Lessons from fMRI about Mapping Cortical

in the mam-malian cortex are often clustered into sub-millimeter-scale columns, which span the entire cortical plate (Mountcastle 1957; Hubel and Wiesel 1962). Although a direct relationship between this clustering and function is repeatedly questioned (Swindale 1990; Purves and others 1992; Horton and Adams 2005), the structure, function, and plasticity of cortical columns have no doubt been of great interest. Thus, many efforts are dedicated to visualizing functional structures. Initially, the existence of cortical columns was determined by single-unit recordings (Hubel and Wiesel 1962, 1963). Because unit recordings measure on a point-by-point basis, it takes an enormous time to record a sufficient number of neurons to obtain a partial picture of columnar arrangement. To map cortical columns effectively, techniques must be capable of acquiring high resolution and of covering large cortical areas; 2-deoxyglucose (2-DG) autoradiography and optical imaging meet these requirements. The 2-DG autoradiography measures increases in the glucose consumption rate during stimulation (Kennedy and others 1976). However, in vivo imaging is not possible, and only the map corresponding to one stimulus can be obtained. The first high-resolution mapping of functional columns in live animals was accomplished by Blasdel and Salama (1986) using a video camera and a voltage-sensitive dye. Their seminal study triggered the use of optical signals for mapping functional columns (Tso and The optical imaging of intrinsic signals detects a change in reflection of the incident light through the cortex, induced by a change in light-absorbing hemoglobin contents (Grinvald and others 1986; Frostig and others 1990). Because endogenous hemoglobin is used as a tracer, it allows mapping during repeated measurements. Even though this technique has been used extensively for mapping columnar structures, its application is limited to the upper layers of exposed cortex due to a limited penetration of light and to animal models due to the required invasive surgical preparation. An extension of mapping functional columns to humans awaits the progress of recently developed fMRI. Using high-resolution fMRI, ocu-lar dominance column–like maps in humans have been obtained by subtracting images acquired during right-eye-only and left-eye-only stimulation Setting aside the biological meaning of functional columns, the columnar structure is an ideal biological model to evaluate the spatial resolution of functional imaging techniques. There has been a persistent concern as to whether the high-resolution fMRI signals actually mark the sites of increased neural activity. This skepticism is based not only on a lack of a direct confirmation by other …