Timing of cortical activation: a latency-resolved event-related functional MR imaging study

The time course of cortical activations of different anatomic areas has been demonstrated to reflect, to some degree, the temporal dynamics of the brain network. The purpose of this study was to determine the temporal sequence of the hemodynamic response in the visual, supplemental motor (SMA), and primary motor cortical areas by using a visuomotor reaction time task.

The reaction times (RTs) of 26 right-handed subjects were recorded in response to a visual cue during an event-related functional MR imaging (fMRI) experiment. Statistical parametric mapping (SPM99) was used, and activation maps were produced for each subject. This was followed by a random-effects group analysis. Using a weighted least-squares approach, we recorded the time at onset of the hemodynamic response of the fMRI activation in four regions of interest: the right occipital (RO) and left occipital (LO) visual cortices, the SMA, and the left sensorimotor area (LM1). Linear regression analysis was done between the RTs and the mean latencies for the four areas.

Using the group analysis, the results showed that the first regions to activate were the visual occipital cortices (RO and LO) with mean latency +/- standard error of mean (SEM) of 1.74 +/- 0.05 s and 1.85 +/- 0.08 s, respectively. The visual occipital areas were followed by the SMA of 2.07 +/- 0.16 s and finally the LM1 with a mean latency of 2.1 +/- 0.15 s. There were significant differences in the mean onset of latencies between RO and LO, RO and SMA, and RO and LM1 (P <.05). On performing regression analysis between the RTs and the mean latencies by using the group analysis, there was no significant correlation with any of the four areas. By using an individual subject analysis, the results again showed that the first regions to activate were the visual occipital cortices (RO and LO) with mean latency +/- SEM of 1.75 +/- 0.06 s and 1.84 +/- 0.12 s, respectively, followed by the SMA with a mean latency of 2.19 +/- 0.25 s and finally the LM1 of 2.26 +/- 0.38 s. There was no significant difference between the mean onset latencies.

The onset of the hemodynamic response started first in the visual cortex (input) followed by the SMA and primary motor cortical area (output). The onset of activation showed no direct correlation with the overall RTs of the subjects, leading one to suggest that the peripheral motor unit may have a greater impact on RT than the central contribution.