Independent component analysis and the functional architecture of the inferior parietal cortex of behaving monkey

Optical imaging of the intrinsic signal from deoxyhemoglobin at 605 shows that the IPL has a functional topography for the eye position gain field (Siegel et al, 2001). In order to extract the maximal information for the spatial (30 um) and temporal information (2 Hz) available in the intrinsic signal data, independent component analysis was utilized (Bell and Sejnowski, 1995). Regions of the cortical surface could be segregated; often these cortical patches had signals that preceded optical signals in veins that drained them. The tuning of the patches and the draining vessels matched. Surprisingly the strength of the signal in the vessels greatly exceeded that of the patches of cortex. Vessels that were in close proximity could drain different cortical patches and as a result have drastically different dependence on eye position. We hypothesize that the incoming neural activity first activates cortex; the modified blood then is collected and concentrated in vessels. The interrelationships between the functional architecture and the angioarchitectonics remain unexplored. These results substantially impact interpretation of fMR, as these changes are likely spatially low-pass filtered. Thus the tuning of a vessel, which should predominate in the fMR signal, may not correctly indicate the tuning of the nearby cortex. Possibly erroneous conclusions may be drawn from such data. Higher temporal resolution data and measurement of the EKG will further clarify the contribution of the cortex and circulation to the optically measured signals and ultimately to the slower and lower spatial resolution fMR measurements.