BINAURAL AND FREQUENCY REPRESENTATION IN THE PRIMARY AUDITORY-CORTEX OF THE BIG BROWN BAT, EPTESICUS-FUSCUS

This study examines the binaural and frequency representation in the p rimary auditory cortex (AC) of the big brown bat, Eptesicus fuscus, by using an ear-phone stimulation system. All 306 cortical neurons studi ed were excited by contralateral sound stimulation but they were eithe r excited, inhibited or not affected by ipsilateral sound stimulation. These cortical neurons were columnarly organized according to their b inaural and frequency-tuning properties. The excitation-excitation col umns which occupy about 15% of the AC are mainly aggregated within an oval-shaped area of the central AC. The excitation-inhibition neurons and binaural neurons with mixed properties are distributed in the rema ining 85% of the surrounding primary AC. Although the best frequency ( BF) of these neurons shows a tendency to decrease from high to low alo ng the anteroposterior axis of the primary AC, systematic variation in BF is not always consistent across the entire mapping area. In partic ular, BFs of cortical neurons isolated in the anterior AC vary quite u nsystematically such that neurons with similar BFs are aggregated in i solated patches. Isofrequency and binaural columns are segregated into bands that intersect each other.