VISUAL-MOTION RESPONSE PROPERTIES OF NEURONS IN DORSOLATERAL PONTINE NUCLEUS OF ALERT MONKEY

In this study we sought to characterize the visual motion processing that exists in the dorsolateral pontine nucleus (DLPN) and make a comparison with the reported visual responses of the middle temporal (MT) and medial superior temporal (MST) areas of the monkey cerebral cortex. The DLPN is implicated as a component of the visuomotor interface involved with the regulation of smooth-pursuit eye movements, becuase it is a major terminus for afferents from MT and MST and also the source of efferents to cerebellar regions involved with eye-movement control. Some DLPN cells were preferentially responsive to discrete (spot and bar) visual stimuli, or to large-field, random-dot pattern motion, or to both discrete and large-field visual motion. The results suggest differential input from localized regions of MT and MST. The visual-motion responses of DLPN neurons were direction selective for 86% of the discrete visual responses and 95% of the large-field responses. Direction tuning bandwidths (full width at 50% maximum response amplitude) averaged 107° and 120° for discrete and large-field visual motion responses, respectively. For the two visual response types, the direction index averaged 0.95 and 1.02, indicating that responses to stimuli moving in preferred directions were, on average, 20 and 50 times greater than responses to discrete or large-field stimulus movement in the opposite directions, respectively. Most of the DLPN visual responses to movements of discrete visual stimuli exhibited increases in amplitude up to preferred retinal image speeds between 20 and 80°/s, with an average preferred speed of 39°/s. At higher speeds, the response amplitude of most units decreased, although a few units exhibited a broad saturation in response amplitude that was maintained up to at least 150°/s before the response decreased. Over the range of speeds up to the preferred speeds, the sensitivity of DPLN neurons to discrete stimulus-related, retinal-image speed averaged 3.0 spikes/s per deg/s. The responses to large-field visual motion were less sensitive to retinal image speed and exhibited an average sensitivity of 1.4 spikes/s per deg/s before the visual response saturated. DPLN and MT were quantitatively comparable with respect to degree of direction selectivity, retinal image speed tuning, and distribution of preferred speeds. Many DPLN receptive fields contained the fovea and were larger than those of MT and more like MST receptive fields in size. The DLPN visual motion responses to discrete visual stimuli were just the type of neural signal that models of the smooth-pursuit eye-movement system employ and that psychophysical results indicate are important in the regulation of pursuit.