Gödel, Escher, Bach An Eternal Golden Braid by Douglas R. Hofstadter

neurons would reliably create specific images or sensations in the patient.
These artificially provoked impressions ranged from strange but indefin-
able fears to buzzes and colors, and, most impressively of all, to entire
successions of events recalled from some earlier time of life, such as a
childhood birthday party. The set of locations which could trigger such
specific events was extremely small-basically centered upon a single
neuron. Now these results of Penfield dramatically oppose the conclusions
of Lashley, since they seem to imply that local areas are responsible for
specific memories, after all.
What can one make of this? One possible explanation could be that
memories are coded locally, but over and over again in different areas of
the cortex-a strategy perhaps developed in evolution as security against
possible loss of cortex in fights, or in experiments conducted by
neurophysiologists. Another explanation would be that memories can be
reconstructed from dynamic processes spread over the whole brain, but
can be triggered from local spots. This theory is based on the notion of
modern telephone networks, where the routing of a long-distance call is
not predictable in advance, for it is selected at the time the call is placed,
and depends on the situation all over the whole country. Destroying any
local part of the network would not block calls; it would just cause them to
be routed around the damaged area. In this sense any call is potentially
nonlocalizable. Yet any calljust connects up two specific points; in this sense
any call is localizable.

Specificity in Visual Processing

Some of the most interesting and significant work on localization of brain

processes has been done in the last fifteen years by David H ubel and

Torsten Wiesel, at Harvard. They have mapped out visual pathways in the

brains of cats, starting with the neurons in the retina, following their

connections towards the rear of the head, passing through the "relay

station" of the lateral geniculate, and ending up in the visual cortex, at the

very back of the brain. First of all, it is remarkable that there exist well-

defined neural pathways, in light of Lashley's results. But more remarkable

are the properties of the neurons located at different stages along the

pathway.

It turns out that retinal neurons are primarily contrast sensors. More

specifically, the way they act is this. Each retinal neuron is normally firing at

a "cruising speed". When its portion of the retina is struck by light, it may

either fire faster or slow down and even stop firing. However, it will do so

only provided that the surrounding part of the retina is less illuminated. So

this means that there are two types of neuron: ·"on-center", and "off-

center". The on-center neurons are those whose firing rate increases

whenever, in the small circular retinal area to which they are sensitive, the

center is bright but the outskirts are dark; the off-center neurons are those

which fire faster when there is darkness in the center and brightness in the

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