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.