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 thes
e fluctuations we have recorded sequences of positive (''seen'') and n
egative (''not seen'') responses, stimulating at 10 sec intervals for
50 min. These records, reflecting the time course of detectability, sh
ow periodic fluctuations. The frequencies of these fluctuations have b
een estimated by autocorrelation and Fourier analysis of the records.
For 19 subjects they yielded spectra with preferential period duration
s between 0.5 and 2, respectively 5 and 15 min in 15 cases, and some s
pectra are noisy. (C) 1997 Elsevier Science Ltd.