Feature-based integration of orientation signals in visual search

We have measured orientation discrimination in the presence of a variable n umber of neutral distracters for two distinct tasks: identification of the orientation of a tilted target and location of its position. Both tasks wer e performed in the presence of visual noise of variable contrasts. Under a range of conditions, subjects could identify the direction of target tilt a t thresholds well below those necessary to locate its position. The locatio n thresholds showed only weak dependency on set-size, consistent with a sti mulus uncertainty of parallel search of the output of independent orientati on analysers, while the identification thresholds showed a much stronger de pendency, varying with the square root of set-size over a wide range noise contrasts. The square root relationship suggests perceptual summation of ta rget and distracters. Manipulating the spread of visual noise suggests that the summation is feature-based, possibly operating on the outputs of first -stage orientation analysers. Pre-cueing the target eliminates the effects of set-size, showing that the summation is under rapid attentional control; the visual system can choose between high performance over a limited area and poorer performance over a much larger area. (C) 2000 Elsevier Science L td. All rights reserved.