Asymmetries of the motor cortex during the lateral judgment in fMRI
In front of a photograph of hand, how do you decide that it depicts a right hand or a left hand? It is admitted that this judgment of laterality relies on a simulation of movement of your own hands, a mental rotation to align your hand on the hand to be identified. We demonstrate for the first time that this simulation mobilizes the motor cortex contralateral to the hand to identify. It thus exhibit the same asymmetries that the movement actually performed, confirming the involvement of motor processes in this judgment.
Using functional magnetic resonance imaging (fMRI), we measured the brain activity in 295 participants, including 150 left-handers while they judged the laterality of hands’ pictures with different orientations.
Asymmetry of activation in the sensori-motor cortex depends on the hand to be identify
A first result is that the right motor, premotor and parietal regions are more activated when participants identify a left hand. Conversely, the left homologous regions are more activated when they recognize a right hand. This asymmetry is therefore identical to that of the movement actually performed, for which the right motor cortex mobilizes the left hemicorps and vice versa. Our result is a very convincing indication that laterality judgment is based on motor simulation.
Averaged activation maps during the hand laterality judgement task. Green: regions more activated by the identification of a right hand than a left hand. Red: regions more activated by a left hand than a right hand. The yellow dotted line indicates the Rolandic sulcus at the level of its genu, where lies the hand motor representation. L: left, R: right
Primary motor cortex is involved in the hand laterality judgment
A second result is that the region for which this pattern of asymmetry is most significant corresponds to the part of the primary motor cortex that codes for the hand movement. While the involvement of the primary cortex in lateral judgment is debated, our result demonstrates its participation. This stresses further the similarity between actual movement and laterality judgment.
No difference between left- and right-handers in the motor cortex
The third result is that the manual preference has no effect on the asymmetry of the activations in the motor cortex: whether one is left-handed or right-handed, the asymmetry of the activations depends only on the laterality of presented the hand, Whether or not it corresponds to the dominant hand of the participant. Here again, this last result conforms to what is observed in the actual movement: the level of activation in the hand contralateral motor cortex that is moved is identical in the left and right handers.
Clinical implications
The cerebral functional similarity between movement performed and hand laterality judgment validates the proposal to include the mental simulation of movement in the therapeutic strategy of motor rehabilitation programs. In this context, the lateral hand judgment, easy to implement, could be an interesting alternative to the executed movement.
Reference
Mellet, E., Mazoyer, B., Leroux, G., Joliot, M., and Tzourio-Mazoyer, N. (2016). Cortical Asymmetries during Hand Laterality Task Vary with Hand Laterality: A fMRI Study in 295 Participants. Frontiers in Human Neuroscience 10.