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An unusual form of contrast adaptation: shifting of a contrast-comparison level. Nicknamed Buffy adaptation) Much of our research since mid-2005 focuses on some unusual and dramatic effects of contrast adaptation in human pattern vision. (We serendipitously came across these effects while trying to study the dynamics of the contrast-gain process of the normalization type revealed in our previous research.)
As the observer adapts to different levels of contrast, the visibility of some contrast-defined patterns greatly increases and that of others greatly decreases. Oddly, visibility is poor for patterns containing contrasts both above and below the recent average contrast.
With help from an overdose of watching Buffy the Vampire Slayer (brought home by a visiting child) we realized that these effects could be explained by a new kind of adaptation process acting in concert with a known contrast-gain control of the normalization type.
At each spatial position in a visual pattern, the new process compares the current contrast to a contrast-comparison level. This comparison level continually and rapidly adapts to equal the recent time-averaged contrast. (The integration time is less and perhaps much less than a second.)The signal going upstream from this comparsion process tells the size of the difference between the current contrast but loses information about the direction of change. Is this an unfortunate side-effect of thePatterns of contrasts above and below this comparison level
Publications about the adaptation of a contrast comparison level
ARTICLES:
Two contrast adaptation processes: Contrast normalization and shifting, rectifying contrast comparison
Wolfson, S. and Graham, N. (2009)
Journal of Vision, Vol 9, Num 4, Article 30.
Abstract: We present psychophysical results demonstrating the interaction of two contrast adaptation processes in human vision: (1) A contrast-gain-control process of the normalization type and (2) a recently-discovered shifting, rectifying contrast-comparison process. Observers adapted (for 1 s) to a grid of Gabor patches at one contrast, then a brief (94 ms) test pattern was shown, and then the adapt pattern was shown again (1 s). The test pattern was the same as the adapt pattern except that the Gabor patches had two different contrasts arranged to create vertical or horizontal contrast-defined stripes. Observers identified the orientation of the test pattern's stripes. Performance is a complicated ("butterfly shaped") function of the average test contrast, centered at the adapt contrast. This shape is a consequence of the interaction of the two contrast adaptation processes. At the ends of the function are "Weber zones" in which the contrast-gain-control process dominates, and at the center of the function is a "Buffy zone" in which the recently-discovered contrast-comparison process dominates.Page 14 and 15 of this paper refers reader to Fig. 14 of Graham and Sutter 2000. Please also see the errata for that paper.
Exploring contrast-controlled adaptation processes in human vision (with help from Buffy the Vampire Slayer)
Graham, N. and Wolfson, S. (2007)
In Computational Vision in Neural and Machine System, eds Michael Jenkin & Laurence Harris, pp. 9-47.
From the introduction to this chapter:
Two sets of psychophysical experiments and the models they were used to generate and test are described in this chapter. The first is described briefly and the second at length.
The first set was designed to investigate behaviorally the dynamics of luminance-controlled processes like light adaptation in the retina or LGN. Strictly speaking, these processes are lower than the level that we have been most interested in (and were done with a third major collaborator, Don Hood, who is very interested in that level). Further, this set is already published for the most part. Thus we will describe it quite briefly. However, we do describe it because it both inspired the second set and also gave us distinct expectations about how the second set would turn out.
The second set of experiments was designed to investigate the dynamics of contrast-controlled processes. We started out to study one such process that had proved necessary to explain our previous results with textured patterns (done in collaboration with other investigators, in particular Jacob Beck and Anne Sutter). But the results of this second set of experiments ended up suggesting the existence of an entirely different contrast-controlled process, and one that we had not previously even imagined. This second set of experiments and the new process they suggested will be the focus of most of this chapter.
An unusual kind of contrast adaptation: shifting a contrast-comparison level
Wolfson, S. and Graham, N. (2007)
Journal of Vision, Vol 7, Num 8, Article 12.
Abstract: We have found an unusual kind of contrast adaptation in human pattern vision that seems fundamentally different from previously reported effects. 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. Oddly, visibility is poor for patterns containing contrasts both above and below the recent average contrast. To explain these effects, we hypothesize a new kind of process acting in concert with a known contrast-gain-control of the normalization type. The new process compares current contrast to an adaptable comparison level; this level reflects the recent average contrast. Such a process existing at an early stage of visual processing is likely to have widespread effects at higher stages.
PDF & HTML of this article.
PRESENTATIONS WITH PUBLISHED ABSTRACTS
Wolfson, S. and Graham, N. (2005) Dynamics of contrast-gain controls in Human Vision Nournal of Vision 5(8), abstract 760. Vision Sciences Society, May 2005
Graham, N. and Wolfson, S. (2006) Complex channels become more complex: Modeling a contrast adaptation process.. Journal of Vision, 6(6), abstract 694. Vision Sciences Society, May 2006
Wolfson, S. and Graham, N (2007). More about "Buffy adaptation". Vision Sciences Society, May 2007.
Wolfson, S., Graham, N., and Pan, S. (2008). Two contrast-adaptation processes: One old, one new. Vision Sciences Society, May 2008.
Wolfson, S., Pan. S., Wable, G., and Graham, N. (2009). Contrast-modulated noise shows an adaptable, rectifying, contrast-comparison process ("Buffy adaptation"). Vision Sciences Society, May 2009.