Photoreceptor coupling and boundary detection.

Electrical coupling between photoreceptors results in the extensive spreading of output potentials along the syncytium of photoreceptor terminals. This smoothing of output potentials seems to make spatial resolution worse. However, the photoreceptor noise that is considered to be non-correlated both in space and time is smoothed to the greater extent than the correlated potential difference across the boundaries between areas of different brightness. This improves the signal-to-noise ratio more for more extended boundaries and favours lowering the threshold so that they can be detected more easily during the subsequent processing. The results have a striking parallel with a well known dependence of contrast threshold on stimulus size as measured psychophysically.