Imbalance between excitation and inhibition in the somatosensory cortex produces postadaptation facilitation

Adaptation is typically associated with attenuation of the neuronal response during sustained or repetitive sensory stimulation, followed by a gradual recovery of the response to its baseline level thereafter. Here, we examined the process of recovery from sensory adaptation in layer IV cells of the rat barrel cortex using in vivo intracellular recordings. Surprisingly, in approximately one-third of the cells, the response to a test stimulus delivered a few hundred milliseconds after the adapting stimulation was significantly facilitated. Recordings under different holding potentials revealed that the enhanced response was the result of an imbalance between excitation and inhibition, where a faster recovery of excitation compared with inhibition facilitated the response. Hence, our data provide the first mechanistic explanation of sensory facilitation after adaptation and suggest that adaptation increases the sensitivity of cortical neurons to sensory stimulation by altering the balance between excitation and inhibition.