Effects of attention on multiunit responses to vibration in the somatosensory regions of the monkey's brain

Abstract

Multiple unit responses to a 40 Hz cutaneous vibration delivered to the contralateral hand were studied in 6 hemispheres of 3 alert monkeys ( Macaca speciosa) trained to press a lever with the ipsilateral hand at the end of a vibratory stimulus lasting 4 sec. The detection task was called relevant when a yellow light was on; the monkey was then rewarded with fruit juice for correct performance. When a red light was on no reward was given and the task was called irrelevant. The monkeys refrained totally from responding during the irrelevant condition. During the performance of this task transdural recording was performed with movable metal semimicroelectrodes in the VPL nucleus of the thalamus, in the middle layers of the primary somatosensory cortex (SI = areas 3b, 1 and 2), in the second comatosensory cortex (SII), and in the motor cortex (area 4). In the VPL strong responses to vibration were recorded. The relevance of the task did not influence the activity of any of the recording sites studied: the responses were similar under all behavioral conditions. In the middle layers of the primary somatosensory cortex (Brodmann's areas 3b, 1 and 2) strong responses were also recorded. The responses were clear and highly synchronous with the stimulus, but the relevance of the task did not have a significant influence on the responses. In the second somatosensory cortex (SII) two kinds of recording site were observed. One was characterized by clearly localized, contralateral receptive fields and the other by diffuse, bilateral receptive fields. In the former the vibratory responses were similar to those in SI and moderately augmented by the relevance of the task. In the latter the vibratory responses were less clearly synchronous with the stimuli but they were significantly stronger during the relevant part of the task. Clear and synchronous vibration responses to cutaneous 40 Hz stimuli were observed in the motor cortex (area 4). Vibratory responses were not as clear as in SI but resembled the second group of responses obtained in SII. They were strongly modified by attention toward the stimuli, being enhanced prior to the correct performance of the relevant task. These results show that cutaneous vibration is transmitted in the brain in a thalamocortical circuitry that involves at least the ventro-basal complex, the primary somatosensory cortex, the second somatosensory cortex and the motor cortex. When the hand is held immobile, behavioural influence on the responses is significant only in the motor cortex and in SII.