One notes that the relationship between cortical changes and the characteristics of hearing loss is relatively difficult to study in practice as noise-induced hearing loss is generally variable Loeb and Smith, ; Atherley et al. In conclusion, our study suggests that the cortical changes produced by acute hearing loss could be sensitive to the sharpness, depth and width of hearing loss.
Moreover, while the cortical changes observed in the present study are short-term, it is possible that a more prolonged exposure to the AFB stimulus could induce long-lasting changes such as those produced by chronic hearing loss or reported in previous studies Robertson and Irvine, ; Rajan et al. The bandwidth of suppressed sidebands derived from this study 0. We also show that lateral suppression is asymmetric as a function of frequency with a stronger and wider suppression produced toward high frequencies suppression was significant up to around 0.
This particular pattern of asymmetric inhibition is consistent with the results of Zhang et al. As the pattern of vibration of the basilar membrane is asymmetric slope is shallower on the basal side of the cochlea compared to the apical side , leading to the corresponding asymmetric pattern of excitation in the cochlear nerve, it has been suggested that the asymmetry of central inhibition stronger inhibition from low to high frequencies may further refine the central representation of spectral edges Suga, ; Okamoto et al.
The present study shows that the cortical representation of spectral edges is enhanced more neurons are dedicated to the representation of spectral edges. A putative link between stimulus importance and its representational size in the primary auditory cortex has been suggested Rutkowski and Weinberger, Our study further suggests that the representational size of spectral cues may be dynamically enhanced in cortex.
This may improve the processing of relevant spectral cues edges within the ever changing acoustic environment. It has been suggested that the responsiveness gain of sub-cortical and cortical neurons could be dynamically adapted to the statistics mean and variance of stimuli. This mechanism provides an elegant solution to the dynamic range problem Viemeister, by adjusting the input—output function of neurons to the distribution of input levels Dean et al. These studies varied the mean and variance of stimulus level across conditions but the mean stimulus level was fixed for single pure tone or uniform for noise bursts or multi-tone pips over frequency for a given condition.
Our study can be considered as an extension of these studies as the mean level was varied over frequency mean level was low in the AFB, and high elsewhere. While the hypothesis of gain control predicts a decrease of gain for high contrast stimuli and the reverse at low contrast stimuli—neurons become more sensitive to small level variations , our results suggest the opposite: the firing rate difference between edge-out and edge-in frequencies are maximally enhanced for sharp and deep contrast. These results emphasize the importance of considering the effects of the spectral dimension spectral envelope in future studies investigating contrast gain control.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Assmann, P.
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