The Effect of Attentional Load on Auditory Cortical Representation of Temporal Edges in Sound

The emergence of an auditory object from a background is signalled by the existence of temporal edges, or transitions, in the properties of the ongoing input to the ears. Often, the background and the object possess substantial non-stationary statistics, and the task is therefore to detect a transition in the pattern of ongoing statistics. In certain cases, edges are manifest as a violation of a previously-acquired representation of the scene (eviolation of regularityi - VR edges), e.g. when an auditory object, against some background, disappears or changes its properties. In other cases, the transition that the system must detect is the emergence of regularity, or eorderi, from disorder (eemergence of regularityi - ER edges). We have recently demonstrated (Chait et al, 2008) that auditory cortical responses to VR and ER edges exhibit different temporal dynamics and that they recruit distinct neural substrates. Sensitivity to temporal edges plays a key role in auditory scene analysis and an important issue is whether this sensitivity is affected by the attentional load of the listener. To address this question, we use MEG to measure early auditory cortical responses to transitions between constant-frequency and random-frequency tone-pip sequences (VR edges) and vice versa (ER edges) while manipulating listenersi attentional load. Simultaneously with the eauditory edgei stimuli, listeners were presented with streams of rapidly-alternating auditory and visual signals and had to perform high- and low- attentional load tasks on these streams. The data reveal that auditory attentional load does not affect the cortical representation of VR edges, but significantly reduces responses to ER edges. Visual attentional load in the current experiment had no effect on either transition response. These results suggest that, under high attentional load, the representation of certain kinds of sound events in the environment is impaired while that of others is unaffected.