“Mach Bands” in the Orientation Dimension: An Illusion Due to Inhibition of Nearby Orientations

Abstract

Lateral inhibition in the space domain is well known to produce bands of greater/lesser perceptual magnitude flanking a luminance transition (i.e., Mach bands). In the present study we demonstrate an analogous perceptual illusion in the orientation domain. Specifically, when broad-spectrum visual noise patterns containing a range of spatial frequencies and a limited range of orientations (e.g., 45 deg) are viewed, a band of orientations near the edge of the range of orientations in the stimulus's spectrum are perceived as being much stronger perceptually even though there is equal energy across the range of orientations presented. Stimuli were constructed from broadband visual noise patterns (with random phase spectra and isotropic amplitude spectra with amplitude spectra with a slope of 1/f). The noise patterns were filtered in the Fourier domain (with a flat ‘bow-tie’ filter) to contain amplitude within a 45 deg band centered at one of four orientations (0, 45, 90, or 135 deg.). The filtered stimuli were then de-convolved with the inverse Fourier transform of this filter to eliminate filter artifacts. In one task participants indicated the most salient orientations in a test pattern. In a magnitude estimation task, perceived magnitude as a function of orientation was obtained. All participants indicated that the most salient orientations were those approximately +/− 20 deg from the central orientation despite the fact that there was equal energy across all orientations in the filtered noise pattern. We suggest that patterns containing a range of orientations result in orientation channels inhibiting other neighboring channels which leads to the illusory bands in the orientation domain near a transition of physical amplitude — an analog to the Mach bands that occur due to lateral inhibition in the space domain. This illusion may also reflect contrast normalization pooling constrained to orientations neighboring an output channel.

Footnotes

 Supported by grants from the Office of Naval Research (# N00014-03-1-0224) and the Kentucky Space Grant Consortium (KSGC — NASA EPSCOR).