During neocortical development, distinct yet related subtypes of neurons assemble through intracortical connections to form the circuits controlling mammalian high-skilled behaviors. These intracortical projections arise from two subtypes of excitatory neurons: layer (L) 4 neurons, which receive thalamocortical (TC) input and project locally within a cortical area, and L2/3 neurons, which receive L4 input and have long-range connections to other cortical areas. While the developmental mechanisms distinguishing intracortically-projecting from subcortically-projecting neurons are increasingly understood, how distinct subtypes of intracortically-projecting neurons emerge is unknown. Given the exquisite sensitivity of L4 neurons to TC synaptic activity during postnatal development, one possibility is that activity-dependent processes also act earlier during corticogenesis to delineate the identity of L4 neurons from that of next-born L2/3 neurons. To test this hypothesis, we selectively manipulated the excitability of L4 neurons during embryonic development. For this ...