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Detecting the position of the gap in a Landolt C is adversely affected by black bars placed tangential to the C and at a certain distance from it. The maximum bar separation affording interaction is proportional to the minimum angle of resolution, even in cases of amblyopia where resolution is presumably not limited by optical spread of the image. It is suggested that this contour interaction is related to the size of the receptive field (and hence to the resolving capacity) associated with the retinal region used to fixate the target.
Flom et al. (Sun,) studied this question.