Analysis of oscillations of the visual sensitivity

At the subjective threshold, repeated flash test stimuli are perceived by the observer on only 50% of occasions, since the detectability of a single light stimulus fluctuates with time. For the analysis of these fluctuations we have recorded sequences of positive ("seen") and negative ("not seen") responses, stimulating at 10 sec intervals for 50 min. These records, reflecting the time course of detectability, show periodic fluctuations. The frequencies of these fluctuations have been estimated by autocorrelation and Fourier analysis of the records. For 19 subjects they yielded spectra with preferential period durations between 0.5 and 2, respectively 5 and 15 min in 15 cases, and some spectra are noisy.