Location-probability profiles for the mouth region of human primary motor-sensory cortex: model and validation

The mouth representation of the human, primary motor cortex (M1) is not reliably identified by surface anatomy but may be reliably localized by means of spatial coordinates. For this report, three quantitative metanalyses were performed which jointly described the mean location, location variability and location-probability profiles of the human M1-mouth representation. First, a literature metanalysis of intersubject functional-area variability was performed using eleven, per-subject studies, each of which reported a coordinate-referenced measure of intersubject variability for one or more brain areas. From these data, a weighted-mean value for intersubject variability was computed, which proved to be small (5.6 mm, standard deviation), consistent across coordinate axes (x, y, z), and consistent across brain areas. Second, a literature metanalysis of the location of M1-mouth was performed using seven, coordinate-referenced, group-mean studies (71 subjects in all), each of which reported a grand-average location for M1-mouth. From this, a weighted-mean location and weighted values for total variability (interlaboratory plus interindividual) were determined. Using these two literature metanalyses as input data, location-probability profiles were computed for the cardinal axes (x, y, and z) of the reference space, using the functional volumes modeling (FVM) statistical model. Third, an original-data metanalysis was performed on in-house PET data from 30 normal subjects performing overt-speech tasks. M1-mouth's mean location, location variability, and location-probability profiles were consistent with those conjointly modeled by FVM from the two literature metanalyses. Collectively, these observations provide a detailed, consensus probabilistic description of the location of the human M1-mouth representation in standardized coordinates.