Neural auditory contrast enhancement in humans

Abstract

The perception of sensory events can be suppressed or enhanced by the surrounding spatial and temporal context in ways that help in detecting novel objects and establishing perceptual constancy. In the auditory system, the phenomenon known as auditory enhancement reflects a general principle of contrast enhancement, where a target sound embedded within a background sound becomes perceptually more salient if the background is presented first by itself. This effect is highly robust, producing an effective enhancement of the target of between 5 and 25 dB (more than two orders of magnitude in intensity), depending on the task. Despite their ubiquity in vision, neural correlates of auditory contrast enhancement have yet to be identified in humans. Here we used the auditory steady-state response to probe the neural response to a target sound under conditions of enhancement. The probe was simultaneously modulated in amplitude with two modulation frequencies, to distinguish cortical from subcortical contributions to this phenomenon. We found robust auditory cortical, but not subcortical, enhancement that correlates with behavior and is consistent with an early theoretical model that postulates neural adaptation of inhibition. Our findings provide empirical support for a previously unverified theory of auditory enhancement and point to new approaches for improving sensory prostheses for hearing loss, such as hearing aids and cochlear implants. Significance StatementA target sound embedded within a background sound becomes perceptually more salient if the background is presented first by itself. This phenomenon, where the target "pops out", is known as auditory enhancement. It reflects a general principle of contrast enhancement, and helps in the detection of new acoustic events in the environment and in establishing the perceptual constancy of speech and other biologically relevant sounds under varying acoustic conditions. We use EEG in humans to reveal a cortical correlate of this perceptual phenomenon that provides empirical support for a longstanding but previously unverified theoretical account.