We have applied functional MRI (fMRI) based on blood oxygenation level-dependent (BOLD) imagecontrast to map odor-elicited olfactory responses at the laminar level in the rat olfactory bulb (OB) elicited by iso-amyl acetate (10 ؊2 dilution of saturated vapor) with spatial and temporal resolutions of 220؋220؋1,000 m and 36 s. The laminar structure of the OB was clearly depicted by highresolution in vivo anatomical MRI with spatial resolution of 110؋110؋1,000 m. In repeated BOLD fMRI measurements, highly significant (P < 0.001) foci were located in the outer layers of both OBs. The occurrence of focal OB activity within a domain at the level of individual glomeruli or groups of glomeruli was corroborated on an intra-and inter-animal basis under anesthetized conditions with this noninvasive method. The dynamic studies demonstrated that the odorelicited BOLD activations were highly reproducible on a time scale of minutes, whereas over tens of minutes the activations sometimes varied slowly. We found large BOLD signal (⌬S͞ S ‫؍‬ 10-30%) arising from the olfactory nerve layer, which is devoid of synapses and composed of unmyelinated fibers and glial cells. Our results support previous studies with other methods showing that odors elicit activity within glomerular layer domains in the mammalian OB, and extend the analysis to shorter time periods at the level of individual glomeruli or groups of glomeruli. With further improvement, BOLD fMRI should be ideal for systematic analysis of the functional significance of individual glomeruli in olfactory information encoding and of spatiotemporal processing within the olfactory system.A critical problem in the field of olfaction is the role played by spatial activity patterns in the neural basis of olfactory discrimination. Interest in this problem has focused in particular on the olfactory bulb (OB), which is the first site of synaptic processing in the vertebrate olfactory pathway. In the mammal, a variety of functional studies have provided evidence that different odor stimuli elicit different patterns of activity within the glomerular layer (GL) of the OB where axons of the olfactory sensory neurons (OSNs) terminate (see refs. 1-8). However, the methods used to date have been limited in either temporal or spatial resolution, rendering it difficult to assess the contributions of these patterns to odor discrimination. A key to further progress is to develop methods that allow for repeated measurements of odor-induced OB activity with high spatial and temporal resolution throughout the OB in the same animal.Functional MRI (fMRI) is a noninvasive method that can provide multiple measurements in the same animal. We have developed fMRI protocols, with blood-oxygenation level dependent (BOLD) contrast (9), in the rat brain at a magnetic field strength of 7 Tesla. Using BOLD fMRI, we have mapped the response in the rat brain during electrical forepaw stimulation (10) and observed the activation of single whisker barrels (11). The spatial location and dimensions of the BOLD...

The previously established cortical representation of rat whiskers in layer IV of the cortex contains distinct cylindrical columns of cellular aggregates, which are termed barrels and correlate in a one-to-one relation to whiskers on the contralateral rat face. In the present study, functional magnetic resonance imaging (fMRI) of the rat brain was used to map whisker barrel activation during mechanical up-down movement (±2.5 mm amplitude at 8 Hz) of single/multiple whisker(s). Multislice gradient echo fMRI experiments were performed at 7 T with in-plane image resolution of 220 x 220 ,um, slice thickness of 1 mm, and echo time of 16 ms. Highly significant (P < 0.001) and localized contralateral regions of activation were observed upon stimulation of single/multiple whisker(s). In all experiments (n = 10), the locations of activation relative to bregma and midline were highly correlated with the neuroanatomical position of the corresponding whisker barrels, and the results were reproducible intra-and interanimal. Our results indicate that fMRI based on blood oxygenation level-dependent image contrast has the sensitivity to depict activation of a single whisker barrel in the rat brain. This noninvasive technique will supplement existing methods in the study of rat barrel cortex and should be particularly useful for the long-term investigations of central nervous system in the same animal.The cortical representation of rat whiskers, located in the posteromedial barrel subfield (PMBSF) part of the somatosensory cortex, contains distinct cylindrical columns of cellular aggregates throughout layer IV of the cortex. These columns (diameter, 300-500 ,um) are termed barrels because of their shape, and a one-to-one correlation between barrels and whiskers on the contralateral rat snout has been demonstrated (1, 2). Rat whiskers are sensitive tactile organs and selectively activate a predictable area in the cortex. These properties of whisker barrels make this system a unique model for study of the central nervous system.Response to whisker stimulation has been studied by various invasive methods-e.g., electrophysiological methods (1, 2), autoradiography (3), optical reflectance (4), video microscopy (5), and laser Doppler flowmetry (6). Functional magnetic resonance imaging (fMRI), based on blood oxygenation leveldependent (BOLD) image contrast (7), provides a noninvasive method to map neuronal activity. The BOLD-fMRI brain mapping method relies on physiologically induced changes in the magnetic properties of blood as determined by the oxygenation state of hemoglobin (7) and this technique has been used for mapping sensory, motor, and cognitive functions in the human brain (for review, see ref. 8). Recently, at a magnetic field strength of 7 T we used fMRI to map activation of the motor and somatosensory areas in the rat cortex during forepaw stimulation (9).The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance ...

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Functional MRI (fMRI) provides a noninvasive method for mapping brain functional activity based on blood oxygenation level dependent (BOLD) image contrast that is primarily due to localized increases in perfusion. Recently, Malonek and Grinvald (Science 272:551-554, 1996) suggested that during sustained functional activation, the increases in perfusion were spread over a much larger area than the localized electrical activity. In this study, it is demonstrated that the spatial distribution of the BOLD fMRI signal during sustained stimulation of rat whiskers has the same spatial pattern and dimension as that of neuronal electrical activity in the rat whisker barrels.