Abstract
Biological tissues are permeable to near-infrared light (NIR), and the changes in its absorbance are mostly due to hemoglobin (Hb). Spectroscopy (NIRS) of the head exposed to weak NIR can noninvasively detect Hb changes that accompany cerebral activation, and it can be performed in a sound-attenuated room without any interference by cochlear implants or hearing aids. Although most NIRS signals come from the gray matter at the surface of the brain, stable responses have been obtained to band-passed noise at the levels equal or close to the individuals' hearing thresholds, and to phonemic and intonational contrasts. The responses to the two kinds of linguistic contrasts diverged significantly to the left and to the right, respectively, in the 11-12 months and older age groups and can serve as an indicator of receptive linguistic development. Good correspondence between the brain responses and subjective audio-linguistic percepts was observed in postlingually deaf adults with cochlear implants, while behavioral discrimination appeared several months later than positive brain responses in a congenitally deaf child with a cochlear implant, and there was a concomitant increase in the amplitude of the evoked response. This suggests that NIRS can be used to objectively assess the auditory linguistic development of hearing impaired children.
