The Journal of Neurophysiology Vol. 83 No. 4 April 2000, pp. 1856-1863
Copyright ©2000 by the American Physiological Society
Department of Biology, Washington University, St. Louis, Missouri 63130
Chowdhury, Syed A. and
Nobuo Suga.
Reorganization of the Frequency Map of the Auditory Cortex Evoked
by Cortical Electrical Stimulation in the Big Brown Bat. J. Neurophysiol. 83: 1856-1863, 2000. In a
search phase of echolocation, big brown bats, Eptesicus
fuscus, emit biosonar pulses at a rate of 10/s and listen to echoes. When a short acoustic stimulus was repetitively delivered at
this rate, the reorganization of the frequency map of the primary auditory cortex took place at and around the neurons tuned to the
frequency of the acoustic stimulus. Such reorganization became larger
when the acoustic stimulus was paired with electrical stimulation of
the cortical neurons tuned to the frequency of the acoustic stimulus.
This reorganization was mainly due to the decrease in the best
frequencies of the neurons that had best frequencies slightly higher
than those of the electrically stimulated cortical neurons or the
frequency of the acoustic stimulus. Neurons with best frequencies
slightly lower than those of the acoustically and/or electrically
stimulated neurons slightly increased their best frequencies. These
changes resulted in the over-representation of repetitively delivered
acoustic stimulus. Because the over-representation resulted in
under-representation of other frequencies, the changes increased the
contrast of the neural representation of the acoustic stimulus. Best
frequency shifts for over-representation were associated with
sharpening of frequency-tuning curves of 25% of the neurons studied.
Because of the increases in both the contrast of neural representation
and the sharpness of tuning, the over-representation of the acoustic
stimulus is accompanied with an improvement of analysis of the acoustic stimulus.
(auditory system / descending system / learning and memory / plasticity / tonotopic map)
Department of Biology, Washington University, One Brookings Drive, St. Louis, MO 63130
Peripheral auditory neurons are tuned to single frequencies of sound. In the central auditory system, excitatory (or facilitatory) and inhibitory neural interactions take place at multiple levels and produce neurons with sharp level-tolerant frequency-tuning curves, neurons tuned to parameters other than frequency, cochleotopic (frequency) maps, which are different from the peripheral cochleotopic map, and computational maps. The mechanisms to create the response properties of these neurons have been considered to be solely caused by divergent and convergent projections of neurons in the ascending auditory system. The recent research on the corticofugal (descending) auditory system, however, indicates that the corticofugal system adjusts and improves auditory signal processing by modulating neural responses and maps. The corticofugal function consists of at least the following subfunctions. (i) Egocentric selection for short-term modulation of auditory signal processing according to auditory experience. Egocentric selection, based on focused positive feedback associated with widespread lateral inhibition, is mediated by the cortical neural net working together with the corticofugal system. (ii) Reorganization for long-term modulation of the processing of behaviorally relevant auditory signals. Reorganization is based on egocentric selection working together with nonauditory systems. (iii) Gain control based on overall excitatory, facilitatory, or inhibitory corticofugal modulation. Egocentric selection can be viewed as selective gain control. (iv) Shaping (or even creation) of response properties of neurons. Filter properties of neurons in the frequency, amplitude, time, and spatial domains can be sharpened by the corticofugal system. Sharpening of tuning is one of the functions of egocentric selection.