Wednesday, February 27, 2008

Moar test material!

My neuroscience prof this semester* has done some work in the intraparietal sulcus, so not only do we students get to read some of her own published papers, but we get to hear the information straight from the source.

The intraparietal sulcus, just as it sounds, is in the middle of the parietal lobe. A sulcus is just a wrinkle in the gray matter, an in-folding of cortex that increases the surface area (and thus computational ability) of the brain. The parietal lobe can generally be said to do spatial processes, with different areas specialized for different processes.

There's a whole big pile of intraparietal areas (in the sulcus) that we know about. But today's test only covers three of 'em, so you're only going to get to hear about three of 'em. But if you're interested, don't just take my word for it! Go look some stuff up!

Neuroscientists aren't necessarily the most creative at naming things. So when they opened up the intraparietal sulcus, the area closest to the middle of the brain was called the Medial Intraparietal Area (MIP), the lowest area was called the Ventral Intraparietal Area (VIP), and the outermost area was called the Lateral Intraparietal Area (LIP). Crazy, I know, but at least it's really easy to remember where each area is. Another handy thing to know is that intraparietal areas are multi-modal, that is, they respond to more than one kind of sensory input.

Area MIP responds to both visual and somatosensory (touch and body position) input. Specifically, cells in this area respond to the sight of objects within reach, the feel of objects touching the arm and hand, the feel of the hand actively exploring for objects, and the feel of the arm and hand moving. Individual cells in area MIP respond best to complex stimuli. For example, a cell in the right MIP might respond best when you (or a monkey) are reaching with your left hand for an object on your left side when the light is on (so you can see yourself reaching, and the object you're reaching for). It will still respond if you're reaching in the dark, or with the right hand, or even just when you're moving your eyes to look at the object within reach, but it won't respond as well. And it won't respond at all if you are looking at something outside of your reach. Given these things, it's pretty clear that area MIP is used for reaching and grasping things around you, which is an important thing for a person to be able to do.

One very cool thing that this guy Dr. Iriki found is that the receptive field (the area around you that is considered "within reach" and thus sets off a response) in MIP can change with tool use. He sat some monkeys down in a room with food scattered all over the floor, and marked all of the spots where the sight (and reaching) of food activated MIP. Seeing as MIP only responds for stuff within reach, all of the activating spots were pretty close to the monkey, within an arm's length. Then he trained monkeys to use little rakes to pull food closer to them. After this, he checked out MIP's activation again, and he found out that while the monkey was using the rake, MIP responded to bits of food that were further away, within a rake's reach of the monkey. That's kinda crazy cool. Anyhoo, MIP sends signals to area F5, which is a fairly high-level motor area in your brain that has motor programs for reaching and grasping things.

VIP has some very different response properties. Cells in VIP also respond to vision and touch, but it's centered around the face instead of the arm or hand. Each cell in VIP responds when you touch a specific area of your face in a certain way. For example, a cell might respond when you run something across your chin from right to left. However, the same cell will respond if you merely see something move across the space right in front of your chin from right to left, as well. Each cell's receptive field includes a patch of skin on your face or head or upper torso, and the area of space in front of that patch of skin. VIP receives input from the visual area of the brain that detects movement (MT), and it's thought to be the area that determines if you can either reach an object with your mouth or avoid it with your head. If someone throws a dodgeball at your face, this area is what tells you to MOVE MOVE MOVE! If you're bringing a bite of tasty pie to your mouth, this area is what helps you lean down and nom on it. If someone's wiggling their fingers an inch in front of your face, chanting, "I'm not TOUCHING you! I'm not TOUCHING you!" this area is why you get so annoyed.

So far we've covered "arm-centered grasping" with MIP, and what my prof* calls "mouth-centered grasping" with VIP. Area LIP is related to something that was termed "eye-centered grasping," which is just another term for bringing something from the periphery of your vision to the center of your vision. LIP cells respond to both visual and eye-movement signals. LIP cells, like all visual cells, have a specific receptive field - an area on your retina that it responds to if something interesting shows up there, like a flashing light. However, LIP cells are not only stimulated by something showing up in the receptive field, but also when you're about to move your eye so that something will show up on it, and when something was on it a second ago. Whenever you move your eye, a copy of the motor command called the corollary discharge signal is sent back to the brain. Your brain uses this signal to let your visual system know, "Ok, everything on the eye is going to shift this much," so that the world around you seems to stay still when your eyes move. About a third of your LIP cells "re-map" in advance of your eye movement -- they start firing as if they're already seeing what your retina is about to see. The rest of the cells re-map afterwards.

The cells in this area also respond to "memory traces". If a light flashes in an LIP cell's receptive field, the cell will fire for a while even after the light goes out. It's thought of as a spatial marker of sorts, saying to your brain, "There was something interesting here, here, here." The cool thing about LIP is that is also re-maps the memory traces. If a light goes on outside of the cell's receptive field and goes off, and right away you move your eye so that the light would have been in the cell's receptive field, the cell fires even though there's no light anymore. Because the memory trace of that light has been re-mapped to its new position in space - which happens to be in that cell's receptive field.

Your brain is frakkin sweet, is all I'm saying.

And that's about three lectures worth of studying, right there. I'm going to go cleanse my think-palette with some blog reading and web gaming. Hope someone read and enjoyed all this, and if not, oh well.

*In case you were wondering, my prof is Dr. Carol L. Colby, of the Department of Neuroscience and Center for the Neural Basis of Cognition at the University of Pittsburgh.

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