Neural substrates of phasic alertness: A functional magnetic resonance imaging study

The “warning effect” refers to the decrease in motor response reaction times to a target when its presentation is preceded by a stimulus indicating that the target will appear shortly. We hypothesized that cue presentation phasically enhances alertness, which in turn facilitates the preparation of a motor response. To test this hypothesis, we conducted functional magnetic resonance imaging during a Go/NoGo task with a warning stimulus. Fifteen subjects completed a visual Go/NoGo task, and 12 completed an analogous task in the auditory modality. After a warning stimulus was presented, a Go or NoGo stimulus was presented with equal probability. Both auditory and visual warning stimuli activated the midbrain, thalamus, and the anterior cingulate cortex (ACC) extending to the pre-supplementary motor area (pre-SMA). The warning-related activation in the pre-SMA and thalamus was greater when warnings were followed by Go events with faster reaction times than when followed by events with slower reaction times. The midbrain, thalamus, and ACC are known to be associated with vigilance or intrinsic alertness, and the pre-SMA is involved in movement selection and preparation. Thus, the warning effect may be partly mediated by the potentiation of the pre-SMA through the midbrain-thalamus-ACC alerting network.