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1. Using the threshold for a small, brief test flash as a measure, the effect of light stimulation of small adjacent retinal regions of the human retina was investigated during adaptation to a medium scotopic illuminance.

2. Within about ° of the tested area, light causes a desensitization gradually decreasing with distance. Beyond that there is a sensitizing zone which in turn gives way to an outer desensitizing zone. The sign of these effects is reversed if the peripheral stimulus is a patch of darkness.

3. Evidence for non-linearity of summation of individual patches was found. Occlusion of each other's sensitizing influence is observed if two regions are closely adjacent to each other.