1. Two previous papers (Hallett, 1969a, b) were concerned with the visual threshold appropriate to a peripheral retinal point and in particular with the rapid changes in point threshold produced by illuminating the dark-adapted eye with large brief duration backgrounds. This paper extends this approach to testing signals of larger subtense.2. Spatial integration for the rods can rapidly diminish and recover within a few hundred milliseconds.3. A brief duration background which raises the threshold at a retinal point as much as some steady background is more effective than the steady background in reducing the extent of spatial integration.4. Hysteresis can be substantial; the extent of spatial integration varies, even for a given point threshold, according to whether the threshold is rising or falling, or temporarily or permanently steady.5. A possible type of non-linear model is discussed in the next paper (Hallett, 1971).
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