Data from: Layer 6 corticocortical neurons are a major route for intra and interhemispheric feedback

The neocortex comprises anatomically discrete yet interconnected areas
that are symmetrically located across the two hemispheres. Determining the
logic of these macrocircuits is necessary for understanding high level
brain function. Here we have mapped the areal and laminar organization of
the ipsilateral and contralateral cortical projection onto the primary
visual, somatosensory and motor cortices in mice. We find that although
the ipsilateral hemisphere is the primary source of cortical input, there
is substantial contralateral symmetry regarding the relative contribution
and areal identity of input. Laminar analysis of these input areas show
that excitatory Layer 6 corticocortical cells (L6 CCs) are a major
projection pathway within and between the two hemispheres. Analysis of the
relative contribution of inputs from supra- (feedforward) and
infra-granular (feedback) layers, reveals that contra-hemispheric
projections reflect a dominant feedback organization compared to their
ipsi-cortical counterpart. The magnitude of the interhemispheric
difference in hierarchy was largest for sensory and motor projection areas
compared to frontal, medial or lateral brain areas due to a proportional
increase in input from L6 neurons. L6 CCs therefore not only mediate
long-range cortical communication but also reflect its inherent feedback
organization.