Three experiments examined how the difference of color and/or luminance between line segments and background affect perceptual completion at the blind spot. When a pair of line segments was presented on the both sides of the blind spot while varying length of the segments, the observers reported whether the line segments appeared ‘a continuous line’ or not. We measured the minimum length of the line segments for perceptual completion, or filling-in, to occur. Experiment 1 examined how different colors of the line segment (red, green, blue, and gray) affect the minimum length of the line segments. Experiment 2 examined how different luminance of the line segments affect the minimum length of the line segments. Experiment 3 examined how the equiluminant chromatic, equichromatic luminance, and both chromatic and luminance difference between line segments and background affect the minimum length of the line segments. The results of the three experiments showed that the minimum length of line segments for perceptual completion to occur at the blind spot (a) did not change irrespective of different colors of line segments with large luminance difference between line segments and background, (b) was shorter, as luminance difference between line segments and background was increased, (c) was longer in both chromatic and luminance condition than that in the equichromatic luminance condition, and (d) was shorter in both chromatic and luminance condition than that in the equiluminant chromatic condition. The results suggest that both color and luminance affect perceptual completion at the blind spot, and that the luminance difference between line segments and background facilitates perceptual completion greater than color difference.

A visual target distance changes with the target’s retinal size as well as its binocular parallax, which induce vergence eye movements. In the study, we investigated the effects of retinal size changes of stimulus image on vergence eye movements induced by binocular parallax change. To do this, we used stereoscopic stimulus that changes retinal size and binocular parallax independently both modulated in a single cycle of sinusoidal at four different maximum speeds (as binocular parallax: 9.6, 4.8, 2.4, 1.2 deg/s; or those corresponding in size, 2.4, 1.2, 0.60, 0.30 deg/s, respectively). The results showed that when the stimulus changes were the faster two speeds, velocity of vergence eye movements was higher for in-phase condition, in which both the retinal size and parallax changes provided corresponding information of distance changes, than for reversedphase condition, in which they provided 180 deg out of phase distance information. When the stimulus changes were slow, however, the differences between the two conditions were diminished. Moreover, size changes without parallax changes did not elicit corresponding vergence eye movements regardless of the size-change velocity. These results suggest that changes in retinal size have the effects on fast mechanism of vergence eye movements but not on slow mechanism, both mechanisms that were reported by Semmlow et al. (1986)¹⁾ based on measuring vergence eye movements.