The impact of mild closed head injury on involuntary saccadic adaptation: evidence for the preservation of implicit motor learning.

Objective: Mild closed head injury (CHI) can impair performance on volitional saccades (fast eye movements), with poorer saccade accuracy being one of the principal deficits. Assessing a patient group with known deficits of volitional saccades, the authors investigated whether mild CHI similarly impairs the implicit adaptation of visually-guided (reflexive) saccades, an important process which maintains saccadic accuracy.

Methods: Within 2 weeks following mild CHI, 30 patients and 30 matched controls were compared on a computerized paradigm, which artificially induced saccadic adaptation. In response to an initial stimulus, subjects made a saccade during which the stimulus was displaced centripetally causing the initial (primary) saccade to be inaccurate. While these intra-saccadic changes remained unnoticed by the subjects, the displacements gradually caused adaptive saccadic hypometria.

Results: No differences in adaptation were found between the CHI group and the controls (F(1, 29) = 0.51, p = 0.48). This finding indicates that mild CHI does not impair implicit reflexive saccade adaptation and suggests that cerebellar function and functions of deeper brain structures such as the thalamus, superior colliculus and the basal ganglia may be largely preserved following mild CHI. The current results support the notion that the profile of oculomotor function after mild CHI reflects a centripetal gradient of impairment and relates closely to the functional integrity of the injured brain.