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Two Contrast-Adaptation Processes -- One Old and One New
Monday, November 10, 2008, 01:00pm - 02:00pm
Department of Psychology, Columbia University
We serendipitously found an unusual effect of contrast adaptation in human pattern vision. As the observer adapts to different levels of contrast, the visibility of some contrast-defined (second-order) patterns dramatically increases and that of others dramatically decreases. In particular, test patterns containing two contrasts, one below and one above the recent averaged contrast (straddling test patterns), are extremely difficult to identify. It is as if some process compares the current contrast at each spatial position to an adaptable comparison level (the recent averaged contrast) and then loses information about the sign of the difference.
This new effect (nicknamed Buffy adaptation) seems fundamentally different from effects previously reported. The old effects showed Weber-law type behavior and could be modeled as a contrast-gain control process of a normalization type.
In experiments covering the entire range of adapt and test contrasts, we have found that the performance of the observer after adapting to any given adapt contrast is a a complicated "butterfly-shaped" function that can be explained as the joint action of the old and new processes.
We still find this new effect surprising. We wonder: Is the very bad performance on straddling test patterns of some possible use in natural vision? Or is it just a not-too-harmful side effect of other desirable characteristics? We wonder whether it is relevant to what occurs within an eye fixation, or between fixations, or both. (Buffy adaptation is largely complete within 100 ms.) We wonder as well about mechanistic issues, e.g. at what level(s) in the visual neural system are the old and the new processes producing their perceptual effects.
This work was done jointly with Sabina Wolfson.
Short paper on beginning of work to be presented (Can be accessed at: http://journalofvision.org/7/8/12/)
Wolfson, S. and Graham, N. (2007). An unusual kind of contrast adaptation: shifting a contrast-comparison level. Journal of Vision, 7(8):12, 1-7
Background reading (Can be accessed at http://www.columbia.edu/~nvg1/BiblioWithAbstracts)
Graham, N. (1992) Breaking the visual stimulus into parts. Current Directions in Psychological Science, 1, 55-61.
Graham, N. and Sutter, A. (2000) Normalization: Contrast-gain control in simple (Fourier) and complex (non-Fourier) pathways of pattern vision. Vision Research, 40, 2737-2761.
Graham, N., and Sutter, A. (1998) Spatial summation in simple (Fourier) and complex (non-Fourier) channels in texture segregation. Vision Research, 38, 231-257.