Image Stabilization in Central Vision Loss: The Horizontal Vestibulo-Ocular Reflex

Abstract

For patients with central vision loss and controls with normal vision, we examined the horizontal vestibulo-ocular reflex (VOR) in complete darkness and in the light when enhanced by vision (VVOR). We expected that the visual-vestibular interaction during VVOR would produce an asymmetry in the gain due to the location of the preferred retinal locus (PRL) of the patients. In the dark, we hypothesized that the VOR would not be affected by the loss of central vision. Nine patients (ages 67 to 92 years) and 17 controls (ages 16 to 81 years) were tested in 10-s active VVOR and VOR procedures at a constant frequency of 0.5 Hz while their eyes and head movements were recorded with a video-based binocular eye tracker. We computed the gain by analyzing the eye and head peak velocities produced during the intervals between saccades. In the light and in darkness, a significant proportion of patients showed larger leftward than rightward peak velocities, consistent with a PRL to the left of the scotoma. No asymmetries were found for the controls. These data support the notion that, after central vision loss, the preferred retinal locus (PRL) in eccentric vision becomes the centre of visual direction, even in the dark.

Keywords: age-related macular degeneration; central vision loss; eye movements; preferred retinal locus; vestibulo-ocular reflex.

Figure 1

Right (RE), left (LE) eye and head position traces in the light (VVOR, above) and in the dark (vestibulo-ocular reflex (VOR), below) for the complete testing period for C8, one of the control participants. The values of the head’s position coordinates are inverted so that the eye and head curves can be overlapped.

Figure 2

For patient P6, right (RE), left (LE) eye and head position traces in the light (VVOR, above) and in the dark (VOR, below) for the complete testing period. The values of the head’s position coordinates are inverted so that the eye and head curves can be overlapped. Fundus photographs show that the preferred retinal locus (PRL) is located on the left of the scotoma in the visual field for both eyes.

Figure 3

For the AMD (solid lines) and control groups (dashed lines), VVOR and VOR gain derived from the peak velocity of leftward and rightward eye movements between saccades. Error bars are ±1 SE.

Figure 4

Mean gain values (average of rightward and leftward eye movements) obtained from the eye’s peak velocity between saccades. All the data were obtained from the participants’ right eye with the exception of control participant C17 who reported diplopia and whose left eye datum is shown as a black open square and the right eye’s as a grey filled square. For all the other participants in both the control and AMD groups, there were no significant differences in gain between the two eyes. The dashed lines show the expected average gain of 1.17 (Equation (1)).

Figure 5

Mean number of saccades per cycle made by the AMD and control groups during VOR testing. Error bars are ±1 SE.

Figure 6

Mean amplitude (in deg) of the forward saccades made by the AMD (N_AMD = 9) and control groups (N_control = 15) during the 20 s of VOR testing. Error bars are ±1 SE.