Bootstrap assessment of the reliability of maxima in surface maps of brain activity of individual subjects derived with electrophysiological and optical methods

The last decade has seen an enormous increase in the attention that is paid to the relative contributions of various areas of the brain to perception, cognition, and movement planning and execution. This has been accompanied by the development and/or increased use of several noninvasive brain imaging techniques, such as PET, fMRI, and, more recently, transcranial optical imaging (Toga & Mazziotta, 1996). In electrophysiology, thanks also to electronic and computational advances, this emphasis has corresponded to a large increase in the number of scalp locations used to record event-related brain potentials (ERPs) and the magnetoencephalogram (MEG). When ERP data are collected from dense electrode arrays, surface maps of the scalp distribution of potentials can be generated (e.g., Duffy, 1982; Gevins, 1996). One of the main uses of these maps is to examine possible differences in the surface localization of brain potentials at a glance, assuming that different intracranial generators and/or different cognitive processes may be active when scalp distribution differences are observed. One of the features of surface maps that researchers pay attention to is the location of foci of maximum activity, either within a time window that may encompass an entire ERP component (such as P3) or within a smaller segment of the average waveform. For example, investigators may focus on a change in the location of the maximum of the P3 component from a frontal to a parietal focus, over the course of a task, or for different groups of subjects (e.g., Fabiani & Friedman, 1995; Friedman, Simpson, & Hamberger, 1993). Because of this increased emphasis on scalp distribution, the issue of the within-subject reliability of some of the scalp distribution features (such as the location of maxima) becomes important. However, there are no established methods for assessing the reliability of these features in individual subjects. A major problem is that assumptions about the normality of the distribution of values observed at each electrode are often violated. This may limit the validity of parametric techniques. Some investigators (e.g., Karniski, Blair, & Snider, 1994) have proposed the use of distribution-free approaches in the analysis of scalp The research reported in this paper was supported by Grant AG05213 from the National Institute of Aging to D.F. and by NIMH Grant 5R01MH57125-01 to G.G. A preliminary report of these data was presented at the 35th Annual Meeting of the Society for Psychophysiological Research, Toronto, Canada. Correspondence should be addressed to M. Fabiani, Department of Psychology, University of Missouri, 210 McAlester Hall, Columbia, MO 65211-2500 (e-mail: [email protected]). Bootstrap assessment of the reliability of maxima in surface maps of brain activity of individual subjects derived with electrophysiological and optical methods