A further application of the active time model to multiple concurrent variable-interval schedules

In this experiment we show that the active time model (ATM) accurately predicts probe data from multiple concurrent VI VI schedules. Subjects were trained under a concurrent VI 30-s VI 60-s and a concurrent VI 60-s VI 120-s schedule. Two types of unreinforced probes were then conducted. The first paired the two VI 60-s stimuli. These stimuli, while equivalent in their associated absolute rates of reinforcement, differed in their relative rates of reinforcement. The second probe paired the VI 30-s stimulus with the relatively rich VI 60-s stimulus. In contrast with the first probe, these stimuli differed in their absolute rates of reinforcement, while being similar in their relative rates. During the first set of probes, birds preferred the VI 60-s stimulus trained with the VI 120-s schedule. During the second set of probes, birds were indifferent to the two stimuli. These results are less extreme than others reported in the literature. Nonetheless, we found that ATM accurately fit individual subject data in both sets of probes. In contrast a variant of scalar expectancy theory did not fit the data at either the individual or group level.     

Rapid acquisition in concurrent chains: effects of initial-link duration

Pigeons were trained in a concurrent chains procedure in which the terminal-link schedules in each session were either fixed-interval (FI) 10s FI 20s or FI 20s FI 10s, as determined by a pseudorandom binary series. The initial-link was a variable-interval (VI) 10-s schedule. Training continued until initial-link response allocation stabilized about midway through each session and was sensitive to the terminal-link immediacy ratio in that session. The initial-link schedule was then varied across sessions between VI 0.01 s and VI 30s according to an ascending and descending sequence. Initial-link response allocation was a bitonic function over the full range of durations. Preference for the FI 10-s terminal-link at first increased as programmed initial-link duration varied from 0.01 to 7.5s, and then decreased as initial-link duration increased to 30s. The bitonic function poses a potential challenge for existing models for steady-state choice, such as delay-reduction theory (DRT) [Fantino, E., 1969. Choice and rate of reinforcement. J. Exp. Anal. Behav. 12, 723-730], which predict a monotonic function. However, an extension of Grace and McLean's [Grace, R.C., McLean, A.P., 2006. Rapid acquisition in concurrent chains: evidence for a decision model. J. Exp. Anal. Behav. 85, 181-202] decision model predicted the bitonic function, and may ultimately provide an integrated account of choice in concurrent chains under both steady-state and dynamic conditions.     

Effect of contingent auditory stimuli on concurrent schedule performance: an alternative punisher to electric shock

This study explored whether load auditory stimuli could be used as functional punishing stimuli in place of electric shock. Three experiments examined the effect of a loud auditory stimulus on rats’ responding maintained by a concurrent reinforcement schedule. In Experiment 1, overall response rate decreased when a concurrent 1.5 s tone presentation schedule was superimposed on the concurrent variable interval (VI) 180-s, VI 180-s reinforcement schedule. On the contrary, response rate increased when a click presentation schedule was added. In Experiment 2, the extent of the response suppression with a 1.5 s tone presentation varied as a function of the frequency of the reinforcement schedule maintaining responses; the leaner the schedule employed, the greater the response suppression. In Experiment 3, response suppression was observed to be inversely related to the duration of the tone; response facilitation was observed when a 3.0-s tone was used. In Experiments 1 and 2, a preference shift towards the alternative with richer reinforcement was observed when the tone schedule was added. In contrast, the preference shifted towards the leaner alternative when the click or longer duration stimulus was used. These results imply that both the type and duration of a loud auditory stimulus, as well as the reinforcement schedule maintaining responses, have a critical role in determining the effect of the stimuli on responding. They also suggest that a loud auditory stimulus can be used as a positive punisher in a choice situation for rats, when the duration of the tone is brief, and the reinforcement schedule maintaining responses is lean.     

Dishabituation with component transitions may contribute to the interactions observed during multiple schedules

Rates of responding by rats were usually higher during the variable interval (VI) 30-s component of a multiple VI 30-s fixed interval (FI) 30-s schedule than during the same component of a multiple VI 30-s VI 30-s schedule (Experiment 1). Response rates were also usually higher during the FI 30-s component of a multiple VI 30-s FI 30-s schedule than during the same component of a multiple FI 30-s FI 30-s schedule (Experiment 2). The differences in response rates were not observed when the components provided VI or FI 120-s schedules. These results were predicted by the idea that differences in habituation to the reinforcer between multiple schedules contribute to behavioral interactions, such as behavioral contrast. However, differences in habituation were not apparent in the within-session patterns of responding. Finding differences in response rates in both experiments violates widely-held assumptions about behavioral interactions, including that behavioral contrast does not occur for rats and that improving the conditions of reinforcement decreases, rather than increases, response rate in the alternative component.     

