The purpose of this study is to investigate the effects of contour-line thickness of a test figure (TF) on hue contrast in Koffka-ring type patterns from the viewpoint that the contour-line thickness affects the perception of the TF's unity. Six subjects were instructed to adjust a comparison figure (CF) to match the apparent hue of the two TF sections overlapping a red-green inducing figure (IF) with a gray background. By using a color-graphic system, the following stimulus conditions were manipulated; 1) the contour-line thickness of the TF (four degrees), 2) the separation of the TF (three degrees), and 3) the employment of control stimuli in which the IF was either completely red or green in order to check the contour-line thickness as a distance between the TF and the IF. The results revealed that the magnitudes of hue contrast (MHCs) decreased as the contour-line thickness increased. In addition, a questionnaire to rate the unity of the TF supported the view that a contour-line thickness intensifies the perceived unity of the TF. Consequently, it is concluded that the contour-line thickness affects the increase of the TF's unity and that the MHCs of the TF surrounded by thick contour-line decrease.

Key words: Koffka-ring type patterns, hue contrast, joint-ratio, contour-line thickness, unity