CER suppression, passive-avoidance learning, and stress-induced suppression of drinking in the Syracuse high- and low-avoidance strains of rats (Rattus norvegicus)

The Syracuse strains of Long-Evans rats were selectively bred for good (SHA) or poor (SLA) avoidance learning in a two-way shuttle box, which resulted in a phenotypic difference that is correlated with behavior patterns indicative of emotional reactivity, SLA animals showing evidence of greater emotional reactivity than SHA animals. The first three experiments examined conditioned suppression of bar pressing and compared paired and unpaired conditioned- and unconditioned-stimulus presentations to evaluate the influence of conditioning versus primary aversive stimulation on baseline responding. SLA animals acquired conditioned suppression faster than SHA animals and also showed greater suppression of baseline responding than SHA animals. In Experiment 4, SLA animals learned a passive-avoidance task faster than SHA animals. In Experiment 5, SLA animals showed greater stress-induced suppression of drinking a weak quinine solution than SHA animals. These data are consistent with the hypothesis that SLA animals are more emotionally reactive than SHA animals.     

Conditioned taste and taste-potentiated odor aversions in the Syracuse high- and low-avoidance (SHA/Bru and SLA/Bru) strains of rats (Rattus norvegicus).

Syracuse high- and low-avoidance Long-Evans rats (Rattus norvegicus; SHA/Bru and SLA/Bru) were selectively bred for good and poor active-avoidance learning. However, SLA/Bru animals are superior to SHA/Bru rats in conditioned suppression and passive avoidance learning. In this experiment, saccharin taste and almond odor were the components of a compound conditioned stimulus (flavor) in an illness-induced aversive conditioning paradigm. SLA/Bru rats (n = 17) showed stronger conditioned flavor, taste, and odor aversion than did SHA/Bru animals (n = 18). Unselected Long-Evans rats (n = 18) were intermediate between the selected strains. SLA/Bru and Long-Evans rats showed taste-potentiated odor aversions in this experiment, whereas SHA/Bru animals did not. The results provide evidence that genetic factors, as exemplified by the different strains, are importantly involved in the mechanisms underlying interoceptive and exteroceptive aversive conditioning.     

The effects of control and lack of control on active and passive avoidance in rhesus monkeys

Most studies which have examined the effects of lack of control have utilized test tasks in which active responding is required, and generally they have found impaired learning. Those few studies which have required passive responding in the test task generally have found facilitation of learning. The present two experiments examined the effects of lack of control in both active and passive avoidance tasks in a primate species (Macaco mulatta) not previously used in this research area. In Experiment 1, although the group without control (IE) tended to be inferior at active and superior at passive avoidance in comparison to the group with control (E), there were no significant differences. In Experiment 2, utilizing a difficult discrimination task in which subjects were required to learn when and when not to respond actively to avoid aversive stimulation, greater group differences were found. Two monkeys from Group IE failed to escape in active avoidance acquisition and, as a whole. Group IE was somewhat slower to respond than Group E. At passive avoidance, however. Group IE was superior to Group E and, as a consequence, more efficiently solved the discrimination problem. Implications of the present results for interpretation of the effects of lack of control as deficits are discussed.     

Effects of signaled and unsignaled shock on schedule-controlled lever pressing and schedule-induced licking: Shock intensity and body weight

Schedule-controlled lever pressing and schedule-induced licking were studied in rats under a multiple fixed-interval fixed-interval schedule of food reinforcement. Following acquisition of stable rates of pressing and licking, a multiple variable-time variable-time schedule of electric-shock delivery was superimposed upon the baseline schedule. In only one component of the multiple schedule, a 5-sec stimulus preceded each shock (signaled shock). In the other component shock was unsignaled. Several shock intensities (Experiment 1) and body weights (Experiment 2) were studied. Lever pressing and licking were affected similarly by experimental manipulations, although with parametric differences. Depending upon shock intensity and body weight, rates of lever pressing and licking were hardly suppressed, suppressed primarily in the unsignaled shock component (differential suppression), or markedly suppressed in both components. Differential suppression during components with signaled and unsignaled shock and conditioned suppression of responding during the preshock stimulus appeared not to be functionally related. Differential suppression depended more on the discriminability of shock-free time, and on shock intensity, body weight, and the type of response than on the “preparatory” behavior preceding shock.     

