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Eli Brenner, Alex Holcombe; Time to contact does not pop out. Journal of Vision 2011;11(11):1213. doi: 10.1167/11.11.1213.
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In visual search, items differing markedly from the others in a basic visual feature are quickly localized, irrespective of the number of distracters. Is time to contact such a basic visual feature? This question cannot be answered with conventional search tasks because time to contact necessarily changes continuously. We therefore developed two alternative tasks in which items converged towards a single point. In the first, before reaching the point, all items disappeared simultaneously. Participants indicated which they thought would have reached the point first. The items were assigned random speeds, with initial distance set so all except the target would have reached the point at the same time. The number of items strongly influenced the difference in time to contact required for the target to be picked reliably. Performance was only slightly better than if participants had simply picked the item that was nearest to the point when the items disappeared. On half the trials of the second experiment one item had a shorter time to contact than the others and on the other half all items had the same time to contact. Participants indicated as quickly as possible whether all items would arrive at the same time. Reaction time hardly depended on the number of items, but on average participants did not respond until the target's time to contact was less than half that of the other items. For trials with no target, they usually did not respond until when a target would have arrived. The results were similar for simulated motion towards the participant. When the speed heterogeneity was eliminated to make proximity a reliable cue, search was much faster. Apparently having a shorter time to contact does not make an item easy to detect. How then do we cross a busy intersection or negotiate a busy plaza?
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