Effects of Changing-Size on Vergence Eye Movements Elicited by Binocular Parallax

Abstract

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.