Theoretical Basis of fMRI Signals
Our main focus concerns the origin of fMRI signals, which are mediated by a combination of neurovascular and neurometabolic coupling. Our laboratory has developed a detailed mathematical model for the fMRI signal based on fluid mechanics and oxygen transport. The results provide excellent descriptions of both oxygen measurements within activated brain tissue, and of the fMRI response evoked by the activation. The models are presently being expanded to provide a detailed picture of oxygen transport throughout the neuropil of cerebral cortex.
We are also interested in the use of fMRI to characterize human spatial vision. To this end, we have developed a powerful new method that uses computed tomography to directly and non-invasively characterize receptive fields in early visual cortex. We aim to utilize these methods to determine how top-down signals such as visual attention affect spatial vision.
Sensation and Attention in the Human Brainstem
Another line of investigation concerns the use of high-resolution structural and functional MRI methods to quantify and visualize functional activity in human brainstem. One set of experiments concern the processing of both bottom-up and top-down signals in the human superior colliculus (SC), a tiny structure that forms millimeter-scale functional maps of visual, auditory, and somatosensory inputs. SC is located near the top of the human brainstem. One goal is to examine how top-down attention affects the nature and spatial distribution of these signals. A second set of experiments have delineated how eye movements are represented in SC. Finally, a new set of experiments examine activity in a different part of brainstem, the vestibular nuclei associated with processing the sense of balance mediated by the semi-circular canals. This work is aimed at understanding the mechanisms of spasticity induced by stroke or other disease. Part of this brainstem research aims to develop new MRI methods that optimize functional data acquisition in this hard-to-image brain region.