Abstract
We investigated the modulation of lateral inhibition in the human auditory cortex by means of magnetoencephalography (MEG). In the first experiment, five acoustic masking stimuli (MS), consisting of noise passing through a digital notch filter which was centered at 1 kHz, were presented. The spectral energy contrasts of four MS were modified systematically by either amplifying or attenuating the edge-frequency bands around the notch (EFB) by 30 dB. Additionally, the width of EFB amplification/attenuation was varied (3/8 or 7/8 octave on each side of the notch). N1m and auditory steady state responses (ASSR), evoked by a test stimulus with a carrier frequency of 1 kHz, were evaluated. A consistent dependence of N1m responses upon the preceding MS was observed. The minimal N1m source strength was found in the narrowest amplified EFB condition, representing pronounced lateral inhibition of neurons with characteristic frequencies corresponding to the center frequency of the notch (NOTCH CF) in secondary auditory cortical areas. We tested in a second experiment whether an even narrower bandwidth of EFB amplification would result in further enhanced lateral inhibition of the NOTCH CF. Here three MS were presented, two of which were modified by amplifying 1/8 or 1/24 octave EFB width around the notch. We found that N1m responses were again significantly smaller in both amplified EFB conditions as compared to the NFN condition. To our knowledge, this is the first study demonstrating that the energy and width of the EFB around the notch modulate lateral inhibition in human secondary auditory cortical areas. Because it is assumed that chronic tinnitus is caused by a lack of lateral inhibition, these new insights could be used as a tool for further improvement of tinnitus treatments focusing on the lateral inhibition of neurons corresponding to the tinnitus frequency, such as the tailor-made notched music training.
Highlights
The amplitude of the N1m response evoked by a pure tone stimulus is reduced when a preceding masking noise with a digital notch filter centered at the frequency of the pure tone is presented [1], [2], [3]
Because neurons with characteristic frequencies (CF) corresponding to frequencies within the notch (NOTCH CF) are not excited during stimulation with notch-filtered noise (NFN), the neurons with CF corresponding to the edge frequency bands around the notch (EFB) are not inhibited by collaterals of neurons with NOTCH CF
A second experiment was conducted in order to investigate this question in more detail. In this experiment we investigated the impact upon lateral inhibition of amplified EFB with two different bandwidths, both narrower than in the first experiment, in comparison to NFN without any additional spectral contrasts
Summary
The amplitude of the N1m response evoked by a pure tone stimulus is reduced when a preceding masking noise with a digital notch filter centered at the frequency of the pure tone is presented [1], [2], [3]. The auditory cortex is organized tonotopically [4], [5], [6] When it receives afferent input from lower levels of the auditory system, excitatory neurons with the same characteristic frequencies (CF) are activated. The result is an enhanced activation of neurons with CF corresponding to the EFB (EFB CF) and, a pronounced lateral inhibition of neurons with NOTCH CF (cf Figure 1 A). In this experiment we investigated the impact upon lateral inhibition of amplified EFB with two different bandwidths, both narrower than in the first experiment, in comparison to NFN without any additional spectral contrasts. A consistent decrement in the amount of lateral inhibition was assumed, with the most lateral inhibition occurring in the narrowest EFB amplification condition and the least lateral inhibition in the NFN condition
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