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MECHANISMS OF THE RESPONSE SELECTIVITIES OF CELLS IN THE MEDIAL SUPERIOR TEMPORAL (MST) AREA OF THE MACAQUE MONKEY

KEIJI TANAKA, HIDE-AKI SAITO and YOSHIRO FUKADA NHK Science and Technical Research Laboratories, Kinuta, Setagaya-ku, Tokyo.

We have studied response properties of cells in the DSR region of the macaque's MST area, using monkeys (M. fuscata) anesthetized with N20/O 2 and immobilized with gallamine triethiodide. The DSR region was defined by a clustering of three types of directionally selective cells with large receptive fields; D-cells responding to a straight movement of patterns in the frontoparallel plane, S-cells selectively responding to a straight movement in the depth, and Rf-cells selectively responding to a rotation in the frontoparallel plane (Saito et al., J. Neurosci. 6: 145-157). Since MST receives strong fiber projection from the MT area where the direction of movements within a small receptive field is systematically analyzed, we hypothesized that the receptive fields of D-, S- and Rf-cells were constructed by integrating regional direction signals provided by MT cells. In the present study, we tested this hypothesis especially for the S- and Rf-cells of the Field-type. The Field-type cells respond to a m69ement of a wide textured field, irrespective of the shape of the texture's components, but are~almost insensitive to a movement of a single simple object. Since a size change and rotat&on of a wide textured field contain several factors other than the arrangement of directions of regional movements, i.e., the difference in speed between central and peripheral parts of the stimulus (for both size change and rotation), the size change of individual components of the texture (for size change only) and the accerelation of movements toward the center (for rotation only), it is possible that these non-directional cues are used to make up the selectivities of the Field-type cells. We prepared artificial motion patterns consisting of four quadrants in which dots moved straight in parallel in each quadrant. The movements in each quadrant were directed so that the combination of the movements in four quadrants simulates either radially arranged movements of dots which occur when the dot pattern changes its size or circularly arranged movements which occur when the ~at~er n rotates. Although the motion patterns did not contain the non-directional cues contained in the real size change or rotation, the S- and Rf-cells responded to them as strongly as to the real size hhange or rotation of the dot pattern. We, therefore, conclude that (i) appropriate arrangement of directions of regional movements + (2) wide extent of the movements at a moment are enough for the activation of the Field-type S- and Rf-cells.

FUNCTIONAL MAPPING OF THE ANTERIOR BANK OF THE SUPERIOR TEMPORAL SULCUS (STS) OF THE MACAQUE MONKEY

KAZUO HIKOSAKA*, MASAO YUKIE*, EIICHI IWAI, HIDE-AKI SAITO, KEIJI TANAKA and YOSHIRO FUKADA. Dept. of Behav. Physiol., Tokyo Metropolitan Inst. for Neurosci., Fuchu, Tokyo 183, and NHK Sci. and Tech. Res. Labs., Setagaya, Tokyo 157

Using monkeys (M. fuscat~) anesthetized with a gas mixture of N~O:O~(70:30) and immobilized with gallamine triethiodide, single unit recordings were made in the anterior bank of the caudal half of the STS. Systematic recordings showed that the cortex of the anterior bank could be divided into three regions from medial to lateral; the medial superior temporal (MST) area in which cells were activated exclusively by visual stimuli and mostly showed directional selectivities (Saito et al., J. Neurosci., 1986); a region in which most cells could not be activated by visual, auditory or somesthetic stimuli; a polysensory area in which cells responded to visual, auditory and/or somesthetic stimuli. The polysensory area may be continuous to that in the rostral STS (Bruce et al., J. Neurophysiol., 1981), but this caudal part of the polysensory area is characterized by a dominancy of cells responding to visual and auditory stimuli. Out of 100 cells recorded in this area, 43 were unimodal (20 visual, 20 auditory and 3 somesthetic cells), 19 were bimodal (15 visual+auditory, 2 visual+somesthetic and 2 auditory+somesthetic cells), 4 were trimodal, and the rest (34) were unresponsive to the stimuli of the three modalities. Visual receptive fields were large (mean size, 60°x60 °) and most cells had selectivity for direction of motion. Auditory responses, which could be elicited by various sounds including pure tones, white noise, human voices and clapping hands, were sensitive to the position of a sound source in space. For some of visual+auditory cells receptive fields for the two modalities overlapped largely. The auditory source had to move in a paticular direction in space for strong activation of some cells, and this direction of motion coincided with the preferred direction of visual responses. Somesthetic receptive fields were large, usually including the whole back of the body, and most cells were activated by a light touch on the skin or by bending of the hairs.