Understanding signal processing in the brain requires that we know the neural circuitry that carries the signals. Teasing out brain circuit details has been difficult. Especially showing the connections of inhibitory interneurons, which are relatively sparse and have diverse morphologies. Optogenetic techniques provide excellent new tools for studying the inhibitory interneuron’s place in cerebral cortical circuits. The new paper “The columnar and laminar organization of inhibitory connections to neocortical excitatory cells” (published January 2011 in Nature Neuroscience) reports on studies that use optogenetic techniques to map inhibitory input onto excitatory pyramidal neurons in the primary visual, somatosensory, and motor cortex of the mouse.

Note: The maps described in the paper are defined by the somatic locations of presynaptic inhibitory neurons and postsynaptic excitatory cells and the strengths of the connections between them.

Across the cortical areas they observed lateral, intralaminar inhibition of excitatory neurons by interneurons located in either the same cortical column or an immediate neighbor, which supports the traditional view that inhibition is largely local, intralaminar, and uniform across areas. However, the research team also observed nine significant translaminar inhibitory inputs:

• Layer 4 to Layer 2/3
• Layer 4 to Layer 5A
• Layer 5A to Layer 2/3
• Layer 5A to Layer 4
• Layer 5B to Layer 2/3
• Layer 5B to Layer 4
• Layer 5B to Layer 5A
• Layer 5B to Layer 6
• Layer 6 to Layer 5B

They found that these nine motifs of inhibitory-to-excitatory connectivity recur in most of the cortical areas they examined. They determined that it was the frequency with which these structural elements were present, not their configuration, that varied between regions. The differences seen in interlaminar inhibitory strength across primary visual, somatosensory, and motor cortex was striking.