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.