This is where we are, in 
1995, in the step-by-step 
analysis of the visual path. In 
terms of numbers of synapses 
(perhaps eight or ten) and 
complexity of transformations, 
it may seem a long way from the 
rods and cones in the retina to 
areas MT or visual area 2 in the 
cortex, but it is surely a far 
longer way from such processes 
as orientation tuning, end-
stopping, disparity tuning, or 
color opponency to the 
recognition of any of the shapes 
that we perceive in our 
everyday life. We are far from 
understanding the perception 
of objects, even such 
comparatively simple ones as a 
circle, a triangle, or the letter 
A--indeed, we are far from even 
being able to come up with 
plausible hypotheses.

    We should not be 
particularly surprised or 
disconcerted over our relative 
ignorance in the face of such 
mysteries. Those who work in 
the field of artificial 
intelligence (AI) cannot design 
a machine that begins to rival 
the brain at carrying out such 
special tasks as processing the 
written word, driving a car 
along a road, or distinguishing 
faces. They have, however, 
shown that the theoretical 
difficulties in accomplishing 
any of these tasks are 
formidable. It is not that the 
difficulties cannot be solved--
the brain clearly has solved 
them--but rather that the 
methods the brain applies 
cannot be simple: in the lingo 
of AI, the problems are 
"nontrivial". So the brain 
solves nontrivial problems. The 
remarkable thing is that it 
solves not just two or three but 
thousands of them.

    In the question period 
following a lecture, a sensory 
physiologist or psychologist 
soon gets used to being asked 
what the best guess is as to how 
objects are recognized. Do cells 
continue to become more and 
more specialized at more and 
more central levels, so that at 
some stage we can expect to 
find cells so specialized that 
they respond to one single 
person's face--say, one's 
grandmother's? This notion, 
called the grandmother cell 
theory, is hard to entertain 
seriously. Would we expect to 
find separate cells for 
grandmother smiling, 
grandmother weeping, or 
grandmother sewing? Separate 
cells for the concept or 
definition of grandmother: 
one's mother's or father's 
mother? And if we did have 
grandmother cells, then what? 
Where would they project?