Figure 3. fMRI alertness index differentiates between auditory task response (hit) and misses.

(A) Event-locked EEG spectrogram for hits and misses. During hits, the elevated power in the alpha band may signify higher alertness; during misses, lower levels of alpha and increased power in lower frequencies suggest lower alertness. Time = 0 corresponds to stimulus onset. (B) Event-locked fMRI alertness index. Within each subject, the fMRI alertness index surrounding each stimulus was averaged across trials with hits (red) and misses (blue), and the across-subject means are represented by solid lines (shaded area is standard error; n = 12 scans for hits, n = 9 scans for misses). Across subjects, the fMRI alertness index predicted significantly greater levels of alertness in hits compared to misses, both within the pre-stimulus interval (5 s prior to stimulus onset; light gray region) and post-stimulus interval (10 s after stimulus onset; striped gray region). A wider time interval is shown for more complete visualization. (C) Visualization of individual subjects’ effects. Within individual scans, the fMRI alertness index was averaged within the 5 s pre-stimulus interval of each trial. Scans with only ‘hits’ are marked as gray circles; no scans had only misses. Within subjects who had both hits and misses, a significant difference in the pre-stimulus fMRI alertness index was found (p=0.01, paired t-test; d = 1.11).