The effect of shock intensity upon responding under a multiple-avoidance schedule

The effect of two shock intensities (1.00 and 2.00 mA) were studied in the acquisition, maintenance, and extinction of unsignalled avoidance by albino rats. Single and multiple avoidance schedules were employed, with shock intensity being the principal condition that differed between schedule components. The higher shock intensity was generally more effective in producing avoidance. Higher response rates and lower shock rates were observed under high-intensity shock when performance stabilized. When the multiple schedule was introduced, the six rats trained under a single shock intensity all showed poorer performance under the new shock intensity, whether it was higher or lower than the training intensity. Performance under the original shock intensity did not change substantially with the introduction of a different shock intensity in the other multiple schedule component. Performance under the new shock intensity showed gradual improvement with continued exposure to it. All of the rats showed persistent “warm-up”, receiving approximately 40% of the total session shocks in the first one-sixth of the session. The degree of warm-up was unrelated to avoidance shock intensity.     

Rats (Rattus norvegicus) selectively bred to differ in avoidance behavior also differ in response to novelty stress, in glycemic conditioning, and in reward contrast

The behavior of the Syracuse high avoidance (SHA) and Syracuse low avoidance (SLA) rats, selectively bred by Brush (F. R. Brush, J. C. Froehlich, & P. Sakellaris, 1979, Behavior Genetics, 9, 309-316) to differ in avoidance behavior, was examined in several different tasks. The SLA rats showed a greater elevation in plasma glucose when exposed to a novel environment; after 7 days of exposure to this environment there was evidence of habituation in the SHA rats but not in the SLA rats; the SHA rats showed a hyperglycemic conditioned response in a glycemic conditioning procedure, the SLA rats showed no evidence of conditioning but had higher overall levels of plasma glucose; both strains showed reliable successive negative contrast effects in consummatory behavior when shifted from 32 to 4% sucrose, but the contrast was larger in the SLA rats; the administration of chlordiazepoxide eliminated negative contrast in the SLA rats but had no effect on contrast in the SHA rats; and the SLA rats were reliably heavier than the SHA rats. The behavioral differences were considered in the context of differences in emotional reactivity between the two strains.     

Behavioral contrast in rats with different reinforcers and different response topographies

Experiment I demonstrated positive behavioral contrast in rats when one of two qualitatively different reinforcers (milk and pellets) was removed from a component of a multiple schedule. The contrast effect was larger and more enduring when milk was removed. Experiment II showed that the rats spent more time on the side of a shuttle-box on which milk was freely available than on the side on which pellets were freely available. Experiment III, a partial replication of Experiment I, failed to demonstrate the contrast effect of Experiment I. Experiment IV demonstrated contrast when two topographically distinct responses, nose-key poking and lever pressing, were required in different components of a multiple schedule. These results extend the conditions that generate behavioral contrast in rats.     

Differential reinstatement predicted by preextinction response rate

Reinstatement refers to the recovery of previously extinguished responding by the responseindependent delivery of a stimulus that was a reinforcer in training. Two experiments were conducted to examine relative reinstatement following the training of differential preextinction response rates, either with equal (Experiment 1) or unequal (Experiment 2) preextinction reinforcement rates. In Experiment 1, each of 3 pigeons first pecked at relatively high rates in the tandem variable-time 117-sec fixed-interval 3-sec component of a multiple schedule and at lower rates in a separate tandem variableinterval 117-sec fixed-time 3-sec component. Reinforcement rates were equal between components. Pecking then was extinguished in each component, before being reinstated under a multiple variabletime 120-sec variable-time 120-sec schedule. Greater reinstatement occurred in the component previously correlated with higher rates of pecking. In Experiment 2, in an initial condition, the mean rate of lever pressing for one group of 8 rats was significantly higher under a fixed-ratio 3 schedule than for another group of 8 rats under a fixed-ratio 1 schedule. Mean reinforcement rate was significantly higher for the group exposed to the fixed-ratio 1 schedule. For each group, lever pressing then was extinguished, before being reinstated under a variable-time 30-sec schedule. Significantly greater mean reinstatement occurred for the group previously exposed to the fixed-ratio 3 schedule. These results suggest that differential reinstatement may be predicted by preextinction response rate, perhaps independently of preextinction reinforcement rate.     

