MR-FOCUSS localizations of language processing and speech onset based on significance thresholds

This research attempts to determine if localization precision is enhanced by applying a significance level threshold to MR-FOCUSS images of the initial cortical locations of speech onset during word naming and color identification. Six normal control subjects were monitored with 148 MEG channels (4D Neuroimaging Magnes WH2500). Task 1 involved visual presentation of a colored block and asking the subject to name the color. Task 2 involved visual presentation of a printed color name, e.g. “green”, displayed in that color. Each subject was asked to say the word out loud. 40 epochs of data were averaged and filtered 1–50 Hz. A data segment from 100 ms prior to stimulus onset until 500 ms after was analyzed with MR-FOCUSS. Statistically significant regions of activation were identified by transforming the time sequence of MR-FOCUSS imaged activity to significance levels based on the distribution of maximal pre-stimulus imaged activation. The locations and timing of imaged functional activity were determined using a significance threshold of 0.05 and co-registered to a normal MRI scan. During Task 1, initial statistically significant cortical activation (Significance level, α = 0.05) was seen at 114 ± 12 ms in the occipital region in all 6 subjects. This area of initial activations was also seen at the same latency in Task 2. This was followed by activation in the right angular gyrus at 245 ± 23 ms for Task 1. For Task 2, however, activation was seen in the left superior temporal region at 234 ± 21 ms. Activation in the left inferior frontal gyrus (Broca's areas) was seen at 400 ± 31 ms in both tasks for all 6 subjects. Cortical location differences of speech onset between reading words and naming colored blocks were easily identified from significance level images. Because significance levels are calculated separately for each cortical location, localization precision was dramatically increased from that obtained using a single amplitude threshold.