Differences in the effect of Pavlovian contingencies upon behavioral momentum using auditory versus visual stimuli.

We examined the role of Pavlovian and operant relations in behavioral momentum by arranging response-contingent alternative reinforcement in one component of a three-component multiple concurrent schedule with rats. This permitted the simultaneous arranging of different response-reinforcer (operant) and stimulus-reinforcer (Pavlovian) contingencies during three baseline conditions. Auditory or visual stimuli were used as discriminative stimuli within the multiple concurrent schedules. Resistance to change of a target response was assessed during a single session of extinction following each baseline condition. The rate of the target response during baseline varied inversely with the rate of response-contingent reinforcement derived from a concurrent source, regardless of whether the discriminative stimuli were auditory or visual. Resistance to change of the target response, however, did depend on the discriminative-stimulus modality. Resistance to change in the presence of visual stimuli was a positive function of the Pavlovian contingencies, whereas resistance to change was unrelated to either the operant or Pavlovian contingencies when the discriminative stimuli were auditory. Stimulus salience may be a factor in determining the differences in resistance to change across sensory modalities.     

Polydipsia induced in rats by second-order schedules of reinforcement.

Three rats were exposed to a second-order schedule in which fixed-interval components ended either with food or with a brief stimulus that was never paired with food. Food and the brief stimulus occurred in a random sequence (variable-ratio 2 overall schedule). Another three rats were exposed to a similar second-order schedule, the only difference being that the food or the stimulus was presented independently of operant behavior (fixed-time components). The three rats exposed to the fixed-interval components licked at a water spout after each food presentation. These rats also licked in the intervals after the brief stimulus. Although the discriminative properties of food and of the brief stimulus were identical in relation to subsequent reinforcement, licking after the stimulus was less than after food. The three rats exposed to the second-order schedules with fixed-time components also licked at the water spout after food, but these rats did not lick consistently after brief stimulus presentations.     

Selective punishment early and late in fixed-ratio schedules of food reinforcement

Pigeons key pecked for grain on a fixed-ratio 100 schedule; electric shocks occurred intermittently at the fifteenth or eighty-fifth response in the ratio. In Experiment I, shock was at the fifteenth response for two birds, and at the eighty-fifth response for two others, in every sixth, twelfth, or eighteenth ratio. Rate of responding decreased as frequency of shock increased, and the pattern of responding included an increased initial pause and low rates or pause-run sequences that extended further into the ratio when shock was at the fifteenth response than when it was at the eighty-fifth response. Shock early in the ratio engendered longer initial pauses than shock late in the ratio. In Experiment II, four birds responded on a two-component multiple schedule in which shock occurred at the fifteenth response of the third ratio in the presence of a white keylight and at the eighty-fifth response of the third ratio in the presence of a green keylight. The overall rates of responding decreased as shock intensity increased. All four birds responded differentially to the white and green keylights, but with a pattern that varied between birds. In general, punishment reduced the probability of responses that preceded it, regardless of the ordinal position of those responses. Both studies confirm that the probability of responding is reduced less by aversive stimuli produced late in a fixed-ratio than by aversive stimuli produced early in a fixed-ratio.     

Response summation to a compound stimulus in a context of choice

Key pecks by two groups of pigeons were reinforced on concurrent schedules. For group Ē, pecks were reinforced during either a visual or an auditory stimulus; for group E, an additional, extinction component was available, during which both visual and auditory stimuli were absent. After training, both groups were given a compound test to measure preference among four stimuli, the three used in training plus a compound of the visual and auditory stimulus. Group E showed preference for the compound, emitting more pecks and spending more time in this stimulus than in other stimuli. Group Ē showed no preference between the compound and visual stimulus, nor between the auditory stimulus and the absence of both stimuli, but preferred the former pair over the latter pair of stimuli.     

Dependency, temporal contiguity, and response-independent reinforcement

A comparison was made of the effects of variable-interval, variable-time, and tandem variable-interval fixed-time schedules on key-peck responding of pigeons. The variable-interval component of the tandem schedule retained the response-reinforcement dependency; the fixed-time component allowed the temporal proximity between responding and reinforcement to vary, constrained only by the duration of the fixed-time interval. Response rates were highest during the variable-interval and lowest during the variable-time schedule. Intermediate response rates occurred during the tandem schedule. The results of a yoked control condition showed that the effects of the tandem schedule were not due simply to changes in reinforcement distribution or frequency. The results suggest that substantial reductions in responding occur when reinforcement is response-dependent but not necessarily contiguous with the response required to produce reinforcement.     

