Sensitization by annular surrounds: the effect of test stimulus size

Studies of scotopic spatial sensitization have shown that the threshold for a small test probe is influenced by the diameter of the background disk against which it is presented. The present study indicates that the diameter of the test probe is also important. As the diameter of the probe is increased from 4.8′ to 2°, sensitization can still be demonstrated. But with increasing test probe size, the magnitude of sensitization decreases and the peak of the sensitization function shifts to a larger diameter of background disk, such that with probes larger than about 40′, the background producing the highest threshold tends to be the same size as the probe. Several possible explanations for the data are considered, including the hypothesis that edge interactions determine thresholds and the alternative suggestion that thresholds are set by size-tuned channels.