A rate measure of the relative aversiveness of signalled vs unsignalled shock

Five rats were trained on a two-component multiple schedule with each component consisting of a two-link chain schedule. Differential response suppression in the initial links of the chain schedules was used as a measure of the relative aversiveness of events introduced into the subsequent terminal links. When unsignalled shock was scheduled in one terminal link and signalled shock in the other (in addition to equal numbers of food reinforcers), responding was suppressed to a greater degree in the initial link preceding the unsignalled-shock condition. Reversing the terminal-link positions of unsignalled shock and signalled shock led to a reversal of the differential response suppression in the initial links. These results confirm previous findings that signalled shock is less aversive than unsignalled shock and extend the generality of this phenomenon from choice measures to rate measures of aversiveness.     

Homogeneous chains, heterogeneous chains, and delay of reinforcement

Three pigeons responded on two-component chain schedules in which the required response topography in the initial and terminal links was similar (a homogeneous chain) or dissimilar (a heterogeneous chain). Key-peck responding in the initial link under a variable-interval 60-second (VI 60) schedule produced a terminal link in which, in different conditions, either key pecking or foot treadling was reinforced according to a VI 60 schedule. Multiple VI 60 VI 60 schedules, in which the responses required in the chain schedules were maintained by primary reinforcement in the two components, preceded and followed each type of chain. These multiple schedules were used to ensure that both responses occurred reliably prior to introducing the chain schedule. Key-peck response rates in the initial link of the chain consistently were higher during the homogeneous chain than during the heterogeneous chain. These results illustrate that intervening events during a period separating an operant response from primary reinforcement influence that operant, independently of the delay between the response and reinforcement.     

Conditioned reinforcement versus time to reinforcement in chain schedules.

Pigeons were trained on three-component chain schedules in which the initial component was either a fixed-interval or variable-interval schedule. The middle and terminal components were varied among fixed-interval fixed-interval, variable-interval variable-interval, and an interdependent variable-interval variable-interval schedule in which the sum of the durations of the two variable-interval components was always equal to the sum of the fixed-interval fixed-interval components. At issue was whether the response rate in the initial component was controlled by its time to primary reinforcement or by the temporal parameters of the stimulus correlated with the middle terminal link. The fixed-interval initial-link schedule maintained much lower response rates than the variable-interval initial-link schedule regardless of the schedules in the middle and terminal links. Nevertheless, the intervening schedules played some role: With fixed-interval schedules in the initial links, response rates were consistently highest with independent variable-interval schedules in the middle and terminal links and intermediate with the interdependent variable-interval schedules; these initial-link differences were predicted by the response rates in the middle link of the chain. With variable-interval schedules in the initial links, response rates were lowest with the fixed-interval fixed-interval schedules following the initial link and were not systematically different for the two types of variable-interval variable-interval schedules. The results suggest that time to reinforcement itself accounts for little if any variance in initial-link responding.     

Second-order schedules of token reinforcement: effects of varying the schedule of food presentation

In the initial link of a complex schedule, one discriminative stimulus was presented and lever pressing produced tokens on fixed-ratio schedules. In the terminal link, signalled by a second discriminative stimulus, deposits of the tokens produced food. With two rats, the terminal link was presented after each sixth component schedule of token reinforcement was completed. With the other two rats, the terminal link was presented following the first component schedule completed after a fixed interval. During the terminal link, each token deposit initially produced food. The schedule of food presentation was subsequently increased such that an increasing number of token deposits in the terminal link was required for each food presentation. Rates of lever pressing in the initial link were inversely related to the schedule of food presentation in the terminal link. These results are similar to those of experiments that have varied schedules of food presentation in chained schedules. Rates and patterns of responding controlled throughout the initial link were more similar to those ordinarily controlled by second-order brief-stimulus schedules than to those controlled by comparable extended chained schedules.     

Resistance to change produced by access to fixed-delay versus variable-delay terminal links

Pigeons' responding was reinforced on a multiple schedule consisting of two two-link chain schedules presented in regular alternation. Responding in initial links (always variable-interval 60-s) produced a key-color change and access to a terminal link. The terminal link for one chain provided food after a fixed delay (fixed-interval or fixed-time); the terminal link for the other provided food after a variable delay (variable-interval or variable-time). The average duration of the terminal-link schedules was varied across conditions, but in every condition the arithmetic mean of the variable-delay terminal-link schedule was equal to the duration of the fixed delay. Response rates were higher in the initial links of the chains with the variable-delay terminal links. Response-decreasing operations (satiation, extinction) were used after performances reached asymptote. Response rates maintained by access to variable-delay terminal links tended to be more resistant to change than were rates maintained by access to fixed-delay terminal links. These results are consistent with the preference for variable- over fixed-interval terminal links observed with concurrent-chains schedules, suggesting (1) that immediacy of reinforcement influences the conditioned reinforcing potency of access to a terminal link and (2) that choice in concurrent chains and resistance of responding to change may be manifestations of the same effect of reinforcement.     

