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Vision Res. 2003 Jun;43(14):1575-87.

How many positions can we perceptually encode, one or many?

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McGill Vision Research, Department of Ophthalmology, McGill University, 687 Pine Avenue West (H4-14), Montreal, Que. H3A 1A1, Canada.


Here we show that our sensitivity for discriminating relative position across the visual field is limited. In experiment 1 we show that we are much worse at detecting a texture defined by the relative position of elements within an array than would be expected if we had access to multiple estimates of relative position across the visual field. In experiment 2 we show that human performance is impaired for positional judgments when there is uncertainty as to which of a number of possible elements is misaligned. This impairment is greater than one would expect from an ideal observer model and greater than that found for a comparable task involving orientation. It is consistent with positional thresholds being determined by only one estimate of relative position. In experiment 3 we estimate the number of suprathreshold positional signals that can be pooled at the same time across the visual field using a standard summation variance paradigm. The results suggest that the human visual system is limited to one estimate of position, but additional estimates can be built up serially over time; however, this process is slow and probably cognitive in nature. These experiments taken as a whole suggest that only one estimate of relative position (i.e. relative to a predefined reference) at a time is accessible at the perceptual level.

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