ECoG observations of power-law scaling in the human cortex

We report the results of our search for power-law electrical signals in the human brain, using subdural electrocorticographic recordings from the surface of the cortex. The power spectral density (PSD) of these signals has the power-law form $ P(f)\sim f^{-\chi} $ from 80 to 500 Hz. This scaling index $\chi = 4.0\pm 0.1$ is universal, across subjects, area in the cortex, and local neural activity levels. The shape of the PSD does not change with local cortex activity, only its amplitude increases. We observe a knee in the spectra at $f_0\simeq 70$ Hz, implying the existence of a characteristic time scale $\tau=(2\pi f_0)^{-1}\simeq 2-4$ msec. For $f<f_0$ we find evidence for a power-law with $\chi_L\simeq 2.0\pm 0.4$.