Choice of higher density signalled shock over lower density unsignalled shock

Unsignalled, inescapable shocks were presented to four albino rats in Experiment 1. By pressing a lever subjects could change the condition to signalled shock for 3-min periods after which unsignalled shock was automatically reinstated. All subjects changed from unsignalled to signalled shock when shock density was the same or when the density of signalled shock was two times greater than unsignalled shock. When the density of signalled shock was four times that of unsignalled shock, three subjects changed to the higher density schedule. One subject changed to a density of signalled shock eight times that of unsignalled shock. The second study showed that the two shock schedules most similar in Experiment 1 were discriminably different because subjects chose lower over higher shock densities when both densities were unsignalled. An analysis stressing safe (signal absent) and unsafe (signal present) periods was discussed.     

Elicited responding in chain schedules.

An omission procedure was employed to study elicited pecking in the first component of a two-component chain schedule. Both components were fixed-interval schedules correlated with colored keylights. The first response following the initial-link schedule produced a second fixed-interval schedule. We studied several fixed-interval lengths in two conditions: a standard response-dependent condition and an omission-contingent condition. The omission-contingent condition differed from the response-dependent condition in that responses during the initial fixed interval terminated the trial (omitting the terminal component and grain). If the terminal component was not omitted, a response following the terminal link's requirement produced 4-s access to grain. Pigeons responded during more than 70% of the initial links in the omission-contingent condition and responded during more than 90% of the initial links in the response-dependent condition. In general, rates of responding were consistent with the percentage data. The responding in the omission condition suggests that there may be elicited pecking, in chain schedules using pigeons, that is not the result of contingent conditioned reinforcement.     

Preference for signalled reinforcement

Key pecking was reinforced on a two-component multiple schedule. A variable-interval schedule controlled reinforcement in both components. During one component, access to reinforcement was preceded by a tone; in the other component, a standard unsignalled schedule was in effect. After performance stabilized, subjects were given a choice between the signalled and unsignalled schedules. They were placed in the chamber with the unsignalled schedule in effect on the right key. A single response on the left, or changeover, key produced the signalled schedule for 1 min. Both pigeons in Experiment I pecked the changeover key at a rate sufficient to remain under the signalled schedule for over 90% of the session. Removing and reintroducing the tone demonstrated that the changeover-key responses were due to the occurrence of the tone. In Experiment II, when pecking the changeover key produced the unsignalled schedule, pecking the changeover key declined. The results may be explained either in terms of Hendry's information hypothesis or as escape from an intermittent positive reinforcement schedule.     

An appraisal of preference for multiple versus mixed schedules

Pigeons' choice responses on either of two keys occasionally produced entry into a terminal link associated with that key. During the terminal links, responses produced access to grain according to mixed- or multiple-interval schedules. The multiple schedules provided stimuli correlated with the interval of time preceding reinforcement whereas the mixed schedules did not. The two subjects reliably preferred the multiple schedules to the mixed schedules throughout a series of replications. Preference for the multiple schedule was much smaller than suggested by earlier work comparing multiple and mixed schedules that had much higher rates of entry into the terminal links. Preference for the multiple schedule was greatly increased in this study when the rate of entry into the terminal schedules was increased. As in previous studies, these high preferences may have been the result of a sharp increase in the number of reinforcements on the multiple (as opposed to the mixed) schedule. The reliable but smaller preferences for the multiple schedule found with lower rates of entry into the terminal links were unconfounded by differences in the number of reinforcements obtained in the two terminal links.     

Choice between two-component chained and tandem schedules

Pigeons were trained on a two-key choice procedure in which a pair of equal and concurrently available variable-interval schedules (initial links) arranged entry into one or the other of two mutually exclusive schedules (terminal links) that ended in primary reinforcement. The terminal links were two-component chained or tandem schedules. Responses during the initial links were distributed equally on the two keys whenever the terminal links were associated with the same sets of interreinforcement intervals. Whether or not the terminal-link interreinforcement intervals were the same on the two keys, initial-link responding was affected by neither the presence nor relative durations of differentially signalled components within a terminal-link schedule. The simplest interpretation of these results is that initial-link responding is maintained directly by delayed primary reinforcement, rather than conditioned reinforcement afforded by the stimuli correlated with the terminal-link schedule components. This finding suggests that aspects of chained schedule performance usually attributed to conditioned reinforcement might best be reinterpreted in terms of delayed primary reinforcement and various discriminative functions served by the component-correlated stimuli.     

