Risk-sensitive foraging in rats: the effects of response-effort and reward-amount manipulations on choice behavior

The literature on risk-sensitive foraging theory provides several accounts of species that fluctuate between risk-averse and risk-prone strategies. The daily energy budget rule suggests that shifts in foraging strategy are precipitated by changes in the forager's energy budget. Researchers have attempted to alter the organism's energy budget using a variety of techniques such as food deprivation, manipulation of ambient temperatures, and delays to food reward; however, response-effort manipulations have been relatively neglected. A choice preparation using a wheel-running response and rats examined risk-sensitive preferences when both response effort and reward amounts were manipulated. Concurrently available reinforcement schedules (FI/60 and VI/60) yielded equivalent food amounts per unit time in all treatments. Two levels of response effort (20 or 120 g tangential resistance) and two levels of reward amount (three or nine pellets) were combined to form four distinct response-effort/reward-amount pairings. Increasing reward amounts significantly shifted choice toward the FI schedule in both response-effort conditions. The incidence of choice preference and the magnitude of shifts in choice were greater for the high response-effort conditions than for the low response-effort conditions. Implications of the significant interaction between response effort and reward amount are discussed in terms of a general energy-budget 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.     

Learning to stop or reset the internal clock

In the present experiments, after training rats in a standard fixed interval (FI) 30 s schedule, we induced a change in the strategy employed during gap trials, by presenting during FI with gaps training, 9-s interruptions of the FI discriminative stimulus in 40% of the trials; in one type of interruption, after the discriminative stimulus resumed, the FI was re-started; in the second type of interruption, the FI had to be completed considering the time before the interruption. The effect of these manipulations was tested in a peak-interval with gaps procedure. The main result was that the strategy employed during gap trials depended on the type of interruption experienced during the training phase, both in a comparison between subjects (experiment 1) and within subjects (experiment 2).     

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.     

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.     

Effects of unsignaled delays of reinforcement on fixed-interval schedule performance

Key pecking of pigeons was maintained by a fixed-interval (FI) 61-s schedule. The effects of resetting and nonresetting unsignaled delays of reinforcement then were examined. The resetting delay was programmed as a differential-reinforcement-of-other-behavior schedule, and the nonresetting delay as a fixed-time schedule. Three delay durations (0.5, 1 and 10 s) were examined. Overall response rates were decreased by one and 10-s delays and increased by 0.5-s delays. Response patterns changed from positively accelerated to more linear when resetting or nonresetting 10-s delays were imposed, but remained predominantly positively accelerated when resetting and nonresetting 0.5- and 1-s delays were in effect. In general, temporal control, as measured by quarter-life values, changed less than overall response rates when delays of reinforcement were in effect. The response patterns controlled by FI schedules are more resilient to the nominally disruptive effects of delays of reinforcement than are corresponding overall response rates.     

Dynamics of temporal control in rats: the effects of a brief transition in interval duration

Five rats lever-pressed for liquid reinforcers delivered according to a fixed-interval (FI) reinforcement schedule, where the interval requirement changed at an unpredictable point within a session. In a short square wave (SSW) condition, eight 30-s intervals were intercalated in a series of 120-s intervals so that the intervals changed from 120 to 30 s then back to 120 s. In a long square wave (LSW) condition the intervals changed from 120 to 480 s then back to 120 s. We observed rapid temporal control of post-reinforcement wait time duration by the IFI duration in the SSW condition only: Wait times decreased significantly during a transition to shorter (30 s) intervals; whereas wait times did not reliably increase during a transition to longer (480 s) intervals. Furthermore, in the SSW condition, wait time in post-transition intervals was shorter than that observed during pre-transition intervals. The results show that rats' wait times are sensitive to moment-to-moment changes in interval duration and that the dynamics depend on the direction in which the intervals change.     

