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Abstract:
In daily life, our auditory system detects and segregates sounds, derived from complex auditory scenes. Yet, limited processing capacities allow only a small subset of these sounds to enter awareness. This MEG study used informational masking to investigate the neural mechanisms that enable auditory awareness. On each trial, subjects indicated whether they detected a target that was embedded in a multi-tone background in 67% of the trials. Targets were defined as a pair of two 40 Hz amplitude-modulated tones, presented sequentially with a fixed SOA of 1050 ms. Hence, target detection required subjects to perceive both tones within a pair (Fig. 1B). We compared MEG activity for hits and misses separately for target tone 1 and 2 both in sensor and source space (Fig. 1A). Successful target detection was associated with changes in transient evoked source activity in bilateral auditory cortices at 3 stages:
(1) an enhanced M50 component for tone 1,(2)a negative component at ∼ 150ms for tone 2 and (3) a later, long-latency negativity for both tone 1 and 2 at ∼ 300ms (Fig. 1C). Moreover, subjects’ perceptual sensitivity (d) positively correlated with the magnitude of the M150 component. In addition, we investigated whether steady-state activity was modulated by awareness. Indeed, even though all target tones elicited 40 Hz steady-state responses, the amplitude of 40Hz activity was significantly enhanced when subjects became aware of tone 1 and 2.
In conclusion, our results suggest that awareness of a two-tone pair relies on a cascade of processes that segregate this pair from a complex auditory scene. (1) The processing of detected tones is enhanced as indicated by an increased M50 and steady-state response. (2) The sequential integration of the target pair after the 2nd tone then elicits an awareness related negativity at ∼ 150ms.
(3) Finally, aware signals may elicit additional attentional processes, which may be reflected in the enhanced long-latency negativity.