Preference for fixed-interval schedules: effects of unequal initial links

Six homing pigeons were trained on concurrent chain schedules in which the terminal links were fixed-interval schedules of 5 sec or 15 sec. One initial-link schedule was always VI 27-sec; the other was varied over conditions from VI 27-sec to VI 181-sec. Preference measured in the initial links varied as a joint function of the initial- and terminal-link schedules. When the initial links were varied with constant, but unequal, terminal links, the slope of the function relating the logarithm of the initial-link response ratio to the logarithm of the terminal-link entry ratio differed from that obtained with equal terminal links. This result indicates that biases attributable to the terminal-link schedules were not constant. The rate of change of preference, or degree of undermatching, in the initial links depended on whether the shorter initial link led to the shorter or the longer terminal link. These results raise the question of whether bias and undermatching in concurrent schedule performance are independent measures.     

The psychological distance to reward

Pigeons' responses in the presence of two concurrently available (initial-link) stimuli produced entry into one of two different and mutually exclusive terminal link stimuli according to identical but independent variable-interval schedules. In one experiment, a two-component chained fixed-interval schedule produced food in one terminal link while a simple fixed-interval schedule produced food in the other terminal link. When the interreinforcement intervals were equal in the two terminal links (i.e., the simple fixed-interval was twice the size of each of the components in the chained schedule) pigeons preferred the simple fixed-interval as measured by their relative rates of responding in the concurrently available initial links. This preference increased as the duration of the terminal links increased. The preference could be reversed by making the simple fixed-interval schedule sufficiently longer than the chained schedule. In the second experiment, the terminal links consisted of two- vs three-component chained fixed-intervals, again with equal interreinforcement intervals. Pigeons preferred the two-component chain to the three-component chain, although these results were less consistent and less dramatic than those in the first experiment. Again, preference increased as the duration of the terminal links increased. The results show that an organism's choice for a schedule will be substantially lowered by the chaining operation even when the interreinforcement interval remains constant.     

Preference for mixed-interval versus fixed-interval schedules

Pigeons were trained on a two-link concurrent chain schedule in which responses on two keys were reinforced according to independent variable-interval schedules by the production of a change in key color. Further responses on the key on which the stimulus change had been produced gave a single food reinforcement and a return to concurrent variable-interval conditions. On one key the terminal link was a two-valued mixed-interval schedule, while on the other, the terminal link was a fixed-interval schedule. When the mixed-interval values were kept constant and the fixed-interval values varied, relative response rates in the initial concurrent links matched relative reinforcement rates in the terminal links when these were computed from cubic transformations of the reciprocals of the intervals comprising the terminal link schedules.     

Choice for signalled over unsignalled shock as a function of shock intensity

Choice between a signalled shock schedule and an unsignalled one was examined at various shock intensities. Three rats were given the opportunity to change from the unsignalled schedule to the signalled one at intensity values between 0.15 mA and 1.0 mA. Steps were usually 0.15 mA and both ascending and descending series were given. For two other rats, shock intensity increased from 0.20 mA to 1.0 mA in 0.20-mA increments; for two additional rats, shock intensity was first 3.0 mA and was then reduced to 1.0 mA. Subjects tended to remain in the unsignalled schedule at the lower shock intensities, but spent most of each session under the signalled schedule at the higher intensities (1.0 mA and 3.0 mA). In addition, the time spent in the signalled schedule tended to vary systematically with shock intensity over at least part of the range of intensity values. It was concluded that the relationship between shock intensity and choice behavior is similar to the relationship between intensity and behavior in procedures involving avoidance, escape, and punishment.     

Choosing between fixed time and variable time shock

Two experiments are reported dealing with choice of fixed or variable time shock schedules with and without signals. Response independent shock was presented on either a fixed-time or a variable-time schedule and subjects could change from one condition to the other for short periods after which the initial condition was automatically reinstated. In the first experiment shocks for both schedules were either unsignalled (6 subjects) or signalled (3 subjects) and all subjects changed from a schedule of variable-time shock to a fixed-time one. Subjects that were given the opportunity to change in the reverse direction did not do so. Opposite findings were obtained in the second experiment (7 subjects) comparing unsignalled fixed-time shock with signalled variable-time shock. Six of the seven subjects changed from the fixed-time schedule to the signalled variable-time one but not the reverse. The data were considered consistent with an analysis of choice based on the reinforcing value of discriminable shock-free periods.     

Inverse relation between choice and local response rate with a schedule of response-produced blackouts

Pigeons were exposed to a two-key concurrent chains schedule in which identical frequencies and distributions of food presentations generated different response rates in the terminal links. An inverse relation between local rate of response in the terminal links and relative frequency of response in the initial links was observed. The high response rate was produced in one terminal link by a second-order schedule in which responding produced brief blackouts of the response key. Responding under the same schedule in the other terminal link did not produce blackouts. Under initial training and after spatial reversal of the terminal-link schedules, two of three pigeons had lower relative frequencies of response in the initial member of the chain with the higher terminal link response rate. The third pigeon showed no change in preference at reversal.     

