MEG reveals different contributions of somatomotor cortex and cerebellum to simple reaction time after temporally structured cues

Abstract

Magnetoencephalography (MEG) was used to measure brain activity while participants performed a simple reaction to targets after either a random interval (uncued targets) or a series of isochronous warning stimuli with 200-ms intervals that acted as a countdown. Targets could arrive "on time" or "early" relative to the preceding warning stimuli. Cerebellar activity before any stimulus onset predicted uncued simple reaction time. Onset of activity in somatomotor cortex relative to the target predicted reaction time after two warning stimuli when the target arrived on time or early. After three warning stimuli, when the target arrived on time and was certain to occur, prestimulus cerebellar activity and somatomotor onset were significant predictors of reaction time. When the target arrived early after three warning stimuli, prestimulus cerebellar and cingulate activity were predictive. The cerebellar results may reflect a number of possible factors, including a role in timing, response readiness, prediction and attention.

Figure 1

Illustration of experimental conditions.

Figure 2

Mean reaction times and 95% confidence intervals. A: Means for the three‐level ANOVA testing the effect of warning stimuli (WS). B: Means for the 2 (number of WS) × 2(target inter‐onset interval [IOI]) ANOVA testing the effect of target IOI. Circles, early targets (target IOI = 100 ms); diamonds, on‐time targets (target IOI = 200 ms).

Figure 3

Responses determined from the MEG data for activity in left auditory cortex (A; P50m), left auditory cortex (B; N100m), somatomotor cortex (C), midline cerebellum (D), and midline cingulate cortex (E). MR images on the left show source locations and orientations for a single subject. Waveforms on the right show response amplitudes averaged across all nine subjects for the “uncued” condition and for “on time” trials with two and three warning stimuli (WS). The dotted vertical lines indicate onset times for the auditory warning stimuli. The solid vertical line indicates target onset time. The first WS occurs at −600 ms for the three‐WS on‐time condition, and at −400 ms for the two‐WS on‐time condition. Target onset is at 0 ms for each condition.

Figure 4

Scatter plots of significant correlations between somatomotor and cerebellar waveform measures reported in Table II.