Daren H. Kaiser

“work ethic” in pigeons: Reward value is directly related to the effort or time required to obtain the reward

Stimuli associated with less effort or with shorter delays to reinforcement are generally preferred over those associated with greater effort or longer delays to reinforcement. However, the opposite appears to be true of stimuli thatfollow greater effort or longer delays. In training, a simple simultaneous discrimination followed a single peck to an initial stimulus (S+FR1 S−FR1) and a different simple simultaneous discrimination followed 20 pecks to the initial stimulus (S+FR20 S−FR20). On test trials, pigeons preferred S+FR20 over S+FR1 and S−FR20 over S−FR1. These data support the view that the state of the animal immediately prior to presentation of the discrimination affects the value of the reinforcement that follows it. This contrast effect is analogous to effects that when they occur in humans have been attributed to more complex cognitive and social factors.

TIMING IN PIGEONS : THE CHOOSE-SHORT EFFECT MAY RESULT FROM PIGEONS' CONFUSION BETWEEN DELAY AND INTERTRIAL INTERVALS

In conditional discriminations, when samples differ only in duration, pigeons typically show a choose-short effect (i.e., higher matching accuracy on short-duration-sample than on long-durationsample trials with increasing delay between sample and comparison stimuli). That this effect depends on the similarity of retention interval (RI) and intertrial interval (ITI) houselight illumination conditions has been taken as evidence that pigeons judge duration relative to a temporal background. In the present experiment, pigeons trained with duration samples and with the ITI either illuminated or not showed a choose-short bias only when the RI illumination on test trials was the same as the ITI illumination had been in training. The results support the hypothesis that the choose-short effect results from the pigeons’ confusion between the ITI and the RI.

Timing in Pigeons: Effects of the Similarity Between Intertrial Interval and Gap in a Timing Signal

Previous research suggests that when a fixed interval is interrupted (known as the gap procedure), pigeons tend to reset memory and start timing from 0 after the gap. However, because the ambient conditions of the gap typically have been the same as during the intertrial interval (ITI), ambiguity may have resulted. In the present experiment, the authors found that when ambient conditions during the gap were similar to the ITI, pigeons tended to reset memory, but when ambient conditions during the gap were different from the ITI, pigeons tended to stop timing, retain the duration of the stimulus in memory, and add to that time when the stimulus reappeared. Thus, when the gap was unambiguous, pigeons timed accurately.

Event-duration discrimination by pigeons: The choose-short effect may result from retention-test novelty

Pigeons trained on a conditional event-duration discrimination typically “choose short” when retention intervals are inserted between samples and comparisons. In two experiments, we tested the hypothesis that this effect results from ambiguity produced by the similarity of the novel retention intervals and the familiar intertrial interval by training pigeons with retention intervals from the outset and, for one group, in addition, making retention intervals distinctive from the intertrial intervals. In Experiment 1, when the retention intervals (0–4 sec) were not distinctive from the intertrial intervals, the pigeons did not show a clear choose-short effect even when extended retention intervals (8 sec) were introduced. When the retention intervals were distinctive, the pigeons showed a choose-long effect (they appeared to time through the retention interval), but it was relatively weak until the retention intervals were extended to 8 sec. In Experiment 2, when pigeons were discouraged from timing through the retention intervals by making the intertrial intervals and retention intervals salient distinct events and using long (up to 16-sec) retention intervals in training, parallel retention functions were found. It appears that when ambiguity is removed, forgetting by pigeons does not occur by the process of subjective shortening. These experiments suggest that the accurate interpretation of results of animal memory research using differential-duration samples must consider the novelty of the retention intervals on test trials as well as their similarity to other trial events.

Perceptual learning in pigeons: Decreased ability to Discriminate samples mapped onto the same comparison in many-to-one matching

Humans often treat two stimuli that are associated with a common response as similar in other contexts. They do so presumably because those stimuli become conceptually or perceptually more similar to each other (perceptual learning). An analogous phenomenon may occur in pigeons when they are trained with a matching-to-sample procedure in which more than one sample is mapped onto the same comparison. In the present research, pigeons were trained to select one comparison following either of two samples (S1 or S2) and to select the other comparison following either of two different samples (S3 or S4). When the samples were then presented as positive and negative stimuli in a simple successive discrimination, samples that had been associated with the same comparison during original training (e.g., S1 vs. S2) were more difficult to discriminate than were samples that had been associated with different comparisons (e.g., S1 vs. S3). Thus, it appears that perceptual learning occurs in pigeons as well.

