Spatial encoding of visual and somatosensory stimuli by single neurons in area 7B of the macaque parietal cortex

Abstract

Lesions in certain areas of the parietal cortex in Macaque monkeys produces deficits in visual and somatosensory perception, as well as, eye and arm activity associated with exploration of extrapersonal space. This study focused on recording from neurons in area 7b of the posterior parietal association cortex with visual or somatosensory receptive fields (RFs) that were modulated by eye or arm position. Two Macaque (Maccaca mulatta) monkeys were trained to maintain a steady gaze and arm position while a visual stimulus (illuminated circle) or a tactile stimulus (brushing of the dorsal side of the hand) was delivered. Microelectrodes were advanced into cortical area 7b of the monkeys and action potentials were recorded extracellularly from neurons while the stimuli were being delivered. When neurons with visual or tactile RFs were identified, the eye or arm position was altered, and the phasic responses to sensory stimuli and tonic activity was evaluated. Modulated phasic and tonic activity in neurons provide information on the spatial encoding of stimuli in extrapersonal space. Previous studies of area 7b have documented a sparse number of neurons with visual RFs, but in the present study, fifty percent of the neurons in the middle and posterior thirds of area 7b had visual RFs. During the eye position task, neurons with craniocentric responses, visual RFs that were dependent upon the head's position, were observed, and in more than half of the trials, neurons with tonic angle of gaze activity discharged when the eyes were directed into the contralateral hemi-field. Eighty-six percent of the neurons with tactile RFs, sensitive to passive arm positioning, increased activity when the hand moved into the contralateral hemi-field. The majority of those neurons increased activity when the arm angle approached forty-five degrees contralateral, a position that coincides with one of the most responsive visual angles in the eye position task. In contrast, fifty-six percent of the neurons with tonic angle of arm signals increased activity when the arm was moving further into ipsilateral hemi-space. All of these neuronal types describe a region of the brain that contributes to the spatial encoding of stimuli in extrapersonal space.