Previously, we have shown that spatial attention to a visual stimulus
can spread across both space and modality to a synchronously presented
but task-irrelevant sound arising from a different location, reflected
by a late-onsetting, sustained, negative-polarity event-related
potential (ERP) wave over frontal-central scalp sites, probably
originating in part from the auditory cortices. Here we explore the
influence of cross-modal conflict on the amplitude and temporal dynamics
of this multisensory spreading-of-attention activity. Subjects attended
selectively to one of two concurrently presented lateral
visually-presented letter streams to perform a sequential comparison
task, while ignoring task-irrelevant, centrally presented spoken letters
that could occur synchronously with either the attended or unattended
lateral visual letters and could be either congruent or incongruent with
them. Extracted auditory ERPs revealed that, collapsed across
congruency conditions, attentional spreading across modalities started
at approximately 220 ms, replicating our earlier findings. The
interaction between attentional spreading and conflict occurred
beginning at approximately 300 ms, with attentional-spreading activity
being larger for incongruent trials. Thus, the increased processing of
an incongruent, task-irrelevant sound in a multisensory stimulation
appeared to occur some time after attention has spread from the attended
visual part to the ignored auditory part, presumably reflecting the
conflict detection and associated attentional capture requiring accrual
of some multisensory interaction processes at a higher-level semantic
processing stage.