RANGE OF ELECTROLOCATION - COMPARISON OF ELECTRORECEPTOR RESPONSES AND RESPONSES OF CEREBELLAR NEURONS IN A GYMNOTID FISH

Metal and plastic objects moved near to the animal''s (Apteronotus albifrons) body (5 mm lateral) evoked changes in receptor activity consisting primarily of increases in the case of metal and decreases due to plastic. No receptors showed similar responses to both types of objects. Objects moved parallel to the long axis of the fish at sequentially greater distances evoked responses which decayed as a power function of distance. The exponents describing this response fall-off ranged from -1.1 to -2.5 and were in good agreement with calculated changes in trans-epidermal voltage due to the presence of objects at increasing distances from the fish. Increasing or decreasing object size did not markedly alter the rate of response decay but shifted the response-distance relationship so that larger or smaller responses occurred at a given distance. Responses of cerebellar cells to moving objects were more complex than those of the single receptors; they usually consisted of both increases and decreases in spike frequency as objects passed through a cell''s receptive field. Although a given cell usually gave different responses to plastic and metal objects, no predictable response (i.e., increase or decrease in spike frequency) was associated with object quality. Most cerebellar cells showed responses dependent on the direction of object movement. Responses of some cerebellar cells decayed in a fashion similar to responses of single receptors as objects moved at sequentially greater distances. Others showed constant response amplitude out to a distance of about 25 mm. The responses of this later cell type were also independent of object size. The population of cerebellar cells studied could not be divided into discreet categories according to the characteristics of response fall-off. They seem to be distributed along a continuum, varying degrees of constant response at increasing distance being present in different cells.