Attentional modulation of neural responses to illusory shapes: evidence from steady-state and evoked visual potentials.

Our perception of illusory shapes, such as the classic Kanizsa triangle, is thought to reflect the visual system's capacity to fill in missing information associated with fragmented or partially occluded objects. Previous work has suggested that such 'modal' filling-in arises at relatively early stages of visual processing, prior to the allocation of focused attention, but few studies have examined whether attention influences neural responses to illusory shapes. Here we asked whether spatial attention affects neural activity evoked by illusory shapes by having participants focus on stimuli presented in one visual field (left or right), while ignoring those on the other side. In two separate experiments, we used electroencephalography (EEG) to measure steady-state visual evoked potentials (SSVEPs; Experiment 1) and event-related potentials (ERPs; Experiment 2), elicited by a Kanizsa square in one hemifield and a competing control stimulus, matched for all relevant physical features, in the other. In Experiment 1, the inducers used to construct the Kanizsa and control stimuli flickered at unique frequencies (10 and 13.33 Hz) to elicit SSVEPs. In Experiment 2, segments were removed briefly from static circular placeholders within the left and right visual hemifields to induce ERPs. In both experiments, participants were instructed to attend covertly to the stimulus on the left or right side to detect brief contrast changes of the inducers. SSVEP analyses revealed that illusory shapes that were ignored yielded a significantly larger neural response than those elicited by non-illusory control stimuli. By contrast, when the stimuli were attended, neural responses to the illusory and control stimuli were statistically indistinguishable. Consistent with these steady-state findings, results of the ERP analyses revealed that evoked responses to illusory shapes emerged earlier relative to control stimuli when they were ignored than when they were attended. Taken together the findings suggest that neural activity associated with modal completion is reliably modulated by spatial attention. Specifically, we show that neural responses to actively ignored illusory shapes are elicited earlier and are sustained at a higher amplitude than responses to control stimuli. This advantage for ignored shapes might reflect adaptive signalling of the presence of an object available for future goal-directed behaviour.