Effective contrast constancy and linearity of contrast sensation

Apparent contrast of gratings and edges was evaluated either by matching the contrast of an adjustable pattern to a reference contrast or by subjective setting a fraction of the reference contrast. In the matching task, patterns appeared to have the same contrast when their suprathreshold contrasts (physical minus threshold) were equal. This means effective contrast constancy of various patterns of high contrasts, since at these levels large just noticeable differences in contrast exceed the differences in contrast thresholds. The contrast “halving” technique failed to demonstrate a sense of absolute contrast for inexperienced observers. However, observers familiar with contrast were able to assess contrast linearly, and according to its suprathreshold value. These results were compared with recent physiological findings.

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