Resistance to change varies inversely with reinforcement context

We report two experiments which test whether resistance to prefeeding and satiation for a variable-interval (VI) schedule that delivers a constant rate of reinforcement varies inversely with the reinforcement rate for an alternative schedule. In Experiment 1, eight pigeons responded in a multiple schedule in which the red key was always associated with a VI 90-s schedule and the green key with either a richer (VI 18s) or leaner (VI 540s) schedule in different conditions. After baseline training in each condition, prefeeding test sessions were conducted in which 10g, 20g, 30g, 40g, and 50g food were provided one-hour prior to test. Additional baseline training was given between each test session. In Experiment 2, two groups of pigeons responded in a multiple schedule similar to Experiment 1. After baseline training, pigeons were exposed to a 5-h satiation test session in which the VI 90-s schedule was available continuously. Test sessions were conducted when pigeons were maintained at 85%, 95%, and 85% of their body weights in an ABA design. Results of both experiments showed that responding in the VI 90-s schedule that alternated with a leaner schedule during baseline was more resistant to prefeeding and satiation. These data rule out alternative explanations for results of previous studies, and confirm that resistance to change varies inversely with reinforcement context.     

The effect of intruded events on peak time: the role of reinforcement history during the intruded event

Pigeons were studied in an extension of a study by Aum et al. [Aum, S., Brown, B.L., Hemmes, N.S. 2004. The effects of concurrent task and gap events on peak time in the peak procedure. Behav. Process. 65, 43-56] on timing behavior under a discrete-trial fixed-interval (FI) procedure during which 6-s intruded events were superimposed on peak-interval (PI) test trials. In Aum et al., one event consisted in termination of the timing cue (gap trial); the other was a stimulus in the presence of which subjects had been trained to respond under an independent random-interval (RI) schedule of reinforcement (concurrent task trial). Aum et al. found a disruption of timing on concurrent task trials that was greater than that on gap trials. The present study investigated history of reinforcement associated with intruded events as a possible explanation of this earlier finding. After training to peck a side key on a 30-s PI procedure, discrimination training was conducted on the center key in separate sessions; red or green 6-s stimuli were associated with RI 24s or EXT (extinction) schedules. During testing under the PI procedure, three types of intruded events were presented during probe trials--the stimulus associated with the RI (S+) or EXT (S-) schedule during discrimination training, or a gap (termination of the side-keylight). Intruded events occurred 3, 9, or 15s after PI trial onset. Effects of reinforcement history were revealed as substantial disruption of timing during the S+ event and relatively little disruption during the S- event. Intermediate effects were found for the gap event. Results indicate that postcue effects are at least partially responsible for the disruptive effects of the S+ event.     

60 Hz electric fields and incandescent light as aversive stimuli controlling the behavior of rats responding under concurrent schedules of reinforcement

Several reports have shown that animals will sometimes engage in behaviors that reduce their exposure to a 60 Hz electric field (E-field). The field, therefore, can function as an aversive stimulus. In other studies, the E-field at equivalent strengths failed to function as an aversive stimulus. The present experiment, using rats, demonstrates how factors other than field strength can influence whether a subject engages in behavior that reduces field exposure. The general design consisted of giving the rat a choice between two alternatives, one of which sometimes included an added stimulus. Each subject was trained to press each of two levers to obtain food. Pressing one lever was reinforced intermittently under a variable interval 2 min schedule (VI 2); pressing the other lever was reinforced by a second VI 2 schedule operating independently of the first. Under this concurrent schedule the rat spent 50% of the daily 50 min session responding to each of the levers, indicating that they were equally “valued.” Next, while the schedules remained in effect, the first response to one of the levers turned on a 100 kV/m E-field which remained on until the rat pressed the other lever. The time spent responding under the schedule associated with the field was reduced by about 5–10%. When the procedure was changed so that no lever presses produced food, i.e., extinction, but the added stimulus contingency remained, the rats spent even less time in the presence of the field. Similar outcomes were observed during both the concurrent food or extinction schedules when incandescent light was used. Thus, both an E-field and incandescent light functioned as aversive stimuli, but the magnitude of the aversiveness was small. Aversiveness depended not only on stimulus intensity, but also on behavioral factors. Bioelectromagnetics 19:210–221, 1998. © 1998 Wiley-Liss, Inc.     

