Summary:We have examined the relationship between cerebral blood volume (CBV) and electrophysiology over primary somatosensory cortex (S-I) in the rat. We did this by comparing the spatial characteristics and time course of activity-related changes in plasma fluorescence, intrin sic optical reflectance signals, and single unit electro physiology in S-I to identical stimuli. S-Is of urethane anesthetized male Sprague-Dawley rats were exposed, and fluorescent Texas Red dextran dye (MW 70,000) was administered intravenously. Subsequently, foredigit elec troshock or vibrissal deflection was associated with fluo rescence increases over contralateral forelimb or postero medial barrel subfield cortex. Fluorescence was delayed and prolonged, indicating that CB V increases at 1-1.5 s and peaks 2-2.5 s after the onset of stimulation in both regions. When stimulus intensity was adjusted to produce barely detectable fluorescence foci (10% above back ground), significant electrophysiologic spiking was seen. At these parameters, fluorescence change overlay areas Cerebral metabolic activity and vascular perfu sion are functionally linked. Regional increases in perfusion have been used to map cerebral activation (Phelps and Mazziotta, 1985), and the characteriza tion of that functional perfusion has provided in sights into cerebral metabolism and neurovascular relationships. Positron emission tomography perfu sion studies have demonstrated the physiological uncoupling between oxygen supply and metabolic Received July 25, 1994; final revision received November 3, 1994; accepted November 5, 1994. Abbreviations used: CBV, cerebral blood volume; CCD, charge-coupled device; FL, forelimb cortex; PMBSF, postero medial barrel subfield; PSTH , peristimulus time histogram; S-I, primary somatosensory cortex; SNR, signal-to-noise ratio. 754of increased cortical layer III cell firing on single unit recordings. However, surface boundaries of the smallest observable fluorescence foci at their peak spatial extents consistently overspilled electrophysiologic center recep tive fields. Corresponding intrinsic optical reflectance de creases were seen at 610 and 850 nm, exhibiting similar timing and colocalizing closely with fluorescence increase at both wavelengths after identical stimuli. These signals similarly overspilled electrophysiologic activity. Thus, we observed delayed increases in vascular fluorescence (related to CBV) over activated cortex. The smallest de tectable fluorescence changes overspilled the center re ceptive field boundaries and were associated with appre ciable electrophysiologic firing. In addition, the striking spatial and temporal similarity between intrinsic optical reflectance and fluorescence activity suggests that changes in intrinsic cortical reflectance are strongly re lated to changes in CBV. Key Words: Cerebral blood vol ume-Electrophysiology-Somatosensory cortex.oxygen usage Fox et ai., 1986), as well as providing evidence for a human visual area V5 (Watson et ai., 1993). However, the spatial and temporal rel...