Previous research on attentional selection of features has yielded seemingly contradictory results: many experiments have found a ‘global’ facilitation of attended features across the entire visual field, whereas classic event related potential (ERP) studies reported an enhancement of attended features at the attended location only. To test the hypothesis that these conflicting results can be explained by temporal stimulus differences, we compared the time-course of feature-selective attention inside and outside the spatial focus of attention. We presented fields of randomly moving purple dots on either side of fixation. Participants were audio-visually cued to attend to either red or blue dots on either the left or right side in order to detect brief coherent motion targets. After a delay, which allowed participants sufficient time to shift attention to the cued location, the purple dots on both sides changed color simultaneously so that half of them became blue and the other half red. Each of these four dot populations flickered at a different frequency, thereby eliciting distinguishable steady-state visual evoked potentials (SSVEPs). This allowed us to concurrently measure the time-course of feature-selective attentional enhancement of stimulus processing in visual cortex after onset of the attended feature on both the attended and the unattended side. The onset of feature-selective attention on the attended side occurred over 100 ms earlier than on the unattended side. The finding that feature-selective attention is not spatially global from the outset, but that its effect spreads to unattended locations with a temporal delay resolves previous contradictions between studies that found global selection of features and studies that failed to find such global selection because they used briefly flashed stimuli. We speculate that the observed delay might be caused by the time needed to coordinate attentional control signals between hemispheres, although the exact mechanisms are still unknown.

Meeting abstract presented at VSS 2014