Response–reinforcer dependency and resistance to change

The effects of the response–reinforcer dependency on resistance to change were studied in three experiments with rats. In Experiment 1, lever pressing produced reinforcers at similar rates after variable interreinforcer intervals in each component of a two‐component multiple schedule. Across conditions, in the fixed component, all reinforcers were response‐dependent; in the alternative component, the percentage of response‐dependent reinforcers was 100, 50 (i.e., 50% response‐dependent and 50% response‐independent) or 10% (i.e., 10% response‐dependent and 90% response‐independent). Resistance to extinction was greater in the alternative than in the fixed component when the dependency in the former was 10%, but was similar between components when this dependency was 100 or 50%. In Experiment 2, a three‐component multiple schedule was used. The dependency was 100% in one component and 10% in the other two. The 10% components differed on how reinforcers were programmed. In one component, as in Experiment 1, a reinforcer had to be collected before the scheduling of other response‐dependent or independent reinforcers. In the other component, response‐dependent and ‐independent reinforcers were programmed by superimposing a variable‐time schedule on an independent variable‐interval schedule. Regardless of the procedure used to program the dependency, resistance to extinction was greater in the 10% components than in the 100% component. These results were replicated in Experiment 3 in which, instead of extinction, VT schedules replaced the baseline schedules in each multiple‐schedule component during the test. We argue that the relative change in dependency from Baseline to Test, which is greater when baseline dependencies are high rather than low, could account for the differential resistance to change in the present experiments. The inconsistencies in results across the present and previous experiments suggest that the effects of dependency on resistance to change are not well understood. Additional systematic analyses are important to further understand the effects of the response–reinforcer relation on resistance to change and to the development of a more comprehensive theory of behavioral persistence.     

Extended pausing by humans on multiple fixed-ratio schedules with varied reinforcer magnitude and response requirements

We conducted three experiments to reproduce and extend Perone and Courtney's (1992) study of pausing at the beginning of fixed-ratio schedules. In a multiple schedule with unequal amounts of food across two components, they found that pigeons paused longest in the component associated with the smaller amount of food (the lean component), but only when it was preceded by the rich component. In our studies, adults with mild intellectual disabilities responded on a touch-sensitive computer monitor to produce money. In Experiment 1, the multiple-schedule components differed in both response requirement and reinforcer magnitude (i.e., the rich component required fewer responses and produced more money than the lean component). Effects shown with pigeons were reproduced in all 7 participants. In Experiment 2, we removed the stimuli that signaled the two schedule components, and participants' extended pausing was eliminated. In Experiment 3, to assess sensitivity to reinforcer magnitude versus fixed-ratio size, we presented conditions with equal ratio sizes but disparate magnitudes and conditions with equal magnitudes but disparate ratio sizes. Sensitivity to these manipulations was idiosyncratic. The present experiments obtained schedule control in verbally competent human participants and, despite procedural differences, we reproduced findings with animal participants. We showed that pausing is jointly determined by past conditions of reinforcement and stimuli correlated with upcoming conditions.     

Tests of behavior momentum in simple and multiple schedules with rats and pigeons.

Four experiments examined the relationship between rate of reinforcement and resistance to change in rats' and pigeons' responses under simple and multiple schedules of reinforcement. In Experiment 1, 28 rats responded under either simple fixed-ratio, variable-ratio, fixed-interval, or variable-interval schedules; in Experiment 2, 3 pigeons responded under simple fixed-ratio schedules. Under each schedule, rate of reinforcement varied across four successive conditions. In Experiment 3, 14 rats responded under either a multiple fixed-ratio schedule or a multiple fixed-interval schedule, each with two components that differed in rate of reinforcement. In Experiment 4, 7 pigeons responded under either a multiple fixed-ratio or a multiple fixed-interval schedule, each with three components that also differed in rate of reinforcement. Under each condition of each experiment, resistance to change was studied by measuring schedule-controlled performance under conditions with prefeeding, response-independent food during the schedule or during timeouts that separated components of the multiple schedules, and by measuring behavior under extinction. There were no consistent differences between rats and pigeons. There was no direct relationship between rates of reinforcement and resistance to change when rates of reinforcement varied across successive conditions in the simple schedules. By comparison, in the multiple schedules there was a direct relationship between rates of reinforcement and resistance to change during most tests of resistance to change. The major exception was delivering response-independent food during the schedule; this disrupted responding, but there was no direct relationship between rates of reinforcement and resistance to change in simple- or multiple-schedule contexts. The data suggest that rate of reinforcement determines resistance to change in multiple schedules, but that this relationship does not hold under simple schedules.     

