On the surface these findings support a hypothesis that timing, delayed matching and classical conditioning are related. However, careful examinationof the effect of IT1 reinforcement on classical conditioning and delayed matching reveals differences suggesting that conditioning and matching are affected in different ways (Spetch, 1985). The present results suggest that there may be differences between classical conditioningand timing. Holder and Roberts (e.g., 1985) have clearly demonstrated that manipulationsthat affect classical conditioningalso affect event duration discriminationand in a very specific way, namely by decreasing accuracy on long-sample trials. In our studies we found little evidence that IT1 reinforcementselectively affected long-sample trials; rather, long- and short-sampletrials seemed to be equally affected by non-contingentIT1 reinforcers. It should be noted that our procedures and those used by Holder and Roberts are somewhat different and these differencesmay be important in our failure to find a specific type of disruption. However, other aspects of our data also suggest different modes of action of ITI reinforcers in the two paradigms. Intertrial interval
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reinforcers affect classical conditioning at a global rather than a local level. That is, the deleterious effect of IT1 reinforcers is not due to local interference by events immediately surrounding a given trial (cf. Jenkins, Barnes, and Barrera, 1981). Rather it is the overall rates of reinforcement associated with the CS and the background or context that seems to be important. Our results, in contrast, show local effects. Although timing accuracy was reduced on trials preceded by empty ITIs in sessions in which IT1 reinforcers occur, performance was much more disrupted on trials preceded by ITI reinforcers. Local effects of ITI reinforcementon delayed matching were also seen by Spetch (1985). Our results also clearly support the emerging conclusion in the reinforcement priming literature that pre-trial reinforcement interfereswith performance on more complex
tasks.
ACKNOWLEGEUENT This research was supported by the Natural Sciences and Engineering Research Council of Canada. M. Hana and C.K. Kim assisted with the experiments.
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