SummaryAnatomical and physiological experiments have outlined a blueprint for the feed-forward flow of activity in cortical circuits: signals are thought to propagate primarily from the middle cortical layer, L4, up to L2/3, and down to the major cortical output layer, L5. Pharmacological manipulations, however, have contested this model and suggested that L4 may not be critical for sensory responses of neurons in either superficial or deep layers. To address these conflicting models we reversibly manipulated L4 activity in awake, behaving mice using cell-type specific optogenetics. In contrast to both prevailing models, we show that activity in L4 directly suppresses L5, in part by activating deep, fast spiking inhibitory neurons. Our data suggest that the net impact of L4 activity is to sharpen the spatial representations of L5 neurons. Thus we establish a novel translaminar inhibitory circuit in the sensory cortex that acts to enhance the feature selectivity of cortical output.

Inhibitory neurons play a fundamental role in cortical computation and behavior. Recent technological advances, such as two photon imaging, targeted in vivo recording, and molecular profiling, have improved our understanding of the function and diversity of cortical interneurons, but for technical reasons most work has been directed towards inhibitory neurons in the superficial cortical layers. Here we review current knowledge specifically on layer 5 (L5) inhibitory microcircuits, which play a critical role in controlling cortical output. We focus on recent work from the well-studied rodent barrel cortex, but also draw on evidence from studies in primary visual cortex and other cortical areas. The diversity of both deep inhibitory neurons and their pyramidal cell targets make this a challenging but essential area of study in cortical computation and sensory processing.

Summary: Understanding how cortical activity generates sensory perceptions requires a detailed dissection of the function of cortical layers. Despite our relatively extensive knowledge of their anatomy and wiring, we have a limited grasp for what each layer contributes to cortical computation. We need to develop a theory of cortical function that is rooted solidly in each layer’s component cell types and fine circuit architecture, and produces predictions that can be validated by specific perturbations. Here, we briefly review progress toward such a theory, and suggest an experimental roadmap toward this goal. We discuss new methods for the all-optical interrogation of cortical layers, for correlating in vivo function with precise identification of transcriptional cell type, and for mapping local and long range activity in vivo with synaptic resolution. The new technologies that can crack the function of cortical layers are finally on the immediate horizon.

The neocortex is functionally organized into layers. Layer four receives the densest bottom up sensory inputs, while layers 2/3 and 5 receive top down inputs that may convey predictive information. A subset of cortical somatostatin (SST) neurons, the Martinotti cells, gate top down input by inhibiting the apical dendrites of pyramidal cells in layers 2/3 and 5, but it is unknown whether an analogous inhibitory mechanism controls activity in layer 4. Using high precision circuit mapping, in vivo optogenetic perturbations, and single cell transcriptional profiling, we reveal complementary circuits in the mouse barrel cortex involving genetically distinct SST subtypes that specifically and reciprocally interconnect with excitatory cells in different layers: Martinotti cells connect with layers 2/3 and 5, whereas non-Martinotti cells connect with layer 4. By enforcing layer-specific inhibition, these parallel SST subnetworks could independently regulate the balance between bottom up and top down input.

The connectivity patterns of excitatory and inhibitory microcircuits are fundamental to computation in the neocortex. Highly specific excitatory projections form a stereotyped microcircuit linking the six cortical layers, but it is unclear whether inhibitory circuits are structured according to a similar layer-based logic or instead wire up non-selectively across the different layers. Understanding principles of inhibitory wiring is critical, since they constrain the computational operations that cortical inhibition can perform. If subnetworks of inhibitory neurons target specific functional components of cortical circuits (e.g. cortical input and output layers), these targets could be independently modulated, enabling a richer repertoire of inhibitory computations. Here we use one and two photon optogenetic circuit mapping techniques to demonstrate that two distinct subtypes of spatially intermingled Layer 5 (L5) somatostatin (SOM) interneurons form exquisitely selective and complementary intracortical circuits. One subtype connects predominantly with L4 and L6 -the primary cortical input layers, while a second subtype connects nearly exclusively with L2/3 and L5 -the primary cortical 2 output layers. This highly specific architecture suggests that separate SOM networks could differentially modulate processing at the input and output stages of the neocortical microcircuit.