The neocortical column has been considered as an elementary unit for information processing. Spatially arranged columns interact with each other through horizontal connections. In order to understand how the horizontal interaction regulates the activity of columns in motor control, we developed a model of the primary motor cortex (M1) considering realistic spatial distribution of the neural connectivity. The model consists of four layers: 2/3, 5A, 5B, and 6. Each layer has excitatory neurons and inhibitory neurons. The connection probability among neurons across/within layers was based on experimental reports using laser scanning photo stimulation on M1 [1].
First, to investigate the contribution of horizontal connection in the model, we stimulated neurons within a 90 micrometer diameter sphere centered in layer 2/3 and recorded synaptic currents from the cells at different horizontal distances. The synaptic currents decreased with the distance from the stimulus site, with less than 50 % at 400 micrometer, which was similar to the experimental result in the barrel cortex.
We further investigated interaction among neural populations by stimulating two neighboring sites (diameter 90 micrometer) in layer 2/3 simultaneously. When we set different strengths of stimulus to two sites, inhibition from strongly stimulated site prevented the neurons in the other site from firing. The result suggests that the neural population in layer 2/3 comparable to column size may have the ability to generate lateral inhibition.
Finally, assuming sensory input to layer 2/3, and top-down input to layer 5B, we simultaneously stimulated layer-2/3 and layer-5B cells of two neighboring columnar populations, with different intensity for layer-2/3 subpopulations and the same subthreshold intensity for layer-5B subpopulations. Lateral inhibition in layer 2/3 led selective activation of neurons in a column with strong stimulus, which resulted in selective activation of a layer-5B subpopulation including corticospinal neurons. The results suggest that the horizontal connections in M1 may enable selection of motor outputs by lateral inhibition among columns.

[1] Katzel D. et al. 2011. Nature Neuroscience 14: 100-107.