Laminar excitability cycles in neocortex.

1. Laminar field potentials produced by paired electrocortical stimuli were recorded with a linear microelectrode array inserted perpendicular to the surface of rat somatosensory cortex. Current source-density (CSD) distributions of the direct cortical response (DCR) were computed from the potential profiles. Principal component analysis (PCA) was used to estimate the time course of evoked transmembrane currents of putative pyramidal cell populations in the supragranular and infragranular layers. 2. Both supra- and infragranular cells displayed an initial period after the conditioning stimulus in which test stimuli produced subnormal evoked response amplitudes. This was followed in both layers by a long period of supernormal then subnormal responses and a second period of supernormal responses. 3. The main laminar difference encountered was a general shortening of all phases of the excitability cycle in the supragranular cells. 4. Excitability cycles in the supra- and infragranular layers closely followed the morphology of average evoked responses to the conditioning stimulus alone. These results and physiological support to the validity of lamina-specific evoked response waveforms derived from combined CSD and PCA analysis of extracellular potential measurements. 5. The relationship between evoked potential amplitude changes and cortical excitability is discussed.