Two-Stage Processing of Sounds during Attention Figure 6. Spatial extent of M100 activity and population-level frequency tuning. Auditory and peri-auditory cortex were divided into roughly equal-sized cortical patches to investigate possible areal differences in frequency tuning and attention effect. The attention effect did not show any significant within- or across-hemisphere asymmetry, but there was a hemispheric interaction effect suggesting asymmetry in the frequency tuning between left and right hemisphere. doi:10.1371/journal.pone.0046872.g006 tion (F18,234~1:735, p~0:049, e~0:81), thus tentatively suggest- ing a differential tuning curve as a function from the start of 2- minute block. This effect was in addition to the same effects of attention and masker reported earlier. There was a trend of stronger M100 amplitudes in the first segment within the 2-minute block, but this effect did not reach significance. This indication of general long-term N100/M100 habituation during the course of stimulus presentation [31] did not reveal anything novel about the time course of the observed attention effect. In fact, the magnitude of the attention effect seemed more or less constant within the block (Fig. 10A). The same partial-averaging analysis for the sustained responses (Fig. 10B), however, showed that the attention effect was dependent on the time from the two-minute block start. Initially, the attention effect was significant but not very clear in magnitude (0–30 s in Fig. 10B), then nearly vanished after this (30–60 s), and increased towards the end of the block (60–120 s in Fig. 10B). Due to this, ANOVA showed a significant ATTENTION|SEG- SEGMENT interaction (F3,39~3:94, p~0:017, e~0:95), in addition to previously reported masker and attention interaction effects. Further, a significant ATTENTION|HEMISPHERE effect was found (F1,13~10:82, p~0:0059), but this may be an artifact of poor signal-to-noise ratio from partial averaging, as selection of different overlapping segments (segments offset by +15 s) removed the effect. Discussion Recent studies have suggested that selective attention enhances both gain and feature selectivity of the human auditory cortex neurons [18,19]. Here, we show that M100 was modulated bilaterally in such a fashion that most robust attentional enhancements were observed during intermediate masker notch widths (Fig. 4). The longer-latency sustained response, occurring at 300–400 ms range after sound onset was, in turn, even more robustly enhanced by attention in the left hemisphere (Fig. 5B). This enhancement was most prominent with narrow notches within the critical band (i.e., with narrower notches than in case of M100). This finding tentatively suggests that there could be neuronal population-level receptive field tuning that facilitates perception of target sounds embedded in noise especially at the latency of the sustained response. The attentional modulation of the M100 appeared to occur right after switching of the auditory task, thus supporting the recent findings of Ahveninen et al. [7]. Furthermore, as a novel finding, the sustained response modulation was more dynamic as there was a dependency on the time elapsed from the beginning of the block (Fig. 10), suggesting that differential neural mechanisms underlie these effects. While there were no differences in left and right hemisphere response magnitudes, activity in the left auditory cortical areas exhibited more pronounced correlations with behavioral measures of performance (see Results and Fig. 9), thus PLOS ONE | www.plosone.org 7 October 2012 | Volume 7 | Issue 10 | e46872