Short-term memory for time intervals

LEARNING

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208-219 (1980)

Short-Term Memory for Time Intervals RUSSELL M. CHURCH Brown

University

The effect of intertrial interval, preset interval, and retention interval on the performance of rats in a time estimation task was described. On each trial a signal was presented for a duration of 2 to 8 sec. Eighteen rats were trained to press one lever (the short response) if the signal was shorter than 4 set, and another lever (the long response) if the signal was longer than 4 sec. When trials were massed (Experiment I), the percentage long response was affected by the classification of the previous signal, but not by its actual duration. This suggests that the animals remembered the response made on the previous trial, but not the signal duration. If a response was not permitted on the previous trial (Experiment 2), the duration or classification of the previous signal had no effect on performance. This supports the conclusion from the first experiment and suggests that an animal can reset its internal clock in less than 2 sec. In Experiment 3, the difference limen of the psychophysical function increased with the duration of the retention interval, but the point of subjective equality did not change. This suggests that resetting of the internal clock occurs on a non-time dimension.

Animals are able to discriminate between signals differing in duration. For example, an animal can be trained to press one lever after a standard signal duration and to press another lever after a longer signal duration (Church, Getty, & Lemer, 1976). The purpose of these experiments is to assess the effects of signal duration on performance after some interval. When the intertrial interval is varied following trials on which a response can occur (Experiment I), or following trials on which a response cannot occur (Experiment 2), performance is improved if the animal is not affected by the previous signal duration. For example, when multiple trials are given in the same session, performance is better if the animal does not confuse the current signal duration with the signal duration on previous trials. In contrast, in the case of the retention interval (Experiment 3), performance is improved if the animal remains affected by the previous signal duration. After signal termination, the memory representation of the signal duration (the clock setting) may change in some regular way. This research was supported by Research Grant GM 23247 from the National Institutes of Health. Requests for reprints should be sent to Russell M. Church, Department of Psychology, Brown University, Providence, RI 02912. 208 0023-%90/80/020208-12$02.00/O Copyright @ 1980 by Academic Press, Inc. All rights of reproduction in any form reserved.

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The purpose of the present experiments is to describe how the memory representation of time changes with time. EXPERIMENT 1: THE INTERTRIAL INTERVAL When trials of a discrimination task are massed, performance often deteriorates. For example, in a delayed matching-to-sample test performance is worse with a S-set inter-trial interval than with intertrial intervals of 15, 30, or 60 set (Jarrard & Moise, 1971). The interfering effect of massed trials may be due to various factors. Some of these are unrelated to the specific conditions of the previous trial, but some of the interfering effects are related to the conditions of the previous trial (Maki, Moe, & Bierley, 1977). For example, the animal on a given trial may be affected by the stimulus of the previous trial and give a response appropriate to it. Alternatively, the animal on a given trial may be affected by its response on the previous trial and repeat this response. The purpose of the first experiment was to determine whether performance in a time estimation task was positively related to the intertrial interval (2, 8, and 32 set) and, if so, whether the proactive interference from the previous trial was due to the duration of the previous signal or the occurrence of the previous response. The method was to train rats to respond on a “short” lever for signals of 2 and 3.2 set, and to respond on a “long” lever for signals of 5 and 8 sec. The major problem was to determine whether the amount of interference was related to signal duration when the nature of the response (short or long) was held constant. Method Subjects The subjects were six male albino Norway rats (Charles River CD). They were experimentally naive and about 90 days old at the start of training. Throughout the experiment each rat received a daily 14-g ration of ground Purina chow mixed with about 25 ml of water. Apparatus Six lever boxes (23.2 x 20.3 x 21.9 cm) were used in the experiment. The roof and the two side panels of each box were made of transparent acrylic; the front and back walls were aluminum; the floor was composed of 16 parallel stainless-steel bars. Two retractable stainless-steel levers (1.1 x 3.4 cm) projected through the front panel, 3.6 cm above the floor (Coulborn Instruments, Model E23-05). Being solenoid activated, they could be extended into the box or retracted in less than 0.2 sec. A pellet dispenser delivered 45-mg Noyes Precision food pellets through an opening in the front panel to a food tray. A 7.5-W houselight was the only source of light. Each lever box was enclosed in an insulation board chamber designed to attenuate sound and block visual stimuli. Each chamber was equipped with a fan for ventilation and an acrylic window to

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to observe the rats. A PDP-12 computer equipment and recorded the responses.

