The effect of sensory uncertainty due to amblyopia (lazy eye) on the planning and execution of visually-guided 3D reaching movements

Abstract

Background: Impairment of spatiotemporal visual processing in amblyopia has been studied extensively, but its effects on visuomotor tasks have rarely been examined. Here, we investigate how visual deficits in amblyopia affect motor planning and online control of visually-guided, unconstrained reaching movements.

Methods: Thirteen patients with mild amblyopia, 13 with severe amblyopia and 13 visually-normal participants were recruited. Participants reached and touched a visual target during binocular and monocular viewing. Motor planning was assessed by examining spatial variability of the trajectory at 50-100 ms after movement onset. Online control was assessed by examining the endpoint variability and by calculating the coefficient of determination (R(2)) which correlates the spatial position of the limb during the movement to endpoint position.

Results: Patients with amblyopia had reduced precision of the motor plan in all viewing conditions as evidenced by increased variability of the reach early in the trajectory. Endpoint precision was comparable between patients with mild amblyopia and control participants. Patients with severe amblyopia had reduced endpoint precision along azimuth and elevation during amblyopic eye viewing only, and along the depth axis in all viewing conditions. In addition, they had significantly higher R(2) values at 70% of movement time along the elevation and depth axes during amblyopic eye viewing.

Conclusion: Sensory uncertainty due to amblyopia leads to reduced precision of the motor plan. The ability to implement online corrections depends on the severity of the visual deficit, viewing condition, and the axis of the reaching movement. Patients with mild amblyopia used online control effectively to compensate for the reduced precision of the motor plan. In contrast, patients with severe amblyopia were not able to use online control as effectively to amend the limb trajectory especially along the depth axis, which could be due to their abnormal stereopsis.

Figure 1. Experimental set up.

(A) Participants fixated on a cross displayed on a computer monitor, with their index finger placed on a force sensitive resistor (FSR). The target was a high contrast circle (visual angle 0.25°) shown after a random delay (range 1.5–3 sec) at ±5° or ±10°. (B) The 3D reach vector, defined as a straight line connecting the initial and end position of the finger, was calculated from the finger trajectory data. For each trial, we also computed the angle between the reach vector and the target vector (defined as a straight path connecting the initial position of the finger and target location).

Figure 2. Representative reach trajectory.

Typical data showing the reach vector trajectory to the +10° target in a representative control subject (left column), patient with mild amblyopia (middle column), and severe amblyopia (right column) during binocular viewing (top row), fellow eye (right eye in control) viewing (middle row), and amblyopic eye (left eye in control) viewing (bottom row). Patients with mild and severe amblyopia had greater variability in spatial limb position during reaching in comparison to the control subject.

Figure 3. Reach precision during acceleration.

Patients with mild and severe amblyopia had significantly reduced precision of the vector angle at 50 ms following movement onset (A), and 100 ms after the onset of movement (B). For control subjects, fellow eye is the right eye and amblyopic eye is the left eye. Error bars = ±1 standard error.

Figure 4. End-point precision.

Mean endpoint precision (variable error) of the reaching movement along the azimuth (A), elevation (B), and depth axes (C). Patients with severe amblyopia had reduced precision during amblyopic eye viewing along azimuth (p<0.0001) and elevation axes (p<0.05), and during all viewing conditions along the depth axis (p<0.01). For control subjects, fellow eye is the right eye and amblyopic eye is the left eye. Error bars = ±1 standard error.

Figure 5. Proportion of explained variance.

R² values (Fisher z scores) relating the spatial location of the finger at 10% intervals (normalized to movement time) to the overall movement amplitude during binocular (left column), fellow eye (middle column) and amblyopic eye (right column) along the azimuth (top row), elevation (middle row), and depth axes (bottom row). There was no significant difference between control participants and patients with mild amblyopia in all viewing conditions in all three axes. However, patients with severe amblyopia had significantly higher R² values at 70% of movement time along elevation (p<0.05) during amblyopic eye viewing, and along the depth axis (p<0.01) in all viewing conditions. The higher R² values in the latter half of the trajectory indicate that movements relied heavily on pre-programmed responses.