Role of the pretectal nucleus of the optic tract in short-latency ocular following responses in monkeys
When a large-field image is suddenly moved in front of an observer, an ocular following response (OFR) with short latency (<60 ms in monkey and <85 ms in human) is observed. Previous studies have shown that neurons in the pretectal nucleus of the optic tract (NOT) of the monkey respond to movements of large-field visual stimuli. To understand the potential role of the NOT in the OFR, we first recorded single-unit activity in the NOT of four monkeys (Macaca fuscata). Sixty-six NOT neurons preferred large-field ipsiversive visual motion. In 86% (49/57) of the neurons, optimal directions were distributed over ±30° from ipsilateral. NOT units were sensitive to the speed of the visual motion; 54% (27/50) preferred slow (≤20°/s), 22% (11/50) preferred fast (≥80°/s) and the remainder intermediate speeds. Their response latencies to the moving visual scene were very short (~51 ms), and 44% of them led the onset of the OFR by 10 ms or more. To characterize the response properties of these neurons, we reconstructed the temporal firing patterns of 17 NOT neurons, using the acceleration, velocity, position and bias components of retinal image slip or eye movements during the OFR by a least squares error method. For each stimulus speed fitting condition, using either retinal slip or eye movements, their firing patterns were matched to some extent although the goodness of fit was better using retinal slip than when eye movements were used. Neither of these models could be applied independently of stimulus speed, suggesting that the firing pattern of the NOT neurons represented information associated with retinal slip or eye movements during the OFR, over a limited range. To provide further evidence that the NOT is involved in generating the OFR, we placed unilateral microinjections of muscimol into the NOT. Following the muscimol injection, we observed a ~50% decrease in eye velocity of the OFR toward the side of injection regardless of stimulus speed, while only a weak effect was observed in the OFR during contraversive or vertical image motion. These results suggest that the NOT may play a role in the initiation and support of the short-latency ocular following response.
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