Wednesday, December 17, 2008

Consciousness (3): Mr B's first look at consciousness

Now we'll look at how your garden-variety biologist (Mr B) approaches the phenomenon of consciousness. For now, we'll have him treat it as he would any other biological phenomenon.

Recall that Mr B takes a naturalistic, empirical approach to things. His first order of business is to determine what variables are correlated with conscious states (just as he did with action potential generation). We'll focus mostly on conscious perception of external events (e.g., seeing a sunset), so as to avoid the complexity of things like consciousness of one's thoughts (e.g., the experience of thinking about a chess move).

Mr B does not focus narrowly on experiments that tell us about the neural basis of consciousness, but also on experiments that reveal important details of the structure of consciousness itself and its relationship to external stimuli (i.e., psychophysics). The more empirical constraints, the better. It is possible to learn a great deal about respiration without knowing anything about the respiratory system: you can learn how the inputs (composition of air breathed) and outputs (exhaled air) relate to one another, and to other variables such as the blood pressure and breathing rate of the organism. Hence, learning about a biological mechanism doesn't mean focusing in on that mechanism exclusively: much can be learned by studying its products, how it is perturbed by inputs, etc..

The brain is necessary for conscious experience
At the grossest and most obvious level, Mr B notes that the only organ necessary for consciousness is the brain. Contrary to the Greeks' heart-based theory of mind, he knows people have literally lived without hearts, perfectly conscious, for months (article here). You can lose kidneys, arms, your stomach, etc, and while you may not be healthy or happy, you will still be conscious. Conversely, if you inactivate a brain with an anesthetic, the loss of consciousness will be quite dramatic.

The brain is sufficient for conscious experience
Take a powerful hallucinogen and entire new experiences are evoked endogenously. Something similar seems to happen while dreaming: a world is experienced that is largely independent of present sensory inputs. Amputees often feel that the removed limb is still present, moving around, making gestures, in the well-known 'phantom limb' phenomenon (this has been shown to not be due to irritation of the nerves at the end of the severed limb).

In all such cases, we experience a world that is not actually there. So the brain in effect constructs the experience. Some might like to say that the brain builds a 'representation' or 'simulation' or 'virtual reality model' of the world, and this is what we experience. Mr B may slip into such (often metaphorical) language, but for now he just means that experience is a neural construction, which is a more neutral way to put things (though note by saying it is a 'construction' he doesn't mean to imply it is a "mere construct" with no validity).

Note this hypothesis already generalizes beyond the data: Mr B is assuming that perceptual experience during normal waking periods is generated by similar mechanisms to those used during sleep, hallucinations, and phantom limbs. Mr B realizes this could be a mistake, but as a provisional hypothesis, it seems reasonable, especially given the existence of illusions generated even in healthy brains (we will have more to say about illusions later).

Implicit in the hypothesis that experience is a neural construct is the claim that neural processes of a certain sort (to be determined) are not just necessary, but sufficient for experience. Given his general biological approach, it seems a conclusion almost forced upon Mr B.

In the next post, we'll continue to follow Mr B in his quest to understand consciousness. He'll see just how complicated a problem he has taken on.

Tuesday, December 09, 2008

Consciousness (2): Introducing the garden-variety biologist

This is the second of my ongoing discussion of biological approaches to consciousness and creationists' recent attacks on such approaches. In this post I sketch a portrait of a fictional character, a garden-variety biologist we'll call 'Mr B.'

Let's assume Mr B doesn't understand how neurons fire action potentials. In the rest of this post we'll examine his general approach to the problem. In a future post we'll consider how he approaches the problem of consciousness.

He believes that neuronal excitability is likely complex, but that it will ultimately be explained in terms of individually innocuous mechanisms, a complicated orchestra of proteins, lipids, carbohydrates, and other ingredients standardly found in cells. The mechanisms should all conform to physical principles, even if many of them cannot strictly be derived from the laws of physics. For instance, if there are untethered chemicals in a neuron, he expects their diffusion to follow the rules laid out in physical chemistry.

Mr B takes an empirical approach to his subject matter. He is likely to sit down at the lab bench with an example of what he is studying (a model system), and poke and prod at it to see how it behaves.

For instance, to get a bead on how neurons are activated, he may prepare a single neuron in a dish and treat it with various chemicals (e.g., sodium, potassium, neurotoxins), expose it to different temperatures, different light and oxygen levels, etc while measuring the voltage across its membrane. Such experiments will reveal how the behavior of the neuron depends on different variables in the preparation.

The experiments, guided by his best guess at how neurons work, will help him form new ideas or refine his old ideas. For instance, when he removes sodium from a neuron's bath, he finds that the neuron stops firing action potentials. This suggests to him that the action potential is caused by an influx of sodium into the neuron.

Mr B will usually write out equations to summarize what he has observed. However, he doesn't just want to describe his observations. He will attmpt to come up with new experiments to test his ideas about the action potential (e.g., if his sodium-based theory is true, then increasing the concentration of sodium in the neuron's bath should result in a larger action potential). The desire to turn his ideas into predictions often involves translating his words into mathematics so the concepts can be more clearly expressed, make his assumptions explicit, and provide a basis for precise predictions.

So far, Mr B is not much different than a physicist or chemist. All take an empirical approach to their subject matter, prefer mathematical to word-based models, and value empirical tests of their theories.

