Under free-field stimulation conditions, corticofugal regulation of auditory sensitivity of neurons in the central nucleus of the inferior colliculus of the big brown bat, Eptesicus fuscus, was studied by blocking activities of auditory cortical neurons with Lidocaine or by electrical stimulation in auditory cortical neuron recording sites. The corticocollicular pathway regulated the number of impulses, the auditory spatial response areas and the frequency-tuning curves of inferior colliculus neurons through facilitation or inhibition. Corticofugal regulation was most effective at low sound intensity and was dependent upon the time interval between acoustic and electrical stimuli. At optimal inter-stimulus intervals, inferior colliculus neurons had the smallest number of impulses and the longest response latency during corticofugal inhibition. The opposite effects were observed during corticofugal facilitation. Corticofugal inhibitory latency was longer than corticofugal facilitatory latency. Iontophoretic application of gamma-aminobutyric acid and bicuculline to inferior colliculus recording sites produced effects similar to what were observed during corticofugal inhibition and facilitation. We suggest that corticofugal regulation of central auditory sensitivity can provide an animal with a mechanism to regulate acoustic signal processing in the ascending auditory pathway.
This study examines the binaural and frequency representation in the primary auditory cortex (AC) of the big brown bat, Eptesicus fuscus, by using an ear-phone stimulation system. All 306 cortical neurons studied were excited by contralateral sound stimulation but they were either excited, inhibited or not affected by ipsilateral sound stimulation. These cortical neurons were columnarly organized according to their binaural and frequency-tuning properties. The excitation-excitation columns 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 remaining 85% of the surrounding primary AC. Although the best frequency (BF) of these neurons shows a tendency to decrease from high to low along the anteroposterior axis of the primary AC, systematic variation in BF is not always consistent across the entire mapping area. In particular, BFs of cortical neurons isolated in the anterior AC vary quite unsystematically such that neurons with similar BFs are aggregated in isolated patches. Isofrequency and binaural columns are segregated into bands that intersect each other.
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