B - 61 Increased Cortical Efficiency in the Absence of Behavioral Improvement on Working Memory Task Revealed by Functional Near-Infrared Spectroscopy.

Abstract

Functional near-infrared spectroscopy (fNIRS) is a non-invasive functional neuroimaging method that indirectly measures cortical activation via task-related changes in oxygenated hemoglobin (HbO). We used fNIRS during a working memory task to assess learning effect over time by assessing brain activity (fNIRS signal) and task performance. We hypothesized that in later blocks of the task, learning (better accuracy) would be correlated to less increase in HbO in prefrontal regions, indicating improved cognitive efficiency. Eighteen healthy adults [mean age = 24.9 (SD = 4.2); 14 female] engaged in 8 blocks of serial-3 subtraction for 30seconds each followed by 20seconds of rest. fNIRS data were collected in 8 cortical regions of interest (ROI) broadly covering the frontal lobe. fNIRS signal in each ROI and task-performance data were compared for the first 4 and last 4 blocks to examine learning. fNIRS signal was significantly greater for the first 4 than last 4 blocks (z = -2.1, p < 0.05) in only the right dorsolateral prefrontal cortex ROI. No learning effects appeared for any task-performance variables. These results indicate a dissociation between brain activity and task performance during a working memory task in healthy adults. There was less activity in the right dorsolateral prefrontal cortex during later than earlier trials, indicating an increase in this region's efficiency, without a change in task performance. The results suggest that fNIRS may be sensitive to change in brain activity before it appears clinically, which may be useful in studying people with conditions such as preclinical Alzheimer's disease, and in assessing subtle effects of interventions.