Psychophysical evidence for contraction of the range of spatial integration as a mechanism for filtering out spatial noise in a random dot motion display.

Human judgment is frequently impaired by distracters extending across our field of view. How we extract relevant information from a spatially restricted region in a complex scene in spite of this impairment is an important issue in vision. Recently, it has been shown that this impairment can be reduced by increasing the number of surrounding distracters without changing the density, thus increasing the total area covered by the distracters. Little, however, is known regarding the underlying mechanism(s). Here, we tested the hypothesis that visual impairment by distracters is due to integration of irrelevant information across space, and that further addition of distracters produces contraction of the spatial integration field. Human subjects were instructed to judge the direction of motion within a center disk and to ignore motion noise in the surrounding annulus in a random dot kinematogram. We observed a non-monotonic effect of the size of the annulus, in which the subjects' discrimination thresholds at first increased, and then decreased as the size of the annulus became larger. We further investigated how weak coherent motion in the surrounding annulus interferes with the subjects' performance. Importantly, we found that the amount of interference decreases with the addition of surrounding motion noise, consistent with the hypothesis that the addition of distracters produces contraction of the range of spatial integration. Our results suggest that integration within a visual receptive field causes impairment by distracters across our visual field, and that contraction of the range of integration can counteract this impairment.