The mechanisms controlling the opening and closing of critical periods is not known, but Bochner et al. found that in adult mice, genetic disruption of paired immunoglobulin-like receptor B (PirB) increased neural plasticity of the visual cortex following monocular deprivation. Furthermore, acute blockade of PirB in layer 5 cortical pyramidal neurons increased spine density and mini excitatory postsynaptic currents, indicative of formation of new functional synapses, and this suggests that PirB contributes to the active repression of plasticity in the mature nervous system.