Cortical representation of release from auditory masking

The aim of the present study was to find a functional MRI correlate in human auditory cortex of the psychoacoustical effect of release from masking, using amplitude-modulated noise stimuli. A sinusoidal target signal was embedded in a bandlimited white noise, which was either unmodulated or (co)modulated. Psychoacoustical thresholds were measured for the target signals in both types of masking noise, using an adaptive procedure. The mean threshold difference between the unmodulated and the comodulated condition, i.e., the release from masking, was 15 dB. The same listeners then participated in an fMRI experiment, recording activation of auditory cortex in response to tones in the presence of modulated and unmodulated noise maskers at five different signal-to-noise ratios. In general, a spatial dissociation of changes of overall level and signal-to-noise ratio in auditory cortex was found, replicating a previous fMRI study on pure-tone masking. The comparison of the fMRI activation maps for a signal presented in modulated and in unmodulated noise reveals that those regions in the antero-lateral part of Heschl's gyrus previously shown to represent the audibility of a tonal target (rather than overall level) exhibit a stronger activation for the modulated than for the unmodulated conditions. This result is interpreted as a physiological correlate of the psychoacoustical effect of comodulation masking release at the level of the auditory cortex.