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Procedure Pretraining. Each rat received at least one session of continuous reinforcement training. During this session a pellet of food was delivered each minute for 30 min and, in addition, each lever press was reinforced by a pellet of food. Initially, the left lever was inserted and 10 responses were reinforced; then only the right lever was inserted and 10 responses were reinforced. This alternation between the left and right levers continued until the rat had pressed each lever 30 times. Training (Days 1-20). Half the rats were trained to press the left lever following a 2-set signal and the right lever following an 8-set signal; the other rats were trained to press the right lever following a 2-set signal and the left lever following an S-set signal. For convenience, a response on the lever that was correct for a 2-set signal will be called the “short” response; a response on the lever that was correct for an 8-set signal will be called the “long” response. On each trial the house light went off for some duration (the signal); then the correct lever was inserted at the end of this period of time. When the rat pressed the lever, a pellet of food was delivered, and the lever retracted. After a 30-set intertrial interval, another trial was begun. Each signal duration was presented with a probability of 5 on each trial. A daily session lasted for 1 hr and 50 min. Training at various signal durations (Days 11-30). On each trial the signal (light off) lasted either for 2, 3.2, 5, or 8 sec. Then both levers were inserted. The “short” response was correct for signals of 2 and 3.2 set; the “long” response was correct for signals of 5 and 8 sec. If a correct response was made, food was delivered and both levers were withdrawn; if an incorrect response was made both levers were withdrawn. The intertrial interval was 30 sec. Sessions 11-20 were 50 min, but all subsequent sessions in this phase were 1 hr and 50 min. Testing at various intertrial intervals (Days 3140). The procedure was the same as during Days 11-30 except for the intertrial interval and the session length. On a given day, the intertrial interval was 2, 8, or 32 set, and each rat had 10 days at each intertrial interval. Each rat had the three intertrial intervals in each 3-day block and two rats were tested on each intertrial interval on each day, Within these restrictions, the treatments were administered randomly. A session ended after 1 hr and 50 mitt, or when 100 reinforcements (after 10 warm-up trials) had been received, whichever came first.

Results Figure 1 shows the mean percentage long response as function of (a) duration of the signal on the current trial (N), (b) duration of the signal on the previous trial (N-l), and (c) the length of the intertrial interval (I). It

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FIG. 1. Intertrial interval. Mean proportion long response as a function of signal duration, and duration of the signal on the previous trial. The intertrial intervals were 2 (left panel), 8 (center panel), and 32 set (right panel). In the schematic representation of the procedure the wide bar represents the signal, the vertical lines represent the lever insertion, and the triangle represents a response.

does not include the first 10 trials of warmup, and it does not include trials following errors. Thus, the response, as well as the signal durations on trial N-l and N, is known. Since each rat was observed under each condition, a four-factor analysis of variance was performed. The (Ylevel for all statistical tests was set at 5%, two tailed. The results were as follows: (a) The mean percentage of long responses was positively related to the duration of the signal on the current trial [F(3, 15) = 48.31. This effect of signal duration was increased with longer intertrial intervals [F(6, 30) = 30.11. (b) The mean percentage of long responses was positively related to the duration of the signal on the previous trial [F(3, 15) = 11.41. If the previous signal was long (5 or 8 set) the rat was more likely to estimate the duration of the next signal as long than if the previous signal was short (2 or 3.2 set). The two short-duration signals had equivalent effects when they were given on Trial N- 1; the same was true of the two long-duration signals. This effect of signal duration decreased with longer intertrial intervals [F(6, 30) = 9.21. Thus, performance on the time discrimination task improved as the intertrial interval increased. (c) There were reliable individual differences in overall percentage long responding, and in the interaction of individual differences with several variables. These differences were due primarily to the magitude of the effects of the variables on the percentage of choice of the long response. Discussion The percentage long response was positively related to the signal duration on the current trial, and there was a nonspecific interference with massed trials, i.e., signal duration on the current trial had less influence when trials were massed. In addition, when trials were massed behavior appropriate to the preceding trial was often repeated. This has also been reported by others (Medin, 1969; Spear, 1971).

