Abstract

Increment and decrement thresholds expressed as Weber fractions were measured from the center to the edge of a square control field. The luminance necessary for a brightness match at each of these locations was also recorded and used to provide an additional index of discriminability for purposes of comparison. Discriminability was also measured across a square experimental field which was centered in a high-luminance surround. The surround produced no change of either increment or decrement threshold luminances from the center of the square experimental field to 10′ of visual angle from its edge, although the apparent brightness of the field diminished. The experiment was repeated at two levels of luminance in which the contrast ratio between the surround and the experimental field was kept constant. Weber fractions derived from brightness matches yielded similar results with respect to location and surround luminance.

© 1974 Optical Society of America

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