A sensitization effect with rectilinear stimuli

Psychophysical end-stopping associated with line targets

Increment threshold for a small (e.g. 1' x 5') line target superimposed on backgrounds of various shapes and sizes was measured to provide a detailed map of the spatial interactions about line targets. This modified "Westheimer paradigm" indicated sensitization in the length direction as well as in the width direction around the line target. The effect of the adaptation field summed over an elongated, end-tapered central region, and showed strong end-zone antagonism beyond the ends of the elongated summation area, as well as flank antagonism to the sides. Secondary disinhibitory and inhibitory areas outside of the antagonistic surround were also demonstrated. When length of the test line was varied, the length of the summation region increased concomitantly, while the length of the end-zones remained fixed. End-zone antagonism was slightly weaker at oblique orientations. These results demonstrate a perceptual analog to neurophysiological end-stopping, and suggest a multilobed y-dimension weighting profile appropriate for models of spatial visual abilities.

Areas of spatial interaction for a hyperacuity stimulus

We report that the ability to detect a small vernier offset (less than 5 sec of arc in many individuals) between two small spots of light separated by a narrow gap can be disrupted by presenting additional targets in close proximity to the vernier stimulus. A rectangular background of light, centered on the vernier stimulus, elevates offset threshold by a factor of two when the total width of the background is 3-4 min arc. Backgrounds narrower or wider than 3-4 min have less or no effect on vernier threshold. These areas of spatial interaction extend to either side of a vertically-oriented vernier stimulus, or above and below a horizontally-oriented stimulus, and their extent is dependent upon the gap size of the two-dot stimulus. The effect of the presence of the background on vernier threshold cannot be accounted for by spatial interval cues nor by changes in the visibility of the stimulus. Two alternative interpretations of the results are presented to develop a description of the underlying mechanisms which produce hyperacuity spatial interactions.