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On each trial of the present experiment, the rat presumably formed a memory representation of the signal duration (a clock setting) and compared it to some representation of a standard duration (a criterion). If the representation of the signal duration was less than the criterion, a short response was made; otherwise a long response was made. One possibility is that the animal had a memory representation of the signal duration on Trials N- 1 and N, and sometimes based its choice on the basis of the signal of the previous trial. If this were the case, then the probability of a long response on a given trial would be a positive function of the duration of the signal on the previous trial. Another possibility is that the animal was affected by its decision on the previous trial as well as the signal duration on the current trial. If so, the probability of a long response on a given trial would be greater if the last signal were either of the long ones (and the animal made the long response) than if the last signal were either of the short ones (and the animal made the short response). This is what happened. The classification of the signal as long or short on the previous trial was important, but whether the short signal was 2 or 3.2 set (or whether the long signal was 5 or 8 set) did not matter. If the animal was affected by its decision on the previous trial but not the actual duration of the signal, then this should also be seen following error trials. Unfortunately, these data were not collected. Apparently, in this experiment, the rats were affected by (remembered) their decision on the previous trial (a two-category classification), not the subjective representation of the actual duration of the signal (a multivalued classification). The effect dissipated with time, and could not be detected at 32 sec. EXPERIMENT

2: THE EFFECT OF PRESETTING THE INTERNAL CLOCK On the basis of Experiment 1, it appears that the interfering effects of massed practice in a time estimation experiment is related to the response on the previous trial, not to the representation of the signal duration (clock setting). The present experiment provides a more direct test of this notion. It is essentially a repetition of the inter-trial interval experiment except, on some trials, no response is permitted. On regular trials, a signal occurs for some duration, the levers are inserted, and the rat chooses to make the short or long response. On preset trials, a signal occurs for some duration (the preset signal), the levers are not inserted, there is an intertrial interval, and then another signal is presented. The rat’s task is to estimate the duration of this most-recent signal, without influence from the previous signal. To be consistent with the interpretation of Experiment 1, the elimination of the response requirement from the procedure of Experiment 1 should eliminate the deleterious effect of massed trials.

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Method

Subjects and Apparatus The same subjects used in the previous experiment the same apparatus was employed.

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Procedure The treatment for the first 60 days has been described in Experiment 1. Variation in preset signal duration, intertrial interval, and signal duration occurred on Days 61-90. There were some trials in which a signal occurred (the preset signal) and then, after an intertrial interval, a signal whose duration was to be judged was presented. On each trial the signal (light off) lasted either for 2, 3.2, 5, or 8 sec. On half the trials, randomly selected, the signal was presented and the levers inserted to permit the rat to make the long or short response, as usual. On the remaining trials the levers were not inserted at signal termination. Instead there was another intertrial interval and then another signal followed by insertion of the levers. The restrictions on the random assignment of the three intertrial intervals (2,8, and 32 set) to the animals were the same as those described in the previous experiment. Results

Figure 2 shows the percentage long response on preset trials as a function of (a) duration of the signal on the current trial (N), (b) duration of the signal on the previous trial (N-1) when no response was permitted, and (c) the length of the intertrial interval (I). (It does not include the first 10 trials of warm-up.) The percentage long response was a function of the duration of the signal on the current trial, but it was unrelated to the duration of the preset signal, A four-factor analysis of variance similar to the one described in Experiment 1 was performed. The results were as

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FIG. 2. Preset interval. Mean proportion long response as a function of signal duration, and duration of the previous signal (when no response was permitted). The intertrial intervals were 2 (left panel), 8 (center panel), and 32 set (right panel). The elements of the schematic representation of the procedure are defined in the caption of Fig. 1.

