A consensus definition for deep layer 6 excitatory neurons in mouse somatosensory, visual, and motor cortex

To understand neocortical function, it is helpful to define cortical cell types. Recent studies indicate that neurons in the deepest cortical layer play roles in mediating thalamocortical interactions and modulating brain state and are implicated in neuropsychiatric disease. However, understanding the functions of deep layer 6 (layer 6b [L6b]) neurons has been hampered by the lack of agreed-upon definitions for these neuron types. We compared commonly used methods for defining L6b neurons, including molecular, transcriptional, and morphological approaches and transgenic mouse lines, and identified a core population of L6b neurons. This population does not innervate the thalamus, unlike layer 6 corticothalamic neurons (L6CThNs) in more superficial layer 6. Rather, single L6b neurons project ipsilaterally between cortical areas. Their intrinsic electrophysiological properties were stable after the first two postnatal weeks. The four methods we identify for defining L6b neurons enable comparisons across studies testing their contributions to cortical function.