Broadband changes in the cortical surface potential track activation of functionally diverse neuronal populations.

We illustrate a general principal of electrical potential measurements from the surface of the cerebral cortex, by revisiting and reanalyzing experimental work from the visual, language and motor systems. A naive decomposition technique of electrocorticographic power spectral measurements reveals that broadband spectral changes reliably track task engagement. These broadband changes are shown to be a generic correlate of local cortical function across a variety of brain areas and behavioral tasks. Furthermore, they fit a power-law form that is consistent with simple models of the dendritic integration of asynchronous local population firing. Because broadband spectral changes covary with diverse perceptual and behavioral states on the timescale of 20-50 ms, they provide a powerful and widely applicable experimental tool.

Cortical electrode localization from X-rays and simple mapping for electrocorticographic research: The "Location on Cortex" (LOC) package for MATLAB.

Medically refractory epilepsy accounts for more than 30% of the epilepsy population. Scalp EEG electrodes have limited ability to localize seizure onset from deep structures and implantation of subdural electrodes with long term monitoring provides additional information. Apart from clinical application, this patient population provides a unique opportunity for acquiring electrocorticography data in research paradigms. We present a method for rapid localization of electrodes using lateral and anterior-posterior X-rays. Skull landmarks and proportions are used for co-registration with the standardized Talairach coordinate system. This MATLAB-based "Location on Cortex" (LOC) package facilitates rapid visualization of clinical and experimental data in a user-friendly manner.

Task-related principal component analysis: formalism and illustration.

We address the application of a modified form of Principal Component Analysis (PCA) to data which is characterized by sparse, but known, event times. The sparsity of such event times makes it unlikely that they have a major contribution to the overall covariance in the data, and standard PCA components generated from this covariance may not give us useful insight about the task. A simple method is shown here which generates an orthogonal, "task-related PCA" (trPCA) transform based upon correlations between non-simultaneous, event-time locked, subsets of data. Non-simultaneity is the constraint that epochs of data are only compared to epochs of data from other points in time, which explicitly selects for reproducible effects. The prescription for trPCA is presented within the context of a fusiform face area experiment for illustration. In this experiment, a reproducible, face-stimulus specific, negative potential deflection is observed 200ms (N200) after presentation. We demonstrate how this N200 phenomenon, initially distributed across a subtemporal electrocorticographic (ECoG) array, may be isolated in a single component using trPCA.

Real-time functional brain mapping using electrocorticography.

We demonstrate the feasibility of real-time cortical mapping from arrays of subdural electrodes using the electrocorticographic signal power in the higher spectral frequencies (76-200 Hz, or "chi-index"). Hand area was mapped offline in eight individuals using brief baseline and hand-movement measurements. In one patient, hand sensorimotor cortex was identified online during a handshake. We propose that this high-frequency component of the electrocorticogram provides a generic, reliable, clinically useful correlate of local cortical function.