Nicotine and the behavioral mechanisms of intertemporal choice

Nicotine has been found to produce dose-dependent increases in impulsive choice (preference for smaller, sooner reinforcers relative to larger, later reinforcers) in rats. Such increases could be produced by either of two behavioral mechanisms: (1) an increase in delay discounting (i.e., exacerbating the impact of differences in reinforcer delays) which would increase the value of a sooner reinforcer relative to a later one, or (2) a decrease in magnitude sensitivity (i.e., diminishing the impact of differences in reinforcer magnitudes) which would increase the value of a smaller reinforcer relative to a larger one. To isolate which of these two behavioral mechanisms was likely responsible for nicotine's effect on impulsive choice, we manipulated reinforcer delay and magnitude using a concurrent, variable interval (VI 30 s, VI 30 s) schedule of reinforcement with 2 groups of Long-Evans rats (n = 6 per group). For one group, choices were made between a 1-s delay and a 9-s delay to 2 food pellets. For a second group, choices were made between 1 pellet and 3 pellets. Nicotine (vehicle, 0.03, 0.1, 0.3, 0.56 and 0.74 mg/kg) produced dose-dependent decreases in preference for large versus small magnitude reinforcers and had no consistent effect on preference for short versus long delays. This suggests that nicotine decreases sensitivity to reinforcer magnitude.     

Differential resurgence and response elimination

Resurgence refers to the transient recovery of previously reinforced, but presently not reinforced, responding when more recently reinforced responding is extinguished. The primary purpose of our research was to determine how differential resurgence results from the procedures used to eliminate that responding. There were three conditions in each of five experiments. In Condition 1, key pecking by pigeons was maintained under a two-component multiple variable-interval (VI) 30-s VI 30-s schedule. In Condition 2, this pecking was eliminated in different ways across components. In Condition 3, extinction was in effect for all responses, and resurgence of key pecking was compared across components. These three conditions were repeated for most pigeons, and the procedures used to eliminate responding in Condition 2 varied across experiments. In Experiment 1, there was greater resurgence, and an earlier onset of it, after a differential-reinforcement-of-other-behavior (DRO) schedule than after a VI schedule was correlated with pecking an alternative key. Experiments 2 and 3 showed that the differential resurgence in Experiment 1 probably was not due to conditional stimulus control or the periodicity of food delivery, respectively. In Experiment 4, there was no systematic difference in resurgence after either a DRO schedule or a VI schedule correlated with treadle pressing. In Experiment 5, there was greater resurgence, and/or an earlier onset of it, after a VI schedule correlated with treadle pressing than after a VI schedule correlated with pecking an alternative key. Taken together, the results showed that the reinforcement of an alternative key-peck response was the most effective means of reducing subsequent key-peck resurgence. The relation of these results to an understanding of resurgence is discussed.     

Effects of d-amphetamine on behavior maintained by signaled and unsignaled delays to reinforcement

Four pigeons responded under a progressive-delay procedure. In a signaled-delay condition, a chained variable interval (VI) 30-s progressive time (PT) 4-s schedule was arranged; in an unsignaled-delay condition, a tandem VI 30-s PT 4-s schedule was arranged. Two pigeons experienced a signaled-unsignaled-signaled sequence; whereas, two pigeons experienced an unsignaled-signaled-unsignaled sequence. Effects of saline and d-amphetamine were determined under each condition. At intermediate doses (1.0 and 1.78 m/kg) delay functions were shallower, area under the curve was increased, and, when possible, break points were increased compared to saline; these effects were not systematically related to signaling conditions. These effects on control by delay often were accompanied by decreased response rates at 0 s. These results suggest that stimulus conditions associated with the delay may not play a crucial role in effects of d-amphetamine and other stimulants on behavior controlled by reinforcement delay.     

Effects of Physiological Internal Noise on Model Predictions of Concurrent Vowel Identification for Normal-Hearing Listeners

Previous studies have shown that concurrent vowel identification improves with increasing temporal onset asynchrony of the vowels, even if the vowels have the same fundamental frequency. The current study investigated the possible underlying neural processing involved in concurrent vowel perception. The individual vowel stimuli from a previously published study were used as inputs for a phenomenological auditory-nerve (AN) model. Spectrotemporal representations of simulated neural excitation patterns were constructed (i.e., neurograms) and then matched quantitatively with the neurograms of the single vowels using the Neurogram Similarity Index Measure (NSIM). A novel computational decision model was used to predict concurrent vowel identification. To facilitate optimum matches between the model predictions and the behavioral human data, internal noise was added at either neurogram generation or neurogram matching using the NSIM procedure. The best fit to the behavioral data was achieved with a signal-to-noise ratio (SNR) of 8 dB for internal noise added at the neurogram but with a much smaller amount of internal noise (SNR of 60 dB) for internal noise added at the level of the NSIM computations. The results suggest that accurate modeling of concurrent vowel data from listeners with normal hearing may partly depend on internal noise and where internal noise is hypothesized to occur during the concurrent vowel identification process.     