I've been painting Mr B as a bit myopic, focusing exclusively on how this little mechanism works. This leaves out his broader uniquely biological perspective. By focusing in on mechanisms, Mr B might be able to explain how a sperm locomotes, but that will tell him nothing about its function, about its role in the biological system in which it is embedded. This biofunctional orientation is what tends to distinguish Mr B from his colleagues in physics and chemistry.

Focusing in our our example, Mr B wants to know why neurons fire action potentials. What do action potentials contribute to the nervous system's higher-order goal of controlling behavior? Are action potentials involved in signalling from neuron to neuron? Could he be studying an epiphenomenon? It could be that the mechanisms he found in the dish are not even used in vivo. For instance, is there enough extracellular sodium in the nervous system for his sodium-based theory to work? Such questions will haunt Mr B and suggest new experiments.

Some might be tempted to insist that another facet of Mr B's approach is that he takes an evolutionary perspective on what he is studying. This is certainly possible, but not essential. Mr B realizes that brains are organs that evolved to help organisms navigate the world. But this doesn't necessarily help him understand how individual neurons work, or even their functional role in an intact animal. Evolution will indirectly color his perspective on the system he is studying, and certainly he has no patience with creationists who would say that the mechanism of action potential generation could not have evolved without divine intervention. Mr B realizes he doesn't even understand the mechanisms involved yet, and that is an important prerequisite to constructing a phylogenetic history.

Before taking leave of Mr B, we should note that he believes his ignorance of neural excitability is a relatively boring psychological fact about himself, not a deep fact with profound metaphysical implications (this is a point Patricia Churchland likes to make about consciousness, but right now we're leaving aside consciousness). He knows, as he approaches the problem of neuronal excitability, that he might be like the biologists in 1900 trying to understand the mechanisms of inheritance, that it might be a long time before he succeeds. A novel conceptual and empirical infrastructure might be required before the problem can be solved, or even posed in a way that yields results. His ignorance spurs his curiosity and creativity, it doesn't make him think there is something fundamentally wrong with biology. He stubbornly resists creativity sinks such as claims that neuronal excitability is forever beyond our understanding, or that supernatural beings are required to explain the strange animal electricity observed in nervous systems.

In the next post Mr B will examine the neural basis of consciousness at an abstract level, considering what types of processes in the brain are most likely to be conscious. He will also see how daunting his task is.

Wednesday, December 03, 2008

Creationists take aim at neuroscience (1): defining their target

A recent opinion piece in New Scientist, Creationists declare war over the brain, discusses the natural alignment between antievolutionists and those that think the human mind (in particular consciousness) is forever outside the explanatory reach of neuroscience. The topic of consciousness tends to bring out the nutballs, and creationism ties people's knickers in knots, so the article has received a good deal of attention from the internet commentariat.

Since I've thought about this topic way too much, I thought I'd throw my crap into the ring too. I'll discuss the arguments of the neodualists indirectly at first, dividing my discussion of consciousness into multiple posts. Because 'consciousness' is a dirty word in some neuroscience quarters, in this post I'll clear the air by clarifying what I mean by the term.

What is consciousness?
What are you experiencing right now? For instance, are you aware of hunger pangs in your gut, words on a screen, the deep red hues of a freshly picked rose? 'Consciousness' is just another word for this ability to perceive or be aware of the world. Indeed, for those who want to avoid the C-word, 'awareness' is a perfectly good synonym.

The canonical instances of conscious awareness are moments when we are awake, alert, and attending to something interesting such as a sunset. However, even while dreaming we are conscious of something, perhaps a sort of neuronal simulation of the world.

Should scientists bother with consciousness?
Over beers many neuroscientists are dismissive when consciousness comes up. They treat it as a "philosophical" problem, a waste of time for real scientists. I find this attitude strange. New data fuel conceptual progress in science, so it seems an empirical approach is the best way to make headway on something that is clearly a real and important phenomenon. Avoiding the topic leaves it in the hands of the philosophers, a fate just a little better than death.

I suppose one could argue that there is no way to study consciousness experimentally because it is inherently subjective or something. This argument doesn't work, though, as there already exist fairly straightforward experimental probes of consciousness. For example, binocular rivalry. If you show a different image to each eye (see example rivalrous stimulus below), you don't see a fusion of the two images. Rather, you perceive the images one at a time (a dog then a cat, not a dog-cat). Neuroscientists can compare the bits of the brain that track the eye-locked stimuli (which stay the same) with those that oscillate with the visual percept. This has provided a useful roadmap that tells us which parts of the brain are locked to the stimulus, and which shift with the object of conscious awareness.

The dismissive types are typically either unfamiliar with such experimental paradigms, or they tend to be skeptical of all research with a psychological component. For the former, Koch's book The Quest for Consciousness gives a nice summary of many experiments. For those skeptical of all cognitive neuroscience, there isn't much to be done (frankly, I am sympathetic to general skepticism toward cognitive neuroscience, which is a very speculative discipline right now). Hence, my take-home argument is that consciousness is just as legitimate (or illegitimate) a research topic as more mainstream psychological phenomena like attention and memory.

I should add one caveat. I have been writing as if all uses of the term 'consciousness' refer to the same thing. This may be false. Perhaps there are separate mechanisms for different sensory modalities. Or even within a modality: for instance, there could be different mechanisms for awareness of things in the center versus the periphery of our visual field. Maybe the mechanisms that underlie dreaming have little overlap with waking awareness. It could be that 'consciousness' is a mongrel term like 'memory,' and it will splinter as the science progresses.

My next post will begin describing what a biological approach to consciousness would look like.