Neural basis of localized and delocalized fMRI patterns

What connections can we draw between fMRI and neural firing patterns? The basic requirement of differencing in neuroimaging has limitations because subtraction of baseline activity removes an important part of the total activity 1. Our prior studies have quantitatively revealed that most neurons in the ensemble (in a voxel) contribute differently before, during, and after the stimulation 2. Since the ensemble activity of pyramidal neurons in layer 4 depends on sensory (or localized) input by the thalamus and global (or delocalized) input from other cortical regions, delocalized signals from a global workspace may affect localized stimulus-induced responses. To test the hypothesis that delocalized signals, which have been proposed to include subjective contributions 3, can modulate localized signals we conducted fMRI and electrophysiological studies in rats under varied conditions of anesthesia and forepaw stimulation. The fMRI experiments of -chloralose (40 mg/kg/hr) and halothane (0.7%) anesthetized rats were conducted on 7.0 T or 9.4 T systems using calibrated fMRI 1, 2. The electrical activity was measured by Tungsten microelectodes. The extracellular signals were filtered to separate field and action potentials (FP, AP), where the AP data were binned for spiking frequency. While the fMRI data from both baselines (i.e., halothane and -chloralose) showed sensory-induced changes in the contralateral primary somatosensory and motor areas, there were increased delocalized activities observed with (light) halothane anesthesia. These regions were ipsilateral primary somatosensory, contralateral secondary somatosensory, as well as perirhinal and retrosplenial agranular areas. In contrast signals from delocalized regions were notably absent with (deep) -chloralose anesthesia. These results were supported by correlated changes in AP and FP time courses. The spiking frequency distributions measured from the most activated foci in the contralateral (and ipsilateral) primary somatosensory area(s). The contralateral distributions with (deep) -chloralose anesthesia showed a significant shift from low to high frequency values upon stimulation. The contralateral distribution shift was less noteworthy with (light) halothane anesthesia. The distributions for the baselines with halothane and -chloralose were significantly different, in which the degrees of global inputs assigned to higher frequencies dominated under (light) halothane anesthesia. The contralateral distributions upon stimulation with halothane and -chloralose were also significantly different, suggesting that under the (light) halothane anesthesia level the global inputs dominated whereas local changes were dominated under the (deep) -chloralose anesthesia. Since neuronal populations in different regions do not function as modules, the resultant fMRI or electrophysiological activity patterns arise from interactions of afferent and efferent connections – the latter of which was modulated by anesthesia in this study. These results provide the initial steps to explore the interactions of localized and delocalized underpinnings of the concept of the global workspace model 3 (See Figure 1).