Long-term exposure to occupational noise alters the cortical organization of sound processing

Abstract

Long-term exposure to noise may cause an altered hemispheric lateralization of speech processing even in silent conditions. We examined whether this lateralization shift is speech specific or occurs also for other sounds. Brain responses from 10 healthy noise-exposed workers (>5 years) and 10 matched controls were recorded with a 32-channel electroencephalogram in two conditions, one including standard and deviant speech sounds, the other non-speech sounds, with novel sounds in both. The deviant-sound elicited mismatch negativity (MMN) was larger to non-speech than speech sounds in control subjects, while it did not differ between the sound types in the noise-exposed subjects. Moreover, the MMN to speech sounds was lateralized to the right hemisphere in exposed workers, while it was left-hemisphere predominant in control subjects. No group topography difference was found for non-speech sounds. The deviant sounds that were close in formant space to the standards elicited a longer MMN latency in both speech and non-speech conditions in exposed subjects than controls. No group differences were found for cortical responses to novel sounds. Long-term noise exposure altered the strength and the hemispheric organization of speech-sound discrimination and decreased the speed of sound-change processing. Subpathological changes in cortical responses to sounds may occur even in subjects without a peripheral damage but continuously exposed to noisy auditory environments.