ROLE OF PRIMARY SOMATIC SENSORY CORTEX IN THE CATEGORIZATION OF TACTILE STIMULI - EFFECTS OF LESIONS

We lesioned the right primary somatic sensory (SI) cortex in two monke ys trained to categorize the speed of moving tactile stimuli. Animals performed the task by pressing with the right hand one of two target s witches to indicate whether the speed of a probe moving across the gla brous skin of the left hand was low or high. Sensory performance was e valuated with psychometric techniques and motor behavior was monitored by measuring the reaction (RT) and movement (MT) times before the exp eriment and throughout the 60 days after the ablation of SI cortex. Af ter the lesion, there was a slight increase in the RTs but no change i n the MTs, indicating that removal of SI cortex did not affect the ani mals' capacity to detect the stimuli. However, monkeys lost their abil ity to categorize the stimulus speeds. This effect was observed from t he Ist day after the lesion until the end of the study. We conclude th at somatosensory areas outside SI can by themselves process tactile in formation in a limited way and that the extraction of higher-order fea tures that takes place during the categorization task requires the int ervention of SI cortex.