Eye, Brain, and Vision by David H. Hubel

Author:David H. Hubel [Hubel, David H.]
Language: eng
Format: epub
Tags: Neuroscience, Physiological Psychology, Physiology
ISBN: 9780716750208
Google: IglhQgAACAAJ
Amazon: 0716750201
Publisher: W H Freeman & Co
Published: 1988-02-15T07:00:00+00:00

In the two figures on the previous page, a typical experiment is illustrated for part of a close-to-horizontal penetration through area 17, in which 23 cells were recorded. The eyes were not perfectly aligned on the screen (because of the anesthetic and a muscle-relaxing agent), so that the projections of the foveas of the two eyes were separated by about 2 degrees. The color circles in the figure above represent roughly the sizes of the receptive fields, about a degree in diameter, positioned 4 degrees below and to the left of the foveal projections—the records were from the right hemisphere. The first cell, 96, was binocular, but the next 14 were dominated strongly by the right eye. From then on, for cells in to 118, the left eye took over. You can see how regularly the orientations were shifting during this sequence, in this case always counterclockwise. When the shift in orientation is plotted against track distance (in the graph on the previous page), the points form an almost perfect straight line. The change from one eye to the other was not accompanied by any obvious change either in the tendency to shift counterclockwise or in the slope of the line. We interpret this to mean that the two systems of groupings, by eye dominance and by orientation, are not closely related. It is as though the cortex were diced up in two completely different ways. In such penetrations, the direction of orientation shifts may be clockwise or counterclockwise, and most penetrations, if long enough, sooner or later show shifts in the direction of rotation; these occur at unpredictable intervals of a few millimeters. The graph on the next page shows an example of a sequence with several such reversals.

We see in some experiments a final peculiarity called a fracture. Just as we are becoming mesmerized by the relentless regularity, observing shift after shift in the same direction, we see on rare occasions a sudden break in the sequence, with a shift of 45 to 90 degrees. The sequence then resumes with the same regularity, but often with a reversal from clockwise to counterclockwise. The graph on page 25 shows such a fracture, followed a few tenths of a millimeter later by another one.

The problem of learning what these groupings, or regions of constant orientation, look like if viewed from above the cortex has proved much more difficult than viewing ocular-dominance columns from the same perspective. Until very recently we have had no direct way of seeing the orientation groupings and have had to try to deduce the form from microelectrode penetrations such as those I have shown here. The reversals and fractures both suggest that the geometry is not simple. On the other hand, the linear regularity that we see, often over millimeter after millimeter of cortex, must imply a regularity at least within small regions of cortex; the reversals and fractures would then suggest that the regularity is broken up every few millimeters. Within these regions of regularity, we can predict the geometry to some extent.

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