Temporal Eye-Hand Coordination During Visually Guided Reaching in 7- to 12-Year-Old Children With Strabismus

Abstract

Purpose: We recently found slow visually guided reaching in strabismic children, especially in the final approach. Here, we expand on those data by reporting saccade kinematics and temporal eye-hand coordination during visually guided reaching in children treated for strabismus compared with controls.

Methods: Thirty children diagnosed with esotropia, a form of strabismus, 7 to 12 years of age and 32 age-similar control children were enrolled. Eye movements and index finger movements were recorded. While viewing binocularly, children reached out and touched a small dot that appeared randomly in one of four locations along the horizontal meridian (±5° or ±10°). Saccade kinematic measures (latency, accuracy and precision, peak velocity, and frequency of corrective and reach-related saccades) and temporal eye-hand coordination measures (saccade-to-reach planning interval, saccade-to-reach peak velocity interval) were compared. Factors associated with impaired performance were also evaluated.

Results: During visually guided reaching, strabismic children had longer primary saccade latency (strabismic, 195 ± 29 ms vs. control; 175 ± 23 ms; P = 0.004), a 25% decrease in primary saccade precision (0.15 ± 0.06 vs. 0.12 ± 0.03; P = 0.007), a 45% decrease in the final saccade precision (0.16 ± 0.06 vs. 0.11 ± 0.03; P < 0.001), and more reach-related saccades (16 ± 13% of trials vs. 8 ± 6% of trials; P = 0.001) compared with a control group. No measurable stereoacuity was related to poor saccade kinematics.

Conclusions: Strabismus impacts saccade kinematics during visually guided reaching in children, with poor binocularity playing a role in performance. Coupled with previous data showing slow reaching in the final approach, the current saccade data suggest that children treated for strabismus have not yet adapted or formed an efficient compensatory strategy during visually guided reaching.

Figure 1.

Visually guided reaching experimental set up. Children held on to a stick placed 5 cm in front of them as they fixated a cross displayed on a computer monitor with both eyes open at a viewing distance of 35 cm. The cross then disappeared and a small white dot appeared on the left or right displaced 5° or 10° from fixation. The child was instructed to reach out and touch the dot with their index finger as quickly and accurately as possible, and then return to the stick. The EyeLink 1000 recorded eye movements and the Leap Motion Controller system (LMC) recorded hand movements.

Figure 2.

Violin plots displaying the distribution of saccade kinematic measures for strabismic children compared with controls. For each violin plot, the embedded boxplot represents the interquartile range, the black cross represents the mean, and black horizontal lines represent the median. Strabismic children were similar to controls for primary saccade PV (B), primary saccade gain (C), and final saccade gain (E), but had significantly longer primary saccade latency (A), and decreased primary saccade gain (D) and final saccade gain (F).

Figure 3.

Example eye traces showing increased saccade variability (i.e., decreased precision) in a child with strabismus (top) compared with a control child (bottom) for each target position.

Figure 4.

Violin plots displaying the distribution of the percentage of corrective saccades and reach-related saccades for strabismic children compared with controls. For each violin plot, the embedded boxplot represents the interquartile range, the black cross represents the mean, and black horizontal lines represent the median. Strabismic children had a similar frequency of corrective saccades as controls (A), but more reach-related saccades than controls (B).

Figure 5.

Examples of a typical visually guided reaching trial for a child with strabismus (top) and a control child (bottom). The dotted line indicates primary saccade latency (SL). Included in both examples are the saccade-to-reach planning interval (S-R) and the saccade-to-reach-PV interval (S-PV), and a reach-related saccade (RRS) in the strabismus example only. An asterisk (*) in the strabismus example indicates group mean is significantly different than controls.

Figure 6.

Violin plots displaying the distribution of the saccade-to-reach planning interval (A) and the saccade-to-reach PV interval (B) for strabismic children compared with controls. For each violin plot, the embedded boxplot represents the interquartile range, the black cross represents the mean, and black horizontal lines represent the median. No group differences were found.