The following schedule of reinforcement as a fundamental determinant of steady state contrast in multiple schedules

Two experiments investigated whether steady-state interactions in multiple schedules depend exclusively on the following schedule of reinforcement. Experiment 1 used a four-component multiple schedule in which two components were associated with the same constant schedule of reinforcement, and where rate of reinforcement was varied in the component that followed one of these. Contrast effects were reliable only in the component that preceded the point of reinforcement variation, although some contrast did occur otherwise. In those instances where contrast other than the following-schedule effect did occur, it was accounted for by the effect of the preceding schedule, an effect for which there were consistent individual differences among subjects, and which varied with component duration. Experiment 2 used a three-component schedule, in which reinforcement rate was varied in the middle component. The results were consistent with Experiment 1, as the following-schedule effect was the only consistent effect that occurred, although an effect of the preceding schedule did occur for some subjects under some conditions, and was especially evident early in training. The conclusion from both experiments is that there is no general effect of relative rate of reinforcement apart from the sum of the effects of the preceding and following schedules, and that the following-schedule effect is the fundamental cause of steady-state interactions.     

Active and passive avoidance performances following bilateral olfactory bulbectomy

Three experiments were performed to study the effects of bilateral olfactory bulbectomy upon active and passive avoidance conditioning. In the first experiment, olfactory bulbectomized rats were found not to be impaired on a one-way active avoidance task but showed a fear retention deficit. In Experiment 2, sham-operated rats demonstrated a degree of fear retention that was a positive linear function of the number of shock presentations; olfactory bulbectomized rats showed a total absence of fear retention. In the third experiment olfactory bulbectomized rats failed to demonstrate the inverse relationship between shock intensity and two-way avoidance performance normally evidenced. The pattern of data obtained suggests that while bulbectomized rats do not show any general avoidance impairment they do seem to be unaffected by the suppressive effects of shock upon ongoing behavior.     

Marking effects in instrumental performance on DRH schedules

Three experiments investigated the effect of presenting a brief stimulus after a response sequence on the rate of lever-pressing by rats on differential reinforcement of high rate (DRH) schedules. In Experiment 1 enhanced responding was produced by a visual stimulus presented during a 500-msec delay of reinforcement compared to a condition in which no stimulus was presented. In Experiment 2 rats responded on a multiple DRH DRH schedule in which the DRH contingency was reinforced on a 50% schedule in each component. Equivalent levels of responding occurred in the components when reinforcement was signalled in one component and when the signal was presented following the non-reinforced schedules in the other components. A further group of rats received the stimulus presented after non-reinforced schedules in one component but not at all in the other component; responding was enhanced in the former component relative to the latter component. In Experiment 3 brief stimuli presented after the completion of DRH components on a second-order VR (DRH) schedule elevated response rates irrespective of whether the signal was presented paired or unpaired with reinforcement. The present data support the view that a brief signal may serve to mark a response sequence in memory and facilitate instrumental performance.     

Second-order schedules: discrimination of components

Pigeons were exposed to a series of second-order schedules in which the completion of a fixed number of fixed-interval components produced food. In Experiment 1, brief (2 sec) stimulus presentations occurred as each fixed-interval component was completed. During the brief-stimulus presentation terminating the last fixed-interval component, a response was required on a second key, the brief-stimulus key, to produce food. Responses on the brief-stimulus key before the last brief-stimulus presentation had no scheduled consequences, but served as a measure of the extent to which the final component was discriminated from preceding components. Whether there were one, two, four, or eight fixed-interval components, responses on the brief-stimulus key occurred during virtually every brief-stimulus presentation. In Experiment 2, an attempt was made to punish unnecessary responses on the brief-stimulus key, i.e., responses on the brief-stimulus key that occurred before the last component. None of the pigeons learned to withhold these responses, even though they produced a 15-sec timeout and loss of primary reinforcement. In Experiment 3, different key colors were associated with each component of a second-order schedule (a chain schedule). In contrast to Experiment 1, brief-stimulus key responses were confined to the last component. It was concluded that pigeons do not discriminate well between components of second-order schedules unless a unique exteroceptive cue is provided for each component. The relative discriminability of the components may account for the observed differences in initial-component response rates between comparable brief-stimulus, tandem, and chain schedules.     

