Saccade-related responses of reticular thalamic neurons in monkey

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IN>IONKEY.MAKOTO KATO AND OKIHIDE HIliOSAK.4, National Institute for Physiological Sciences, Myodaiii, Okazaki 444, Japan. It is well established that the basal ganglia participate in the control of saccadic eye (SNr) which pr0ject.s to the superiol movements through the substantia nigra pars reticulata of this projection is colliculus. The SNr also projects to the thalamic nuclei, but the function not clear. The reticular thalamic nucleus (NRT) is known to connect, mutually with the thalamic nuclei and to exert widespread inhibitory effects on thalamic neurons. Recent studies have shown that the NRT receives projections from the SNr and the external pallidum, suggesting participation of the YRT in the control of eye movements. To test this hypothesis, we recorded extracellular single unit activities from neurons in the anterior portion of the NRT in the monkey performing visuo-oculomotor tasks. Location of the NRT was determined using MRI and physiological landmarks NRT neurons typically showed irregular st,ructures. obtained by unit recordin g in the neighboring tonic background discharges of medium to high rates (20-70 spikes/s). Out of 60 cells recorded, 27 showed saccade responses time locked to saccades to visual or remembered targets. The saccadic responses were classified into 4 types: a decrease in discharge rate (n=15), a decrease followed by an increase (n=4), an increase (n=7j, and an increase followed by a decrease (n=l). One third of saccadic cells (n=8) showed the preferred direct.ion or amplitude of saccades; the others (n=lY) showed no preference. The onset of t,he saccadic responses usually preceded that. of a saccade (50100 ms). Twelve cells exhibited visual responses as well which, unlike the saccadic responses, were always an increase in discharge rate. These results suggest, that the NRT neurons tend to disinhibit with the thalamocortical neurons before and during saccadic eye movements and, possibly together disinhibition derived from the SNr, may play a role in the initiation or facilitation of saccadic eye movements.

S~CCADE-RELATEDRESPONSESOFRETICULARTHALAMICNEURONS

MODULATION OF VISUALLY-TRIGGERED SACCADES BY ELECTRICAL MICROSTIMULATION OF THE MONKEY PREFRONTAL CORTEX. MASAO AZUMA, YASUSHI KODAKA, AND HISAO SUZUKI, Department of Physiology Hirosaki University School of medicine, Hirosaki 036, Japan. We investiqated the effects of electrical stimulation of the frontal eve field (FEF) and its adjacent area upon visually-triggered saccades. For brain stimulation, a train of electrical pulses was applied through a microelectrode. The visually-triggered saccades were obtained in the monkeys whose head was tightly fixed. They were trained to gaze a dim light spot at the center of the visual field, and to conduct saccades to another light spot. To examine the effects of stimulation, it was given around the onset of the visual saccades. The microstimulation to the medial site in the FEF elicited large saccades toward the contralateral site to stimulating hemisphere. When the stimulating site became more laterally, the saccade amplitude became smaller, and finally no saccades were obtained. The stimulation in the medial FEF delayed the onset of the visual saccades. During the delay period, the electrical saccades were observed. Delaying was also seen even in the lateral site where no saccades was electrically evoked. During the delay period, the eyes stayed fixed. Length of the delay period depended on the intensity and duration of the train pulses. On considering that the lateral area contains the neurons activated during animal's gazing at a light spot, present results suggested that the area may subserve fixation of the eyes upon a visual target with inhibiting the saccade generation.

EYE MOVEMENTS EVOKEDBY STIMULATIONOF AN EXTRASTRIATECORTEXIN THE CAT TAKAGIe2.

HARUO TODAl,

TOYOHISAYOSHIZAWA’2, TOM00 AND01 AND TAKEHIKOBANDO’. DeDartments of

20Dhthalmoloqv. NiiqataUniversitySchool of Medicine,Asahi-Machi,Niiqata,Niiqata The properties evoked

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and Japan.

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of evoked DENS were 40-100

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