Abstract
Smooth pursuit eye movements are crucial for tracking moving visual stimuli. This capacity is subserved by a bilateral network of brain regions including the frontal eye fields, basal ganglia, and occipitotemporal cortex (Sharpe, 2008, Lencer et al., 2008). Previous studies have demonstrated deficits in which adults with unilateral frontal or posterior lesions, or hemispherectomy execute saccades rather than smooth pursuit ipsilesionally (Morrow et al., 1995, Thurston et al., 1988, Troost et al., 1972). It is not known to what extent the smooth pursuit system is affected in those with childhood hemispherectomy for the treatment of drug-resistant epilepsy. We recorded eye movements using an EyeLink 1000 Plus during sinusoidal smooth pursuit in individuals with childhood hemispherectomy (n = 14, 10 left [LH] and 4 right [RH] hemispherectomies, age at surgery: < 1 month-8 years, age at test: 12-32 years). Participants tracked a target moving in a horizontal sinusoidal pattern (frequency = 0.3 Hz, amplitude = 10 degrees) for 4-12, 10 second trials. Qualitative visual analysis of sinusoidal trace plots shows that the participants generally followed the target, but had large variability in smooth pursuit, with many exhibiting frequent saccades, blinks, or other positional deviance from target position. To investigate possible asymmetries in this atypical pattern, we compared pursuit movements in the ipsilesional and contralesional direction for each participant. Every participant showed statistically significant differences between ipsilesional and contralesional movement variance (p < 0.05). To test whether these asymmetries can be explained by saccadic interruption of smooth pursuit, we computed the number of saccades during ipsilesional and contralesional movement from smoothed eye velocity traces. All 10 LH and one RH participant exhibited more ipsilesional than contralesional saccades. Surprisingly, the remaining RH participants did not show this atypical pattern. Overall, our results elucidate smooth pursuit asymmetries in individuals with childhood hemispherectomy.