Slabs and Blobs in Striate Cortex

A striking technical advance in the functional-MRI technique made it possible to visualize individual orientation columns in the visual cortex of cats (D.-S. Kim et al., 2000), and the results showed excellent agreement with data from electrical recording and optical imaging studies.

Also within the ocular dominance slabs of primate visual areas are vertical blobs (sometimes called pegs) that can be seen when the tissue is stained to reveal the enzyme cytochrome oxidase. Early experiments suggested that the blobs were related to color vision (Hendrickson, 1985; Livingstone and Hubel, 1984), but later work has cast doubt on this hypothesis. Quantitative studies show that neither chromatic tuning (Lennie et al., 1990) nor orientation tuning (Leventhal et al., 1995) readily distinguishes neurons in the blobs from neurons outside the blobs. In addition, even nocturnal primates (which have few cones in their retinas) and cats (which have little color vision) have blobs; and cone-rich rodents, such as ground squirrels, do not have blobs.

Blobs extend above and below cortical layer IV but are not seen in layer IV itself. Figure 1 diagrams the organization of the primate primary visual cortex, including the large ocular dominance slabs, the orientation columns, and the blobs. Along the centers of the ocular dominance slabs, the orientation columns are arranged radially, like the “pinwheels” in Figure 10.22d of the printed text, with blobs at the centers of the pinwheels; but elsewhere the orientation columns are laid out in a rectangular arrangement, like city blocks that cross the borders of the ocular dominance slabs.

Figure 1  Organization of the Primate Primary Visual Cortex
These two partial ocular dominance slabs represent the left and right eyes, respectively. Blobs extend vertically through layers I–III and V–VI, located in the centers of ocular dominance slabs. Small columns represent the preferred orientations of groups of cells; these orientation columns radiate from the centers of the blobs. The orientation is color-coded here as in Figure 10.22d of the textbook. For simplification, this diagram does not represent spatial frequency (higher spatial frequencies occur at the edges of the blocks and lower frequencies in the centers), nor does it represent the spectrally opponent cells that occur irregularly in the columns.