Neonatal cocaine exposure, activity, and responsivity to cocaine in a rodent model

This study examined the effects of neonatal cocaine exposure on running wheel activity and subsequent responsivity to cocaine using a rodent model. Subjects were artificially reared from postnatal (PND) days 4-10 via an intragastric cannula. The four treatment groups included two cocaine doses (20 mg/kg/day and 40 mg/kg/day), an artificially reared control and a normally reared suckled control. Subjects were tested at either PND 21 through PND 24 (Experiment 1) or PND 60 through PND 70 (Experiment 2) for 2 consecutive days. Testing consisted of a 30-min habituation period followed by injection of either saline (Day 1) or cocaine (Day 2) and an additional 60-min test session. Neonatal treatment had little effect on baseline activity or activity following saline injection at either age. All subjects showed an activation with cocaine injections, however, the activation was more pronounced in juveniles. Again, neonatal treatment did not interact with response to cocaine. These findings suggest that neonatal cocaine exposure does not alter activity or long-term responsivity to 20 mg/kg cocaine as measured in the running wheel apparatus.

Presence/Absence-Sample Matching by Pigeons: Divergent Retention Functions May Result From the Similarity of Behavior During the Absence Sample and the Retention Interval

Divergent choose-absence retention functions typically found in pigeons following presence/absence-sample matching have been attributed to the development of a single-code/default coding strategy. However, such effects may result from adventitious differential responding to the samples. In Experiment 1, retention functions were divergent only when differential sample responding could serve as the basis for comparison choice. In Experiment 2, when pecking did not occur during the retention interval, a choose-absence bias was found, but when pecking occurred during the retention interval, a choose-presence bias resulted. In Experiment 3, positive transfer was found when a stimulus associated with the absence of pecking replaced the absence sample but not when a stimulus associated with pecking replaced the presence sample. Thus, presence/absence-sample matching may not encourage the development of a single-code/default coding strategy in pigeons.

Neonatal cocaine exposure and activity rhythms in rats

This study looked at the effects of neonatal cocaine exposure on activity rhythms over a 48-h period in rats. Subjects were artificially-reared from postnatal days (PN) 4-10 via intragastric cannulas. The four treatment groups included two cocaine doses (20 and 40 mg/kg per day), an artificially-reared control and a normally reared suckled control. Subjects were tested at PN 38-40 in an automated running wheel. Neonatal cocaine exposure did not alter activity rhythms over the 48-h test period. However, there was a gender-specific effect of neonatal cocaine exposure on response to the novel test chamber and to the experimenter. The 20 mg/kg cocaine-exposed females showed increased running wheel activity relative to all other groups after placement in the running wheel. During the second 24-h period, cocaine-exposed females from both cocaine groups showed increased activity relative to controls following the entry of an experimenter to the test room. These findings suggest that female rats exposed to cocaine neonatally show an increased response to novel environments and stimuli.

Value transfer in a simultaneous discrimination by pigeons: The value of the S + is not specific to the simultaneous discrimination context

During simultaneous discrimination training, there is evidence that some of the value of the S+ transfers to the S−. When the value of the S+ is altered outside the context of the simultaneous discrimination, two very different predictions are made concerning its effect on its S−, depending on whether one views the S+ as an occasion setter or as a stimulus capable of transferring value. In four experiments, pigeons were trained with two similar simultaneous discriminations, A+B− and C+D−, and two single-stimulus trial types, A and C, (in which A always had greater nominal value than C). According to value transfer theory, on test trials, B should always be preferred over D, because B and D should be affected by the net values of A and C, respectively. According to an occasion setting account, however, D should be preferred over B because the presence of D signals a higher probability of reinforcement for responding to C than when C is alone, and/or the presence of B signals a lower probability of reinforcement for responding to A than when A is alone. In all four experiments, the pigeons preferred B over D, a result consistent with value transfer theory. Thus, an S− can acquire value from an S+ even when that value is conditioned in a “context” different from that of the simultaneous discrimination.

Problem solving following neonatal exposure to cocaine, ethanol, or cocaine/ethanol in combination in rats

This study examined the effects of neonatal drug exposure on performance in a digging maze. Subjects were Sprague-Dawley rats, artificially reared (AR) and fed through a gastrostomy tube from postnatal days (PND) 4-10. The AR groups included a cocaine group (20 mg/kg/day cocaine hydrochloride), an ethanol group (4 g/kg/day ethanol), a cocaine/ethanol group (20 mg/kg/day cocaine and 4 g/kg/day ethanol), and an AR control group. A suckled control raised by its dam was also included. At approximately PND 55, subjects were tested in a digging maze paradigm. The digging maze required subjects to use a species typical behavior (digging) to solve a novel problem (gaining access to water). While neonatal treatment had no effect on acquisition of a simple runway task for water reward, neonatal exposure to cocaine and ethanol in combination resulted in impaired performance on the digging maze task. None of the other neonatal treatment groups showed impairments on this task. These findings suggest that exposure to these doses of cocaine and ethanol during neonatal development may have more serious effects on problem solving tasks in rats than exposure to either drug alone.