Mechanisms responsible for the assessment of visual location: theory and evidence.

Vernier acuity demonstrates a fine sense of relative visual location, which can be exploited to examine the question of mechanisms for the extraction of position information. By determining the appearance and location of composite stimuli consisting of a number of unresolved parallel bars of different luminances, we have demonstrated that vernier acuities of the order of 5 sec arc are due to mechanisms with an inflexible manner of encoding local intensity changes in the retinal image. The results are compared and contrasted with current theoretical approaches to the problems of early visual coding, and are best described by a model which encodes only the occurrence and location of zero-crossings in the second derivative of the retinal light distribution.