Each stimulus was presented at one of six fixed brightnesses, five of them differing in steps of approximately 0.14 log units, and the sixth was zero. The values remained unchanged over the eight experimental days. The brightnesses of successive stimuli were presented in random order, but with the restriction that each one followed itself and each other brightness 10 times in a full day's session. The same random series was used each day. Each step represents 0.14 log units.
All functions are straight lines visually fitted to the experimental points.
P R as a function of Stimulus Brightness Alone Absolute threshold functions for each of the four S s, determined without regard to the preceding response or stimulus, are shown in Figure 1. Figure 1 Absolute threshold functions for each S . Each point represents the mean of eight values, one for each of the experimental days. The curves for successive subjects are shifted two steps to the right. By sign test this result would occur far less than 1 per cent of the time if the preceding response bore no systematic relationship to frequency of response.