Localizing Electrical Source Generators in the Human Brain

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LOCALIZATION OF VEP SOURCES USING A COMMON TIME FUNCTION
T. Carney*#, S. Slotnick#, S. A. Klein# & S. Dastmalchi#
*Neurometrics Institute, Oakland, CA; #University of California, Berkeley

Purpose Visual evoked potentials (VEP) have the potential of providing us with the location and time-course of visual information processing in the human brain. However, while VEP and MEG temporal sensitivity is unrivaled, its spatial localization accuracy has been poor at best. A new method is needed to avoid local minima in the non-linear least squares search for source locations. We have integrated several technologies and developed novel algorithms to improve the spatial localization accuracy of the VEP. Methods The visual stimulus was a dartboard array of patches with cortical magnification factor scaling. Each patch was independently modulated using m-sequence noise. This multi-input stimulus has the advantage of stimulating small cortical regions, minimizing eye movements to peripheral targets and allowing us to study response non-linearity. Several 16 min recordings on each subject were combined to achieve high signal-to-noise. An array of 49 electrodes was placed over the occipital region with a spacing of about 2.5 cm. Our method of analysis makes use of the known retinotopic organization of cortical areas to increase the ratio of data points to search parameters. We assume that sources for stimulus patches at the same eccentricity will have the same temporal function. This assumption enables us to identify sources for patches where the signal-to-noise ratio is low. Results As expected, sources corresponding to the stimulus patches were localized to the opposite brain hemisphere and the general dorsal-ventral inversion was upheld. Conclusions Multi-input stimulus methods, multi-electrode recording and data analysis that is cognizant of known cortical organization offers an improved method of identifying and localizing the cortical sources of the VEP.

From: Investigative Opthalmology & Visual Science, 1998, 39(4), S182