Ocular motor stability of foveation periods

Abstract

Individuals with congenital nystagmus (CN) usually do not experience oscillopsia. We hypothesized that the ability to suppress oscillopsia in CN is the result of perceptual mechanisms that differentiate retinal image motion caused by the oscillation itself from that of real target motion. In previous studies we showed that: (1) by inducing oscillopsia in CN subjects who did not experience it under normal conditions, retinal image stability was insufficient to suppress oscillopsia in the presence of nystagmus; (2) the presence of repeatable, well-developed (retinal error position ≤ 0.5° and error velocity ≤ 4°/sec) foveation periods was necessary for visual constancy. The present study is of a subject with diagonal CN whose direction of perceived oscillopsia varied with the eye used for fixation; the oscillopsia corresponded to the absence of well-developed foveation periods in either (horizontal or vertical) or both planes. Thus, poorly developed foveation periods in the horizontal, vertical or both planes resulted in horizontal, vertical or diagonal oscillopsia respectively; except for the diagonal condition, the direction of oscillopsia was not directly linked to that of the CN. Based on this and our previous studies of CN foveation dynamics during the perception and suppression of oscillopsia, we hypothesize that the necessary and sufficient conditions for visual constancy in either plane are the presence of repeatable, well-developed foveation periods of sufficient time duration in that plane (perhaps as little as 15 msec), and that these conditions must be simultaneously satisfied in both planes to preclude oscillopsia in either plane.

Key Words:

• oscillopsia
• congenital nystagmus
• foveation
• spatial constancy