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By Root 1459 0

discovery about form perception. In the late 1950s, David Hubel and Torsten Wiesel, two brilliant neurophysiologists at Harvard Medical School, were testing the responses of visual cortical cells in cats. They would implant a microelectrode in a cell of a cat’s visual cortex; although they could not pick a particular cell, by inserting the probe at about the right spot and right angle they knew what area they were reaching. Wiesel once likened the process to spearing cherries from a bowl with a toothpick; you may not be able to see which cherry you’re spearing, but you’re sure to hit something. The cat would be restrained in a harness while the researchers shone spots or bars of light and other figures on a screen. By securely fixing the position of the cat’s head, the researchers could know which part of the retina the image fell on and link this with the cortical area being probed. Through an amplifier and loudspeaker, they could hear the cell fire; at rest, it might produce a few “pops” per second, but chatter away at fifty or a hundred pops per second when stimulated.34

Since both the retina and the cortex are complicated structures, it took great patience to discover which cells, at what location and in which layer of the cortex, respond to messages from different areas of the retina.35 One day in 1958, this excruciatingly fine-detailed work yielded an astonishing and half-accidental finding. Hubel and Wiesel had positioned an electrode tip in a cell but for hours couldn’t induce rapid firing. As Hubel recalled, a few years ago:

We tried everything short of standing on our heads to get it to fire. (It did fire spontaneously from time to time, as most cortical cells do, but we had a hard time convincing ourselves that our stimuli had caused any of that activity.) To stimulate, we were using mostly white circular spots and black spots. After about five hours of struggle, we suddenly had the impression that the glass with the [black] dot was occasionally producing a response, but the response seemed to have little to do with the dot. Eventually we caught on: it was the sharp but faint shadow cast by the edge of the glass [slide] as we slid it into the slot that was doing the trick. We soon convinced ourselves that the edge worked only when its shadow was swept across one small part of the retina and that the sweeping had to be done with the edge in one particular orientation.36

In short, the cell responded strongly to a horizontal line or edge but only weakly or not at all to a dot, a tilted line, or a vertical line.

Hubel and Wiesel (and other researchers) went on to show that some other cells are specifically responsive to lines at an angle or to vertical lines or to right angles or to distinct edges (where there is a contrast between an object and what surrounds it). It became clear that the cells of the visual cortex are so specialized that they respond only to particular details of images on the retina. Hubel and Wiesel won a Nobel Prize in 1981 for this and related brain research.

One bizarre offshoot of the Hubel and Wiesel work was the notion of the “grandmother cell”—a parody, by J. Y. Lettvin, at the time of Hubel and Wiesel’s work, of what he considered the simplistic notion that single neurons in the brain might detect and represent every object, including one’s grandmother. The parody had enough appeal to be seriously considered by some perception specialists but actually became shorthand for all the overwhelming practical arguments against a one-to-one object coding scheme.37

In any case, Hubel and Wiesel’s line-detector cells are a proven reality. Interestingly, this response is partly acquired, even though it is neurological. In a 1970 experiment kittens were reared in a vertical cylinder lined with vertical stripes and never saw horizontal lines. When they were tested for vision, at five months, they were blind to horizontal lines or objects. The neural explanation is that the cortical cells that respond to horizontal lines had failed to develop during the early stages of the kittens’ lives.38 Similarly,