Feature processing asymmetry in a colour and orientation conjunction-search task

Abstract

Distinctive visual cortical areas process specific visual features of objects. Does this imply that individual features are also processed independently? To investigate this, visual-search performance for individual features was compared with performance for these same features in a conjunction-search task. Subjects searched for a target among 12 distractors while their eyes were tracked. Accuracy and latency of the initial saccade in a trial were measured. In a feature-search task, colour contrast and orientation difference for 70% correct discrimination performance was determined for each individual subject. Next, with stimuli set to these individually determined threshold levels, search performance was measured for each feature separately as well as during a conjunction task. Colour-discrimination performance was slightly better in conjunction search than in feature search. In contrast, performance on orientation discrimination dropped dramatically in conjunction search compared to feature search. Importantly, in conjunction search, saccadic latency in correct colour and correct orientation trials was approximately equal, ruling out a speed - accuracy trade-off explanation for this finding. We conclude that, when information needs to be combined to perform a task, processing of one feature can be dependent on the processing of another one, resulting in asymmetric performance.