outer ring. If an on-center pattern is shown to an off-center neuron, the
neuron will slow down in firing (and vice versa). Uniform illumination will
leave both types of retinal neuron unaffected; they will continue to fire at
cruising speed.
From the retina, signals from these neurons proceed via the optic
nerve to the lateral geniculate, located somewhere towards the middle of
the brain. There, one can find a direct mapping of the retinal surface in the
sense that there are lateral-geniculate neurons which are triggered only by
specific stimuli falling on specific areas of the retina. In that sense, the
lateral geniculate is disappointing; it seems to be only a "relay station", and
not a further processor (although to give it its due, the contrast sensitivity
seems to be enhanced in the lateral geniculate). The retinal image is coded
in a straightforward way in the firing patterns of the neurons in the lateral
geniculate, despite the fact that the neurons there are not arranged on a
two-dimensional surface in the form of the retina, but in a three-
dimensional block. So two dimensions get mapped onto three, yet the
information is preserved: an isomorphism. There is probably some deep
meaning to the change in the dimensionality of the representation, which is
not yet fully appreciated. In any case, there are so many further un-
explained stages of vision that we should not be disappointed but pleased
by the fact that-to some extent-we have figured out this one stage!
From the lateral geniculate, the signals proceed back to the visual
cortex. Here, some new types of processing occur. The cells of the visual
cortex are divided into three categories: simple, complex, and hyper-
complex. Simple cells act very much like retinal cells or lateral geniculate
cells: they respond to point-like light or dark spots with contrasting sur-
rounds, in particular regions of the retina. Complex cells, by contrast, usu-
ally receive input from a hundred or more other cells, and they detect light
or dark bars oriented at specific angles on the retina (see Fig. 67). Hyper-
complex cells respond to corners, ban., or even "tongues" moving in specific
directions (again see Fig. 67). These latter cells are so highly specialized
that they are sometimes called "higher-order hypercomplex cells".
A "Grandmother Cell"?Because of the discovery of cells in the visual cortex which can be triggered
by stimuli of ever-increasing complexity, some people have wondered if
things are not leading in the direction of "one cell, one concept"-for
example, you would have a "grandmother cell" which would fire if, and
only if, your grandmother came into view. This somewhat humorous
example of a "superhypercomplex cell" is not taken very seriously. How-
ever, it is not obvious what alternative theory seems reasonable. One possi-
bility is that larger neural networks are excited collectively by sufficiently
complex visual stimuli. Of course, the triggering of these larger mul-
tineuron units would somehow have to come from integration of signals
emanating from the many hypercomplex cells. How this might be done,
nobody knows. Just when we seem to be approaching the threshold where(^344) Brains and Thoughts