FIG. 1.

(A) Total activity represented by neuronal population histograms. Neuronal activity for baseline and stimulated states, X and Y, represented in terms of neuronal population histograms (i.e., number of neurons firing at given spike rate). The stimulated states X and Y show very similar distributions, whereas the baseline states X and Y show very different distributions. The baseline state X is lower than baseline state Y when Eq. A1 (see Appendix) is applied to estimate (relative) baseline CMR_O2 values for each state. (B) Dynamic representation of total activity. In vivo time courses of the spiking rate of a neuronal ensemble in vivo for baseline states X and Y (from the same rat) during sensory stimulation. The total activity reached on stimulation for both states is quite similar as suggested from the neuronal histograms. The evoked change in neuronal activity with stimulation is much larger from baseline state X. (C) ΔCMR_O2 map from calibrated fMRI. In vivo averaged ΔCMR_O2 maps for baseline states X and Y (from the same rat) during sensory stimulation. The evoked change in CMR_O2 with stimulation is much larger from baseline state X. All data from (Smith et al., 2002). See Appendix for a detailed explanation. CMR_O2, oxidative energy demand; fMRI, functional magnetic resonance imaging.