| Summary: | Against a plethora of visual input, visual attention filters and selects relevant information and disengages from irrelevant items. One possible mechanism to enable disengagement is reflected in inhibition of return (IOR), the finding that previously visited locations are responded to slower than locations that have not been investigated before. In line with the proposal that IOR reflects a mechanism to facilitate visual search, evidence suggests that IOR is coded in space-based (“spatiotopic”) coordinates, despite the largely retina-based (“retinotopic”) coding scheme of the visual system. For IOR to efficiently facilitate visual search it should be coded solely in spatiotopic coordinates, but recent investigations show retinotopic IOR effects alongside spatiotopic IOR. The putative function of IOR has also been challenged by the observation that the eyes return to previously visited locations more frequently than would be predicted based on chance. The presented program of research examines the factors that influence whether, and how, IOR is implemented in a way that would facilitate visual search. Firstly, it was examined whether it is an efferent signal about an upcoming eye movement or a more general prediction-based mechanism that maintains IOR in spatiotopic coordinates across eye movements. IOR was observed in both retinotopic and spatiotopic coordinates across eye movements, but was observed in a weakened form and only in location-based coordinates when objects, instead of the eyes, were moved. These results suggest that efferent signals about upcoming eye movements contribute to updating and maintaining IOR tags in useful locations when the eyes move. Secondly, the relative strength and robustness of retinotopic and spatiotopic IOR were examined; the relative frequency of cue-target pairings in retinotopic and spatiotopic references frames had no significant effect on the presence of retinotopic IOR, whereas practice with the experimental task strengthened spatiotopic IOR III and eliminated retinotopic IOR. Thirdly, spatiotopic IOR was observed to be more robust than retinotopic IOR for both saccadic and manual responses. Fourthly, for responses to targets appearing in the brief interval before the eye movement (< 150ms), IOR was observed in the future retinotopic location of the target, suggesting that IOR was remapped predictively. Finally, it was demonstrated that IOR is reduced for intermediate locations along pre-planned sequences of saccades. Taken together the findings of the presented series of research suggest that IOR is updated into spatiotopic coordinates across eye movements. Spatiotopic IOR involves the efferent signal of the eye movement and is updated predictively before the saccade, extending the notion that predictive remapping updates attentional pointers to updating of inhibitory effects. Retinotopic IOR was consistently weaker than spatiotopic IOR across all experiments, and was eliminated with practice, consistent with retinotopic IOR being an undesirable, but avoidable, consequence of inhibiting locations while moving the eyes. Finally, the reduction of IOR for intermediate locations along preplanned saccade sequences is consistent with the idea that the degree to which a location was attended can determine how inhibited that location subsequently becomes. It also could explain why refixations are commonly observed in free visual search, which would typically contain many such pre-planned sequences. Taken together, the findings are additional evidence that IOR reflects a mechanism that facilitates visual search under the conditions in which search normally occurs, that is, across overt eye movements and sequences of eye movements.
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