The pattern of functional coupling of brain regions in the awake rat

The functional interactions among many regions of the rat brain were characterized simultaneously by using a new method for analyzing regional cerebral metabolic rates for glucose (rCMRglc). Regional CMRglc, a measure of regional functional activity, was determined in 68 brain regions of 24 awake, male, 3-month-old Fischer rats by the quantitative autoradiographic [14C]deoxyglucose technique. Because rCMRglc for each region varies among subjects, the functional coupling of two regions can be assessed by determining whether their metabolic rate change in a coordinated manner from rat to rat. Positive coupling arises if, for each rat, higher rCMRglc in one region is accompanied by a higher metabolic rate in another area; likewise, regions whose metabolic rates vary inversely are negatively coupled. A pairwise coupling can be quantified by calculating the linear correlation coefficient for points whose coordinates represent rCMRglc values for the two regions in each of the 24 rats. Correlation coefficients were calculated for each possible region pair among the 68 areas studied. The technique of partial correlation was used to weight equally the contribution of each rat to the correlation coefficient, regardless of its overall mean CMRglc. In general, highly positive couplings (R greater than 0.5) were found between adjacent or nearby regions, whereas distant region pairs often were coupled negatively (R less than -0.5). A large number of positive couplings were found between left-right homologous regions and between neocortical areas. Similar findings in humans, obtained by positron emission tomography, have been reported. This technique uses inter-animal variation in rCMRglc to demonstrate functional interactions among brain areas, thus providing a method to explore anatomical-functional linkages in a given experimental state.