The type of decision which a neuron faces-and this can take place up to a
thousand times per second-is this: whether or not to fire-that is, to
release ions down its axon, which e\entually will cross over into the entry
ports of one or more other neurons, thus causing them to make the same
sort of decision. The decision is made in a very simple manner: if the sum
of all inputs exceeds a certain threshold, yes; otherwise, no. Some of the
inputs can be negative inputs, which cancel out positive inputs coming from
somewhere else. In any case, it is simple addition which rules the lowest
level of the mind. To paraphrase Descartes' famous remark, "I think,
therefore I sum" (from the Latin Cogito, ergo am).
Now although the manner of making the decision sounds very simple,
there is one fact which complicates the issue: there may be as many as
200,000 separate entry ports to a neuron, which means that up to 200,000
separate summands may be involved in determining the neuron's next
action. Once the decision has been made, a pulse of ions streaks down the
axon towards its terminal end. Before the ions reach the end, however,
they may encounter a bifurcation--or several. In such cases, the single
output pulse splits up as it moves down the bifurcating axon, and by the
time it has reached the end, "it" has become "they"-and they may reach
their destinations at separate times, since the axon branches along which
they travel may be of different lengths and have different resistivities. The
important thing, though, is that they all began as one single pulse, moving
away from the cell body. After a neuron fires, it needs a short recovery time
before firing again; cha.racteristically this is measured in milliseconds, so
that a neuron may fire 'up to about a thousand times per second.
Larger Structures in the BrainNow we have described the brain's "ants". What about "teams", or "sig-
nals"? What about "symbols"? We make the following observation: despite
the complexity of its input, a single neuron can respond only in a very
primitive way-by firing, or not firing. This is a very small amount of
information. Certainly for large amounts of information to be carried or
processed, many neurons must be involved. And therefore one might guess
that larger structures, composed from many neurons, would exist, which
handle concepts on a higher level. This is undoubtedly true, but the most
naIve assumption-that there is a fixed group of neurons for each different
concept-is almost certainly false.
There are many anatomical portions of the brain which can be distin-
guished from each other, such as the cerebrum, the cerebellum, the
hypothalamus (see Fig. 66). The cerebrum is the largest part of the human
brain, and is divided into a left hemisphere and a right hemisphere. The
outer few millimeters of each cerebral hemisphere are coated with a
layered "bark", or cerebral cortex. The amount of cerebral cortex is the major
distinguishing feature, in terms of anatomy, between human brains and
brains of less intelligent species. We will not describe any of the brain's
suborgans in detail because, as it turns out, only the roughest mapping can(^340) Brains and Thoughts