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SPATIAL & TEMPORAL RESOLUTION of fMRI: Current Research


Functional magnetic resonance imaging (fMRI) techniques provide the capability of visualizing increased neuronal activity with high spatial resolution and specificity. Further, time-resolved single-trial event-related fMRI allows changes in neural activity to be detected at a temporal resolution on the order of one second. However, most neural processes occur on a time scale of milliseconds, rather than seconds. To further improve the temporal resolution of fMRI, it is critical to understand the exact relationship between neuronal activity and the corresponding hemodynamic response. In this application, we aim to elucidate the temporal characteristics of the hemodynamic response induced by neural activity, compare hemodynamic and neural activity in the same subject, and develop a new hemodynamic response model which allows prediction of neural activity from fMRI data.

Temporal Resolution
Single-trial fMRI images were acquired at 4 Tesla during visually-instructed, delayed four-digit movement. Empty circles, corresponding to fingers, were sequentially filled, one every 700 ms, with a total presentation time of 2.1 s, defining the order of finger movements. After a 7.0 sec delay time, the GO sign was indicated and the subject pressed the button switches in the prescribed order. Three coronal slices containing primary visual and motor areas were obtained at 197 ms intervals. Spatiodynamic functional maps displayed using a hotmetal color scale were overlaid on 3-D anatomic figures (top: all slices with transparent anatomical data; lower left: primary visual cortex; lower right: primary motor cortex and cerebellum). Initially, activation was observed at the primary visual cortex. About 9 sec later, motor cortex was activated during the four-digit sequential movements. This figure indicates that dynamic brain activity on a timescale of seconds can be detected using true single-trial fMRI at high magnetic field strength. Higher temporal resolution requires an understanding of the relationship between dynamic neural activity and the fMRI signal.

In order to further understand temporal limits of fMRI, it is critical to examine the relationship between neural activity and hemodynamic responses. For this, we simultaneously measure neural activity using field potentials and hemodynamic responses using intrinsic optical imaging and laser Doppler flowmeter in well-established animal models. The Pittsburgh site investigates the spatiotemporal relationship between neural activity and fMRI during visual stimulation as well as during somatosensory stimulations. The Akita site (sub-contract) is probing the temporal relationship between neural activity and blood flow during hindpaw stimulations in rats.


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