164 nature neuroscience • volume 3 no 2 • february 2000 articles Localization is a principle that is widely used in brain: cytoar- chitectonically distinct areas form the basis for functional spe- cialization 1 . Such parcellation of the cortical tissue into functional subunits is especially prominent at the level of individual cortical columns. In visual cortex of mammals, neurons with similar response properties such as ocular dominance or orientation preference are clustered into columns, spanning the entire cor- tical plate from the pia to white matter 2–4 . Studies of the struc- ture, function and plasticity of cortical columns using a variety of traditional mapping techniques, however, suffer from funda- mental limitations. For example, intra- and extracellular record- ings yield insufficient field of view, and the 2-deoxyglucose 5 technique is not viable for mapping in vivo. The optical imaging of intrinsic signals allows simultaneous recording of neuronal activity over large areas of cortex 6–9 . However, this technique can- not be considered to be noninvasive, and furthermore, its appli- cation is limited to the exposed cortical surface 9,10 . The progress of blood-oxygenation level-dependent func- tional magnetic resonance imaging 11,12 raises hope that the func- tional architecture of the living brain can be visualized noninvasively, avoiding the limitations of the aforementioned techniques. Using the paramagnetic deoxyhemoglobin as an endogenous contrast agent 11,12 , BOLD-based functional images can be obtained in vivo (in contrast to the 2-deoxyglucose (2- DG) technique 5,13 ), do not require extrinsic contrast agents (in contrast to the positron emission technique 14,15 ) and can access activation signals from the entire brain (in contrast to optical imaging 6–9 ). Most importantly, the noninvasiveness of MRI ide- ally suits this technique for studying the human brain during cognitive and perceptual tasks 16–21 . Numerous BOLD studies during cognitive 16 , motor 17 and per- ceptual 18–21 tasks indicate good spatial correlation between neu- ronal and hemodynamic responses at a coarse scale (several millimeter to centimeter), and the BOLD signal pointspread is comparable to that of optical imaging 21 . The ability of BOLD fMRI to map the columnar architecture of the brain, however, is controversial, as the ultimate functional specificity of BOLD is undetermined. Because optical spectroscopy data predicts a ‘biphasic’ BOLD response following neuronal stimulation 22,23 (with each BOLD phase potentially yielding different mapping resolutions), it becomes imperative to determine the limits of the functional specificity that can be achieved with BOLD. The exact temporal kinetics of the BOLD responses in mammalian brains, however, remain vigorously debated (compare refs. 24–26 with refs. 27, 28); thus it remains to be seen whether the func- tional specificity of BOLD is sufficient to map the basic compu- tational units of the brain’s functional architecture, namely, that of cortical columns. RESULTS To resolve the question of whether and to what extent the colum- nar architecture of the brain can be labeled using the BOLD- fMRI technique, we used ultra-high field magnets to obtain MR signals originating from individual orientation columns in cat visual cortex (area 18). Visual stimuli were optimized to drive orientation-selective, complex-type area 18 neurons 29 . In this study, area 18 was used because the distance between two iso- orientation columns is greater in this area than in area 17 (ref. 30). Furthermore, area 18 on the lateral gyrus is essentially flat in the cat and can be covered by a single imaging slice. We carried out a total of ten semi-chronic experiments in ten hemispheres of five different animals. Unless otherwise mentioned, similar results were obtained in all ten experiments. Statistical data for all ten studies are given in parentheses. Figure 1a shows an anatomical MR image of cat visual cor- tex on the lateral gyrus. All activation maps were derived from a plane tangential to area 18 on the lateral gyrus (green box, Fig. 1a). Colored pixels indicating regions of increased BOLD- signal change (Fig. 1c; see Methods) reveal the pattern of cortical activation in response to a moving grating oriented at 45°. As indicated in this panel, robust and homogenous activities were observed in the lateral gyri of both hemispheres. The region of activity extended several millimeters in anterior–posterior and medial–lateral directions. In cat area 18, the average spacing between two neighboring iso-orientation columns is ∼1.2–1.4 mm (ref. 30). Therefore, the nominal in-plane resolution of 156 × 156 μm 2 per pixel achieved in this study (see Methods) should have been sufficient to resolve individual orientation columns. As evident in Fig. 1c, however, a ‘columnar’ layout was not obtained. All four activation maps High-resolution mapping of iso- orientation columns by fMRI Dae-Shik Kim, Timothy Q. Duong and Seong-Gi Kim Center for Magnetic Resonance Research, University of Minnesota Medical School, 2021 6th Street S.E., Minneapolis, Minnesota 55455, USA Correspondence should be addressed to S.-G.K.(kim@cmrr.umn.edu) Blood-oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is an important tool for localizing brain functions in vivo. However, the ability of BOLD fMRI to map corti- cal columnar structures is highly controversial, as the ultimate functional specificity of BOLD remains unknown. Here we report a biphasic BOLD response to visual stimulation in the primary visual cortex of cats. In functional imaging, the initial BOLD signal decrease accurately labeled individual iso- orientation columns. In contrast, the delayed positive BOLD changes indicated the pattern of overall activation in the visual cortex, but were less suited to discriminate active from inactive columns. © 2000 Nature America Inc. • http://neurosci.nature.com © 2000 Nature America Inc. • http://neurosci.nature.com