Single Unit and Population Analysis of Saccade-related Fastigial Nucleus Activity in the Alert Monkey
A remarkable role in saccade control is played by the cerebellar caudal fastigial nucleus, FN (fastigial oculomotor region, FOR), which lies under the cerebellar vermis. FOR bursting activity is related to many saccade properties, such as direction, amplitude and duration. The precise mechanism by which FOR bursting influences a saccade is under debate. It has been assumed on the basis of microelectrode recordings that saccade-related FOR neurones act without receiving information about the eye position. Similarly, discharge activity of P-cells in the oculomotor vermis does not reveal a prominent eye position-dependency. On the other hand, it is proposed that eye position supports saccadic control via at least some FOR neurones, as some authors found that a minority of FOR neurones do react to a change in horizontal eye position. Likewise, studies on saccade metrics in vermal oculomotor areas indicate that the effects of lesions or of electrical stimulations vary with the starting position of the eye. This suggests that these areas are involved in the neuronal compensation for non-linearities in orbital mechanics. In any case, it is generally agreed that there is a consistent difference in the timing of saccade-related bursts produced by FOR neurones, resulting in shorter latencies for contralateral than for ipsilateral saccades. This pattern of burst timing indicates that the bursts are associated with the beginning of contralateral saccades and with the end of ipsilateral ones. We have investigated (1) whether or not the initial eye position has an effect on bursts for ipsilateral and contralateral saccades, (2) if there is a difference between centripetal and centrifugal saccades, and (3) if FOR neurones influence acceleration and deceleration of a saccade and, if so, in what way. For this purpose, a 9-point horizontal starting position training paradigm was applied, using a 3x3 square grid spaced at 16° intervals. Based on observations on 75 saccade-related FOR neurones, our results permit the following conclusions. (1) Ipsilateral saccades clearly differ from contralateral ones. Individual neurone analysis as well as population burst analysis based on averaging the data across neurones, show that neither the vertical nor the horizontal component of the initial eye position substantially influences saccade-related bursting of the FOR neurone population. (2) Centripetal and centrifugal saccades do not differ as to FOR bursting. (3) Although most FOR neurones do not receive information about eye position, they seem to play a crucial role in acceleration and/or deceleration of saccades. Finally, as the discharge patterns of individual neurones are highly variable, with prominent differences in both latency and amplitude, we propose that modification of the relative contributions of individual FOR neurones to the total FOR neurone population response may provide a mechanism for the adaptive control of saccade metrics in the Rhesus monkey.
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