Columns

The second major organizing principle of the neocortex is columns. Some columnar organization is visible in stained images, but most of the evidence for columns is

based on how cells respond to different inputs.

When scientists use probes to see what makes neurons become active, they find that neurons that are vertically aligned, across different layers, respond to roughly the same input.

This drawing illustrates some of the response properties of cells in V1, the first cortical region to process information from the retina.

One of the first discoveries was that most cells in V1 respond to lines or edges at different orientations at specific areas of the retina. Cells that are vertically aligned in columns all respond to edges with the same orientation. If you look carefully, you will see that the drawing shows a set of small lines at different orientations arrayed across the top of the section. These lines indicate what line orientation cells at that location respond to. Cells that are vertically aligned (within the thin vertical stripes) respond to the lines of the same orientation.

There are several other columnar properties seen in V1, two of which are shown in the drawing. There are “ocular dominance columns” where cells respond to similar combinations of left and right eye influence. And there are “blobs” where cells are primarily color sensitive. The ocular dominance columns are the larger blocks in the diagram. Each ocular dominance column includes a set of orientation columns. The “blobs” are the dark ovals.

The general rule for neocortex is that several different response properties are overlaid on one another, such as orientation and ocular dominance. As you move horizontally across the cortical surface, the combination of response properties exhibited by cells changes. However, vertically aligned neurons share the same set of response properties. This vertical alignment is true in auditory, visual, and

somatosensory areas. There is some debate amongst neuroscientists whether this is true everywhere in the neocortex but it appears to be true in most areas if not all.