An Assessment of Fixed Interval Timing in Free-Flying Honey Bees (Apis mellifera ligustica): An Analysis of Individual Performance

Interval timing is a key element of foraging theory, models of predator avoidance, and competitive interactions. Although interval timing is well documented in vertebrate species, it is virtually unstudied in invertebrates. In the present experiment, we used free-flying honey bees (Apis mellifera ligustica) as a model for timing behaviors. Subjects were trained to enter a hole in an automated artificial flower to receive a nectar reinforcer (i.e. reward). Responses were continuously reinforced prior to exposure to either a fixed interval (FI) 15-sec, FI 30-sec, FI 60-sec, or FI 120-sec reinforcement schedule. We measured response rate and post-reinforcement pause within each fixed interval trial between reinforcers. Honey bees responded at higher frequencies earlier in the fixed interval suggesting subject responding did not come under traditional forms of temporal control. Response rates were lower during FI conditions compared to performance on continuous reinforcement schedules, and responding was more resistant to extinction when previously reinforced on FI schedules. However, no “scalloped” or “break-and-run” patterns of group or individual responses reinforced on FI schedules were observed; no traditional evidence of temporal control was found. Finally, longer FI schedules eventually caused all subjects to cease returning to the operant chamber indicating subjects did not tolerate the longer FI schedules.     

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.     

Psychological distance to reward: equating the number of stimulus and response segments

Psychological distance to reward, or the segmentation effect, refers to the preference for a terminal link of a concurrent-chains schedule consisting of a simple reinforcement schedule (e.g. fixed interval [FI] 30s) relative to its chained-schedule counterpart (e.g. chained FI 15s FI 15s). This experiment was conducted to examine whether the segmentation effect is due to the number of terminal-link stimulus and response segments per se. Three pigeons pecked under a concurrent-chains schedule in which identical variable-interval (VI) schedules operated in the initial links. In each session, half the terminal-link entries followed one initial-link key and the other half followed the other initial-link key. The initial-link keys correlated with the different terminal links were manipulated across conditions. In the first three conditions, each terminal link contained a chained fixed-time (FT) FT schedule, and in the final three conditions, each terminal link contained a chained FI FI schedule. In each condition, in one terminal link (alternating), the order of two key colors correlated with the different schedule segments alternated across terminal-link entries, whereas in the other terminal link (constant), the order of two other key colors was identical for each entry. With the chained FT FT schedule terminal links, there was indifference between the alternating and constant terminal links within and across pigeons, as indexed by initial-link choice proportions. In addition, terminal-link response rates were relatively low. With the chained FI FI schedule terminal links, for each pigeon, there was relatively more preference for the alternating terminal link and terminal-link response rates increased relative to conditions with the chained FT FT schedule terminal links. These data suggest that the segmentation effect is not due simply to the number of terminal-link stimulus or response segments per se, but rather to a required period of responding during a stimulus segment that never is paired with reinforcement.     

Investigations of timing during the schedule and reinforcement intervals with wheel-running reinforcement

Across two experiments, a peak procedure was used to assess the timing of the onset and offset of an opportunity to run as a reinforcer. The first experiment investigated the effect of reinforcer duration on temporal discrimination of the onset of the reinforcement interval. Three male Wistar rats were exposed to fixed-interval (FI) 30-s schedules of wheel-running reinforcement and the duration of the opportunity to run was varied across values of 15, 30, and 60s. Each session consisted of 50 reinforcers and 10 probe trials. Results showed that as reinforcer duration increased, the percentage of postreinforcement pauses longer than the 30-s schedule interval increased. On probe trials, peak response rates occurred near the time of reinforcer delivery and peak times varied with reinforcer duration. In a second experiment, seven female Long-Evans rats were exposed to FI 30-s schedules leading to 30-s opportunities to run. Timing of the onset and offset of the reinforcement period was assessed by probe trials during the schedule interval and during the reinforcement interval in separate conditions. The results provided evidence of timing of the onset, but not the offset of the wheel-running reinforcement period. Further research is required to assess if timing occurs during a wheel-running reinforcement period.     

Reward magnitude and timing in pigeons

We investigated the interaction of motivation and timing by manipulating the expected reward magnitude during a peak procedure. Four pigeons were tested with three different reward magnitudes, operationalized as duration of food access. Each stimulus predicted a different reward magnitude on a 5 s fixed-interval schedule. Trials with different reward magnitudes were randomly intermingled in a session. Most pigeons responded less often and started responding later on peak trials when a smaller reward was expected, but showed no differences in response termination or peak times. Reward magnitude was independently corroborated through unreinforced choice trials, when pigeons chose between the three stimuli presented simultaneously. These results contribute to a growing body of evidence that the expected reward magnitude influences the decision to start anticipatory responding in tasks where the reward becomes available after a fixed interval, but does not alter peak times, nor the decision to stop responding on peak trials.     

