| Functional analysis of the brain has progressed much over the past 10 years through the use of functional magnetic resonance imaging (fMRI). Localization of brain activity is made possible by blood oxygenation-level-dependent (BOLD) contrast. Temporal characteristics relating to this change in signal intensity are not fully understood, however.; Many areas of study must be understood before performing fMRI research. Appendix A provides a detailed explanation of the principles of magnetic resonance imaging. Appendix B reviews the research behind the BOLD phenomenon, in which neural activation stimulates the flow of oxygenated blood, changing the local signal intensity in a magnetic resonance image. The temporal response of this intensity change yields the hemodynamic response function (HRF). Appendix C goes through the steps of fMRI data processing, delving into the realms of digital signal processing and statistics.; The main part of the dissertation focuses on the nonlinearity of the BOLD response in relation to stimuli of varying duration. Block design experiments in fMRI stimulate the brain of the subject for a specific time interval. The shape of the HRF varies with the duration of the stimulus. The relationship between increasing stimulus duration and the shape of the resulting HRF has been found to be nonlinear, meaning that the responses to short stimuli cannot be used to accurately predict the responses to long stimuli. This research more clearly defines the extent of the nonlinearity with respect to stimulus duration and examines the differences in this nonlinearity between different primary cortices in the brain (auditory, motor, and visual). |
