Two-Stage Processing of Sounds during Attention Figure 10. Time course of the attention effect. (A) M100 and (B) sustained response amplitudes (+SEM) as a function of time from condition start, averaged across maskers for clarity. The M100 attention effect does not show any dynamics, but the sustained response shows a significant interaction effect with attention and time range from the condition start. Significant differences between AttAud and AttVis are indicated by asterisks between the curves, and significant differences within condition between time ranges are marked either above or below the curves (for AttAud and AttVis, respectively, * pv0:05, ** pv0:01, *** pv0:001). doi:10.1371/journal.pone.0046872.g010 later-latency suppression of non-relevant sounds [10,11]. The dynamics of the attentional effect of both the M100 and sustained response being highest at small and intermediate notches (see Figs. 4 and 5) can be related to lateral inhibition [62] suppressing neural populations differently when attention is focused to sounds compared to baseline (see also [18]). Task difficulty variability (Fig. 8) with notches below +200 Hz can play a role in the presently observed sustained response modulation (Fig. 5), as previous studies have shown that there is an interaction between task difficulty and magnitude of the inhibitory effect [10]. In addition, differences in the temporal dynamics of these two effects (Fig. 10) suggest that attentional modulation of M100 and sustained response might indeed have different underlying mechanisms. In this study, we recorded population-level MEG responses, which are generated mostly by post-synaptic current summation when thousands of neurons accumulate post-synaptic potentials in synchrony [54]. Thus, caution must be exercised when speculating about single-neuron level mechanisms. Our results show similar tendencies of attentional modulation that animal models have shown at the level of single-neuron receptive fields [63–65], where enhancements at the target frequency during task performance are often accompanied by suppression at nearby frequencies, thus increasing the detectability of the target sound. Notably, in these studies, the receptive fields have been quantified by correlating neuronal spiking with spectrotemporal properties of specific type of ongoing background auditory stimulation; thus in these animal models the observed attentional tuning effects reflect more changes in sustained rather than onset responses. The critical band or equivalent rectangular bandwidth (ERB) at 1000 Hz has been estimated to be 162 Hz [66,67]. Here, the range of which M100 amplitude was modulated by notch width was +150–200 Hz (Fig. 4A), and our behavioral data suggests an ERB value of between +200–300 Hz (Fig. 8) where d’ in AttAud condition reaches its plateau. These are well in line with the classical ERB estimate and recent MEG studies [68,69], and slightly lower than ERB of 248 Hz that was estimated in a previous MEG study using a highly similar masking paradigm [70]. Interestingly, the range of notches where attention modu- lated the M100 responses was partly beyond the critical band, whereas the sustained response amplitude modulation showed strongest effects within the classical ERB, with notches below +200 Hz (Fig. 5). In the present study, we contrasted data from two conditions with identical stimuli, and attention was directed to either sounds or visual flankers. Because we omitted passive condition due to problems in controlling the arousal of the subject and in order to keep the experimental time within reasonable limits, it could be argued that the differences between conditions arose from suppression of auditory-evoked response due to visual task processing. However, recent studies have shown that a concurrent visual task with similar difficulty level, irrelevant to sound PLOS ONE | www.plosone.org 10 October 2012 | Volume 7 | Issue 10 | e46872