Sensory determinants of object recognition in a weakly electric fish

Mormyrid weakly electric fish produce discrete pulses of electricity separated by varying intervals of silence. The temporal structure of their electromotor behavior is highly dependent on the behavioral context. While resting, pulse rate is low and sequential variation is high. In this context, the delivery a brief novel stimulus often elicits a transient acceleration in electromotor activity, called a novelty response. The novelty response effectively increases the sampling rate of the electrolocation system and has psychophysical properties akin to orienting responses. During these electromotor burst displays, pulse rate is high and sequential variation is low. While mormyrids actively probe an object in their environment, electromotor activity becomes highly regular.;This dissertation utilized the novelty response to quantitatively test the synergism of multiple senses during the recognition of a moving object. The novelty response provides a rare insight into sensory perception because it is reducible into scalar values and persists under muscular immobilization. Principally, we discovered that the magnitude of multisensory novelty responses were significantly greater than the arithmetic sum of their component unisensory responses. This supralinearity was only evident when the unimodal stimuli elicited weak responses. Linear and sublinear response properties occurred when the unimodal stimuli elicited large responses. The most significant change occurred to novelty response duration. Additional changes occurred to response probability, amplitude, and area. Also, the prevalence of a particular electromotor burst display, known as a 'scallop', significantly increased during multimodal stimulation. Therefore, multisensory integration significantly enhances the recognition of moving objects. Supralinear enhancement may result from a qualitative change in perception that is rooted in the ecological significance of multisensory integration to prey capture.;Despite the rich variation in the temporal structure of electromotor behavior during object recognition, little is known about its affect on sensory processing. We tested the effect of the temporal pattern of electrolocation on the neural integration of fictive object stimuli. We discovered that midbrain neurons are selective to specific intervals of stimulation that predominate either 'burst' or 'rest' modes of electromotor activity. Utilizing the intracellular whole cell technique, we were able elucidate multiple integrative mechanisms that could explain this selectivity. We discovered rate-dependent changes in the synaptic strength of excitation and inhibition. Agonistic center-surround receptive field organization may help shape interval selectivity.