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Jason D. Forte, Colin W. Clifford; Interocular transfer of the tilt illusion shows that monocular orientationmechanisms are colour selective. Journal of Vision 2004;4(8):785. doi: 10.1167/4.8.785.
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Perceptual studies of the tilt illusion (where a vertical test grating appears tilted when surrounded by a tilted inducer grating) provide evidence that orientation processing depends on stimulus colour. Clifford et al. (2003) showed that the tilt illusion (TI) is largest for inducers modulated along the same chromatic direction as the test, with TI strength falling by around 50% when test and inducers have orthogonal chromatic directions. This suggests some orientation mechanisms are selective for chromaticity while others code for form in a chromatically insensitive manner. We investigated the interocular transfer of the TI to determine the contribution of colour specific mechanisms to binocular levels of form processing. Sinewave test gratings with a 3 degree aperture were surrounded by 10 degree inducers with +15 or −15 degrees orientation (both 1 cycle per degree). Gratings were modulated along the same or orthogonal vectors in DKL colour space. Using a mirror haploscope, test and inducer were presented in the same or different eyes. The orientation of the test was varied using an adaptive staircase to find the subjective vertical, and the TI was computed as half the difference in subjective vertical for the +15 and −15 degree inducer conditions. We found that the monocular component of the TI is entirely colour-selective. The TI for orthogonal colour directions is purely binocular and the binocular component shows weak colour-selectivity. The results suggest that colour and orientation are intimately coupled at monocular stages of visual processing, whereas binocular visual mechanisms code for form in a manner that is largely insensitive to chromatic signature. Clifford, C.W., Spehar, B., Solomon, S.G., Martin, P.R., Zaidi, Q. (2003). Interactions between color and luminance in the perception of orientation. Journal of Vision, 3, 106–115.
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