Schedules of food postponement: II. Maintenance of behavior by food postponement and effects of the schedule parameter

In Experiment I, food-deprived, feeder-trained squirrel monkeys pressed a lever to postpone brief electric shocks (Response-Shock=Shock-Shock interval=30 seconds). Forty-one three-hour sessions of shock postponement were followed by 120 sessions of concurrent shock and food postponement. The shock schedule was unchanged and the food schedule was Response-food interval–20 seconds, Food-food interval 10 seconds. After concurrent shock and food postponement, the shock schedule was discontinued and 40 sessions of food postponement ensued, followed by 53 sessions of extinction. After extinction, food postponement was resumed for 11 sessions. Stable responding with low food rates was maintained under food-postponement after the concurrent schedule. Responding decreased to low levels under extinction and recovered immediately to previous levels when the food-postponement schedule was re-instated. In Experiment II, a parameter of the food-postponement schedule was studied sequentially. Using the same subjects, the Response-food–Food-food interval was manipulated from four seconds to 80 seconds with several orders of presentation. Relations of response rates and food rates to the parameter were similar to those seen under shock postponement. Exposure to very short postponement times (four seconds), resulting in very high food rates, decreased but did not abolish subsequent responding at longer postponement times. Results are discussed from the point of view that reinforcing functions of stimuli consequent on responding depend on a prior history of scheduled contact with those stimuli.     

Time-based and count-based measurement of preference

Rats' pressing on two levers was reinforced according to two independent variable-interval schedules that were varied during the experiment. Since the levers were connected directly to the programming equipment, bypassing the standard pulseformers, reinforcement could occur while a lever was held down. Although the time a lever was pressed might, therefore, have varied independently of number of presses, these two measures covaried substantially, because the average duration of the presses remained roughly constant. This rough invariance may have resulted from the rats' tendency to make bursts of brief presses (i.e., to jiggle the levers), even though the contingencies encouraged holding. When duration did vary, presses on the two levers tended to vary together. As a result, relative time spent pressing corresponded closely to relative number of presses. Both of these measures conformed well to the matching law. Absolute behavioral frequency at a lever, measured either way, varied directly with proportion of reinforcement for that lever, in accordance with the generalized version of the matching law. Number of presses seemed, on balance, to be a slightly more reliable measure than pressing time. The substantial interchangeability may prove more significant than the slight disparity, however, because it supports the notion that all behavior can be measured on a common scale of time.     

Behavior of humans in variable-interval schedules of reinforcement

During Phase I, human subjects pressed a button for monetary reinforcement in five variable-interval schedules, each of which specified a different frequency of reinforcement. The rate of responding was an increasing, negatively accelerated function of reinforcement frequency; the data conformed closely to Herrnstein's equation. During Phase II, the same five schedules were in operation, but in addition a concurrent variable-interval schedule (B) was introduced, responses on which were always reinforced at the same frequency. Response rate in component A increased while the response rate in B decreased, as a function of the reinforcement frequency in component A. Relative response rates in the two component schedules matched the relative frequencies of reinforcement. Comparing the absolute response rates in component A during Phase I and Phase II it was found that introduction of the concurrent schedule did not affect the value of the theoretical maximum response rate, but did increase the value of the reinforcement frequency needed to obtain any particular submaximal response rate.     

Performance on variable-interval schedules arranged singly and concurrently

Extensive parametric data were obtained from pigeons responding on variable-interval schedules arranged on three, two, and one response keys. Number of responses on the keys, the time spent responding on the keys, and the number of reinforcements obtained on the keys were measured. Response rates on each key were an increasing function of the reinforcement rate on that key, and an inverse function of the reinforcement rate on the other keys. In terms of preference, both response and time-allocation ratios undermatched ratios of obtained reinforcements, and the degree of undermatching was consistent both within, and between, two- and three-schedule data. When absolute response-rate data were analyzed according to Herrnstein's (1970) quantitative account, obtained values of assumed constants were not consistent either within or between conditions. However, a power-function modification of Herrnstein's account fitted the data well and provided similar exponent values to those obtained for the undermatching of preference ratios.     

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.