To measure the gain of the vestibulo-ocular reflex, subjects must have their head displaced abruptly in the dark, such that moving the head by 10 degrees leftward will induce rightward eye movements with respect to the head by the same amount to yield a gain value of 1: a 10 degree head movement divided by 10-degree counter-rotated eye movements. Nguyen-Vu, Raymond et al. (2013) were able to manipulate the simple spike rate generated by cerebellar Purkinje neurons at the flocculus by either optogenetically exciting the Purkinje neurons or by optogenetically inhibiting these neurons by activation of the climbing fibres to evoke complex spikes, whose presence at a Purkinje neuron subtracts simple spikes. This optogenetic manipulation was done as mice were oscillated sinusoidally left and right at 1 Hz and at +- 10 degrees for a period of 30 minutes, all done in the dark. It was found that if simple spikes were added to the Purkinje neurons on the same side of the cerebellum as the direction of the head displacement, the gain of the vestibular response was decreased, and if simple spikes were subtracted from the Purkinje neurons on the same side of the cerebellum as the direction of the head displacement, the gain was increased. This outcome concurs with the alterations of simple spikes by having monkeys don minimizing or maximizing prisms, which systematically decrease or increase, respectively, the simple spikes of floccular Purkinje neurons thereby inducing a decrease or an increase in the gain of the vestibulo-ocular reflex (Miles et al. 1980a).
Regardless of whether the simple-spike manipulation is done internally with optogenetics or externally with prisms, the direction of the gain change is in correspondence. In the end, it is the addition and subtraction of simple spikes that brings about change to the motor output that is finalized at the cerebellar nuclei (Miles and Lisberger 1981), and under normal circumstances it the presence or absence of complex spikes that subtracts or adds simple spikes to the efference-copy code (of all behaviors: Bell et al. 1997; De Zeeuw 2021; Giovannucci et al. 2017; Loyola et al. 2019; Shadmehr 2020; Tehovnik et al. 2021; Wang et al. 2023), so that when MT/MST is engaged by an abrupt head movement (whose neurons exhibit a minimal visual pursuit latency of 35 ms, Raiguel et al. 1999) the eyes automatically counter-rotate, but at a latency as short as 5 to 6 ms (Hunter and Collen 2002), a behavior that is typically performed unconsciously. But even if not performed unconsciously, one’s potential awareness of the visual pursuit by neocortex as triggered by the vestibular system is delayed by over 30 ms with respect to the vestibular latency. Nevertheless, a neocortex of a returning astronaut must take charge to reset the vestibular gain of the cerebellum for 1G conditions, which can take as much as one week to recalibrate (Carriot et al. 2021). Recalibration would be expected to occur at vestibular latencies of over an order of magnitude greater than the shortest latencies achieved after a week of training.
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