INDEPENDENT CODING ACROSS SPATIAL SCALES IN MOVING FRACTAL IMAGES

We compared observers' ability to discriminate the direction of appare nt motion using images which varied in their spatial characteristics; white or flat spectrum noise, and lif noise which has an amplitude spe ctrum characteristic of natural scenes. The upper spatial limit for di scrimination (d(max)) was measured using a two-flash random dot kinema togram (RDK), which consisted either of a pair of bandpass filtered im ages or of a bandpass filtered image and its broadband counterpart. Si x bandpass central frequencies were used, ranging from 0.25 to 5.66 cy c/deg. Subjects could perform the direction discrimination task for al l six central frequencies in both the bandpass-bandpass and bandpass-b roadband sequences for the 1/f images, and d(max) 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 sc ales is equally salient to the human motion system in images whose amp litude spectra fall as lif. However, for white noise images, informati on at the higher spatial frequencies is more salient and dominates per formance in the direction discrimination task. These results are consi stent with a model in which spatial frequency filters in the input lin es of motion detectors have octave constant spatial frequency bandwidt hs 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. (C) 1997 Elsevier Science Ltd.