Source Modelling of the P300 with Variation of Probability in Two Modalities Using the Dipole Localization Method

Available evidence suggests that the P300, an endogenous activity believed to be an index of cognitive activity, is comprised of functionally distinct multiple sources that overlap spatially and temporally. Specifically, two questions are addressed in the present thesis. Is the P300 a reflection of more than one neural source, and if so, are these multiple sources indexing distinct cognitive processes? The classic oddball paradigm was used with visual and auditory stimuli occurring at two levels of probability. This design was used since the literature suggests that these variables may independently influence the subcomponents of the P300 and therefore distinguish them. As well, if the P300 reflects a unitary source of processing, then this source should be invariant across sensory modalities. A spatio-temporal dipole localization modelling procedure was applied to the data to determine whether the P300 is comprised of a single source or multiple sources and to determine whether the sources varied as the experimental variables of modality, task and probability were varied. The rationale was that functionally distinct sources should produce different localization results across conditions. This localization procedure models sources of neural activity as equivalent dipoles and models the head as a four-sphere volume conductor.The model employs a least-squares iterative procedure to compare observed electrical fields to those produced by the assumed sources in the spherical volume conductor. While this modelling procedure has necessary limitations, the results suggest that it was useful in determining that multiple, distributed sources comprise the P300. Furthermore, the results demonstrate that visual and auditory components are distinct, as are the sources which are involved with processing information about the frequency of events. It is not clear whether target and nontarget events are processed by functionally distinct sources. It is concluded that the umbrella component P300 is actually a manifestation of multiple modes of cognitive processing which overlap in space and time.