Two determinants of the peak shift in human voluntary stimulus generalization

Two hundred and forty college students were divided into two groups, with training stimuli (from the brightness dimension) selected to produce small and large adaptation level shifts between discrimination training (to respond “same” to S+ and “different” to S—) and gen-eralization testing. These were further divided into three groups with discriminations expected to yield positive (toward brighter values), negative (toward dimmer values), or zero post-discrimination peak shifts. Half the subjects received brief discrimination training while half received extended training. A further group of 60 subjects were given exposure to the stimuli comparable to that of the extended training subjects, but were asked to rate the stimuli instead of being given discrimination training. The results indicated that two independent, additive sources of shift were active. One source, occurring in all groups, was interpreted as being due to the change in adaptation level from training to test. The other source of shift, occurring only in the groups with extended discrimination training, was interpreted as due to the establishment of asymmetrical decision criteria; the more traditional interpretation in terms of the interaction between excitatory and inhibitory gradients of response strength was also considered.     

Spatial generalization and peak shift in humans

Using a computer betting game, five experiments tested university students on spatial generalization and peak shift. On each trial, one location was marked and the subject was invited to bet 0-4 points. At the winning location (S+), bets won four times the points betted. At nearby losing locations (S−s), points betted were lost. Generalization gradients were exponential in shape, supporting Shepard's (1987), law (Experiment 1). With peak shift manipulations, three kinds of peak shift or area shift were found. (1) Subjects betted more on the S+ side than on the S− side (Experiments 2-4). (2) When asked if a location was the winning location, subjects responded “yes” more often to locations on the S+ side than to locations on the S− side (Experiments 3-5). (3) When asked to point to the winning location on the screen, subjects' errors indicated peak shift (Experiment 5).     

The effects of signaling stimulus presentation during noncontingent reinforcement

The effects of signaling the return of items or attention during treatment with noncontingent reinforcement were examined. First, functional analyses showed that the problem behavior exhibited by 2 teenagers with developmental disabilities was sensitive to social positive reinforcement. Next, delivery of the stimulus that maintained problem behavior on a fixed-time (FT) schedule was compared to a condition in which the removal of the stimulus during the same FT schedule was immediately preceded by a statement indicating that the stimulus would be returned and the initiation of a digital timer. Results show that the FT schedule reduced problem behavior, and the addition of an informative statement and a timer further decreased problem behavior.     

Stimulus generalization and the peak shift with movement stimuli

Stimulus generalization is suggested as an alternative method for examination of the "novelty" problem in motor learning. These experiments demonstrated that stimulus generalization occurs using simple movements as stimuli. The phenomenon of the "peak shift" in post-discrimination generalization gradients was also examined. The first experiment demonstrated that a peak shift occurred using linear movements as stimuli and that the magnitude of the peak shift increased as the difference between the training stimuli decreased. The second experiment showed similar results when the stimuli consisted of a range of movements rather than single movement length. The final experiment provided evidence that perception of movement length is influenced by the magnitude of an immediately preceding movement. The relevance of these studies to current motor-learning theory is discussed.     

Stimulus generalization, discrimination learning, and peak shift in horses.

Using horses, we investigated three aspects of the stimulus control of lever-pressing behavior: stimulus generalization, discrimination learning, and peak shift. Nine solid black circles, ranging in size from 0.5 in. to 4.5 in. (1.3 cm to 11.4 cm) served as stimuli. Each horse was shaped, using successive approximations, to press a rat lever with its lip in the presence of a positive stimulus, the 2.5-in. (6.4-cm) circle. Shaping proceeded quickly and was comparable to that of other laboratory organisms. After responding was maintained on a variable-interval 30-s schedule, stimulus generalization gradients were collected from 2 horses prior to discrimination training. During discrimination training, grain followed lever presses in the presence of a positive stimulus (a 2.5-in circle) and never followed lever presses in the presence of a negative stimulus (a 1.5-in. [3.8-cm] circle). Three horses met a criterion of zero responses to the negative stimulus in fewer than 15 sessions. Horses given stimulus generalization testing prior to discrimination training produced symmetrical gradients; horses given discrimination training prior to generalization testing produced asymmetrical gradients. The peak of these gradients shifted away from the negative stimulus. These results are consistent with discrimination, stimulus generalization, and peak-shift phenomena observed in other organisms.     

Adaptation level as an explanation of the peak shift in generalization with movement stimuli

Two experiments examine the value of adaptation level theory as an explanation of the peak shift phenomenon in stimulus generalization using linear arm movements. The first experiment manipulates the differences in adaptation levels during training by varying the frequency of experience of the training stimulus. Significant shifts in the peaks of generalization gradients were subsequently identified. The peak shifts were in the direction predicted by adaptation level theory. The second experiment manipulates testing adaptation levels by biasing the generalization test movements. Three groups of subjects performed differentially weighted test sequences in which the relative number of short, medium, or long movements was manipulated. Significant peak shifts of generalization gradients were observed in the direction predicted by adaptation level theory.     

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.

     

The effects of programming common stimuli for enhancing stimulus generalization of academic behavior

Programming common stimuli is a strategy for generalizing behavior across settings (Stokes & Baer, 1977). The present study programmed common stimuli (i.e., goal statement and use of a pictorial icon) to generalize the effects of a reinforcement-based intervention for students identified as either developmentally delayed or emotionally disturbed. Results supported the effectiveness of the strategy in producing generalized responding from training to the generalization setting. The importance of methodological rigor in future research exploring generalization and the need to compare generalization strategies are discussed.     

Behavioral contrast and the peak shift: effects of extended discrimination training

Behavioral contrast and the peak shift, two widely observed phenomena in experiments on successive discrimination learning, were shown to disappear during extended discrimination training. This finding and an analysis of the conditions under which behavioral contrast and the peak shift occur suggested that both phenomena were by-products of emotional responses that resulted from response suppression and that their disappearance after extended discrimination training could be attributed to the dissipation of such emotional responses.     

The interactive effects of stimulus generalization decrement and contextual pre-exposure cues upon fear incubation

Models of fear incubation may be described as explanations for the interference of Pavlovian extinction. From this perspective, the effects of several parameters upon the incubation phenomenon were examined. Fear incubation was found to be attenuated by manipulations associated with stimulus generalization decrement (Experiment 1), as well as pre-exposure manipulations (Experiment 2). These findings were incorporated within existing models of incubation.