The Effects of Amblyopia on Planning (Feedforward) and Execution (Feedback) of Visually-Guided Reaching Movements

Abstract

Purpose: : Impairment of spatiotemporal visual processing in amblyopia has been studied extensively, but its effects on visuomotor tasks have been examined sparsely. We investigated how the spatiotemporal visual deficits in amblyopia affect visually-guided reaching movements.

Methods: : Seventeen patients with anisometropic amblyopia and 17 visually normal subjects were recruited. Participants executed reach-to-touch movements toward targets presented randomly 5° or 10° to the left or right of central fixation during binocular and monocular viewing. Hand movements were recorded using an NDI Optotrak motion capture system. The effects of amblyopia on motor planning (feedforward control) were evaluated by examining peak acceleration, time to peak acceleration, and spatial variability of finger position at peak acceleration during the reaching movement. The effects of amblyopia on reach execution (online feedback control) were assessed by calculating the spatial variability of the finger trajectory, and the coefficient of determination (R²) which relates the spatial position of the finger at 25% (early), 50% (middle) and 75% (late) into the trajectory to the endpoint position. A low R² value indicates that online feedback was used to attenuate errors in the initial motor plan.

Results: : Mean end-point accuracy was comparable between patients and normal subjects in all viewing conditions; however, mean precision was slightly worse (p=0.003) when patients viewed with the amblyopic eye. Feedforward control was affected in patients in all viewing conditions; they exhibited reduced peak acceleration (p=0.006), prolonged time to peak acceleration (p=0.021), and increased variability of finger position at peak acceleration (p=0.037). Feedback control was also affected in patients in all viewing conditions. Although the spatial variability of finger position was increased early in the trajectory (at peak acceleration), it was reduced later in the trajectory (at peak deceleration), when compared to normal subjects. Patients also exhibited higher R² values (p=0.013) in the second-half of the reaching movement, suggesting that they started to implement online corrections earlier in the trajectory.

Conclusions: : Patients with anisometropic amblyopia modified the kinematics of their reaching movement in order to optimize reaching performance. Their strategy may reflect an adaptation to compensate for degraded spatiotemporal vision in amblyopia.

Keywords: amblyopia • vision and action