Accommodative and Vergence Responses to a Moving Stimulus in Concussion

Abstract

Purpose: Concussed adolescents often report visual symptoms, especially for moving targets, but the mechanisms resulting in oculomotor deficits remain unclear. We objectively measured accommodative and vergence responses to a moving target in concussed adolescents and controls.

Methods: Thirty-two symptomatic concussed participants (mean age, 14.4 ± 2.6 years; mean days since concussion, 107 days; range, 36-273 days) and 32 healthy controls (mean age, 12.7 ± 2.1 years) viewed a movie binocularly (closed-loop) and monocularly (vergence open-loop), as well as a Difference of Gaussians (DoG) target binocularly (accommodation open-loop). The movie or DoG target sinusoidally moved toward and away from participants at a 0.1-hertz (Hz) frequency at four separate stimulus amplitudes (1.50 diopters [D], 1.00 D, 0.50 D, 0.25 D) around a 2.50-D midpoint. Accommodation and vergence were continuously measured at 50 Hz using the PowerRef 3. Fourier analysis was used to assess the response amplitudes at the 0.1-Hz frequency. A 2 × 3 analysis of variance with the factors group (concussed, control) and viewing condition (binocular, monocular, DoG) was conducted on response amplitudes.

Results: Across groups, accommodative and vergence responses were significantly higher in binocular than monocular conditions (P < 0.001), but not DoG conditions. Compared to controls, concussed participants had significantly reduced monocular accommodative responses (P < 0.012; e.g., at 1.50 D, controls = 1.09 ± 0.47 D and concussed = 0.80 ± 0.36 D, P = 0.011). No group differences were observed for vergence responses in any viewing condition.

Conclusions: Accommodative and vergence responses to the moving target were largely driven by disparity cues for both groups, with only minimal improvements in the presence of additional blur cues. Concussed participants showed reduced accommodative responses to a 0.1-Hz stimulus in monocular conditions, indicating mild accommodative deficits in the absence of disparity cues.

Figure 1.

Visualization of experimental set-up. Note: Room lights were turned off and the light-shielding enclosure was fully covered during the study.

Figure 2.

(A) Sinusoidal stimulus movement profile with 1.00-D amplitude and 2.50-D midpoint. (B) Corresponding amplitude spectra. (C) Accommodative response to the stimulus movement. (D) Response amplitude showing a peak at the 0.1-Hz stimulus frequency.

Figure 3.

Simulated sinusoidal data (black curve) across a 30-second viewing period is divided into 5-second bins (bins 1 to 6), with each 10-second cycle consisting of two bins. The percentage of available data samples was calculated (with each bin consisting of a total of 250 possible data samples) and is indicated at the top of each bin (≥50% or <50 %). (A) The data quality criterion was met because two of three cycles had at least 50% data available (blue). (B) The data quality criterion failed because only one cycle has sufficient data (blue).

Figure 4.

Smoothed (over five data samples) accommodative (top) and vergence (bottom) responses from a control participant (left) and a concussed participant (right) for a stimulus movement profile with 1.00-D amplitude in the binocular condition. Accommodative lags and associated vergence disparity were observed.

Figure 5.

Distribution of accommodative response amplitudes (in diopters) at each stimulus amplitude (dotted lines; 1.50 D, 1.00 D, 0.50 D, 0.25 D), separately for concussed and control participants as well as viewing condition. Bino, binocular; Mono, monocular. Connecting lines indicate related data points from the same participant.

Figure 6.

Distribution of vergence response amplitudes (in meter angles) at each stimulus amplitude (dotted lines; 1.50 MA, 1.00 MA, 0.50 MA, 0.25 MA), separately for concussed and control participants as well as viewing condition. Connecting lines indicate related data points from the same participant.