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. 2007 May 17;8:33.
doi: 10.1186/1471-2202-8-33.

Asymmetric Lateral Inhibitory Neural Activity in the Auditory System: A Magnetoencephalographic Study

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Free PMC article

Asymmetric Lateral Inhibitory Neural Activity in the Auditory System: A Magnetoencephalographic Study

Hidehiko Okamoto et al. BMC Neurosci. .
Free PMC article

Abstract

Background: Decrements of auditory evoked responses elicited by repeatedly presented sounds with similar frequencies have been well investigated by means of electroencephalography and magnetoencephalography (MEG). However the possible inhibitory interactions between different neuronal populations remains poorly understood. In the present study, we investigated the effect of proceeding notch-filtered noises (NFNs) with different frequency spectra on a following test tone using MEG.

Results: Three-second exposure to the NFNs resulted in significantly different N1m responses to a 1000 Hz test tone presented 500 ms after the offset of the NFNs. The NFN with a lower spectral edge closest to the test tone mostly decreased the N1m amplitude.

Conclusion: The decrement of the N1m component after exposure to the NFNs could be explained partly in terms of lateral inhibition. The results demonstrated that the amplitude of the N1m was more effectively influenced by inhibitory lateral connections originating from neurons corresponding to lower rather than higher frequencies. We interpret this effect of asymmetric lateral inhibition in the auditory system as an important contribution to reduce the asymmetric neural activity profiles originating from the cochlea.

Figures

Figure 1
Figure 1
Hypothesized neural network in auditory system. Left: schematic diagram of hypothesized neural activity corresponding to a stimulus frequency from the peripheral to the central auditory pathway. Neural activity becomes sharper in the more central levels, especially in the high frequency range. Right: hypothetical excitatory and inhibitory neural network from the peripheral to the central auditory pathway. Red lines indicate excitatory neural connections and blue lines indicate inhibitory connections. Solid blue lines projecting from lower to higher frequencies have stronger inhibitory effects than the dashed blue lines projecting from higher to lower frequencies.
Figure 2
Figure 2
Experimental design and representative single subject results. (a): Schematic representation of the stimulus sequence with notch-filtered noise (NFN) of 3 s duration and the following test stimulus (TS) of 150 ms duration with an inter-stimulus interval of 500 ms. (b): Amplitude spectrum of the 3 s NFN measured at the ear-piece. Differences in frequency domain between the TS and the low-pass slope of the NFNs are: 1/6 octave (NFN1), 2/6 octave (NFN2), 3/6 octave (NFN3), 4/6 octave (NFN4), and 5/6 octave (NFN5). Center frequencies of the stop-band regions are: 1260 (NFN1), 1122 (NFN2), 1000 (NFN3), 891 (NFN4), and 794 Hz (NFN5). All NFNs had one-octave stop-band frequency. The bandwidths of the neighbouring dotted vertical lines are 1/6 octave. (c): Superposition of the auditory evoked magnetic fields (AEFs) from all magnetic sensors as recorded in one representative subject. AEFs elicited by the test stimulus (TS) following each NFN (NFN1 to NFN5).
Figure 3
Figure 3
Normalized N1m source strengths. Group means (n = 9) of the normalized N1m source strength obtained by the test stimulus (TS) following each NFN (NFN1 to NFN5). Error-bars denote the 95 % confidence interval limits for the group means. White and gray bars represent responses from the left and right hemispheres, respectively.

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