Temporal whitening: transient noise perceptually equalizes the 1/f temporal amplitude spectrum.

Naturally occurring luminance distributions are approximately 1/f in their spatial and temporal amplitude spectra. By systematically varying the spatio-temporal profile of broadband noise stimuli, we demonstrate that humans invariably overestimate the proportion of high spatial and temporal frequency energy. Critically, we find that that the strength of this bias is of a magnitude that predicts a perceptually equalized response to the spatio-temporal fall off in the natural amplitude spectrum. This interpretation is supported by our finding that the magnitude of this transient response bias, while evident across a broad range of narrowband spatial frequencies (0.25-8 cycles/deg), decreases above 2 cycles/deg, which itself compensates for the increase in temporal frequency energy previously observed at high spatial frequencies as a consequence of small fixational eye movements (M. Rucci, R. Iovin, M. Poletti, & F. Santini, 2007). Additional temporal masking and adaptation experiments reveal a transiently biased asymmetry. Whereas temporal frequencies >4 Hz mask and adapt 1- and 15-Hz targets, lower masking and adaptation frequencies have much less effect on sensitivity to 15-Hz compared with 1-Hz targets. These results imply that the visual system over-represents its transient input to an extent that predicts an equalized temporal channel response to the low-frequency-biased structure of natural scenes.