Sensory processing in the pallium of a mormyrid fish

Abstract

To investigate the functional organization of higher brain levels in fish we test the hypothesis that the dorsal gray mantle of the telencephalon of a mormyrid fish has discrete receptive areas for several sensory modalities. Multiunit and compound field potentials evoked by auditory, visual, electrosensory and water displacement stimuli in this weakly electric fish are recorded with multiple semimicroelectrodes placed in many tracks and depths in or near area dorsalis telencephali, pars medialis (Dm). Most responsive loci are unimodal; some respond to two or more modalities. Each modality dominates a circumscribed area, chiefly separate. Auditory and electrical responses cluster in the dorsal 500 m of rostral and caudolateral Dm, respectively. Two auditory subdivisions underline specialization of this sense. Mechanoreception occupies a caudal area overlapping electroreception but centered 500 m deeper. Visual responses scatter widely through ventral areas. Auditory, electrosensory and mechanosensory responses are dominated by a negative wave within the first 50 ms, followed by 15-55 Hz oscillations and a slow positive wave with multiunit spikes lasting from 200-500 ms. Stimuli can induce shifts in coherence of certain frequency bands between neighboring loci. Every electric organ discharge command is followed within 3 ms by a large, mainly negative but generally biphasic, widespread corollary discharge. At certain loci large, slow ("delta F") waves usually precede transient shifts in electric organ discharge rate. Sensory evoked potentials in this fish pallium may be more segregated than in elasmobranchs and anurans and have some surprising similarities to those in mammals. Key Words: cerebral cortex; corollary discharge; induced rhythms; evoked potential; gamma band; lateral line; mormyrid.