Neuronal mechanisms for object discrimination in the weakly electric fish Eigenmannia virescens

The peripheral sensory basis for object discrimination was investigated in the weakly electric fish Eigenmannia virescens. Single unit recordings were made from the primary afferent fibres in the posterior branch of the anterior lateral line nerve while the local electric field (self-generated and stimulated) was modified by external resistance and capacitance shunts. Both fibre types (probability and phase coders) responded differentially to capacitance and resistance shunts of equivalent impedence. The degree of response differentiation between the two shunting conditions varied with the intensity of the electrical stimulus at the receptor. These data suggest that the primary electroreceptors can discriminatively encode the two electrical characteristics of 'objects'. However, since the response of primary electroreceptors also varied with the spatial orientation of the shunting electrodes, central structures must play an important role in object discrimination.