The influence of age on adaptation of disparity vergence and phoria

Abstract

A paucity of research exists to investigate whether the normal aging process influences the ability to adapt disparity vergence and phoria. Vergence eye movements and dissociated phoria were recorded from 49 healthy subjects (ages 20-70years) using an objective eye movement tracking system. Four-degree vergence responses were modified using a double-step protocol. Dynamics of vergence were quantified via peak velocity. The phoria adaptation experiment measured the magnitude (net change in phoria level) and rate (magnitude divided by the time constant) of phoria adaption during 5min of sustained fixation on a binocular target (40cm/8.44° from midline). The magnitude of phoria adaptation decreased as a function of age (r=-0.33; p=0.04). The ability to adapt vergence peak velocity and the rate of phoria adaptation showed no significant age-related influence (p>0.05). The data suggest that the ability to modify the disparity vergence system and the rate of phoria adaptation are not dependent on age; whereas, the magnitude of phoria adaptation decreases as part of the normal adult aging process. These results have clinical and basic science implications because one should consider age when assessing the changes in the magnitude of phoria adaptation which can be abnormal in those with oculomotor dysfunctions.

Keywords: Adaptation; Age; Eye movements; Phoria; Vergence.

Fig. 1.

Haploscope experimental set-up. Two monitors were used to project visual stimuli to two partially reflective mirrors so that the left and right eye each saw an independent visual image.

Fig. 2.

A: An example of 4° single convergence step position (black) and velocity (gray) traces recorded during the baseline phase (left column) and modification (middle column) phase. Double steps have two high-velocity components termed the first high-velocity component and second high-velocity component (right column). The arrows in plot A indicate the peak velocity. During the conditioning phase, the peak velocity of the 4° vergence steps is greater compared to the baseline responses as a result of the short-term modification experiment. 2B: The phoria adaptation experimental protocol where each arrow represents a phoria measurement.

Fig. 3.

Ensemble of several 4° convergence eye movements per subject who demonstrated slowest eye movements within each age group (plots A (young), B (mid-aged) and C (older)) as well as subjects who exhibited the fastest convergence eye movements (plots D (young), E (mid-aged), and F (older)) during the baseline phase of the vergence modification experiment. Each trace is an individual eye movement. The age of the subject is denoted on the plot.

Fig. 4.

Ensemble of several 4° convergence eye movements from the subject who demonstrated slowest (plots A, B and C) and fastest (plots D, E, and F) peak velocity convergence responses per age group during the modification phase of the vergence modification experiment. Each trace is an individual eye movement. The age of the subject is denoted on the plot.

Fig. 5.

Ensemble of several double 4° convergence eye movements (8° disparity) per subject who demonstrated the slowest (plots A, B and C) and fastest (plots D, E, and F) convergence responses from each age group during the modification phase of the vergence modification experiment. Each trace is an individual eye movement. The age of the subject is denoted on the plot.

Fig. 6.

Examples of phoria measurements (denoted as ‘o’) from six subjects where the first row are subjects who exhibited the slowest rate of phoria adaptation to the 40 cm (8.44°) binocular target for each age group. The second row are subjects who exhibited the fastest rate of phoria adaptation. The first, second and third column are from subjects within the young, mid-aged and older age groups, respectively. The age of the subject is denoted in the upper left portion of each plot. The phoria measurements were fit with an exponential curve denoted using a solid black line.

Fig. 7.

Linear regression analysis of the magnitude or change in the phoria during a phoria adaptation experiment (plot A) and the rate of phoria adaptation (plot B) plotted as a function of age.

Fig. 8.

Change in convergence peak velocity (plot A) and high-velocity ratio (plot B) as a function of rate of phoria adaptation (Δ/min) shown for young (white), mid-aged (gray), and older (black) age groups.