Independent coding across spatial scales in moving fractal images

We compared observers' ability to discriminate the direction of apparent motion using images which varied in their spatial characteristic; white or flat spectrum noise, and 1/f noise which has an amplitude spectrum characteristic of natural scenes. The upper spatial limit for discrimination (dmax) was measured using a two-flash random dot kinematogram (RDK), which consisted either of a pair of bandpass filtered images or of a bandpass filtered image and its broadband counterpart. Six bandpass central frequencies were used, ranging from 0.25 to 5.66 cyc/deg. Subjects could perform the direction discrimination task for all six central frequencies in both the bandpass-bandpass and bandpass-broadband sequences for the 1/f images, and dmax values were found to be approximately equal in these two conditions at all spatial scales. However, for the white noise images, direction discrimination was not possible at the lowest central frequencies in the bandpass-broadband task. These data show that information from a wide range of spatial scales is equally salient to the human motion system in images whose amplitude spectra fall as 1/f. However, for white noise images, information at the higher spatial frequencies is more salient and dominates performance in the direction discrimination task. These results are consistent with a model in which spatial frequency filters in the input lines of motion detectors have octave constant spatial frequency bandwidths and equal peak sensitivity. In line with a number of recent studies, this suggests that the spatial properties of motion sensitive cells are matched to the statistical properties of natural scenes.