Critical band masking in optic flow

Abstract

Visual processing has been widely investigated with narrow band stimuli at low contrasts. We used a masking paradigm to examine how visual sensitivity under these conditions compares with the perception of the direction of heading in real scenes (i.e., with dynamic natural images at high contrasts). We first confirmed and extended previous studies showing biases in the amplitude distribution for spatial frequency, temporal frequency, speed and direction in dynamic natural movies. We then measured contrast thresholds for identification of the direction of motion for an observer traveling at various speeds. In spite of differences in contrast sensitivity and large non-uniformities in the amplitude content of the stimuli, contrast thresholds were relatively invariant of spatial frequency and completely invariant of temporal frequency, speed and direction. Our results suggest that visual processing normalises responses to supra-threshold structure at different spatial and temporal frequencies within natural stimuli and so equates their effective visibility.

Keywords:

• Optic flow
• masking
• motion
• natural images
• driving

Notes

*Generating phase noise from the FFT of a noise image, rather than generating random angles directly, ensures the correct (Hermitian) symmetry of the resulting spectrum which in turns ensures a near-zero imaginary result (i.e., that there is no loss of power) when this is back-transformed into the image domain.