PHOTORECEPTOR COUPLING AND BOUNDARY DETECTION

Electrical coupling between photoreceptors results in the extensive sp reading of output potentials along the syncytium of photoreceptor term inals. This smoothing of output potentials seems to make spatial resol ution worse. However, the photoreceptor noise that is considered to be non-correlated both in space and rime is smoothed to the greater exte nt than the correlated potential difference across the boundaries betw een areas of different brightness. This improves the signal-to-noise r atio more for more extended boundaries and favours lowering the thresh old so that they can be detected more easily during the subsequent pro cessing. The results have a striking parallel with a well known depend ence of contrast threshold on stimulus size as measured psychophysical ly. (C) 1998 Elsevier Science Ltd. All rights reserved.