Nonindependence of successive responses in measurements of the visual threshold.

Abstract

This expt. was performed to determine whether successive responses given in visual threshold measurements are statistically independent of one another, an assumption implicit in several theories of sensory thresholds. 16 subjects were presented, on 4 successive days, with 300 invariant stimuli of that brightness found previously to be seen 50% of the time. On 2 days these stimuli were presented automatically at 5-sec. intervals; on the other 2, the subject presented them to himself. Critical ratios of serial correlation were computed on the time-ordered sequences of responses at lags 1, 3, 5, 7, 9, 11, and 20. The mean 50% absolute visual threshold for each subject elicited, on the avg., 50% frequencies of responses on immediately subsequent days. The subject''s response to the presentation of a discrete, dim flash of light depended not only on the brightness of the stimulus, but also, among other things, on his response to preceding flashes of the same brightness. Each response is dependent upon previous responses, or perhaps both are dependent on a 3d variable which varies in time. This nonindependence is exhibited with statistical significance when as many as 10 responses intervene between the correlated responses. The value of CRr seems to be a monotonically decreasing function of the lag. No statistically significant differences appeared between the data obtained under the 2 conditions of stimulation.