Observation of two distinct spatial-temporal BOLD clusters during sensory stimulation in rats

Neuronal activity evokes changes in local CBF and CBV, whose spatial differences are not fully known. We use the Radial Correlation Contrast (RCC) analysis method with high spatial resolution 100 x 100 x 1000 microm3 data collected with an 11.7 T magnet to differentiate two spatial-temporal BOLD clusters during sensory rat forepaw stimulation and hypothesize that each corresponds to either the CBF or the CBV processes. One cluster, obtained during the time segment of stimulation onset, is characterized by a high positive BOLD signal whereas the other, obtained during the simulation decline time segment, is characterized by a lower positive signal and strong post stimulus undershoot. The average volume of stimulation onset clusters is embedded in the stimulation decline clusters with the latter significantly larger and shifted towards deeper cortical layers. Comparison of amplitude-RCC and cross-correlation analyses performed on equivalent time segments (30 s, 40 images) revealed no differences in cluster size or location, demonstrating that temporal locality is more important than spatial locality in distinguishing between stimulation onset and stimulation decline clusters. We hypothesize that clusters characterized by stimulation onset are highly weighted by local changes in CBF whereas clusters characterized by stimulation decline are more CBV weighted. Moreover, the data suggest that the locations of the highest CBF changes are distinct from the locations of the highest CBV changes. While the former located within stimulation decline clusters and its weight is gradually reduced towards cluster's periphery (mainly ventrally), the highest changes in CBV occur in the cluster's periphery with only modest changes towards its center.