Somatotopic organization of the lateral sulcus of owl monkeys: Area 3b, s‐II, and a ventral somatosensory area

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Abstract
Multiunit microelectrode recordings and injections of horseradish peroxidase (HRP) were used to reveal neuron response properties, somatotopic organization, and interconnections of somatosensory cortex in the lateral sulcus (sylvian fissure) of New World owl monkeys. There were a number of main findings. (1) Representations of the face and head in areas 3b, 1, and S‐II are found on the upper bank of the lateral sulcus. Most of the mouth and lip representations of area 3b were found in a rostral extension along the lip of the lateral sulcus. Adjacent cortex deeper in the lateral sulcus represented the nose, eye, ear, and scalp. (2) S‐II was located on the upper bank of the lateral sulcus and extended past the fundus onto the deepest part of the lower bank. The face was represented most superficially in the sulcus, with the hand, foot, and trunk located in a rostrocaudal sequence deeper in the sulcus. The orientation of S‐II is “erect,” with the limbs pointing away from area 3b. (3) Neurons in S‐II were activated by light tactile stimulation of the contralateral body surface. Receptive fields were several times larger than for area 3b neurons. (4) A 1‐2‐mm strip of cortex separating the face and hand representations in S‐II was consistently responsive to the stimulation of deep receptors but was unresponsive to light cutaneous stimulation. (5) Injections of horseradish peroxidase in the electrophysiologically identified hand or foot representations of area 3b revealed somatotopically matched interconnections with mapped hand and foot representations in S‐II. (6) A systematic representation of the body, termed the “ventral somatic” area, VS, was found extending laterally from S‐II on the lower bank of the lateral sulcus. Within VS, the hand and foot were represented deep in the sulcus along the hand and foot regions of S‐II, and the face was lateral near the ventral lip of the sulcus. (7) Neurons at most recording sites in the VS region were activated by contralateral cutaneous stimuli. However, a few sites had neurons with bilateral receptive fields. Receptive field sizes were comparable to those in S‐II. In addition, neurons in islands of cortex in the VS region had properties that suggested that they were activated by pacinian receptors, while other regions were difficult to activate by light tactile stimuli but responded to stimuli that would activate deep receptors. (8) A few recording sites caudal to S‐II on the upper bank of the lateral sulcus were responsive to somatic stimuli. (9) Finally, some recording sites rostral to S‐II in the vicinity of the granular insular region were responsive to cutaneous stimuli. This cortex had sparse interconnections with area 3b. Overall the results provide a detailed understanding of how the face and head are represented in the lateral part of area 3b, reveal body surface representations in S‐II and an adjoining ventral somatic field (VS), locate neurons with pacinianlike features in or about VS, and demonstrate somatotopic interconnections between S‐II and area 3b (S‐I proper).