Effects of feature pre-cueing in the conjunction search of colour and orientation

While we search for objects in our environment, we often have to combine information from multiple visual modalities such as colour or orientation. Previously, we (Hannus et al, 2004 Perception 33 Supplement, 13) found that despite matching the difficulty of colour and orientation discriminability, in conjunction search, subjects' first saccades went much more often to the correct colour than to the correct orientation. Thus, accuracy of orientation discrimination was found to be contingent on whether or not colour discrimination is required as well, suggesting that features are processed conjunctively, rather than independently. Here, we investigated this same issue by examining the effect of pre-cueing individual features on performance. We asked subjects to search for combinations of colour and orientation while their eye movements were recorded. We manipulated the temporal dissociation of feature processing in conjunction search: information about either colour or orientation was presented either 0, 20, 40, 80, 160, 320, or 640 ms before the other feature. Initial saccades were tracked to determine target selection. Pre-cueing of colour improved target-detection accuracy, whereas orientation pre-cueing did not. For the individual features, colour pre-cueing significantly improved colour-discrimination performance compared with orientation-discrimination performance. Pre-cueing orientation information had no effect on feature-discrimination performance. Current results suggest the existence of an asymmetry in the extent to which pre-cueing features can affect conjunction-search performance. Our findings are consistent with the idea that colour and orientation are not processed fully independently in conjunction search