Cognitive Science: An Introduction/Brain Architecture and Processing

Although the main focus of cognitive science is on studying minds at an information processing level, for natural intelligences (animals such as human beings), we can use constraints from our biology, particularly that of the brain, to constrain theories.

There are several different processes you need to think about to understand how the brain does what it does. The first is the firing of neurons--or, more accurately, changes in the firing rates of neurons. The second is the concentration of neurotransmitters, and the third is the synchrony of firing, as measured in brain waves.

Neuron Firing
Different brain areas are connected and neuron firing allows communication between these areas. Neuron firing rates, and changes in them, represent information in the brain that can be passed from one area to another. There are hundreds of neuron types, and thousands of subtypes, each with their own shape, biochemistry, connections to other types, and electrical properties. In this section we will describe behaviors typical of most neurons.

A neuron can "fire," which means that it sends a signal to other neurons. When a neuron fires, it releases chemicals (called neurotransmitters) into a space between itself and the next neuron. This space is called the synapse. Receptors of this next neuron collect these neurotransmitters. The places where neurotransmitters get picked up are called dendrites. Generally, these neurotransmitters are excitatory or inhibitory. Receiving neurotransmitters changes the electrical properties of the neuron. If it reaches a certain threshold state, then this neuron will fire too. This process sends a signal through a structure called an axon, which leads to synapses with the dendrites of yet more neurons. This is a cartoon sketch of something that is much more complex, but it is basically true.

It has been estimated that human brains can process about 120 bits or sensory information per second.

Let's take a simple example of timing. It takes about a millisecond for a neuron to generate an electrical spike. This means that the higher-level information processes that use spikes as information must work at a slower rate. That is, mind activity that uses neuron spiking has to be slower than a millisecond.

Similarly, we can look at how fast a neuron can transmit a signal. Axons are the part of the neuron that sends messages to other neurons, and axon messages move at speeds ranging from about several meters per second to hundreds of meters per second (that's about 1 mph to 268 mph). The connections between neurons, called synapses, also take time. Chemical synapses take about one half and a few milliseconds. There are also electrical synapses, which reduce this time to almost zero. This is fast, but not that fast, putting significant time delays in processing.

Most information processing in the brain is communicated through neurons. A neuron is a cell that takes information in (usually from other neurons). If it gets enough information, it "fires" and sends information along to something else (again, usually other neurons).



(tk describe neuron firing)

But firing or not firing isn't really the important thing. What's really important is the firing rate. So even though a neuron firing or not is binary, the firing rate can vary continuously. Firing rates range from between 0 to 500 spikes per second (for which there is about a 2-millisecond interval between spikes). There is no such thing as a negative firing rate.

We can see that the firing of a neuron is binary, but the firing rate is analog. In the visual system, for example, the firing rate of many neurons indicates the brightness of light falling on a particular part of the retina. This means that, in a sense, whether or not the brain is digital or analog depends on what level of abstraction you're focusing on.

Neurotransmitters
Neurons communicate with each other by releasing chemicals called "neurotransmitters." There are a whole lot of different neurotransmitters (about 40 in humans), and they all have multiple functions. Some of the ones you'll hear about often are dopamine, serotonin, acetylcholine, and norepinephrine. The chemical concentration of particular neurotransmitters can affect the neuron firing, and thus thinking. For example, if someone has too much dopamine in their system, they are better at detecting patterns, but also more prone to compulsive behaviors.

Some neurotransmitters are also neuromodulators, that change how neurons respond to other neurons, making them more or less sensitive to input from them. These are not momentary effects. This way, the neuromodulators can change brain behavior without rewiring neurons.

Brain Waves
The final way to understand how the brain does what it does is by considering brain waves. Large groups of neurons can fire in synchronicity, and these rhythmic patterns also have an effect on thinking. These can be measured by machines such as EEGs.