Shaping the location of a pigeon's peck: effect of rate and size of shaping steps.

For several pigeons, pecking at particular locations within a ten-inch-wide response area was reinforced by grain presentations. The reinforced locations changed systematically to "shape" response location back and forth across the area. The rate and size of these shifts in reinforced locations were varied in both between-subject and within-subject comparisons to evaluate the influence of these variables on the shaping process. Larger step sizes produced larger shifts in location for all sizes inspected, with all sizes from .5 to 3.0 inches effective in shaping behavior. More rapid steps were approximately as effective as slower steps for all rates of shift inspected from 25 reinforcers to 400 reinforcers per step. These data suggest that shaping peck location proceeds most efficiently with rapid, relatively large shifts in criterion performance.     

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.     

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.     

Performance in concurrent interval schedules: a systematic replication

Five pigeons were trained on a variety of concurrent interval schedules that arranged reinforcements at either fixed or variable times after the last reinforcement. Two measures were obtained: the number of responses on each schedule, and the time spent responding on each schedule. Ratios of response rates on the two schedules did not equal ratios of reinforcement rates when both schedules were variable nor when one was variable and the other fixed. Ratios of times spent responding approximately equalled ratios of reinforcement rates when both schedules were variable, but did not do so when one was fixed.     

Preference in pigeons given a choice between sequences of fixed-ratio schedules: Effects of ratio values and duration of food delivery

Pigeons were exposed to schedules that consisted of two sequential fixed ratios, the completion of each followed by food delivery. When each alternative provided two food deliveries per 100 responses, the schedule with the shorter initial fixed-ratio value was consistently preferred. Subsequent attempts were made to shift this established preference by (1) increasing the ratio requirement in the second fixed ratio of the preferred schedule; (2) increasing the duration of food delivery in the second fixed ratio of the nonpreferred schedule; (3) decreasing the duration of food delivery in the first fixed ratio of the preferred schedule; and (4) shortening the second fixed ratio of the nonpreferred schedule. Preference shifted from the schedule with the shorter initial fixed ratio only when the duration of food delivery associated with the first fixed ratio of that schedule was too brief to allow eating. Under all other conditions, pigeons strongly preferred the schedule with the shorter initial fixed ratio even when, overall, that schedule yielded briefer access to food or required more responses to obtain equivalent access.     

Measures of response bias at minimum-detectable luminance levels in the pigeon

Using an operant analogue of the yes-no detection task, six pigeons were trained to detect luminance changes under two different reinforcement-scheduling procedures. In the first, an uncontrolled reinforcement-ratio procedure, the relative frequency of food reinforcers obtained for correct detections was free to vary with the birds' behavior as luminance levels were changed. In the second, a controlled reinforcement-ratio procedure, changes in preference could not alter the relative distribution of food reinforcers between the two response alternatives. Extreme response biases developed as luminance was decreased to threshold in the uncontrolled procedure. No progressive changes in response bias as a function of decreasing luminance were seen in the controlled procedure. Absolute thresholds for light intensity were lower in the controlled than in the uncontrolled procedure.     

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.     

Stimulus-reinforcer contingencies and local behavioral contrast

Four pigeons were exposed to a series of multiple schedules of variable-interval reinforcement in which pecks were required on one key (operant key) and components were signalled on a second key (signal key). Four additional pigeons experienced identical conditions, except that a yoking procedure delivered food on variable-time schedules, with no key pecks required. One of the components of the multiple schedule was constant throughout the experiment as a variable-interval (or variable-time) 30-second schedule. Operant-key responding during the constant component was uniform throughout the component, uninfluenced by changes in the duration of the variable component, and only slightly influenced by changes in reinforcement frequency correlated with the variable component. By comparison, signal-key response rate during the constant component was highest at the onset of the component, was higher when the variable component was 60-sec long than when it was 1-sec long, and was higher when no reinforcement occurred in the variable component than when reinforcement was scheduled in the variable component. These characteristics of signal-key pecking matched characteristics of local positive behavioral contrast. These data are taken to support the “additivity theory” of behavioral contrast and to suggest that Pavlovian stimulus-reinforcer relations contribute primarily to the phenomenon of local positive contrast.     

Effects of response-independent negative reinforcers on negatively reinforced key pecking

Previous research has shown that presenting response-independent positive reinforcers reduces the response rate of an operant maintained by positive reinforcement. The present experiment investigated a similar effect using shock-free time as a negative reinforcer. Brief shocks were delivered in the presence of a distinctive stimulus, and pigeon's key pecks were reinforced by the occasional presentation of a 2-minute shock-free period. Extra 2-minute shock-free periods were added independently of behavior. For each of three pigeons, response rate during shock-on periods declined with added shock-free periods; the more frequently the extra shock-free periods occurred the greater the decline in response rate. This outcome is predicted by extending the Law of Effect to include negative reinforcement.