Concussed adolescents frequently experience neurocognitive deficits even after symptom resolution. Measurement of cognitive deficits can be obtained using functional near-infrared spectroscopy (fNIRS), a portable, non-invasive imaging technology that measures prefrontal cortical (PFC) activity. The goal of this exploratory study was to quantify differences in neural efficiency, a measure that combines PFC activity and behavioral performance, among concussed and uninjured adolescents during an N-back task, a standard assessment of working memory.

Concussed and uninjured adolescents completed an N-back task which requires participants to react when the current stimulus is the same as the n-th letter before the stimulus letter. There were four trials each of 0-, 1-, and 2-back conditions. Reaction times for each condition were captured, with increased reaction time indicating worse performance. Participants wore a continuous-wave fNIRS device positioned over the forehead to record PFC activity. Neural efficiency metrics were based on normalized reaction time and normalized PFC activity across participants. Linear mixed-effects models were used to examine the fixed effects of task condition (0-, 1-, or 2-back), injury status, and their interaction and random effect of subject on neural efficiency.

Ten concussed adolescents (8 female, age(mean±sd):17.6±0.7 years, days since injury(mean±sd):19.2±9.4), and 16 uninjured controls (8 female, age(mean±sd):17.9±0.6 years) completed the N-back task. Task condition and the interaction of injury status and task condition were statistically significant. Concussed adolescents displayed significantly worse neural efficiency than uninjured controls only during the 2-back condition.

The association of injury status with decreased neural efficiency was limited to the most difficult condition (2-back), indicating that concussed adolescents were able to meet the task demands of the simpler conditions efficiently, but not the complex 2-back condition.

These findings support further study of more complex assessments of cognitive function post-concussion in adolescents.