Scientific Understanding of Consciousness
Motor Cortex Circuit for Motor Planning and Movement
Nature 519, 51–56 (05 March 2015)
A motor cortex circuit for motor planning and movement
Nuo Li, et.al.
Janelia Research Campus, Howard Hughes Medical Institute, Ashburn, Virginia 20147, USA
Laboratory of Systems Neuroscience, National Institute of Mental Health, Bethesda, Maryland 20892, USA
Activity in motor cortex predicts specific movements seconds before they occur, but how this preparatory activity relates to upcoming movements is obscure. We dissected the conversion of preparatory activity to movement within a structured motor cortex circuit. An anterior lateral region of the mouse cortex (a possible homologue of premotor cortex in primates) contains equal proportions of intermingled neurons predicting ipsi- or contralateral movements, yet unilateral inactivation of this cortical region during movement planning disrupts contralateral movements. Using cell-type-specific electrophysiology, cellular imaging and optogenetic perturbation, we show that layer 5 neurons projecting within the cortex have unbiased laterality. Activity with a contralateral population bias arises specifically in layer 5 neurons projecting to the brainstem, and only late during movement planning. These results reveal the transformation of distributed preparatory activity into movement commands within hierarchically organized cortical circuits.
The motor cortex is critical for planning and the execution of voluntary movements. Unilateral lesions in premotor areas of motor cortex disrupt planning of movements into the contra-lesional space. Neurons in premotor cortex have activity anticipating specific movements long before movement onset, a neural correlate of movement planning. But intermingled motor cortex neurons show puzzlingly diverse selectivity for multiple movement directions with complex dynamics. The relationship of this complex preparatory activity to future movements is not understood. A key question is how preparatory activity evolves into commands that descend to motor centres to trigger movement.
In the mouse, the anterior lateral motor cortex (ALM) is involved in planning directed licking. Unilateral inactivation of ALM during movement planning interferes with upcoming tongue movements in the contralateral direction without impairing movements. A large proportion of ALM neurons exhibit preparatory activity that predicts movements, similar to premotor cortex in non-human primates. Despite the lateralized deficit from ALM inactivation, ALM neurons in each hemisphere have a preference for contra- or ipsilateral movements in roughly equal proportions.
To determine how silencing a brain area with non-lateralized selectivity causes a directional movement bias we measured neuronal activity within hierarchically organized ALM circuits. ALM projection neurons include two major classes: intratelencephalic neurons that project to other cortical areas and pyramidal tract neurons that project out of the cortex, including to motor-related areas in the brainstem. Intratelencephalic neurons connect to other intratelencephalic neurons and excite pyramidal tract neurons, but not vice versa. Pyramidal tract neurons are thus at the output end of the local ALM circuit. We show that equal proportions of intratelencephalic neurons have preparatory activity for either ipsi- or contralateral movements. Contralateral population activity in pyramidal tract neurons arises late during movement planning to drive directional licking. Our results reveal the flow of information within motor cortex circuits involved in converting preparatory activity into movements.
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