EXPERIENCE-DEPENDENT CORTICOFUGAL ADJUSTMENT OF MIDBRAIN FREQUENCY MAP IN BAT AUDITORY-SYSTEM

Recent studies of corticofugal modulation of auditory information proc essing indicate that cortical neurons mediate both a highly focused po sitive feedback to subcortical neurons ''matched'' in tuning to a part icular acoustic parameter and a widespread lateral inhibition to ''unm atched'' subcortical neurons. This cortical function for the adjustmen t and improvement of subcortical information processing is called egoc entric selection. Egocentric selection enhances the neural representat ion of frequently occurring signals in the central auditory system. Fo r our present studies performed with the big brown bat (Eptesicus fusc us),we hypothesized that egocentric selection adjusts the frequency ma p of the inferior colliculus (IC) according to auditory experience bas ed on associative learning. To test this hypothesis, we delivered acou stic stimuli paired with electric leg stimulation to the bat, because such paired stimuli allowed the animal to learn that the acoustic stim ulus was behaviorally important and to make behavioral and neural adju stments based on the acquired importance of the acoustic stimulus, We found that acoustic stimulation alone evokes a change in the frequency map of the IC; that this change in the IC becomes greater when the ac oustic stimulation is made behaviorally relevant by pairing it with el ectrical stimulation; that the collicular change is mediated by the co rticofugal system; and that the IC itself can sustain the change evoke d by the corticofugal system for some time, Our data support the hypot hesis.