Extinguished operant responding shows stimulus specificity

The experiment tested for stimulus specificity in extinguished operant responding. Eight pigeons pecked keys for food reinforcers delivered by a variable interval (VI) 60-s schedule. The key was illuminated with red light during some sessions and white light during others. Then, responding was placed on extinction. During some sessions of extinction, the color of the key light remained constant throughout the session (red or white). During other sessions the color changed at 30 min into the session (red to white or white to red). Response rate increased after the change of key color in extinction. If it is assumed that key color is part of the stimulus to which subjects habituate, then these results are consistent with McSweeney and Swindell's [J. Gen. Psychol. 129 (2002) 364] suggestion that responding declines in extinction partly because subjects habituate to the stimuli that support conditioned responding. Habituation is relatively specific to the exact nature of the stimulus presented. Therefore, changes in the stimulus violate stimulus specificity and restore habituated responding. The results are also consistent with other theories that attribute extinction to a reduction of stimulus control [e.g., Psychol. Bull. 114 (1993) 80; J. Exp. Psychol.: Anim. Behav. Process. 16 (1990) 235], but considerations such as parsimony and testability favor the habituation hypothesis over these theories.     

Effects of free-food deliveries and delays on choice under concurrent-chains schedules

Pigeons responded on a concurrent-chains schedule with two equal variable-interval (VI) schedules as initial links and delays to food of 3 and 12s as the two terminal links. In even-numbered sessions, no other reinforcement schedule was present, and all pigeons showed a strong preference for the response key that had the shorter, 3-s terminal-link delay. In odd-numbered sessions, the initial links were interrupted at random times by one of three different types of events. When the interruptions were immediate food deliveries, the response percentages increased on the key that had the 3-s delay. When the interruptions were 30-s delays followed by food, the response percentages remained approximately unchanged. When the interruptions were 30-s delays with no food, the response percentages decreased. The results were used to compare the predictions of different mathematical models of concurrent-chains performance. The results favored models that assume that preference is determined by the relative amount of advantage that is gained when a terminal link is entered.     

Rapid acquisition of preference in concurrent schedules: effects of reinforcement amount

Four pigeons were trained on concurrent variable-interval 30-s schedules. Relative reinforcer amounts arranged across the two alternatives were varied across sessions according to a pseudorandom binary sequence [cf., Hunter, I., Davison, M., 1985. Determination of a behavioral transfer function: white-noise analysis of session-to-session response-ratio dynamics on concurrent VI schedules. J. Exp. Anal. Behav. 43, 43-59]; the ratios (left/right) were either 1/7 or 7/1. Reinforcer amount was manipulated by varying the number of 1.2s hopper presentations. Sessions ended after 30 reinforcers (15 for each alternative). After approximately 30 sessions, response ratios for all pigeons began to track the changes in amount ratio (i.e., subjects' responding showed a moderate increase in sensitivity of responding to reinforcer amount). Characteristics of responding were similar to procedures in which reinforcer rate and immediacy have been manipulated, although sensitivity estimates for amount were lower than those previously obtained with rate and immediacy. This procedure may serve as a useful method for studying the effects of certain environmental manipulations (e.g., drug administration) on sensitivity to reinforcer amount.     

Resistance to extinction, generalization decrement, and conditioned reinforcement

This study investigated generalization decrement during an extinction resistance-to-change test for pigeon key pecking using a two-component multiple schedule with equal variable-interval 3-min schedules and different reinforcer amounts (one component presented 2-s access to reinforcement and the other 8s). After establishing baseline responding, subjects were assigned to one of the two extinction conditions: hopper stimuli (hopper and hopper light were activated but no food was available) or Control (inactive hopper and hopper light). Responding in the 8-s component was more resistant to extinction than responding in the 2-s component, the hopper stimuli group was more resistant to extinction compared to the Control group, and an interaction between amount of reinforcement, extinction condition, and session block was present. This finding supports generalization decrement as a factor that influences resistance to extinction. Hopper-time data (the amount of time subjects spent with their heads in the hopper) were compared to resistance-to-change data in an investigation of the role of conditioned reinforcement on resistance to change.     

