ECoG observations of power-law scaling in the human cortex

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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) f⁻χ from 80 to 500 Hz. This scaling index χ= 4.0± 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 f₀≃ 70 Hz, implying the existence of a characteristic time scale τ=(2πf₀)⁻¹≃ 2-4 msec. For f<f₀ we find evidence for a power-law with χₗ≃ 2.0± 0.4.