Two-Stage Processing of Sounds during Attention Figure 9. Correlations between behavioral data and MNE activity. The correlations were calculated for each vertex at the M100 peak latency for the per-subject data. The correlations shown on the overlaid cortex are thresholded at pv0:01. Scatterplots show the mean amplitude of the selected ROI vs. the behavioral measure (across subjects and stimuli). The detectability index d’ correlated with the vertices showing high M100 amplitude. Reaction time (RT) had a negative correlation in both hemispheres during AttAud condition, whereas significant RT correlations during AttVis condition were probably at least partly artificial due to uneven distribution of data. doi:10.1371/journal.pone.0046872.g009 Our current results are in line with earlier studies showing attentional modulation of transient 100-ms onset responses with concurrent masking paradigm [7,18,19,32–35]. Naturally, we cannot exclude a possible earlier-latency effect, since attentional modulation can be seen at brainstem [36,37], and even lower level (see [12]). We could not, however, reliably quantify the 50-ms response due to poor signal-to-noise with the narrowest notches (see Fig. 2B). Similar to a recent MEG study [35], we failed to observe a significant masker|attention interaction in M100 amplitude, however, given that the significant effects were observed for intermediate notch widths it is possible that the attentional enhancement of M100 reflected a combination of gain and receptive field tuning effects in the present study (see Fig. 4A). Further, an interaction with masker, attention, and hemisphere was evident in the sustained response (Fig. 5), which was significantly modulated especially in the left hemisphere, showing most prominent attention effects with narrow notches (+50 Hz– +200 Hz), but not with the white noise masker. This possibly reflects different feature specificity [38] and response properties [39–41] of distinct M100 and sustained response generators. The sustained response [24,42] or post-M100 response increase in sound-feature specificity [43] and enhancement during selective attention have been shown before [44–46]. To our knowledge this is the first notched-noise masking paradigm study to report significant masker dependent changes in the sustained response magnitude. This long-latency evoked response has sometimes been referred to as processing negativity (PN) [47] and can vary for instance as a function of task difficulty [48]. However, processing negativity or similar separate process cannot alone explain the current sustained response results unless it is defined to possess feature specificity besides task demand relations. Further, there is evidence from intracranial studies in humans that support the notion that N100/M100 and longer-latency components are directly enhanced during attention [49,50] instead of supplemental attention-induced activity explaining the response enhancement. Our results of auditory-evoked M100 and sustained response modulation during a high-load task suggest that selective attention enhances activity in the secondary auditory areas, close to sites where the M100 and sustained responses are localized. M100 is primarily generated posterior to the primary auditory cortex in HG, in posterior supratemporal plane [51,52]. Unlike in our previous EEG study [18], we failed to observe attentional enhancement of M100 with the no-masker sound. As a potential explanation for this, there are auditory selective attention studies where MEG and EEG have been recorded simultaneously, and in such studies MEG has been noted to be less sensitive to M100 enhancement during selective attention than EEG [53], which could be due to radial sources (that MEG is insensitive to [54,55]) contributing to the N100 attentional enhancement recorded with EEG. Thus, EEG and MEG results can differ from each other, even if recorded simultaneously [56]. In the present study, the attentional enhancement of sound feature selectivity was more clear for the sustained than the M100 response. N100/M100 response is related to sound onset or any transient change in auditory environment [57–60], whereas later components can be more sensitive to bottom-up sound features and top-down modulation [61]. This is possibly related to differential mechanisms in initial enhancement of relevant and PLOS ONE | www.plosone.org 9 October 2012 | Volume 7 | Issue 10 | e46872