& Psychophysics 1973. Vol. 13, No.3. 525·526 A new afterimage ( contrast afterimage")

F. SHIVELY University ofManitoba, Winnipeg, Manitoba, Canada

A new type of afterimage is reported in this paper.

The term "afterimage" refers to the image that occurs negative afterimage of the background appears and the after the inducing stimulus is removed. A new type of new center disk afterimage is not seen. The new afterimage is reported below. afterimage was reported by all lOSs tested under the Figure 1 (upper half) depicts the inducing and test following conditions: a center disk of 5 deg and a stimuli for the production of the well-known surround of 45 deg visual angle. The luminance of the phenomena of negative afterimages. Under these inducing and test stimuli was 2.5 and 6.0 fL, conditions (blue center and white surround), if the S respectively. The inducing stimulus was viewed for fixates first the blue center of the inducing stimulus and 15 sec. then shifts his fixation to the center of the white test The following explanation combines the mechanisms stimulus, a yellow center disk, the negative afterimage, of retinal adaptation and lateral inhibition to account appears. However, if the figure-ground relationship of for the new afterimage. the inducing stimulus is reversed (white center and blue When the inducing stimulus is viewed, the sensitivity background; see the lower half of Fig. 1), the afterimage of the blue system of the retinal region stimulated by appearing in the test situation is a blue center disk, a the blue surround is reduced relative to that of the red blue similar in hue to the blue of the background of the and systems. As a consequence, when this retinal inducing stimulus. surround region is subsequently stimulated by the white This new afterimage is easily produced with ordinary light of the test stimulus, its red and green systems are colored papers and lighting. Neither size of the center activated more than its adapted blue system. Assume, disk nor hue of the background is critical (blue is used following Alpern and Rushton (1965), that the red, herein only for ease of exposition) in production of the green, and blue systems of the surround region laterally new afterimage. The extensiveness of the background is inhibit only the red, green, and blue systems, critical, however. That is, the background must be an respectively, of the 's center region. As a extended one. If the background is only an annulus, a consequence of this color-specific inhibition and the reduced activity of the adapted blue system, the center region of the retina will have its blue system inhibited less than its red and green systems. Stated positively, the center region's blue system will be activated more than will the center region's red and green systems. The center region of the test stimulus will therefore be seen WHITE -,, f I BACKGROUND , I as blue. '." The above explanation is incomplete inasmuch as it BLUE CENTER would predict that the white center disk of the inducing stimulus should look colored (yellow), whereas, in fact, INDUCING TEST it looks white. However, the center disk of the inducing' stimulus can be made to look yellow if the contrast ~ BLUE between the center and surround is reduced by AFTERIMAGE overlaying the inducing stimulus with a piece of tissue BLUE paper (Graham & Brown, 1965). The implication of this CKGROUND is that the lateral "flow" of inhibition is prevented or WHITE reduced by the sharp breaks in the intensity gradients ENTER that occur between center and surround of the inducing WHITE~ stimulus. INDUCING BACKGROUND TEST It would be expected. therefore, that if the intensity gradient between center and surround were removed, the effects of lateral inhibition would manifest itself. The Fig, 1. (Upper) The inducing and test stimuli for the negative afterimage, (Lower) The inducing and test stimuli for the new test stimulus used to produce the new afterimage has no afterimage. intensity gradient.

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REFERENCES appearances: Brightness constancy and color constancy. In C. H. Graham (Ed.), Vision and . New York: Alpern, M., & Rushton, W. A. H. The specificity of the cone Wiley, 1965. interaction in the after-flash effect. Journal of Physiology, 1965,176,473-482. (Received for publication December 26, 1972; Graham, C. H., & Brown, J. L. Color contrast and color revision received February 1, 1973.)