A unifying principle underlying the extracellular field potential spectral responses in the human cortex

Abstract

Electrophysiological mass potentials show complex spectral changes upon neuronal activation. However, it is unknown to what extent these complex band-limited changes are interrelated or, alternatively, reflect separate neuronal processes. To address this question, intracranial electrocorticograms (ECoG) responses were recorded in patients engaged in visuomotor tasks. We found that in the 10- to 100-Hz frequency range there was a significant reduction in the exponent χ of the 1/ fχ component of the spectrum associated with neuronal activation. In a minority of electrodes showing particularly high activations the exponent reduction was associated with specific band-limited power modulations: emergence of a high gamma (80–100 Hz) and a decrease in the alpha (9–12 Hz) peaks. Importantly, the peaks' height was correlated with the 1/ fχ exponent on activation. Control simulation ruled out the possibility that the change in 1/ fχ exponent was a consequence of the analysis procedure. These results reveal a new global, cross-frequency (10–100 Hz) neuronal process reflected in a significant reduction of the power spectrum slope of the ECoG signal.

Interpreting the electrophysiological power spectrum

“…These events give rise to the general 1/f background. The flat-spectrum signal, I propose, arises from the aggregate spiking activity of the local population, which has been previously cited as a probable contributor to the LFP (Buzsaki et al 2012;Schomburg et al 2012). In reality, the two signals are likely interrelated, because changes in presynaptic activity, which are possibly reflected in the broadband shift seen in Manning et al (2009) andMiller et al (2007), will certainly cause changes in local population activity.…”

“…The Fourier power spectrum, or frequency representation, can often shed more insight on the phenomenon of interested, as exemplified by the target article, a recent study from Podvalny et al (2015). The target article showed several behaviorally relevant changes in the power spectrum slope that are not captured by the inverse power law model [P(f) ϳ Af ].…”

“…With this, several key findings raised by Podvalny et al (2015) can be explained within a simplified mathematical framework. First, it is observed that the addition of a flatspectrum signal dominates the low-power (high frequency) portions of the power law PSD, flattening the slope at those frequencies while having no effect in the lower frequencies, thus introducing an upward deflection (Fig.…”

“…2C in the Podvalny et al 2015, the target of this Neuro Forum, as well as Buzsáki and Draguhn 2004;Buzsáki et al 2012). This is mathematically equivalent to a negative linear relationship in the log-log domain [log P(f) ϳ log A ϩ log f].…”

“…colleagues (2007, 2009) subsequently supported this observation through computational modeling of the LFP, while also having observed activation-related shifts and a stable slope in power spectra of human ECoG, after accounting for a bend, or knee, at ϳ70 Hz. Recently, Podvalny et al (2015) reported slope changes in the 10-to 100-Hz range in human ECoG PSD during a visuomotor attention task, corresponding to a "rotation" of the spectrum with slopes between Ϫ3 and Ϫ1. Similar observations were made in both rabbit and human ECoG data previously, where Freeman and Zhai (2009) attributed the overall rotation of the spectrum to changes in rise times of the impulse response of a recurrent excitatory network.…”