Neuroanatomists from Cajal on (1) have searched in the cerebral cortex for units of structural organization that transcend the laminar pattern visible even to the untutored eye in Nissl-stained preparations. Many have commented on the vertical column-like arrays of cell bodies running orthogonal to the horizontal laminae that are particularly conspicuous in the temporal cortex of humans and other primates (Fig. 1). These columns have been promoted in the past as the morphological correlates of the functional columnarity of the cortex, known from physiological studies (2). The hypothesis of the column as the fundamental processing unit of the cerebral cortex was formulated by Mountcastle (3) from studies of cells responding to tactile stimuli in the somatosensory cortex of the cat. The hypothesis requires that nerve cells in middle layers of the cortex, in which thalamic afferents terminate, should be joined by narrow vertical connections to cells in layers lying superficial and deep to them, so that all cells in the column are excited by incoming stimuli with only small latency differences. The columns form a series of repeating units across the horizontal extent of the cortex. Figure 1 Nissl-stained section of the upper bank of the superior temporal sulcus from a human brain, showing microcolumns of nerve cells (×40). The verticality of cell–cell connections in the cortex has never been in doubt, but determining the minimal unit of such connectivity and the extent to which it is based on morphologically definable arrays of cells continues to exercise investigators. One problem inherent in the different experimental models customarily invoked as demonstrative of cortical columns is that they differ in scale. A column defined by neurons responding to peripheral stimulation as a vertically oriented microelectrode descends through the cortical layers, is narrower than the layer IV barrels of the rodent somatosensory cortex, and the …