Montag E D
Center for Visual Science, University of Rochester, New York 14627, USA.
J Opt Soc Am A Opt Image Sci Vis. 1997 May;14(5):997-1006. doi: 10.1364/josaa.14.000997.
The integration of color and form to produce a unified percept is a central problem in vision research. We know that the spatial arrangement of colored stimuli influences their detectability. In the gap effect, for example, a small gap between two colored fields enhances their discriminability [Opt. Acta 24, 159 (1977)]. Chromatic thresholds are also reduced when test fields are spatially demarcated. To explore the mechanisms underlying these types of effect, the influence of spatial structure on chromatic sensitivity for gratings was measured. For sine-wave and square-wave gratings modulated in different directions in color space, contrast sensitivity was measured by using a two-alternative forced-choice procedure with a QUEST staircase. Thin lines, of the same orientation as that of the grating, were superimposed on it at half-cycle intervals. The phase of the superimposed lines was varied. For S-cone modulated gratings, dark lines placed at the midpoints between peaks and troughs (90 degrees) increased sensitivity. As the phase of the lines moved toward the peaks (0 degree), their effect on sensitivity declined to zero. A similar but smaller effect was seen for isoluminant L- and M-cone modulated gratings. The superimposed lines always impaired contrast sensitivity for achromatic gratings, especially at a phase of 0 degree. Spatial structure superimposed on gratings can both facilitate and impair contrast sensitivity. In the presence of sharp boundaries, chromatic sensitivity is increased. This effect may depend more on the salience of boundaries, since isoluminant lines superimposed on S-cone modulated gratings and gray lines of similar cone contrast can facilitate detection if they are of sufficient contrast. Achromatic contrast sensitivity is reduced when the boundaries are present. The additional luminance information at the boundaries masks the grating. A simple model in which spatial integration is arrested at the positions of the superimposed lines fits only the isoluminant conditions. For both luminance and chromatic contrast the change in sensitivity depends on phase.
