Dual-stage adaptive filtering of sounds during selective attention

Selectively attending to task-relevant sounds whilst ignoring background noise is one of the most amazing feats performed by the human brain. Here, we studied the underlying neural mechanisms by recording magnetoencephalographic (MEG) responses of 14 healthy human subjects while they performed a nearthreshold auditory discrimination task vs. a visual control task of similar difficulty. The auditory stimuli consisted of parametrically notch-filtered continuous noise masker sounds, and of 1020-Hz target tones occasionally (P = 0.1) replacing 1000-Hz standard tones of 300-ms duration that were embedded at the center of the notches. As a control for masker presence related effects, tone-evoked responses were additionally recorded without masker sound. Selective attention to tones increased the amplitude of the onset N1 response at ∼100 ms to the standard tones during presence of the masker sound. Attention modulated sustained response most clearly at 300–400 ms time range from sound onset, specifically with notches narrower than the critical band, selectively reducing the masker-induced suppression of the toneevoked response. Our results show evidence of a dual-stage filtering mechanism of sensory input in the human auditory cortex: 1) gain increase of attended sounds at early (∼100 ms) latencies bilaterally in posterior parts of the secondary auditory areas, and 2) adaptive filtering of attended sounds from taskirrelevant background masker at longer latency (∼300 ms) in more medial regions, predominantly in the left hemisphere, enhancing processing of near-threshold sounds.