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follows: (a) The mean percentage long response was positively related to the signal duration on the current trial, [F(3, 15) = 180.21. This effect of signal duration increased with long intertrial intervals [F(6, 30) = 6.01. Thus, as in the time discrimination task of Experiment 1, performance improved as the intertrial interval increased. (b) The mean percentage of long responses was not related to the duration of the preset signal [F(3, 15) = 0.3, NS]. (c) There were reliable individual differences in overall percentage of long responding, and in the interaction of individual differences with several variables. These differences were due primarily to the magnitude of the effects of the variables on the percentage of choice of the long response. Discussion

Manipulation of the preset interval in the present experiment led to simpler results than the intertrial interval manipulation of Experiment 1. The percentage long response was positively related to the signal duration on the current trial and there was a small nonspecific interference with massed trials. There was, however, no specific interference by the preset signal duration. Presentation of the preset signal must have preset the clock, since the animal could discriminate between the durations on regular trials when the levers were inserted. If the levers were not inserted, however, the duration of the preset signal did not affect the decision regarding the duration of the next signal. This was true even when the interval between the termination of the preset signal and the onset of the signal to be judged was only 2 sec. Apparently, the internal clock can be reset rapidly. Several properties of the internal clock were reported in a previous study (Roberts & Church, 1978): The internal clock could stop temporarily during a gap in the signal, and then continue; the clock could time up in absolute units; the same clock was used to time signals from different modalities. In that study it was shown that the internal clock could be trained to run during a gap and to appear to time in proportional units. The present study extends the properties of the clock that can be trained: It can also be trained to reset on cummand. EXPERIMENT

3A: THE RETENTION

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On the basis of Experiment 2 it appeared that a rat can reset its clock completely within 2 sec. Because it occurred so quickly, it was difficult to determine whether or not the resetting occurred in the time domain. To investigate this question, it is desirable to choose a situation in which the change in clock setting is more gradual. Thus, instead of dealing with a situation in which it is advantageous to the animal to reset its clock quickly (following a preset signal) we will deal with a situation in which it

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is advantageous to the animal to maintain its clock setting (during a retention interval). An animal exposed to a particular stimulus shows evidence of remembering it after a few seconds, but performance decreases as a function of the retention interval. (For example, this is the case in the delayed matching-to-sample procedure, Roberts, 1972.) In the present experiment, the rat was exposed to a signal duration, followed by a retention interval. After the retention interval the levers were inserted into the box and the animal could report the duration of the signal. The problem was to determine if the subjective representation of the time interval decreased during a retention interval. If the clock is set at 8 set and is reset to 0 set, does it, at some time, pass through a value indistinguishable from 4 set? This would be seen in the present experiment by some decrease in the percentage long responses following a signal of any duration as a function of the retention interval. Method Subjects and Apparatus

Five rats, similar to those used in Experiment 1, were used in this experiment. The same apparatus described in Experiment 1 was used. Procedure Pretraining. The samepretraining described in Experiment 1was used. Training (Days 1-25). The same training described in Experiment 1 was

used, except as follows: At the end of the signal both levers were inserted. If the rat pressed the correct lever a pellet of food was delivered and both levers were retracted; if the rat pressed the wrong lever no food was delivered and both levers were retracted and the same signal duration was presented on the next trial (correction method). Test 1 (Days 26-35). The procedure was the same as during training, except that there were no correction trials and, on some trials, a signal was followed by a retention interval. The duration of the signal (light off) was randomly selected on each trial to be 2 or 8 sec. With a probability of .5 the levers were inserted immediately after the signal termination. (This was the procedure of original training.) On the remaining trials there was a retention interval of S, 2, 8, or 32 set (each equally likely) before the levers were inserted. If the rat made an error on a trial with a 0-set retention interval, the trial was repeated. Test 2 (Days 36-N). The signal duration was either 2,8, or 0 sec. The probability of a 0-set signal was Ys;the probability of a 2- or 8-set signal was equal. As usual, the left response was correct after a 2-set signal, and the right response was correct after an 8-set signal. When the levers were inserted without a signal (0-set signal), neither response was correct; this was used as a test for response bias.

