A bimodal map of space: somatosensory receptive fields in the macaque putamen with corresponding visual receptive fields.

The macaque putamen contains neurons that respond to somatosensory stimuli such as light touch, joint movement, or deep muscle pressure. Their receptive fields are arranged to form a map of the body. In the face and arm region of this somatotopic map we found neurons that responded to visual stimuli. Some neurons were bimodal, responding to both visual and somatosensory stimuli, while others were purely visual, or purely somatosensory. The bimodal neurons usually responded to light cutaneous stimulation, rather than to joint movement or deep muscle pressure. They responded to visual stimuli near their tactile receptive field and were not selective for the shape or the color of the stimuli. For cells with tactile receptive fields on the face, the visual receptive field subtended a solid angle extending from the tactile receptive field to about 10 cm. For cells with tactile receptive fields on the arm, the visual receptive field often extended further from the animal. These bimodal properties provide a map of the visual space that immediately surrounds the monkey. The map is organized somatotopically, that is, by body part, rather than retinotopically as in most visual areas. It could function to guide movements in the animal's immediate vicinity. Cortical areas 6, 7b, and VIP contain bimodal cells with very similar properties to those in the putamen. We suggest that the bimodal cells in area 6, 7b, VIP, and the putamen form part of an interconnected system that represents extra personal space in a somatotopic fashion.