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OptokineSys: an automated method of quantifying visual function, even in people who cannot follow commands
Brain injury may lead to cerebral visual impairment (CVI), a leading cause of disability in children. Diagnosing CVI is difficult, as brain injury often also leads to cognitive impairments that preclude traditional vision assessment methods. To address this, we developed an automated system to measure vision based on the smooth tracking of moving visual stimuli. Our system, called OptokineSys, moves visual stimuli smoothly across a large computer screen while an eye-tracker measures gaze position. Our algorithm determines in real time whether the eyes smoothly follow stimulus movement. It rewards smooth tracking by playing music of the subject's or caregiver's choice, to motivate continued engagement with the task. The algorithm tolerates reverse saccades, catch-up saccades and changes of direction, but interruptions longer than these cause the system to pause the music. Smooth tracking in the correct direction and approximately correct speed is evidence that the subject can see the stimulus. An adaptive procedure uses this evidence to alter contrast and spatial frequency and determine a threshold of visuomotor function. Our use of OptokineSys as a vision assessment tool is based on the principle that, given intact smooth pursuit, these tracking thresholds reflect the subjects' spatial vision. We used OptokineSys to vary the contrast of a band-limited visual noise stimulus, to measure contrast thresholds as a function of spatial frequency. We also adapted spatial frequency at a fixed contrast level, to obtain spatial frequency thresholds as a single acuity measure. In healthy adult subjects, we found: (1) That the system worked robustly and without needing per-subject calibration; (2) that the resulting contrast sensitivity functions had an inverted-U shape, as expected of a true measure of spatial vision; (3) that contrast sensitivity measures were highly repeatable within and across subjects; (4) that spatial-frequency thresholds were well correlated with eye-chart acuity (LogMAR). In children with brain injury, we found (5) that OptokineSys engages children even when they are unable to follow verbal instructions, and (6) that it provides objective information about visual function, including repeatable spatial-frequency thresholds, even in children who cannot communicate and hence cannot otherwise be measured. We conclude that our system is a promising tool for assessing spatial vision, even for people who cannot follow commands. Efficient, objective measurement of visual thresholds in people with brain injury should enable future studies to determine incidence, natural history, and treatment of CVI.