SINGLE‐SAMPLE DISCRIMINATION OF DIFFERENT SCHEDULES' REINFORCED INTERRESPONSE TIMES

Food‐deprived rats in Experiment 1 responded to one of two tandem schedules that were, with equal probability, associated with a sample lever. The tandem schedules' initial links were different random‐interval schedules. Their values were adjusted to approximate equality in time to completing each tandem schedule's response requirements. The tandem schedules differed in their terminal links: One reinforced short interresponse times; the other reinforced long ones. Tandem‐schedule completion presented two comparison levers, one of which was associated with each tandem schedule. Pressing the lever associated with the sample‐lever tandem schedule produced a food pellet. Pressing the other produced a blackout. The difference between terminal‐link reinforced interresponse times varied across 10‐trial blocks within a session. Conditional‐discrimination accuracy increased with the size of the temporal difference between terminal‐link reinforced interresponse times. In Experiment 2, one tandem schedule was replaced by a random ratio, while the comparison schedule was either a tandem schedule that only reinforced long interresponse times or a random‐interval schedule. Again, conditional‐discrimination accuracy increased with the temporal difference between the two schedules' reinforced interresponse times. Most rats mastered the discrimination between random ratio and random interval, showing that the interresponse times reinforced by these schedules can serve to discriminate between these schedules.     

A temporal parameter influencing choice between signalled and unsignalled shock schedules

The present study investigated whether choice of a signalled variable-time shock schedule over an unsignalled one was influenced by the average intershock interval. Eight rats were given a choice between signalled and unsignalled shock schedules in a series of conditions with average intershock intervals of 510, 270, 150, 90, 60, and 45 sec. Each test condition was preceded by a training-baseline condition, and schedule values were arranged in an ascending (four subjects) or descending (four subjects) order. Choice of the signalled conditions was directly related to the average intershock interval of the variable-time schedule for six of the eight subjects. The per cent of time in the signalled condition was highest when the average intershock interval was 150 sec or longer and lowest when the average intershock interval was 45 sec. The findings were interpreted as being due to changes in the safety features of the signalled schedule, rather than to changes in the average intershock interval per se.     

Effects on responding of mixed and multiple schedules of signalled and unsignalled response-dependent electric-shock delivery

Responding in two rats was maintained under mixed and multiple variable-interval 35-sec variable-interval 35-sec food delivery schedules. Similar rates and patterns of responding occurred in each component of the two schedules. Mixed and multiple variable-interval 65-sec variable-interval 65-sec schedules of response-dependent shock delivery were super-imposed on the mixed and multiple baseline food schedules, respectively. In one component, a 5-sec stimulus was presented on the average of once every 65 sec. Offset of the stimulus arranged that the next response would produce shock. In the other component, no stimulus was presented during the 5-sec period. The mixed schedule of signalled and unsignalled dependent shock delivery yielded similar degrees of response suppression in each component, but the multiple schedule of shock delivery revealed differential degrees of response suppression. Considerably more suppression occurred in the component not associated with the preshock stimulus, thus implicating the discriminative functions of the correlated stimulus.     

Choice in the time-left procedure and in concurrent chains with a time-left terminal link.

In two experiments, rats chose between a standard fixed-duration food-associated stimulus and a stimulus whose duration was the time remaining to reinforcement in an elapsing comparison interval. In Experiment 1, 4 rats responded in a time-left procedure wherein a single initial-link variable-interval schedule set up two potential terminal links simultaneously. As time elapsed in the initial-link schedule, the choice was between a standard fixed-interval 30-s terminal link and a time-left terminal link whose programmed interval requirement equaled 90 s minus the elapsed time in the initial link. Rats generally responded more on the lever with the shortest programmed terminal-link duration, but the temporal parameters of the procedure were found to vary with response distributions. Contrary to previous reports, therefore, time-left data were well predicted by choice models that make no assumptions about animal timing. In Experiment 2, 8 rats responded on a concurrent-chains schedule with independent variable-interval initial links and a time-left terminal link in one of the choice schedules. On the time-left lever, the programmed terminal-link delay equaled 90 s minus the elapsed time in the time-left initial link. On the standard lever, terminal-link responses were reinforced according to a variable-interval schedule whose average value varied over four conditions. Relative time-left initial-link responses increased in the elapsing time-left initial-link schedule as the time-left terminal link became shorter relative to the standard terminal link. Scalar expectancy theory failed to predict the resultant data, but a modified version of the delay-reduction model made good predictions. An analysis of the elaboration of scalar expectancy theory for variable delays demonstrated that the model is poorly formulated for arithmetically distributed delays.