Mossy fibres sending retinal-slip, eye, and head velocity signals to the flocculus of the monkey

Abstract

Discharges of mossy fibres were recorded from the cerebellar flocculus of monkeys trained to fixate a small visual target and to track the target when it moved slowly. The experimental paradigms used were designed to study neural responses to retinal-slip velocity, eye velocity, or head velocity, individually or in combination. Among 485 mossy-fibre units recorded from the flocculus, sixty-four units (or 13%) responded to movement of the visual stimulus in the horizontal plane. Two distinct groups of visual mossy fibres were found: they were designated 'visual units' (thirty-nine/sixty-four units or 61%) and 'visuomotor units' (twenty-five/sixty-four units or 39%). The visual units responded exclusively to the retinal-slip velocity. Stationary fixation was necessary for clear cyclic modulation of activity. Their responses declined when the retinal-slip velocity was reduced by eye movements in the same direction. The responses of the visual units were directionally selective and lagged behind the occurrence of 'turnabouts' (changes in direction of stimulus movement) and their peak discharges also lagged the occurrence of peak velocity. Each visual unit had a limited range of velocity sensitivity; in some units the range covered the velocity range of smooth-pursuit eye movements. The visuomotor units had visual receptive fields in the peripheral retina (outside of the central 10 deg); they received also oculomotor and vestibular signals. When the head was stationary, the visuomotor units responded to the target velocity (or visual stimulus velocity) which is the algebraic sum of the retinal-slip velocity and the eye velocity. Their responses reflected the retinal-slip velocity during stationary fixation and the eye velocity during smooth-pursuit eye movements. The responses to stimulus movements were, therefore, almost identical regardless of whether the eyes remained stationary or moved with the stimulus. In response to sinusoidal stimulus movements, the responses of the visuomotor units frequently preceded the stimulus velocity, and the phase lead relative to the velocity curve increased when the frequency of sinusoidal movements was increased. This reflected a relatively constant lead of neural discharges (circa 125 ms) during various frequencies. When the head was moved, the responses of the visuomotor units were dominated by the head velocity, and discharges in response either to the retinal-slip velocity or to the eye velocity (both in the direction opposite to the head velocity) were occluded.(ABSTRACT TRUNCATED AT 400 WORDS)