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Results Figure 3 shows the percentage long response as a function of the retention interval on Test 1 for the 2- and 8-set signal. The percentage long response given the 2-set signal increased as the retention interval increased, and the percentage long response given the B-set signal decreased. At the longest retention interval (32 set) only 31% of the responses were “long” following an B-set signal. This was significantly less than .5 [t(4) = 3.81. On trials when no signal was presented during Test 2, the rats were more likely to make the short response than the long response. (The probability of a long response was .21.) This was significantly less than .5 [t(4) = 4.01. On these sessions the animal continued to discriminate the 2-set from the B-set signal. (The probabilities of a long response were .06 and .97, respectively.) Discussion This experiment gives no evidence that forgetting of a time interval occurs in the time dimension. If the subjective duration of a signal decreased during the retention interval, the percentage long response to the 8-set signal would have decreased, but the percentage long response to the 2-set signal would not have increased. This increase suggests that forgetting occurs on a dimension other than time. One possibility is that performance is produced by some trials on which the response is controlled by signal duration and some trials on which the animal guesses. There are two indications that the guesses are biased. With the longest 100

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FIG. 3. Retention interval. Mean proportion long response as a function of retention interval for a 2- and 8-set signal. The elements of the schematic representation of the procedure are defined in the caption of Fig. 1.

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1(32 set), the probability of a long response was absence of a signal, the probability of a guess of Thus, if the trace of the signal had faded, or it the the animal usually guessed the signal was short.

EXPERIMENT 38: THE RETENTION INTERVAL Although there was no evidence in Experiment 3A that signals that were easily classified as “long” or “short” became subjectively shorter, a more sensitive test would involve signals that are close to the borderline between the short and long categories. The method was to determine the psychophysical function relating probability of a long response to signal duration, and then to determine how a retention interval affects this function. Normally, the duration that the animal equally often classifies as long or short is close to the geometric mean of the two extreme durations (Church & Deluty, 1977). The question is whether or not this point of subjective equality increases as the retention interval increases. Method Subjects and Apparatus Four rats, similar to those used in Experiment 1, were used in this experiment. The same apparatus described in Experiment 1 was used. Procedure Pretraining. The same pretraining procedure used in Experiment 1 was used in the present experiment. Training (Days Z-15). The same training procedure used in Experiment 2 was used in the present experiment. Testing at various signal durations (Days 16-45). The training conditions were maintained except (a) there were no correction trials, and (b) on each trial one of nine signal durations was presented with equal probability. They ranged in equal logarithmic steps from 2 to 8 set (2, 2.5, 3.2, 4.0, 5.0, 6.4, and 8.0 set) plus two extreme durations (1 and 16 set). The “short” lever response was correct for signals under 4 set; the “long” lever response was correct for signals over 4 sec. No response was reinforced following the 4-set signal. Testing with various retention intervals (Days 46-60). The conditions of Days 16-45 were maintained except that, with a probability of .5, there was an interval of time between the termination of the signal and the insertion of the lever. Each rat had five consecutive sessions with a particular retention interval (0.5, 2.0, or 8.0 set); but the order of treatments was randomized. Additional testing with the 8-set retention interval (Days 61-80). The testing conditions were maintained, with an 8-set retention interval. Dur-

