A task-irrelevant sound occurring synchronously with a visual stimulus arising from a different location can lead to increased activity in both the visual and auditory cortices when the visual stimulus is attended vs. unattended, reflecting the object-related spreading of visual attention across both space and modality (Busse et al., PNAS, 2005). Here, we asked if and how ERP brain-activity patterns for this multisensory ’spreading of attention’ would be modulated when visual attention is focused on a lateral vs. a central location, and how this would vary as a function of whether there was object-related conflict between the relevant visual and irrelevant auditory inputs.

In two separate EEG experiments, subjects’ attention was either focused on a central or to a lateral position while discriminating the visual component of congruent vs. incongruent letter-sound pairs. In the lateral task, subjects attended to one of two concurrent streams of visual letters presented to the left and right fields. In the central task, only a central letter stream was presented and was always fully attended. In both tasks, on some trials, a task-irrelevant letter sound was presented centrally. Using appropriate subtractions, we extracted the auditory ERPs to the task-irrelevant auditory stimulus for the congruent and incongruent combinations in the attended central and lateral conditions, as well as in the unattended lateral condition.

Reaction times (RT) to the visual stimuli were slower overall for the lateral task, but the incongruency RT effects were significantly larger, likely indicating increased influence of the task-irrelevant sound with increased task difficulty. In each task, for trials in which the visual stimulus was attended, the extracted auditory ERP showed a negative-polarity, frontally distributed, incongruency effect starting at ~200 ms, consistent with the synchronous incongruent letter sound acting as a distracter that captures attention. Beyond ~300ms, the lateral task showed an absolute increase in the extracted auditory-ERP negativity in the congruent condition as well, although still less than in the incongruent, thus resulting in an overall smaller difference of attended incongruent vs. attended congruent combinations at these longer latencies. The auditory combination with an unattended visual stimulus provided an additional control in the lateral task, showing that these effects occurred mainly when the visual stimuli were attended. One exception was a small negativity at ~500ms for congruent vs. pure visual in the lateral unattended condition, perhaps reflecting some automaticity of speech processing. In summary, the results suggest that if attention is focused laterally vs. centrally, a task-irrelevant but incongruent letter sound has increased influence on discrimination performance of attended visual letters, reflected neurally by modulation in the late, but not early, perception-related ERP activity.