Reaction Time After Spinal Cord Injury

Spinal cord injury (SCI) commonly causes physical disabilities, such as paralysis, weakness, nerve pain, and lack of motor coordination, that can significantly reduce quality of life. In recent years, some researchers have suggested that people with SCI may also struggle with cognitive deficits, such as slowed reaction time, impairing their ability to carry out everyday tasks and skills.

Most of this evidence comes from reaction time tasks, which typically ask participants to manually respond to stimuli by clicking a mouse, pressing a button, or completing some other hand-based motor action. In theory, SCI patients who only experience lower-body paralysis should have unimpaired upper-body motor skills, enabling them to complete manual reaction time tasks as easily as an uninjured person. However, some other researchers suspect that because all motor tasks require some involvement from the spinal cord and central nervous system, even SCI patients with upper-body mobility may experience subtle motor impairment in the hands and fingers. As a result, their relatively poorer outcomes on manual reaction time tasks may be caused by motor impairment rather than impaired cognition.

A research team sought to resolve this problem by using an ocular-activated reaction time task. Ocular tasks, which ask participants to indicate responses based on eye movement rather than hand movement, are designed for patients with full-body paralysis, making them ideal for tetraplegic (fully paralyzed) SCI patients. Unlike body movement, eye movement does not require any involvement from the spinal system. Accordingly, the research team hypothesized that an ocular-activated reaction time task is a fairer, more accurate way to measure reaction time among all patients with SCI, even those whose hand movements appear unaffected.

To test this hypothesis, the team administered an ocular-activated reaction time task to a group of 12 tetraplegic SCI patients and a comparison group of 12 uninjured participants. Participants were instructed to look at a blank screen until a small black target stimulus appeared on the screen, at which point they were to shift their gaze to the target until it disappeared. The research team measured reaction time by analyzing how long each participant took to recognize and fixate on the target stimulus.

After analyzing their data, the researchers found that quadriplegic SCI participants and the uninjured comparison participants performed equally well on the ocular reaction time task. These results contradict findings from previous manual reaction time studies of paraplegic SCI patients, which suggest that SCI impairs reaction time. The researchers concluded that these previous findings do not accurately reflect SCI patients’ reaction times because the manual task requires physical movement, which can be subtly impaired after SCI. They recommended that future reaction time studies involving people with SCI should use ocular-activated tasks rather than manual tasks.

Bloch A, Shaham M, Vakil E, et al. Examining implicit procedural learning in tetraplegia using an oculomotor serial reaction time task. PLoS One. (April 2020).

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