Feature-based integration of orientation signals in visual search.
We have measured orientation discrimination in the presence of a variable number of neutral distracters for two distinct tasks: identification of the orientation of a tilted target and location of its position. Both tasks were performed in the presence of visual noise of variable contrasts. Under a range of conditions, subjects could identify the direction of target tilt at thresholds well below those necessary to locate its position. The location thresholds showed only weak dependency on set-size, consistent with a stimulus uncertainty of parallel search of the output of independent orientation analysers, while the identification thresholds showed a much stronger dependency, varying with the square root of set-size over a wide range noise contrasts. The square root relationship suggests perceptual summation of target 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.
Baldassi
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Istituto di Neurofisiologia del CNR, Via S. Zeno 51, Pisa, Italy.
Burr
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DC
D000328:Adult; D001288:Attention / Q000502:physiology*; D003463:Cues; D004192:Discrimination, Psychological / Q000502:physiology; D006801:Humans; D008960:Models, Psychological; D012684:Sensory Thresholds / Q000502:physiology; D014796:Visual Perception / Q000502:physiology*