Categorization of somaesthetic stimuli: sensorimotor performance and neuronal activity in primary somatic sensory cortex of awake monkeys.
We used psychometric techniques to study the sensorimotor performance of four monkeys trained to classify the speed of moving tactile stimuli. Animals performed the task by pressing one of two target switches to indicate whether the speed of probe movement across the glabrous skin of the hand was low or high. Psychometric curves indicated that animals classified the stimulus speeds irrespective of which finger was stimulated, traverse distance and direction. The mean values of the reaction (RT) and movement (MT) times during the correct categorization of low and high stimulus speeds were similar. However, a slight increase was detected in the mean values of the RT during the incorrect categorization but not in the MT. During the task, activity of single neurones (n = 45) was recorded in primary somatic sensory (SI) cortex. The results indicate that a class of neurones (n = 12) of SI cortex increased their impulse rates as a function of the stimulus speeds. However, the magnitude of their responses was similar during the correct and incorrect categorizations of stimuli. The same neurones also responded when the same set of stimuli used in the categorization task were delivered passively. Neurones of SI cortex responded with a latency of 25.8 +/- 0.6 ms (+/- s.e.m.) relative to the beginning of the moving tactile stimuli during the categorization task. The same neurones (n = 17) also responded with a similar latency (24.6 +/- 4.0 ms) when the stimuli were delivered passively. These results may suggest that, although this evoked neuronal activity may be important for the perception of the moving tactile stimuli, more central structures associated with SI cortex may determine the performance of this learned somaesthetic task.