Within-session changes in responding during concurrent variable interval variable ratio schedules

Rats (Experiment 1) and pigeons (Experiment 2) responded on several concurrent variable interval (VI) variable ratio (VR) schedules. The rate of, but not the time spent, responding in each component usually changed within-sessions. The bias and sensitivity parameters of the generalized matching law (GML) did not change systematically within-sessions. The fit of the GML to the data did not change within-sessions for pigeons, but it was better in the middle than at the beginning or end of the session for some for rats. Both over- and under-matching occurred. These results imply that within-session changes in responding do not usually cause problems for assessing the validity of the GML when subjects respond on concurrent VI VR schedules. The results also suggest that under- and over-matching are not produced by different factors, but rather lie on a continuum.     

Human perceptual learning: The effect of pre-exposure schedule depends on task demands

The effects of the pre-exposure schedule (concurrent, intermixed, and blocked) to two similar visual stimuli were assessed in three different tasks. Participants were more accurate identifying one of two pre-exposed stimuli as the target by means of same/different judgments after concurrent than intermixed or blocked pre-exposures. Regardless of pre-exposure schedule, participants were accurate in identifying the same target stimulus in a subsequent multiple choice task. However, the other pre-exposed stimulus was incorrectly chosen as the target in a greater proportion after blocked than intermixed or concurrent pre-exposure. Finally, participants who received the blocked schedule showed a greater ability to construct the target in a puzzle test than those who received a concurrent or intermixed schedule. These results suggest that the effect of pre-exposure schedule may depend on task-specific demands. But all these results might be explained by a selective attention mechanism like that proposed by Gibson (1969) to account for perceptual learning.     

Reinforcer concentration effects on a fixed-interval schedule

Four rats received training on a mixed FI 30-s FI 150-s schedule, where the different FI values were associated with different levers. During baseline, the reinforcer was a 30% concentration of condensed milk. During subsequent testing sessions, the reinforcer concentration was varied within sessions over values of 10, 30, 50, and 70%. Measures of behaviour were taken from the FI 30-s lever during trials where the reinforcer was delivered for responses on the other lever. Increasing the reinforcer concentration which began the interval (a) increased the time to start responding in the interval, and (b) increased the location of the response peak on the FI 30-s lever (often to values well above 30s). Response rate at the peak, and spread of the response rate versus time function, changed much less with reinforcer concentration. The data are discussed relative to predictions derived from Scalar Expectancy Theory, the Behavioural Theory of Timing, and the Tuned-trace model.     

The effects of concurrent task and gap events on peak time in the peak procedure

The effect of a concurrent task on timing performance of pigeons was investigated with the peak interval procedure. Birds were trained to peck a side key on a discrete-trial schedule that included reinforced fixed-interval (FI) 30-s trials and nonreinforced extended probe trials. Then, in separate sessions, birds were trained to peck a 6-s center key for food. In a subsequent test phase, the FI procedure was in effect along with dual-task probe test trials. On those test trials, the 6-s center key (task cue) was presented at 3, 9, or 15s after probe trial onset. During another test phase, a 6-s gap (the FI keylight was extinguished) was presented at 3, 9, or 15s after probe trial onset. Peak time increased with center key time of onset, and was greater under task than gap conditions. Moreover, peak time under task conditions exceeded values predicted by stop and reset clock mechanisms. These results are at variance with current attentional accounts of timing behavior in dual-task conditions, and suggest a role of nontemporal factors in the control of timing behavior.     

Increased effort requirements and risk sensitivity: a comparison of delay and magnitude manipulations

Reward magnitude and delay to reward were independently manipulated in two separate experiments examining risk-sensitive choice in rats. A dual-running wheel apparatus was used and the tangential force resistance required to displace both wheels was low (50g) for half of the subjects, and high (120g) for the remaining subjects. Concurrent FI30-s and FI60-s schedules delivered equivalent amounts of food reward per unit time (i.e. 5 and 10 pellets of food, respectively), and these conditions served as the baseline treatment for all subjects. Variability, either in reward magnitude or delay, was introduced on the long-delay (60s) schedule during the second phase. All subjects were returned to the baseline condition in the third phase, and variability was introduced on the short-delay (30s) interval schedule during phase four. The subjects were again returned to the baseline condition in the fifth and final phase, ultimately yielding a five-phase ABACA design. Original baseline performance was characterized by a slight short-delay interval preference, and this pattern of performance was recovered with each subsequent presentation of the baseline condition. Overall, the data obtained from the reward magnitude and delay-to-reward manipulations were indistinguishable; subjects experiencing low-response effort requirement behaved in a risk-indifferent manner and subjects experiencing high-response effort requirement preferred the variable schedule. Implications for the daily energy budget rule on risk-sensitive foraging are discussed in light of these findings.