SENSORY PROCESSING IN THE PALLIUM OF A MORMYRID FISH

To investigate the functional organization of higher brain levels in f ish we test the hypothesis that the dorsal gray mantle of the telencep halon of a mormyrid fish has discrete receptive areas for several sens ory modalities. Multiunit and compound field potentials evoked by audi tory, visual, electrosensory, and water displacement stimuli in this w eakly electric fish are recorded with multiple semimicroelectrodes pla ced in many tracks and depths in or near telencephalic area dorsalis p ars 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 dor sal 500 mu m of rostral and caudolateral Dm, respectively. Two auditor y subdivisions underline specialization of this sense. Mechanoreceptio n occupies a caudal area overlapping electroreception but centered 500 mu m deeper. Visual responses scatter widely through ventral areas. A uditory, electrosensory, and mechanosensory responses are dominated by a negative wave within the first 50 msec, followed by 15-55 Hz oscill ations and a slow positive wave with multiunit spikes lasting from 200 to 500 msec, Stimuli can induce shifts in coherence of certain freque ncy bands between neighboring loci, Every electric organ discharge com mand is followed within 3 msec by a large, mainly negative but general ly biphasic, widespread corollary discharge. At certain loci large, sl ow (''delta F'') waves usually precede transient shifts in electric or gan discharge rate. Sensory-evoked potentials in this fish pallium may be more segregated than in elasmobranchs and anurans and have some su rprising similarities to those in mammals.