UC San Diego

Functional magnetic resonance imaging (fMRI) has become a powerful tool to noninvasively localize and measure in vivo brain function, and the growth of the field has exploded. However, the strength and validity of the technique lies in understanding the relationship between the underlying neuronal activity and the fMRI signal. While we know that the fMRI signal increases in response to metabolic demands of increased neuronal activity, we do not understand the exact relationship between neural activity, hemodynamics, and the fMRI signal. The fMRI response is known to be nonlinear and disproportionately large at short stimulus durations. This dissertation examines the nonlinearity of the fMRI signal and how we may utilize a nonlinear response to understand neuronal populations on a subvoxel level. First, the hypothesis of transient neuronal onsets as a source of fMRI nonlinearity at short stimulus durations was examined using MEG and fMRI. We observed that transient neuronal activity is not the sole source of fMRI nonlinearity. Second, the validity of the fMRI adaptation paradigm was assessed by examining a well-known adaptive property (motion) in well-studied visual areas. These results were compared with psychophysical performance on a speed discrimination task and imply that MT+ is direction selective while visual areas V1, V2, V3, V3A, and V4V are orientation selective