University of South Dakota Internet Psychology Laboratory
The Craik-O'Brien contour is a one-dimensional luminance gradient whose light distribution is depicted in Figure 1.
Figure 1. Luminance profile of a Craik-O'Brien stimulus.
An image having the same horizontal luminance profile as Figure 1 is depicted in Figure 2 (below). Inspection of this image reveals a mismatch between its actual luminance distribution and its "perceived" lightness distribution. Rather than perceiving a contour bisecting a common field of gray, one perceives distinctly separate hemi-fields of "white" and "gray" to the left and right sides of the image, respectively. This is one of many curious phenomena of human contrast perception. Now, to prove to yourself that the left and right halves of the image in Figure 2 really have the same average luminance - use a pencil (or your finger) to cover the perceived "border" between the two hemi-fields. Upon occluding this border you will probably be surprised to observe that the field on either side of your pencil/finger now appear to be the same shade of "white".
Figure 2. Example of Craik-O'Brien Countour
Figure 3. Psychological appearance of sample Craik-O'Brien Countour
Figure 4. Complex manifestation of a Craik-O'Brien
See what happens when you occlude the border between the "gray" and "white" objects.
(Source: The Purves-Lab www.purveslab.net)
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