Laminar-specific cortico-cortical loops in mouse visual cortex

Many theories propose recurrent interactions across the cortical
hierarchy, but it is unclear if cortical circuits are selectively wired to
implement looped computations. Using subcellular
channelrhodopsin-2-assisted circuit mapping in mouse visual cortex, we
compared feedforward (FF) or feedback (FB) cortico-cortical synaptic input
to cells projecting back to the input source (looped neurons) with
neighboring cells projecting to a different cortical or subcortical area
(non-looped neurons). Despite having different laminar innervation
patterns, FF and FB afferents showed similar cell-type selectivity, making
stronger excitatory connections with looped neurons versus non-looped
neurons in layer (L) 5 and L6, but not in L2/3. In most cases, these
stronger connections in looped L5 neurons were located on their apical
tufts, but not on their perisomatic dendrites. Our results reveal that
cortico-cortical connections are selectively wired to form monosynaptic
excitatory loops and support a differential role of supragranular and
subgranular neurons in hierarchical recurrent computations.