Judging what came first

You can’t possibly process everything that’s going on around you. Instead you’re armed with an attentional spotlight that selects areas and objects of interest for preferential processing. An anomalous consequence of this, is that we judge objects appearing where our attention is focused to have appeared earlier than objects located elsewhere, even if they really appeared at the same time. It could be that information arising from the attended object is transmitted through the brain’s sensory pathways more quickly than information arising from the unattended object, thus resulting in the perception that the attended stimulus occurred first. But now, using electroencephalography (EEG) to record electrical activity in the brain, John McDonald and colleagues refute this suggestion, arguing instead that selective attention increases the size of the neural signal arising from the attended object, not its speed.

Twenty-eight participants sat through hundreds of trials in which an irrelevant sound first grabbed their attention to the left or right, before two lights flashed up, one on their left, the other on their right. Consistent with past research, the light that flashed up on the same side as the sound (where the participants’ attention had been drawn to), was perceived to have occurred earlier than the other light, even if they had both appeared at the same time. However, EEG recordings of the brain’s electrical activity showed that although the light on the same side as the sound was perceived to have occurred earlier, it wasn’t processed more quickly in the brain. It was, however, associated with a larger neural signal (over the contra-lateral ventral occipital cortex), than the unattended light.

“…attention-induced effects on time-order perception may arise from changes in the strength of neural signals in ventral occipital areas that underlie visual object perception”, the authors said. “It follows from this hypothesis that attention-induced enhancements in signal strength that occur at early stages of visual processing are interpreted as a timing difference by a later comparator mechanism”.
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McDonald, J.J., Teder-Salejarvi, W.A., Russo, F.D. & Hillyard, S.A. (2005). Neural basis of auditory-induced shifts in visual time-order perception. Nature Neuroscience, Advance Online Publication: DOI: 10.1038/nn1512.

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