Are Brains Just Fleshy Computers?

Computers have been beating chess masters for several years now, and just last month, a computer dominated the TV quiz show Jeopardy. What's to stop a computer from achieving human-like consciousness?

Let's back up a bit. Computers can do some things that the human brain can do, like store information and perform rapid calculations, albeit much better. So it is tempting to see the computer as a kind of brain and, reciprocally, the brain as a kind of computer, but ultimately the analogy is misleading. This essay reviews a few elements of brain biology, pointing out some key differences between brains and machines.

The brain does indeed perform computations like a computer. The brain has units in networks (neurons instead of semiconductors) that take in and compile data, operate on the data, and generate output. But brains are far more than fancy calculators. It is no small matter of detail that their circuits are made of biological stuff, not metallic conductors. Their information content is defined not only by the binary logic of a circuit diagram, but also by the dynamic physiological state of the conductors and the context in which the system operates.

Computers typically perform the same operation repeatedly exactly the same way, ad infinitum, whether the end result is the calculation of the fuel consumption of a smart bomb or of the trajectory of a ball in a Wii tennis match. In contrast, the repeated use of a brain circuit alters the performance of the circuit, so the same input on two separate occasions will likely yield a different outcome. This quality is inherent in the makeup of the biological components of the circuit.

Individual neurons in the brain comprise the elements of a vast network. Indeed, the brain is more than a network — it is a network of networks. Some neurons synapse locally with other neurons in the same region; others send out long projections to other parts of the cortex (the outer layers of the brain) or to subcortical structures (clusters of neurons deep within the brain that send projections to and receive projections from the cortex and other subcortical structures).

The wiring in a simple electrical circuit tends to have fixed conductive properties, under standard environmental conditions. However, the conductive properties of neurons change markedly with use. Without getting too much into the biological detail, heavy activity through a network can induce a physical change in the neurons that comprise that network: after heavy use, the same network may be activated with lighter stimulation than was needed to activate it before the heavy usage.

Computer circuits communicate via electrical charges, or electrons. An electron is an electron is an electron, whether it transmits charge in a computer or a toaster. Neurons in a network communicate via a variety of chemicals called neurotransmitters. There are several neurotransmitter systems, each with a unique distribution through the brain and nervous system, and each serves a unique set of functions.

Some neurons primarily encode information. These include both neurons within the cortex and neurons that connect the cortex to subcortical structures, like the hippocampus and basal ganglia. Information-encoding neurons tend to employ neurotransmitters like glutamate that promote depolarization in the receiving neuron and gamma-amino-butyric acid (GABA) that inhibit depolarization. The integration of activating and inhibiting influences on neurons and networks enriches the potential for finely tuned responses. Networks that include glutamate-mediated neurotransmission can alter their properties as described above; that is, they are capable of plasticity.

Other sets of neurons emanate primarily from arcanely named subcortical structures, like the substantia nigra, raphe nucleus, and locus coeruleus, and have activating or inhibiting effects on populations of neurons over various regions of the cortex. These neurons secrete neurotransmitters like serotonin, dopamine, and norepinephrine. These three systems differ in how they radiate to the cortex. Behaviorally, the end result from the arousal of dopamine neurons tends to be heightened mobilization for action, whereas norepinephrine sharpens thinking and perception, and serotonin modulates action.

Altered function in these neurotransmitters is associated with symptoms that reflect their influential role in the regulation of cognition, emotions, and behavior. Suicide and aggression specifically have been associated with perturbations in serotonin, psychosis and addiction with dopamine, anxiety and mood disorders with norepinephrine.

In contrast to the arousing effects on specific regions of the cortex generated by dopamine, serotonin, and norepinephrine, neurons that secrete acetylcholine serve as the messengers to induce arousal in general, both deep in the brain and across the cortex.

A host of other molecules serves to translate the physiological state of the body into adaptively useful information. The hypothalamus sits at the center of this chemical network. Its neurons radiate, emitting a variety of neurotransmitters to points throughout the brain in response to changes in hormone levels, nutritional state, stress, and so on.

So one could argue that although computers can already do repetitive things better, a human brain has the capacity for a rich variety of nuanced responses to the environment, as well as the un-machine-like ability to alter its function in response to experience.