Superficial layers in M1 and beyond: Neural evidence of cortical and spinal activation during imagined movements

Motor imagery is the mental simulation of an action without any overt movement, but shares common neural substrates with motor execution. In particular, the involvement of the primary motor cortex (M1) has been debated, and recently, Persichetti et al. (2020) probed, in an elegant experiment, the activation of different cortical layers within the hand-knob M1 area, during actual and imagined movements. By means of the vascular space occupancy technique, they achieved sub-millimeter spatial specificity without the vasculature bias of the BOLD signal. The main finding was that imagined finger movements activated only the superficial layers II/III with mainly cortico-cortical connections to M1, whereas actual finger movements activated both superficial layers and the deeper layers Vb/VI with descending corticospinal projections. This would nicely explain the absence of muscle activity during motor imagery. However, this exclusive activation of superficial layers in M1 during motor imagery is at odds with numerous observations in the current literature. In this opinion letter, we present several studies with different methodological approaches that found neural modulation downstream the pyramidal cells while imagining. These modulations following motor imagery practice, besides the changes within M1, would also explain improvement in motor learning. We also discussed the results by Persichetti et al. in regards of their protocol.