Background: Colour is known to facilitate visual search although its role as a determinant of target conspicuity is not so clear. People with abnormal colour vision have problems recognising and differentiating colours but the extent to which they may be at a disadvantage in visual search when redundant colour is a target attribute has not been investigated previously.
Method: Six colour normal and 29 colour deficient subjects (7 P, 6 D, 7 PA, 9 DA) were instructed to find a diamond-shaped target embedded in a background of 138 differently shaped distracters and to report its position as quickly as possible. The target was redundantly colour-coded red, yellow, green, blue or white and the distracters had the same colours in various combinations. The target was uniquely colour-coded in some presentations, partially redundant in others and in some presentations the target and distracters had the same colour. In experiment 1 the displays were presented for 20 seconds and search time was measured; in experiment 2 the exposure time was 500 ms and the proportion of targets located correctly was taken as a measure of the target's conspicuity.
Results: Unique colour coding reduced search times significantly and greatly enhanced conspicuity but had no benefit when some distracters had the same colour as the target. Observers with colour vision deficiency had longer search times and the coloured targets were less conspicuous to them compared with colour normal observers. However, unique colour coding assisted their search, especially when the targets were red, blue or white.
Conclusions: Observers with colour vision deficiency are less efficient than colour normal observers at visual search when the target is marked out by colour and coloured targets are less conspicuous for them.