Linking time-on-task, spatial bias and hemispheric activation asymmetry: a neural correlate of rightward attention drift.

Biases of spatial attention may be moderated by non-spatial factors such as attentional load and time-on-task. Although these effects are thought to arise from depletion of right hemisphere processing resources, their neurophysiological bases have yet to be confirmed. We recorded posterior α-band EEG--a marker of cortical excitability linked to spatial attention orienting--from 66 non-clinical participants who detected transient, unilateral visual targets while also monitoring stimuli at fixation. Asymmetry indices were derived for both lateral target reaction times and hemispheric differences in α-activity before and after lateral target onsets. Pre-target α became more prominent over the right, relative to left, hemisphere as the task progressed over 48-min, and this change was correlated with a significant rightward shift in spatial bias. Contrary to past studies of posterior α-asymmetry and orienting, here participants did not receive pre-target cues. Thus we show that asymmetries in the hemispheric distribution of anticipatory α are not only apparent during externally-cued attention orienting, but are also sensitive to decreasing alertness over time. These data are the first to link rightward attention drift over time with change in hemispheric activation asymmetry, providing important implications for our understanding of interacting spatial attention and non-spatial alertness networks.