Cerebellar influence on the time structure of movement in the electric fish Eigenmannia

To determine how cerebellar activity influences motor co-ordination, a simple motor act of an electric fish, Eigenmannia spp, was studied. Whenever the fish encounters an object of conductivity different from water, it bends the caudal two-thirds of its body towards the object. This behavior, called "probing", was analysed in the normal fish and in fish with a small cerebellar lesion. Probing could be described as a wave travelling along the body very similar to what one observes when shaking a rope of moderate elasticity. In normal fish, the wave starts at the tipe of the tail and proceeds towards the head, dying out after about two-thirds of the body length. A lesion in the corpus cerebelli causes this wave to start at the head and move towards the tail, though it is otherwise the same as in normal fish, i.e. it has the same frequency components as revealed by Fourier analysis. In addition to the change in probing, the lesioned fish also undulates its body while swimming whereas it is normally kept straight. This undulating wave turns out to be the same as that during probing where it is executed whilst the fish keeps itself on the spot with its locomotory apparatus, the anal fin, which is driven by muscles independent of the trunk muscles used for probing.(ABSTRACT TRUNCATED AT 250 WORDS)