Proceedings of the Annual Meeting of the Cognitive Science Society

Previous research has shown that stimulus values on a sensorycontinuum are perceived in a categorical manner by human subjects andby rhesus monkeys (Wilson, 1972; Streitfeld & Wilson, in press). Thatis, stimuli which are judged to belong to different perceptualcategories are discriminated more accurately than are stimuli whichare perceived as belonging to the same perceptual category. In theseexperiments, the category boundary was defined as the adaptation level(AL) established by the stimulus series presented to the subject.Wilson and DeBauche (1981) showed that resection of visual"association cortex" in monkey abolished categorical perception ofvisual features and they hypothesized that modality-specific neuralsubstrates that preserve the effects of stimulation provide aninternal referent which determines the manner in which given stimulusvalues are identified and discriminated. In the study described here,the effects of focal brain damage on categorical perception of threestimulus continua was examined in neurological patients. Processingof visual features as members of perceptual categories was doublydissociated from processing of haptic stimuli; posterior lesions inthe right hemisphere selectively impaired categorization of visualstimuli differing in length and orientation while anterior lesions inthe left hemisphere selectively impaired categorical perception ofweight. Implications for the neural dynamics of categorization arediscussed in the context of AL theory and principles of neuralorganization.