Force gradients in stimulus generalization

Pigeons were trained to peck with differing force requirements on a key specially designed to measure and control force of pecking without changes in the mechanical threshold. Following training on a key illuminated by a single wavelength, a generalization test was given. Force and rate gradients were obtained. Force gradients were shown to have a form similar to rate gradients. The slope of the force gradients was a direct function of the force of responding at the conclusion of training. Rate gradients were independent of the force of responding.     

Instructions and stimulus categorizing in a measure of stimulus generalization

In Experiment I, three groups of 20 Ss each were exposed to a light of 550 mμ (yellowish-green) for 60 sec and then viewed a random sequence of wavelengths with instructions to respond only to the original color. The instructions given the three groups were worded differently in an attempt to vary the strength of a set-to-discriminate assumed to be created by this procedure. The three groups produced similar gradients, each with a peak of responding at 540 mμ, in agreement with Kalish's (1958) published gradient for the 550 mμ standard stimulus value. It was suggested that the nature of the task is such that a strong discriminatory set is produced regardless of the wording of the instructions.

A temporal analysis of the gradient as it develops during the testing revealed that initially the peak of responding occurs at 550 mμ; but as testing progresses, it shifts gradually in the direction of the shorter wavelengths (purer greens). Experiment II was performed to test the generality of the phenomenon of regression to the primary color. Two groups of 20 Ss each were tested for generalization following exposure to 510 mμ (bluish-green) and 525 mμ (pure green), respectively. We predicted that the 510 mμ gradient would reveal a progressive shift toward the longer wavelengths (purer greens), whereas the 525 mμ gradient would show no tendency to shift. The results were strikingly in accord with these predictions.

We concluded that although a physiological process could not be ruled out, the verbal labeling of the standard stimulus value may well be responsible for the regression of the gradient toward the primary color.

     

Contextual control of stimulus generalization and stimulus equivalence in hierarchical categorization

The purpose of this study was to determine whether hierarchical categorization would result from a combination of contextually controlled conditional discrimination training, stimulus generalization, and stimulus equivalence. First, differential selection responses to a specific stimulus feature were brought under contextual control. This contextual control was hierarchical in that stimuli at the top of the hierarchy all evoked one response, whereas those at the bottom each evoked different responses. The evocative functions of these stimuli generalized in predictable ways along a dimension of physical similarity. Then, these functions were indirectly acquired by a set of nonsense syllables that were related via transitivity relations to the originally trained stimuli. These nonsense syllables effectively served as names for the different stimulus classes within each level of the hierarchy.     

A generalization of stimulus sampling theory

A modification of stimulus sampling theory is presented. The restriction that each stimulus element is conditioned to one and only one response is replaced with the notion of a scale of conditioning for each element. This variation provides a context in which such variables as reward magnitude and motivation can be viewed as determiners of behavior. Some experimental results on multiple response problems also have a natural interpretation in terms of these ideas.     

Effects of averaging data during stimulus generalization

Rats were trained to press two keys consecutively for reinforcement. During stimulus one (slow clicker) a 6-sec time delay was required between the two responses. During stimulus eight (fast clicker) no time delay was required between the two responses. When tested with intermediate stimuli (intermediate click rates) the median time delays emitted by the animals were intermediate between their performances on the original training stimuli, resulting in typical generalization gradients. Closer examination of the data revealed that the median values were not representative of the behavior of the animals.     

Analysis of response rates during stimulus generalization

In the presence of one click frequency, the presses of two hungry rats on one of two levers were reinforced with food on variable-interval schedules; in the presence of a different click frequency, presses on the other lever were reinforced. In stimulus generalization tests, a variety of click frequencies were presented and reinforcement withheld. The test stimuli were found to exert control over which of the two levers the rats pressed, but not over the rate of pressing the selected lever. The results were interpreted as further evidence that intermediate rates in generalization gradients may be the result of the alternation of several distinct behavior patterns.     

Response patterning during stimulus generalization in the rat

Nine rats were trained to bar press in the presence of a clicking sound of 6.67 cps (S(D)) for 1-min variable-interval food reinforcement randomly alternated with a clicking sound of 20 cps (S(Delta)) signifying extinction. After a criterion of 90% of total responses in the presence of the S(D) was obtained, a generalization test was administered, including values of 6.67, 10.00, 13.33, and 20.00 cps, with responses in the presence of the S(D) continuing to be reinforced during testing. The test yielded a gradient of response strength with rate highest in the presence of the S(D) and decreasing with increasing distance from this value. An interresponse time (IRT) analysis of responding during generalization testing revealed no systematic differences in modal IRT category or in median IRT to the different test stimuli. Mean IRT was lowest in the presence of the S(D) and increased systematically with increasing distance from this value, supporting the hypothesis that the generalization gradient of response rate is primarily the result of an increasing proportion of "long" IRT responses to stimuli increasingly distant from the S(D).