Neural circuits
Nov 21, 2012, 11:33a - Science
Neuroscience, the study of the brain, is absolutely fascinating.
But why choose neuroscience over any other pursuit?
We can try to understand an infinite set of things in our world, from the economic effects of rape to the forces that keep atoms together. But only biology takes a stab at trying to understand the very first thing, *us*. Without us we couldn't ask any other question at all. So the ultimate existential question is to ask, "How do we exist?" When I say "we", I could be referring to our bodies, in which case biology and medicine would be the best places to start. But when I say "we", I'm really referring to our minds, the consciousness that each one of us has. Neuroscience, a small corner of biology, goes much further than biology towards quenching this existential thirst, because it tries to understand our very essence: it tries to understand our minds and how they work.
So once I've chosen neuroscience, which part should I work in?
Neuroscience as a field includes a whole lot more than the study of consciousness. In fact, consciousness is barely even studied, mostly because there isn't a clear observable and also because many researchers are not motivated by the existential question. Most researchers spend their time studying things I find mostly boring, like how the brain develops or how specific molecules function in specific neural events (like how a molecule helps a synapse release neurotransmitter). In principle, development of the brain is absolutely fascinating, as it's the story of how the organism builds its own brain. But in reality development is utterly boring, just a string of molecular and cellular events leading to an outcome, with few interesting principles. I also find molecular neuroscience in general very boring, because so what if molecule X influences whether a synapse secretes neurotransmitter. If it's molecule Y instead of X, would it really matter from a philosophical perspective? The project is one of molecular cataloging and dissection, and neurons just happen to be the cell type of interest. There's nothing inherently neuroscience-y about it - those biologists could be studying a molecular event in any other cell type just as well.
If we increase the scale, we start getting somewhere. "Systems" neuroscience tries to study the functioning of systems of neurons, for example the hippocampus, an area known to be important for forming new memories. And instead of focusing on molecules, researchers focus on electrical activity as the basic phenomenon under observation. I find this more interesting, because our minds are not just the molecular and cellular structures of our brains, but the activity that goes on within these structures. And our minds are not just any activity, but the fast activity, illustrated by our ability to perceive changes in the environment less than a second after they actually happen.
But these system neuroscientists have a different problem than the molecular neuroscientists. While they've identified electrical activity as an element that seems critical for consciousness, they've lost their ability to really dissect the circuit. The hippocampus consists of millions of neurons with elaborate connectivity, and it's buried deep in the functional circuit of the brain, so it's unclear what input it actually receives and what output it distributes.
So is there a middle ground between systems neuroscience and molecular neuroscience?
In fact, there is, though it seems to be under-researched. Maybe I'd call it "circuit" neuroscience. The goal of circuit neuroscience is to identify the signal transformation properties of a neural network from start to finish. As an example: worms move toward food based on its smell. How do they do this? There are specific neurons that directly sense this smell, and they respond electrically in a specific way. This signal is transmitted to another neuron, which sends this signal to yet another neuron, which controls the muscles that enable the worm to move in specific directions. This is an example of a neural circuit. Unlike the hippocampus example above, we can follow the smell signal from sensation all the way to behavior. And now we know how the worm reacts the way it does to the smell of food, on a more mechanistic level.
I posit that consciousness is likewise a result of neural activity. So if I want to understand consciousness, it's best to start understanding neural circuits, and how exactly they transform the signals that they receive.
Surprisingly, there is fairly minimal research on these start-to-finish neural circuits, when compared to molecular, developmental, and systems neuroscience. It's so rare and under-appreciated that a friend and I decided to teach a short class on neural circuits this January at MIT. It's open to the MIT community, so if you're interested in understanding neural circuits from input to output you might want to drop by.
Here's the abstract:
Title: Gap-free Neural Circuits: From Sensory Input to Motor Output
Abstract: Why do people act the way that they do? How sensory input alters the behavioral output of living organisms is a fascinating question in neuroscience. While this is difficult to study in a gap-free manner at the cellular level in mammals, gap-free neural circuits have been identified and their signal transformation properties characterized in simpler organisms. On each day of this class we will discuss a single neural circuit that has been worked out at the cellular level, including how each neuron in the circuit transforms the incoming physiological signal using specific molecules. Circuits will be derived from primary experimental data. We will focus on circuits for which the neurons that sense the stimuli are known, the interneurons are known, and the motor neurons controlling muscle contraction and the resulting behavior are known. Circuits will be drawn from several invertebrate organisms, including the genetic organisms C. elegans and Drosophila, as well as the locust, crayfish and cricket. After this class students will have a precise understanding of several different neural circuits as well as the methods used to identify and analyze these circuits. By providing several examples of real neural circuits, principles for how circuits function in general may become apparent. Students, post-docs and professors welcome.
If you're interested and at MIT come on by.
[This is the second in a series of posts on the study of consciousness. Here's the first post which gives a definition of consciousness.]
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