What central effect of auditory rehabilitation around the bDLF, then, is conceivable?

In our high-frequency sensorineural hearing-loss subjects, the basal part of the cochlea was damaged and consequently unresponsive to high-frequency tones. Thus, before the auditory rehabilitation, the neurons initially encoding this part of the cochlea were already reorganized, in that they were encoding the frequency at the edge of the damaged area. The resulting tonotopic map in the auditory cortex would have an enlarged representation of the lesion-edge frequency. The amplification brought about by the hearing aid meant that some acoustic information was again arriving in the high-frequency region of the cochlea. Thus, the neurons initially encoding these frequencies could get back to their initial function. The tonotopic map in the auditory cortex could thus come to have the same representation as in normal-hearing subjects.

One important feature concerns the time course of recovery: how much time is necessary for such secondary plasticity to be observed? Since the DLF thresholds were normalized for each frequency around the bDLF one month after auditory rehabilitation, it may be hypothesized that the reorganization had already occurred by then, and that the overrepresentation observed at the cut-off frequency was no longer present. This main rehabilitation effect could be observed up to three months post-rehabilitation. The findings at six months may reflect another mechanism of plasticity.

This plasticity may appear to be rather fast, but that is consistent with other studies and other sensory modalities. Indeed, in the somesthetic modality, Giraux et al. (2001) used fMRI to show that, in a double hand-graft patient, only two months were necessary for normal activation of the hands by the sensorimotor cortex to be observed, suggesting that the reorganization induced by the amputation was already reversed by then.

Consequently, if our perceptual changes reflect cortical sensory plasticity, it seems that the first three months of sensory rehabilitation are crucial in the reorganization of sensory pathways.