Tuesday, May 15, 2007

Basal Ganglia? Sucks!

It strikes me that, perhaps foolishly and certainly with a touch of unjustified self-confidence (I think this is what the rest of the world calls arrogance), I think I understand how cortex works (at least at an abstract, algorithmic level). In contrast, I have absolutely no idea what the hell the basal ganglia is doing. Unlike the cortex, where connections are almost entirely bi-directional, the basal ganglia consists of one-directional loops. In this respect, it is like the hippocampus. But whereas the hippocampal cells have been studied ad nauseum and a considerable amount can be said about what each individual cell is doing, the basal ganglia has not been subject to such microscopic scrutiny. I can say with certainty that this hippocampus is essential for the formation of new declarative memories (memories which you can consciously access and render into words, like the capital of France, rather than procedural memories, like how to ride a bike, or a variety of other types of non-declarative memories). If you take the hippocampus out, you can't form new memories. In rats, cells tend to respond when the animal is interacting with a particular combination of stimulus and context. Since rats are very spatial creatures, this usually takes the form of particular places in a room. In a different room, the same cell will respond to a totally different place. The only crazy thing about the hippocampus is the same loop structure which makes the basal ganglia so confusing.

So back to these loops. In the hippocampus, I've long been frustrated by the problem of getting memories out in the same for in which they are pushed in. Since the output cells in the lower layers of the entorhinal cortex are not the same as the input cells in the upper layers of the entorhinal cortex (not to mention other outputs directly from CA1 and subiculum (I think)), I don't understand why these cells should be expected to project to the same regions as the inputs of which the memory consists. This seems like putting all of the stored information through a random permutation function. Every time I ask the hippocampus to remember "cat" it tells me "20 foot tall pile of marshmallows." Eventually, I guess I'd figure out that "20 foot tall pile of marshmallows" is just it's funny way of saying "cat," and I imagine this is what the brain is doing.

The basal ganglia is even worse, though, since I don't even know what the hell it's doing. In Parkinson's disease, disruption of dopaminergic modulation of the basal ganglia induces tremors and rigidity and slowed motion. In sleepy sickness (also known as encephalitis lethargica, or "that disease in that movie with Robin Williams"), similar disruptions of dopaminergic modulation of the basal ganglia render patients almost entirely motionless. What's more, if I recall correctly, when they are briefly revived with L-Dopa, they report perseverative thinking as well. It's as if they don't move because they so occupied counting their fingers that they can't be bothered to interact with the rest of the world. Various summary-type things I've read have suggested that the basal ganglia is necessary for motor planning of various sorts, and a paper I read today claimed that basal ganglia disruptions due to Parkinson's disease prevent patients from learning subtle correlations in the environment.

So I'm left with a part of the brain which seems simultaneously absolutely essential and completely mysterious. Sucks!

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