Increased reinforcement when timeout from avoidance includes access to a safe place

Three experiments investigated the reinforcing value of access to a safe place during timeout from an avoidance schedule. Rats were trained on conjoint schedules in which responding both postponed shock on a free-operant avoidance schedule and produced periods of timeout on fixed-ratio schedules. In some conditions, a shelf was inserted into the operant chamber during timeout, enabling subjects to get off the grid floor. The combination of timeout and shelf maintained substantially higher response rates than the baseline avoidance schedule with ratio requirements as high as 90 (Experiment I). Adding the shelf to timeouts in one component of multiple fixed-ratio schedules of timeout resulted in higher response rates in the component where the shelf was included (Experiment II). When timeouts with and without the shelf were arranged on concurrent schedules, the shelf-timeout combination was preferred, even when of shorter duration than timeout alone (Experiment III). In all three experiments, subjects climbed on the shelf, although all shocks were cancelled during timeout periods. The results could not be accounted for solely in terms of the reinforcing properties of changes in shock rates, but required an interpretation that ascribed conditioned reinforcing value to stimuli associated with such changes.     

Within-subject testing of the signaled-reinforcement effect on operant responding as measured by response rate and resistance to change

Response rates under random-interval schedules are lower when a brief (500 ms) signal accompanies reinforcement than when there is no signal. The present study examined this signaled-reinforcement effect and its relation to resistance to change. In Experiment 1, rats responded on a multiple random-interval 60-s random-interval 60-s schedule, with signaled reinforcement in only one component. Response resistance to alternative reinforcement, prefeeding, and extinction was compared between these components. Lower response rates, and greater resistance to change, occurred in the component with the reinforcement signal. In Experiment 2, response rates and resistance to change were compared after training on a multiple random-interval 60-s random-interval 60-s schedule in which reinforcer delivery was unsignaled in one component and a response-produced uncorrelated stimulus was presented in the other component. Higher response rates and greater resistance to change occurred with the uncorrelated stimulus. These results highlight the significance of considering the effects of an uncorrelated signal when used as a control condition, and challenge accounts of resistance to change that depend solely on reinforcer rate.     

Positive behavioral contrast in 3-month-old infants on multiple conjugate reinforcement schedules.

Positive behavioral contrast was assessed in two experiments with young infants using multiple conjugate reinforcement schedules. Reinforcement was produced by footkicks which activated the objects of an overhead crib mobile in a manner proportional to the vigor and rate of responding. Distinctive color/pattern cues on the sides of the objects served as discriminative stimuli for components of the multiple schedule. In Experiment 1, infants were trained with one cue (S+) only before insertion of S+ into a multiple schedule with an extinction component. A control group received S+ throughout all sessions. In Experiment 2, a multiple schedule was introduced at the outset, and responses in both components were reinforced before the introduction of extinction in the second component. In a final phase, reinforcement was reintroduced into the second component. Positive behavioral contrast occurred in both experiments. Response reduction in the extinction component was seen only in individual relative response curves. In both experiments, negative emotional behaviors accompanied the extinction component, and in Experiment 1, cooing accompanied presentations of S+.     

Instrumental performance on negative schedules

Three experiments examined the performance of rats pressing a lever for food reinforcement on a schedule in which high rates of response resulted in lowered rates of reinforcement (i.e. a schedule with a negative component). In Experiment 1, rats responded on a variable interval (VI) schedule with a conjoint component such that every 30 responses a reinforcement programmed by the VI schedule was cancelled. These subjects generally emitted a lower response rate than rats responding on a VI schedule yoked to the former subjects with respect to the delivery of reinforcement, although response rate differences were sometimes not large. Similar response-rate effects were obtained in Experiment 2 using a within-subject yoking procedure. In Experiment 3, reinforced interresponse times were matched on negative and VI schedules yoked in terms of reinforcement rate, and the response rate emitted in these conditions were similar. These results give support to theories of instrumental conditioning that stress the strengthening and shaping properties of reinforcement.     

Behavioral contrast as differential time allocation

In Experiment I, hooded rats were exposed to multiple variable-interval schedules of reinforcement in which manipulanda and reinforcement magazines at opposite ends of the experimental chamber were associated with the different components. Time allocated to each component was measured by recording the time spent by the subject in the appropriate half of the chamber. Positive behavioral contrast was observed for the comparison between multiple variable-interval 30-second variable-interval 30-second and multiple variable-interval 30-second variable-interval 90-second conditions for both response frequency and time allocation measures, but not for mean local response rate (response frequency per time allocated to a component). In Experiment II, rats were exposed to multiple variable-time schedules in which reinforcement was response independent. Time allocated to each component was measured for two conditions, multiple variable-time 30-second variable-time 30-second and multiple variable-time 30-second variable-time 90-second. Positive behavioral contrast of time allocation was exhibited. The results indicated that time allocation was differentially sensitive to changes in reinforcement probability, and that behavioral contrast may result from the differential allocation of time to the different components of the multiple schedule.