Curbing the Other Side The two hemispheres of the brain are connected via the corpus callosum; however, this pathway and its function are still not fully understood. Palmer et al. (p. 989 ) used a combination of optogenetic, calcium-imaging, and electrophysiological methods to investigate the cellular mechanism of interhemispheric inhibition of the firing frequency of neocortical layer 5 pyramidal neurons in rats in vivo and in vitro. They discovered that this form of inhibition involved interneurons in the top layers of the cortex that suppressed active dendritic currents synergistically recruited by back-propagating action potentials. This mechanism depended upon a γ-aminobutyric acid type B receptor–mediated mechanism acting on specific ion channels in the dendrites of pyramidal neurons.