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Color selectivity and interocular transfer of color specific tilt aftereffects
b’mm Rc,(. Vol. IS. p 157. Pergamon Pres 1975 Printed in Great Britam.
LETTER COLOR
TO THE
EDITORS
SELECTIVITY AND INTEROCULAR TRANSFER SPECIFIC TILT AFTEREFFECI’S (Received 5 Now&w
In a recent paper on visual tilt aftereffects generated by square wave gratings, Lovegrove and Over (1973) reported that the magnitude of tilt aftereffects was larger, during monoptic presentation, when both the inducing and test stimuli (I and T stimuli) were the same (homochromatic) rather than differently colored (heterochromatic) (colors used were red and green). Heterochromatic I and T stimuli produced larger aftereffects during dichoptic presentation. They viewed these results as consistent with their assertion that the processes underlying the generation of figural aftereffects are similar to those involved in the generation of McCullough (1965) effects (color specific orientation aftereffects). They argued that monoptic tilt aftereffects were larger with homochromatic I and T stimuli because “some of the units of the visual system are tuned to both wavelength and contour orientation,. .and the inspection and test stimulus values are being signaled from units with overlapping sensitivities” (Lovegrove and Over, 1973, p. 899). Conversely, dichoptic display produced smaller aftereffects with homochromatic stimuli because color specific orientation detectors are exclusively monocular and while the “tilt aftereffect transfers interocularly, color specificity is no longer maintained” (ibid, p. 900). While their conclusions concerning the similarity between the processes underlying these two phenomena may be valid, the analytical interpretation of their tilt aftereffect data is probably not. In a recent study, Mikaelian (1973) reported that the McCullough (1965) aftereffect does transfer interocularly, thus suggesting that visual units selectively tuned to both wavelength and contour orientation are not exclusively monocular; a speculation for which some electrophysiological support does exist (Gouras, 1972). This finding raises doubts about the theoretical explanations of data that assume monocular specificity of these units, as Lovegrove and Over (1973) do. An aspect of interocular transfer of the McCullough effect does, however, provide an alternative. and a more adequate, explana-
OF COLOR
1973)
tion of Lovegrove and Over’s data. Mikaelian (1973) reported that in addition to transferring interocularly, the colors of the orientation specific aftereffects were, sometimes, organized orthogonally in the contralateral eye. Thus, following monocular exposure, achromatic gratings which in one orientation appeared green in the ipsilateral eye, produced red when viewed by the contralateral eye, and vice versa. Lovegrove and Over’s data now become quite explicable. The dichoptic display generates smaller aftereffects than monoptic with homochromatic I and T stimuli precisely because orientation specific aftereffects transfer interocularly and orthogonally (in terms of color). Although S is viewing homochromatic I and T stimuli dichoptically, functional homochromaticity exists only for the ipsilateral eye. This analysis would also predict that dichoptically generated aftereffects would be largest when the I and T stimuli are heterochromatic. This, of course, is exactly what the authors report (but perhaps do not adequately deal with). As stated earlier, this analysis does not question Lovegrove and Over’s (1973) basic stipulation that tilt aftereffects and McCullough effects are related; if anything, it tends to support their view. Department of Psychology, University ofGeorgia,Athens, Georgia 30602. U.S.A.
H. H.
MIKAELIAN
REFERENCES
Gouras P. (1972) Color opponency from fovea to striate cortex. Invest. Ophthal. 11,427-434. Lovegrove W. J. and Over R. (1973) Colour selectivity in orientation masking and aftereffect. Vision Res. 13, 895 902. McCullough C. (1965) Color adaptation of edge-detectors in the human visual system. Science, N.Y. 149, 1115-l 116. Mikaelian H. H. (1973) Interocular transfer of McCullough effects. Society for Neuroscience, abstracts (79.12).
157
LETTER COLOR
TO THE
EDITORS
SELECTIVITY AND INTEROCULAR TRANSFER SPECIFIC TILT AFTEREFFECI’S (Received 5 Now&w
In a recent paper on visual tilt aftereffects generated by square wave gratings, Lovegrove and Over (1973) reported that the magnitude of tilt aftereffects was larger, during monoptic presentation, when both the inducing and test stimuli (I and T stimuli) were the same (homochromatic) rather than differently colored (heterochromatic) (colors used were red and green). Heterochromatic I and T stimuli produced larger aftereffects during dichoptic presentation. They viewed these results as consistent with their assertion that the processes underlying the generation of figural aftereffects are similar to those involved in the generation of McCullough (1965) effects (color specific orientation aftereffects). They argued that monoptic tilt aftereffects were larger with homochromatic I and T stimuli because “some of the units of the visual system are tuned to both wavelength and contour orientation,. .and the inspection and test stimulus values are being signaled from units with overlapping sensitivities” (Lovegrove and Over, 1973, p. 899). Conversely, dichoptic display produced smaller aftereffects with homochromatic stimuli because color specific orientation detectors are exclusively monocular and while the “tilt aftereffect transfers interocularly, color specificity is no longer maintained” (ibid, p. 900). While their conclusions concerning the similarity between the processes underlying these two phenomena may be valid, the analytical interpretation of their tilt aftereffect data is probably not. In a recent study, Mikaelian (1973) reported that the McCullough (1965) aftereffect does transfer interocularly, thus suggesting that visual units selectively tuned to both wavelength and contour orientation are not exclusively monocular; a speculation for which some electrophysiological support does exist (Gouras, 1972). This finding raises doubts about the theoretical explanations of data that assume monocular specificity of these units, as Lovegrove and Over (1973) do. An aspect of interocular transfer of the McCullough effect does, however, provide an alternative. and a more adequate, explana-
OF COLOR
1973)
tion of Lovegrove and Over’s data. Mikaelian (1973) reported that in addition to transferring interocularly, the colors of the orientation specific aftereffects were, sometimes, organized orthogonally in the contralateral eye. Thus, following monocular exposure, achromatic gratings which in one orientation appeared green in the ipsilateral eye, produced red when viewed by the contralateral eye, and vice versa. Lovegrove and Over’s data now become quite explicable. The dichoptic display generates smaller aftereffects than monoptic with homochromatic I and T stimuli precisely because orientation specific aftereffects transfer interocularly and orthogonally (in terms of color). Although S is viewing homochromatic I and T stimuli dichoptically, functional homochromaticity exists only for the ipsilateral eye. This analysis would also predict that dichoptically generated aftereffects would be largest when the I and T stimuli are heterochromatic. This, of course, is exactly what the authors report (but perhaps do not adequately deal with). As stated earlier, this analysis does not question Lovegrove and Over’s (1973) basic stipulation that tilt aftereffects and McCullough effects are related; if anything, it tends to support their view. Department of Psychology, University ofGeorgia,Athens, Georgia 30602. U.S.A.
H. H.
MIKAELIAN
REFERENCES
Gouras P. (1972) Color opponency from fovea to striate cortex. Invest. Ophthal. 11,427-434. Lovegrove W. J. and Over R. (1973) Colour selectivity in orientation masking and aftereffect. Vision Res. 13, 895 902. McCullough C. (1965) Color adaptation of edge-detectors in the human visual system. Science, N.Y. 149, 1115-l 116. Mikaelian H. H. (1973) Interocular transfer of McCullough effects. Society for Neuroscience, abstracts (79.12).
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