Interval timing in rats: tracking unsignaled changes in the fixed interval schedule requirement

The present experiment examined interval timing in rats under dynamic conditions. A session began with FI60 s intervals, changed to a FI20 s, FI30 s, or FI40 s schedule at an unpredictable point, and then returned to a FI60 s schedule after the rats received 1, 8, or 24 successive short FI intervals. Variations in the duration and number of shorter intervals occurred across sessions and conditions. We observed rapid control of wait time duration by the FI duration of the preceding interval (one-back tracking), and changes in wait time depended on the number and duration of the shorter intervals. Furthermore, we observed proportional and scalar timing effects in overall wait time duration. The results provide information about the relation between interval timing under dynamic and steady state conditions.     

Effect of unsignaled delays between stimuli in a chain schedule on responding and resistance to change

Behavioral momentum theory is an evolving theoretical account of the strength of behavior. One challenge for the theory is determining the role of signal stimuli in determining response strength. This study evaluated the effect of an unsignaled delay between the initial link and terminal link of a two-link chain schedule on resistance to change using a multiple schedule of reinforcement. Pigeons were presented two different signaled delay to reinforcement schedules. Both schedules employed a two-link chain schedule with a variable interval 120-s initial link followed by a 5-s fixed time terminal link schedule. One of the schedules included a 5-s unsignaled delay between the initial link and the terminal link. Resistance to change was assessed with two separate disruption procedures: extinction and adding a variable time 20-s schedule of reinforcement to the inter-component interval. Baseline responding was lower in the schedule with the unsignaled delay but resistance to change for the initial link was unaffected by the unsignaled delay. The results suggest that not all unsignaled delays are equal in their effect on resistance to change.     

Food-exchange with humans in brown capuchin monkeys

To assess how brown capuchin monkeys (Cebus apella) delay gratification and maximize payoff, we carried out four experiments in which six subjects could exchange food pieces with a human experimenter. The pieces differed either in quality or quantity. In qualitative exchanges, all subjects gave a piece of food to receive another of higher value. When the difference of value between the rewards to be returned and those expected was higher, subjects performed better. Only two subjects refrained from nibbling the piece of food before returning it. All subjects performed two or three qualitative exchanges in succession to obtain a given reward. In quantitative exchanges, three subjects returned a food item to obtain a bigger one, but two of them nibbled the item before returning it. Individual differences were marked. Subjects had some difficulties when the food to be returned was similar or equal in quality to that expected.     

Effect of signaling reinforcement on resistance to change in a multiple schedule

This study evaluated the effect of a signal on resistance to change using a multiple schedule of reinforcement. Experiment 1 presented pigeons with three schedules: a signaled delay to reinforcement schedule (a two-link chain schedule with a variable-interval 120-s initial link followed by a 5-s fixed-time schedule), an unsignaled delay schedule (a comparable two-link tandem schedule), and an immediate, zero-delay variable-interval 125-s schedule. Two separate disruption procedures assessed resistance to change: extinction and adding a variable-time 20-s schedule of reinforcement to the inter-component interval. Resistance to change tests were conducted twice, once with the signal stimulus (the terminal link of the chain schedule) present and once with it absent. Results from both disruption procedures showed that signal absence reduced resistance to change for the pre-signal stimulus. In probe choice tests subjects strongly preferred the signal stimulus over the unsignaled stimulus and exhibited no reliable preference when given a choice between the signal stimulus and immediate stimulus. Experiment 2 presented two equal signaled schedules where, during resistance to change tests, the signal remained for one schedule and was removed for the second. Resistance to change was consistently lower when the signal was absent.     

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.     

Fixed interval and fixed time treadle pressing in the pigeon: A comparison with FI and FT keypecking

Experiment I used non-naive pigeons having previously performed on both keypecking and treadlepressing Fixed Interval schedules. In condition IT, treadlepressing was reinforced on successive Fixed Interval 60 seconds, Fixed Time 60 seconds and Fixed Interval 60 seconds schedules. Subsequently (condition IK), the same subjects pecked a key on an identical schedule sequence (FI60, FT60, FI60). In Experiment II, separate groups of naïve subjects were assigned either to treadlepressing (condition IIT) or keypecking (condition IIK) and to the same schedule sequence (FI60, FT60, FI60). Treadle pressing and keypecking decreased greatly in Fixed Time schedules. Curvature indices, pauses and running rates were less sensitive than response rates to the switching from one schedule to the other. Experiments I and II yielded similar results, experimental history accounting only for minor differences. The results were discussed in relation to interspecies differences in the temporal regulation of behavior and operant versus respondent control of the response and schedule-induced behaviour.     

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.