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ing the last 10 days no signal was presented on those trials scheduled to receive the 4-set signal. Results Figure 4 shows the percentage of long response as a function of signal duration for retention intervals of 0.5,2, and 8 sec. The point of subjective equality (PSE) and the difference limen (DL) were calculated as follows: A straight line relating the percentage of long responses to the three central signal durations (3.2,4.0, and 5.0 set) was fitted by the method of least squares. This regression equation was used to find the signal duration corresponding to the probability of 5 of a long response, defined as the PSE; and it was also used to find one-half of the range of the signal duration corresponding to a probability of .75 and .25 of a long response, defined as the DL. There was no significant effect on the PSE or the DL of the 0.5 or 2-set retention interval, but the B-set retention interval did have an effect. It increased the DL [F( 1, 16) = 6.61, and it increased the error rate more on the shortest signal than on the longest signal [t (3) = 4.41. The B-set retention interval did not affect the PSE. With and without the B-set retention interval, the PSE remained near the geometric mean of 4 set (4.0 and 3.9 set, respectively). On trials in which no signal was given the mean percentage of long responses was 15%, with a range for individuals of 0 to 2%. Discussion The subjective duration of a signal did not appear to decrease during a retention interval. In Experiment 3A the B-set retention interval reduced the quality of the time discrimination without producing any marked bias for the short response. In the present experiment the B-set retention interval also decreased the quality of the performance: The psychophysical function was flattened as measured by the increased DL. The main interest, however, was in changes in the PSE. If the subjective duration of the signal had decreased during the retention interval, the psychophysical function after a retention interval would be displaced

FIG. 4. Retention interval. Mean proportion long response as a function of signal duration for a retention interval of 0.5 (left panel), 2 (center panel), and 8 set (right panel). Signal durations were I, 2, 2.6, 3.2, 4, 5. 6.4, 8, or I6 sec.

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horizontally to the right of a control psychophysical function and the PSE would have moved to the right. For example, if the subjective duration of a 4-set signal was decreased by an 8-set retention interval the probability of a long response to the 4-set signal would be less than S. There was no evidence in the present experiment for such a rightward shift of the psychophysical function following a retention interval. In fact, there was some suggestion of a leftward shift. Although the PSE was not measurably changed, there were more errors on the shortest signal than on the longest signal. If the subjective duration of the signal had increased during the retention interval, i.e., the rats did not completely stop their clocks during the retention interval, the psychophysical function for a retention interval would be displaced to the left of a control psychophysical function. For example, if an internal clock continued to run during an 8-set retention interval, the probability of a long response at any signal duration would be increased. There was no evidence that forgetting of a signal duration occurred on the time dimension. Forgetting of quantitive attributes like time and intensity may occur on the same dimension as forgetting of categorical attributes (e.g., wavelength). REFERENCES Church, R. M., & Deluty, M. Z. Bisection of temporal intervals. Journal of Experimental Psychology: Animal Behavior Processes, 1977, 3, 216-228. Church, R. M., Getty, D. J., & Lerner, N. D. Duration discrimination by rats. Journnl of Experimental Psychology: Animal Behavior Processes, 1976, 2, 303-312. Jarrard, L. E., & Moise, S. L. Short-term memory in the monkey. In L. E. Jarrard (Ed.), Cognitive processes of non-human primates. New York: Academic Press, 1971. Maki, W. S., Moe, J. C., & Bierley, C. M. Short-term memory for stimuli, responses, and reinforcers. Journal of Experimental Psychology: Animal Behavior Processes, 1977, 3, 156-177. Medin, D. L. Form perception and pattern reproduction by monkeys. Journal of Comparative and Physiological Psychology, 1969, 68, 412-419. Roberts, W. A. Short-term memory in the pigeon: Effects of repetition and spacing. Journal of Experimental Psychology, 1972, 94, 74-83. Roberts, S., & Church, R. M. Control of an internal clock. Journal of Experimental Psychology: Animal Behavior Processes, 1978, 4, 318-337. Spear, N. E. Forgetting as retrieval failure. In W. K. Honig & P.H.R. James (Eds.), Animal memory. New York: Academic Press, 1971. Received August 1, 1979 Revised January 4, 1980