It's very common for people to wonder whether a brain is a kind of computer. And, by analogy, whether a mind is a program that runs in the brain. I thought I'd jot down some of my thoughts on these questions.
A human brain is comprised of over a hundred billion neurons and over a trillion glial cells. A neuron is a special type of cell that is good at receiving, processing and transmitting information. Like most other kinds of cells, a neuron has a full complement of cellular machinery including DNA and mitochondria. In addition, it has an axon and dendrites that allow it to form complex connections with large numbers of other neurons.
The neurons in a brain are organized into two square sheets called hemispheres, each about 35cm x 35cm in size and 2mm thick. The neurons themselves are organized into 6 layers. The hemispheres are connected by a couple of bundles of neurons, the largest of which is called the corpus callosum.
How does a brain think? Well, it's clear that it's not born with a lot of pre-programmed knowledge. Instead, the neurons seem to embody some general purpose algorithms for learning, creating, evaluating, and acting. These algorithms are implemented on the cellular level, but only achieve their power when operating in a large network of cells.
In computer science terms, a brain is a massively parallel computer whose processors are highly interconnected and running general purpose algorithms. The algorithms contain very little specific knowledge, but instead allow a brain to learn from its environment.
Given that the brain does indeed seem to be a kind of computer, why do so many people react negatively to the analogy? The reason is that a typical computer is very simple and not like the brain at all. Most PCs have just a few processors compared with the brain's hundred billion processors. Similarly, most PC software is written by humans to do a specific task, compared with the brain's processors that run general purpose algorithms for learning.
Another difference is that a PC program is typically written in a computer language like Java or C that runs on a silicon-based CPU. The "program" that a neuron "runs" is written in DNA, proteins, and cellular structures, and its "memory" is methylation and dendritic connections.
The old adage "computers can only do what you program them to do" is thus both true and highly misleading. In the case of the brain, it has been "programmed" (by evolution) to do just a few things, including how to learn. There is no reason why we cannot program digital computers with the same algorithms and thus create machines that can learn, think and act.
One of the cool things about a brain is that its algorithms are capable of learning techniques that in turn make the brain more efficient. For example (as I mentioned in my recent post about Genius), a brain can learn how to speed read and then use this new knowledge to accelerate its own learning. These additional techniques do not change the algorithms running on individual neurons, but affect their connections so that these higher-level algorithms run on the network level. The brain is thu capable of supporting many different algorithms operating on many different layers of abstraction, all bootstrapped from the initial algorithms running at the cell level.
My guess is that the algorithms running on individual neurons are very simple. I bet that when we figure out how to create a digital mind the amount of code running on each cell will be very small, maybe just 100K or so. However, this code will be profound and will embody principles of information processing that we are only currently catching glimpses of.
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