By a most ingenious experiment, illustrated in the diagram on the next page, Murray Sherman and his colleagues went on to establish beyond any doubt the importance of competition in geniculate shrinkage. They first destroyed a tiny part of a kitten's retina in a region corresponding to an area of visual field that receives input from both eyes. Then they sutured closed the other eye. In the geniculate, severe cell atrophy was seen in a small area of the layer to which the eye with the local lesion projected. Many others had observed this result. The layer receiving input from the other eye, the one that had been closed, was also, as expected, generally shrunken, except in the area opposite the region of atrophy. There the cells were normal, despite the absence of visual input. By removing the competition, the atrophy from eye closure had been prevented. Clearly the competition could not be in the geniculate itself, but one has to remember that although the cell bodies and den-
drites of the geniculate cells were in the geniculate, not all the
cell was there:
FURTHER STUDIES IN NEURAL PLASTICITY
The original experiments were followed by a host of others, carried out in many laboratories, investigating almost every imaginable kind of visual deprivation. One of the first and most interesting asked whether bringing up an animal so that it only saw stripes of one orientation would result in a loss of cells sensitive to all other orientations. In 1970 Colin Blakemore and G. F. Cooper, in Cambridge University, exposed kittens from an early age for a few hours each day to vertical black-and-white stripes but otherwise kept them in darkness. Cortical cells that preferred vertical orientation were preserved, but cells favoring other orientations declined dramatically in number.
It is not clear whether cells originally having non-vertical orientations became unresponsive or whether they changed their preferences to vertical. Helmut Hirsch and Nico Spinelli, in a paper published the same year, employed goggles that let the cat see only vertical contours through one eye and only horizontal contours through the other. The result was a cortex containing cells that preferred verticals and cells that preferred horizontals, but few that preferred obliques. Moreover, cells activated by horizontal lines were influenced only by the eye exposed to the horizontal lines, and cells driven by vertical lines, only by the eye exposed to the vertical lines.
Other scientists have reared animals in a room that was dark except for a bright strobe light that flashed once or a few times every second; it showed the animal where it was but presumably minimized any perception of movement.
The results of such experiments, done in 1975 by Max Cynader, Nancy Berman, and Alan Hein, at MIT, and by Max Cyander and G. Chernenko, at Dalhousie in Halifax, were all similar in producing a reduction of movementselective cells. In another series of experiments by F. Tretter, Max Cynader, and Wolf Singer in Munich, animals were exposed to stripes moving from left to right; this led to the expected asymmetrical distribution of cortical direction-selective cells.
It seems natural to ask whether the physiological or anatomical changes produced by any of these deprivation experiments serve a useful purpose. If one eye is sewn closed at birth, does the expansion of the open eye's terrain in layer 4C confer any advantage on that eye? This question is still unanswered.
It is hard to imagine that the acuity of the eye becomes better than normal, since ordinary acuity, the kind that the ophthalmologist measures with the test chart, ultimately depends on receptor spacing (Chapter 3), which is already at the limits imposed by the wavelength of light. In any case it seems most unlikely that such plasticity would have evolved just in case a baby lost an eye or developed strabismus. A more plausible idea is that the changes make use of a plasticity that evolved for some other function—perhaps for allowing the postnatal adjustments of connections necessary for form analysis, movement perception and stereopsis, where vision itself can act as a guide.
Although the deprivation experiments described in this chapter exposed animals to severely distorted environments, they have not involved direct assaults on the nervous system: no nerves were severed and no nervous tissue was destroyed. A number of recent studies have indicated that with more radical tampering even primary sensory areas can be rewired so as to produce gross rearrangements in topography. Moreover, the changes are not confined to a critical period in the early life of the animal, but can occur in the adult.
Michael Merzenich at the University of California at San Francisco has shown that in the somatosensory system, after the nerves to a limb are severed or the limb amputated, the region of cortex previously supplied by these nerves ends up receiving input from neighboring areas of the body, for example from regions of skin bordering on the numb areas. A similar reorganization has been shown to occur in the visual system by Charles Gilbert and his colleagues at the Rockefeller University. They made small lesions in corresponding locations in the two retinas of a cat; when they later recorded from the area of striate cortex to which the damaged retinal areas had projected, they found that, far from being unresponsive, the cells responded actively to regions of retina adjacent to the damaged regions. In both these sets of experiments the cortical cells deprived of their normal sensory supply do not end up high and dry, but acquire new inputs. Their new receptive fields are much larger than normal, and thus by this measure the readjustments are crude. The changes seem to result in part from a heightening of the sensitivity of connections that had been present all along but had presumably been too weak to be easily detected, and in part from a subsequent, much slower development of new connections by a process known as nerve sprouting. Again it is not clear what such readjustments do for the animal; one would hardly expect to obtain a heightening of perception in the areas adjacent to numb or blind areas—
indeed, given the large receptive fields, one would rather predict an' impairment.
In 1974 an experiment by Sherman, Guillery, Kaas and Sanderson demonstrated the importance of competition in geniculate-cell shrinkage. If a small region of the left retina of a kitten is destroyed, there results an island of severe atrophy in the corresponding part of the upper layer of the right lateral geniculate body. If the right eye is then closed, the layer below the dorsal layer, as expected, becomes atrophic—except for the region immediately opposite the upper-layer atrophy, strongly suggesting a competitive origin for the atrophy resulting from eye closure.