An interresponse-time analysis of responding maintained by schedules of response-produced electric shock

The present study investigated ratio contingencies to evaluate factors that may determine the maintenance of responding when electric shock is the consequent event. Initially, squirrel monkeys (Saimiri sciureus) were exposed to a continuous-avoidance schedule to initiate bar pressing. Subsequently, a multiple random-interval variable-ratio yoked schedule of response-produced shock was used to maintain and to compare interval and ratio performance. A microcomputer recorded and stored the number of responses and interresponse times occurring between successive shock presentations during a given random-interval component, and these numbers determined the ratio requirements during the subsequent ratio component. Responding was maintained for more than 80 sessions in two of three monkeys under the multiple schedule with the ratio yoked to the interval component. Responding during the ratio component persisted in only one monkey, however, when the components were no longer yoked. An analysis of the interresponse times immediately preceding shock under the multiple yoked schedule revealed that the terminal interresponse times were longer under the interval schedule than under the ratio contingency. The interresponse-time analysis indicated that differential interresponse-time relationships may be major determinants of the maintenance of behavior controlled by schedules of electric-shock presentation.     

Shock intensity and signaled avoidance responding

Five rats were submitted to a signaled free-operant avoidance contingency. Throughout the experiment, shock intensity was varied from 0.1 to 8.0 mA, with shock duration constant at 200 milleseconds. Results indicate: (a) an all-or-none effect of shock intensity on response and shock rates, on percentage of shocks avoided, and on frequency of occurrence of responding during the preshock stimulus; and (b) no systematic effect of shock intensity on stimulus control, measured either by the percentage of stimulus presentations accompanied by a response or by the percentage of responses that occurred during those preshock stimuli. Such results indicate that for each subject there is a minimum shock intensity necessary to establish and maintain avoidance responding; intensities higher than this minimum value have little or no effect on responding (with an upper limit for those strong intensities with a general disruptive effect on behavior).     

Autocontingencies: Suppressive and accelerative effects of pairs of shocks superimposed on a positively reinforced operant baseline

Previous research has shown that unsignaled shock may accelerate positively reinforced operant responding if each shock signals a subsequent shock-free period. In order to explore the boundary conditions of this effect, two experiments were performed. In Experiment 1, pairs of unsignaled shocks separated by 15, 30, 60, or 120 seconds resulted in suppressed responding during the briefest intershock interval, and in accelerated responding during the longer intervals. When the second shock in each pair signaled a shock-free period of at least 3 minutes, accelerated responding also followed offset of the second shock in all but the 30-second condition. In Experiment 2, the addition of a conditioned stimulus prior to each pair of shocks restored baseline responding, and eliminated accelerative control following the second shock only under the briefest inter-shock interval. The results are discussed in terms of the similarity between autocontingencies (shock/no-shock relations; Davis, Memmott, & Hurwitz, 1975) and recent modifications of the feature-positive procedure (e.g., Reberg & Memmott, 1979), which stress stimulus control by shock/no-shock relationships.     

Reduction of shock duration as negative reinforcement in free-operant avoidance

Rats were trained on a free-operant procedure in which shock duration was controlled by responses within a limited range of interresponse times. Shocks of 1.6-mA intensity occurred randomly with average density of 10 shocks per minute. As long as interresponse times were 15 seconds or less, any shocks received were at the briefer of two durations (.3 second). Whenever interresponse times exceeded 15 seconds, any shocks received were at the longer duration (1.0 second). For six of eight animals, avoidance responding developed quickly and reached levels of better than 90%. Four yoked animals stopped responding within the first few sessions. Shock duration reduction without change in shock probability or intensity was sufficient for the acquisition and maintenance of avoidance responding.     

Responding under schedules combining response-dependent and response-independent shock delivery

Lever pressing of three squirrel monkeys with experience under continuous avoidance schedules was maintained by response-produced shock under a 5-minute variable-interval schedule. Responding decreased when half of the scheduled shocks were delivered independently of lever pressing and decreased further when all shocks were independent of lever pressing. Responding was lowest when all shocks were eliminated. When the proportion of response-dependent shocks increased, responding increased. This relation occurred even though the frequency and temporal distribution of shock delivery remained the same. Responding of two monkeys increased in a graded fashion as the frequency of shock was increased by arranging variable-time 5-minute, 2-minute, and 1-minute schedules jointly with the variable-interval 5-minute schedule. Thus, increasing the proportion of response-independent shocks decreased responding when the overall frequency of shocks stayed the same, but increased responding when the overall frequency of shock delivery increased.     

