THE RESPONSES OF CENTRAL OCTAVOLATERALIS CELLS TO MOVING SOURCES

Mechanosensory lateral line units recorded from the medulla (medial oc tavolateralis nucleus) and midbrain (torus semicircularis) of the bott om dwelling catfish Ancistrus sp. responded to water movements caused by an object that passed the fish laterally. In terms of peak spike ra te or total number of spikes elicited responses increased with object speed and sometimes showed saturation (Figs. 7, 14). At sequentially g reater distances the responses of most medullary lateral line units de cayed with object distance (Fig. 11). Units tuned to a certain object speed or distance were not found. The signed directionality indes of m ost lateral line units was between -50 and +50, i.e. these units were not or only slightly sensitive to the direction of object motion (Figs . 10, 17). However, some units were highly directionally sensitive in that the main features of the response histograms and,or peak spike ra tes clearly depended on the direction of object movement (e.g. Fig. 9C , D and Fig. 16). Midbrain lateral line units of Ancistrus may receive input from more than one sensory modality. All bimodal lateral line u nits were OR units, i.e., the units were reliably driven by a unimodal stimulus of either modality. Units which receive bimodal input may sh ow an extended speed range (e.g. Fig. 18).