The lower visual search efficiency for conjunctions is due to noise and not serial attentional processing

Models of human visual processing start with an initial stage with parallel independent processing of different physical attributes or features (e.g., color; orientation, motion). A second stage in these models is a temporall y serial mechanism (visual attention) that combines or binds information ac ross feature dimensions. Evidence for this serial mechanism is based on exp erimental results for visual search. I conducted a study of visual search a ccuracy that carefully controlled for low-level effects: physical similarit y of target and distractor; element eccentricity, and eye movements. The la rger set-size effects in visual search accuracy for briefly flashed conjunc tion displays, compared with feature displays, are quantitatively predicted by a simple model in which each feature dimension is processed independent ly with inherent neural noise and information is combined linearly across f eature dimensions. The data are not predicted by a temporally serial mechan ism or by a hybrid model with temporally serial and noisy processing. The r esults do not support the idea that a temporally serial mechanism, visual a ttention, binds information across feature dimensions and show that the con junction-feature dichotomy is due to the noisy independent processing of fe atures in the human visual system.