Why do we need definitions?
A neuron responds in the 100s of milliseconds. IOW, a train of pulses
can be repeated about 10 times per second. (Eg, the neurons (light
receptors) in the eye have a cycle time of about 1/10th of a second; the
illusion of motion in movies and videos depends on this.) The question
is how to interpret the neural response as a bit rate. AFAIK, the only
variation in neural response is the rapidity of the pulses within a
train, which suggests that a train carries two bits (fast/slow). Thus a
neuron operates at a bit rate of about 20.
However, the connection topology of the neurons guarantees that the
processing rate of the brain as a whole, or of subsets of it, will be
many times higher. Consider, for example, that small clusters of neurons
in the visual cortex respond to specific features of the visual field
(sometimes, a single neuron does so.) EG, most of us can pick out a
sought for face in a crowd in a matter of a few seconds at most. This
corresponds to a bit rate of several megapixels a second, as measured by
the (linear) processors used in computers. The usual phrase used to
describe this aspect of the brain is "massively parallel," an
unfortunate term IMO, since it implies a two dimensional array or tree.
In fact, the connection topology of neurons is multidimensional, since
many neurons are connected to many neurons. (See the concept of
fractional dimensions to understand how a network embedded in 3-space
can have a dimensionality higher than 3.)
As for what all this has to do with the human capacity for symbolism: I
don't think bit-rates have anything to do with it. It's the topology of
the network that has something to do with it. IMO, at the level of brain
function symbols are specific firing patterns of specific networks,
whose activity is stimulated by inputs from elsewhere in the brain (and
ultimately, in some but not all cases, by input from sensors.) If this
guess is valid, we ought to find symbolic behaviour in any system with
a sufficiently complex neural topology. Some people claim to have
observed evidence symbolic processing in apes (as distinct from
conditioned responses to arbitrary shapes and colours); if true, such
observations imply that apes have brains roughly as complex as humans.
OTOH, it seems no artificial system is complex enough to exhibit
symbolic behaviour.
BTW, mere "connectivity", as measured in average number of connections
per neuron, doesn't tell us much about the topology of the network.
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