High intensity TMS pulses delivered to the motor cortex hand area are known to recruit less excitable corticospinal neurons within the motor hand area and/or neurons spatially further from the hand area [16], [49].
Furthermore, in children with congenital hemiparesis intensive therapy programmes can increase the size of primary hand motor cortex motor contralateral to the paretic hand, with associated functional improvement in the paretic hand [ 17- 20].
To evaluate whether stroke lesion focused on the primary motor cortex hand knob re-localizes the hand motor area in the affected hemisphere.
Indeed, by applying single TMS pulses on the scalp overlying the hand motor cortex, the amplitude of motor evoked potentials (MEPs) recorded from the contralateral target muscles, reflects physiological properties of the motor system [9], [10], either during voluntary reaching and grasping actions [11] or even during motor imagery tasks [12] [17].
Nevertheless, our findings suggest that the contribution of the hand motor cortex to response bias is stronger than that of the articulatory motor cortex, and that these motor regions have opposite effects on response bias.
In brief, a voxel of interest was placed over the hand motor cortex in session 1 and the visual cortex in session 2. A standard PRESS sequence was acquired to assess the creatine and NAA line widths, and a MEGA-PRESS sequence was then acquired to allow simultaneous spectral GABA editing, three-dimensional voxel localization, and water suppression [ 23].
Cartesian distances to the primary motor cortex hand representation, cerebellar gray matter, and cerebellar hand representations were calculated (see Supplementary Methods).
This double dissociation indicates that the hand motor cortex contributes to response bias during demanding discrimination tasks, whereas the articulatory motor cortex contributes to perception of speech sounds.
It is also interesting to note that hand motor cortex is located in between face and leg motor areas, and might therefore capture the overlap of activations produced by all three action word categories. 1 It is therefore still possible that action-relatedness affects hand, face and leg motor areas, but was only detected in the hand area due to this overlap.
These studies demonstrate that, in contrast to hand motor cortex, tDCS at 1 mA for 10 min is unable to excite or inhibit the pharyngeal motor cortex.
Dissociation of sensorimotor deficits after rostral versus caudal lesions in the primary motor cortex hand representation.
