Cerebellar Control of Movement in Fish as Revealed by Small Lesions

Abstract

An attempt was made to quantify the effects of a cerebellar lesion on a specific simple motor task called ”probing” of the electric fish Eigenmannia. Probing serves to improve the “electric image” of objects (Heiligenberg 1975, Bacher 1983). It is a smooth bending of the body towards the object. Using a coordinate system as in Fig. 1 the bending could be described by y (t) = f (x) • sin (9.8t — γ (x), the lesion resulted in a change of the probing such that the above expression differed only in the sign of the phase becoming y (t) = f (x) • sin (9.8t + γ (x)). The difference in the sign of the phase means, that the movement in a normal fish starts at the tip of the tail whereas in a lesioned one at the head. Lesions resulting in this effect had to cover at least part of the region where electrosensitive units have been recorded (Bastian 1974, Behrend 1977). Sparing this region did not alter the normal behavior (see Fig. 2 open triangles and circles). The effect showed no dependency on wether the lesion was unilateral or bilateral nor was there evidence of differential effects for differently located lesions. This is certainly due to the collection of Y-values from a frozen TV picture limiting the accuracy since, as described below, minute changes in segmental timing probably caused by any lesion should manifest quite different overall movements. This order of onset of segmental activity accounting for the movement was calculated using a fish model. It turned out that the movement of a normal animal required a contraction of all segments simultaneously on one side, whereas in the lesioned animal a delay of 0.5 ms between the segments was needed. Considering the cerebellum as a predictive motor space-time metric as formulated by Pellionisz and Llinds (1982) the phase shift is exactly what one would predict in a type of movement where the single components — the segmental muscles — work in a sinusoidal fashion. It results from the missing “lookahead” normally provided by the cerebellar metric.