Although the brain-computer metaphor has served cognitive psychology
well, research in cognitive neuroscience has revealed many important
differences between brains and computers. Appreciating these differences may be
crucial to understanding the mechanisms of neural information processing, and
ultimately for the creation of artificial intelligence. Below, the most
important of these differences are covered .
Difference # 1: Brains are analogue; computers are digital
It's easy to think that neurons are essentially binary, given that they
fire an action potential if they reach a certain threshold, and otherwise do
not fire. This superficial similarity to digital "1's and 0's" belies
a wide variety of continuous and non-linear processes that directly influence
neuronal processing.
For example, one of the primary mechanisms of information transmission
appears to be the rate at which neurons fire - an essentially
continuous variable. Similarly, networks of neurons can fire in relative
synchrony or in relative disarray; this coherence affects the strength of the
signals received by downstream neurons. Finally, inside each and every neuron
is a leaky integrator circuit, composed of a variety of ion channels and
continuously fluctuating membrane potentials.
Difference # 2: The brain uses content-addressable memory
In computers, information in memory is accessed by polling its precise
memory address. This is known as byte-addressable memory. In contrast, the
brain uses content-addressable memory, such that information can be accessed in
memory through "spreading activation" from closely related concepts.
For example, thinking of the word "fox" may automatically spread
activation to memories related to other clever animals, fox-hunting horseback
riders, or attractive members of the opposite sex.
Difference # 3: The brain is a massively parallel machine; computers are
modular and serial
An unfortunate legacy of the brain-computer metaphor is the tendency for
cognitive psychologists to seek out modularity in the brain. For example, the
idea that computers require memory has lead some to seek for the "memory
area," when in fact these distinctions are far more messy. One consequence
of this over-simplification is that we are only now learning that
"memory" regions (such as the hippocampus) are also important
for imagination, the representation of novel goals, spatial navigation, and other diverse
functions.
Difference # 4: Processing speed is not fixed in the brain; there is no
system clock
The speed of neural information processing is subject to a variety of
constraints, including the time for electrochemical signals to traverse axons
and dendrites, axonal myelination, the diffusion time of neurotransmitters
across the synaptic cleft, differences in synaptic efficacy, the coherence of
neural firing, the current availability of neurotransmitters, and the prior
history of neuronal firing. Although there are individual differences in
something psychometricians call "processing speed," this does not
reflect a monolithic or unitary construct, and certainly nothing as concrete as
the speed of a microprocessor. Instead, psychometric "processing
speed" probably indexes a heterogenous combination of all the speed
constraints mentioned above.
Difference # 5 - Short-term memory is not like RAM
Although the apparent similarities between RAM and short-term or
"working" memory emboldened many early cognitive psychologists, a
closer examination reveals strikingly important differences. Although RAM and
short-term memory both seem to require power .Short-term memory seems to hold
only "pointers" to long term memory whereas RAM holds data that is
isomorphic to that being held on the hard disk.
Difference # 6: No hardware/software distinction can be made with
respect to the brain or mind
For years it was tempting to imagine that the brain was the hardware on
which a "mind program" or "mind software" is executing.
This gave rise to a variety of abstract program-like models of cognition, in
which the details of how the brain actually executed those programs was
considered irrelevant, in the same way that a Java program can accomplish the
same function as a C++ program.
"Difference # 7: Synapses are far more complex than electrical
logic gates
Another pernicious feature of the brain-computer metaphor is that it
seems to suggest that brains might also operate on the basis of electrical
signals (action potentials) traveling along individual logical gates.
Unfortunately, this is only half true. The signals which are propagated along
axons are actually electrochemical in nature, meaning that
they travel much more slowly than electrical signals in a computer, and that
they can be modulated in myriad ways. For example, signal transmission is
dependent not only on the putative "logical gates" of synaptic
architecture but also by the presence of a variety of chemicals in the synaptic
cleft, the relative distance between synapse and dendrites, and many other
factors. This adds to the complexity of the processing taking place at each
synapse - and it is therefore profoundly wrong to think that neurons function
merely as transistors.
Difference #8: Unlike computers, processing and memory are performed by
the same components in the brain
Computers process information from memory using CPUs, and then write the
results of that processing back to memory. No such distinction exists
in the brain. As neurons process information they are also modifying their
synapses - which are themselves the substrate of memory. As a result, retrieval
from memory always slightly alters those memories usually making them stronger,
but sometimes making them less accurate.
Difference # 9: The brain is a self-organizing system
This point follows naturally from the previous point - experience
profoundly and directly shapes the nature of neural information processing in a
way that simply does not happen in traditional microprocessors. For example,
the brain is a self-repairing circuit - something known as "trauma-induced
plasticity" kicks in after injury. This can lead to a variety of
interesting changes, including some that seem to unlock unused potential in the
brain (known as acquired savantism), and others that can
result in profound cognitive dysfunction (as is unfortunately far more typical
in traumatic brain injury and developmental disorders).
Difference # 10: Brains have bodies
This is not as trivial as it might seem: it turns out that the brain
takes surprising advantage of the fact that it has a body at its disposal. For
example, despite your intuitive feeling that you could close your eyes and know
the locations of objects around you, a series of experiments in the field
of change blindness has shown that our
visual memories are actually quite sparse. In this case, the brain is
"offloading" its memory requirements to the environment in which it
exists.
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