Imaging brain fatigue from sustained mental workload: an ASL perfusion study of the time-on-task effect

Abstract

During sustained periods of a taxing cognitive workload, humans typically display time-on-task (TOT) effects, in which performance gets steadily worse over the period of task engagement. Arterial spin labeling (ASL) perfusion functional magnetic resonance imaging (fMRI) was used in this study to investigate the neural correlates of TOT effects in a group of 15 subjects as they performed a 20-min continuous psychomotor vigilance test (PVT). Subjects displayed significant TOT effects, as seen in progressively slower reaction times and significantly increased mental fatigue ratings after the task. Perfusion data showed that the PVT activates a right lateralized fronto-parietal attentional network in addition to the basal ganglia and sensorimotor cortices. The fronto-parietal network was less active during post-task rest compared to pre-task rest, and regional CBF decrease in this network correlated with performance decline. These results demonstrate the persistent effects of cognitive fatigue in the fronto-parietal network after a period of heavy mental work and indicate the critical role of this attentional network in mediating TOT effects. Furthermore, resting regional CBF in the thalamus and right middle frontal gyrus prior to task onset was predictive of subjects' subsequent performance decline, suggesting that resting CBF quantified by ASL perfusion fMRI may be a useful indicator of performance potential and a marker of the level of fatigue in the neural attentional system.

Figure 1

a) Means and standard deviations of reaction time (RT) from the first to the last 4-minute quintiles; b) RT time series of four subjects from the first minute to the last minute of the 20-minute PVT. Note that there were robust individual differences in the rate of RT increases. c) Mean mental fatigue scales reported by subjects before and after the PVT. Note both RT and mental fatigue scales showed significant time-on-task effects. d) Self-reported mental fatigue scale changes showed no correlation with RT changes (r = 0.66, p > 0.9)

Figure 2

Quantitative CBF maps from a representative subject.

Figure 3

Brain areas associated with a) the comparison of PVT versus resting baselines; b) the comparison of post-task resting baseline versus pre-task resting baseline, and c) the comparison of the last quintile of PVT versus the first quintile of PVT. The threshold of display was set as FDR or small volume corrected p < 0.05.

Figure 4

Region of interest (ROI) analysis on the functional (a) and structural ROIs (b) showing that RT increases (%) during the PVT negatively correlated with post-task regional CBF changes in the ACC, right MFG, and right IPC.

Figure 5

Region of interest (ROI) analysis on the functional (a) and structural ROIs (b) showing that pre-task resting CBF in the right MFG and thalamus predicted RT increases (%) during the PVT.