Lever attacking and pressing as a function of conditioning and extinguishing a lever-press avoidance response in rats

Six experimental rats were conditioned to press one of two available levers to avoid shock. The levers registered bites as well as presses. For four of these rats, shock was contingent on lever bites when a specified time period had elapsed after the previous shock. An extinction period, in which only periodic noncontingent shocks were presented, followed avoidance training. Six yoked-control rats received the same sequence of shocks as did the corresponding experimental rats in both the conditioning and extinction phases. All six experimental rats repeatedly bit the avoidance lever. Four bit it more than the nonavoidance lever during conditioning, and five bit it more during extinction. Five of the six experimental rats consistently bit the levers many more times during each session than did their respective control rats, suggesting that avoidance conditioning facilitated lever biting. Rates of lever biting and pressing by all of the experimental rats and by some of the control rats were highest immediately following shock throughout both phases. During later portions of the intervals following shock, characteristic effects of conditioning and extinction were observed. This finding suggests that extinction of avoidance behavior by unavoidable shock presentations can be demonstrated more readily when shock-elicited responding is extricated from the data.     

Warmup in free-operant avoidance as a function of the response-shock = shock-shock interval

Warmup effects, the repeated within-session transitions from ineffective to effective avoidance, were examined with rats on free-operant shock-delay procedures. The shock-shock and response-shock intervals were kept equal as they were varied. As measured by both response rates and shock rates, the magnitude of within-session change in performance was inversely related to the size of the manipulated intervals. The duration of warmup tended to decrease as the intervals were increased. This finding, that increased shock frequencies do not shorten the warmup, appears to be inconsistent with all interpretations of the warmup that have been offered to date. Late-session performances replicated general features of prior experiments, but differed with respect to details of secondary conclusions in previous reports. These differences may stem from the selection of especially proficient avoiders for previous experiments.     

Behavior simultaneously maintained by both presentation and termination of noxious stimuli

Lever pressing by two squirrel monkeys was maintained under a 3-minute variable-interval schedule of response-produced electric-shock presentation. At the same time, responding on a second lever was maintained under a 3-minute fixed-interval schedule of termination of the shock-presentation schedule and shock-correlated stimuli. Under the termination schedule, the first response after a 3-minute period produced a 1-minute timeout, during which no events occurred and responding had no scheduled consequence. Relatively high and constant rates of responding were maintained on the lever where responding produced shock. Lower rates and positively accelerated patterns of responding occurred on the lever where responding terminated the shock schedule. Thus, responding was simultaneously maintained by presentation of an event and by termination of a stimulus associated with that event. Rates and patterns of responding on each lever were reversed when the schedules arranged on each lever were reversed on two occasions. When shock intensity was increased from 0 to 10 mA, responding maintained both by presentation of shock and by termination of the shock schedule increased, but responding maintained by shock presentation increased to a greater extent. Positive and negative reinforcement, usually regarded as separate behavioral processes involving different events, can coexist when behavior is controlled by different contingencies involving the same event.     

Biobehavioral effects of extended salt loading and conflict stress in intact baboons.

Behavioral stressors may inhibit sodium excretion, potentially increasing plasma volume and elevating blood pressure during chronic exposure. Blood pressure regulation may be especially deranged during manipulations that further challenge the kidney, such as a diet high in salt content. The effects on blood pressure and other variables of combined behavioral stress (food/shock conflict) and dietary salt (12 g NaCl per day; 218 mEq Na+ per day) were examined in adult male baboons over the course of 1 year. Mean arterial pressure was not significantly elevated over baseline after 5 months of high dietary salt alone (6 +/- 5 mmHg) but was maximally elevated by an average of 17 (+/- 3 SEM) mmHg after 5 months of combined salt and conflict stress. Control baboons showed no significant trends in mean arterial pressure across the same time period. Individual subjects whose blood pressure was "salt+stress resistant" or "salt+stress sensitive" were differentiated by their degree of pressure diuresis and natriuresis, urinary free cortisol, and a behavioral index of stress sensitivity. The data indicate additive effects of chronic high dietary salt intake and behavioral stressors on blood pressure in nonhuman primates that are dependent on renal function and pituitary-adrenocortical activity.     

The initial stage of the international sojourn: excitement or culture shock?

This paper presents findings from an ethnographic study of the adjustment journey of international postgraduate students at a university in the South of England, which involved interviews and participant observation over a 12-month academic year. It was discovered that the initial stage of the sojourn1 was not characterised by feelings of excitement, as suggested by the U-curve model (and its successors): though such feelings were present, they were overwhelmed by negative symptoms more commonly associated with culture shock. The implications of these findings for support structures in higher education are discussed.