Short-latency afferent inhibition varies with cortical state in rat somatosensory cortex

Short-latency afferent inhibition (SAI) can be used to demonstrate experimentally induced or pathological changes in cortical excitability. By recording somatosensory-evoked potentials from rat somatosensory cortex (S1), we can show that SAI further varies with the state of the cortex as deduced from the spectral composition of the electroencephalogram. SAI is strongly increased during episodes of enhanced delta-activity. The amplitude ratio of second /first somatosensory-evoked potential is significantly correlated to the ratio of theta/delta band power, but also depends on the power of higher-frequency bands. We conclude that evoked cortical inhibition is not a constant entity, but varies with the physiological state of the cortical network controlled by the brainstem arousal system.