So I’m settled down to blogging of an evening, with some chocolate (the leftover Easter candy continues), some wine, and Sci-cat. Sci-cat has a wonderful talent for sitting JUST out of reach, and purring loudly, hoping that you, her servant, will show proper deference and move to pet her. At which point she will move JUST out of reach again, purr invitingly, and do her best to look charming. She can do this for hours. Sci, however, has less patience.
But how do I KNOW that Sci-cat is just out of reach? I know that without having to reach out and miss. On the other hand, how do I know that my wine glass is within reach, without having reached to it to test my assumption? It turns out that there is a part of your brain capable of judging near distance, which can tell you if something is within reach or not.
Gallivan, et al. “Is that within reach? fMRI reveals that the human superior parieto-occipital cortex encodes objects reachable by the hand” Journal of Neuroscience, 2009.
And the best thing about this paper? The superior pariety-occipital cortex. You can call it SPOC.
Now, I will admit that fMRI (functional magnetic resonance imaging) can’t tell us everything. People tend to go a little nuts over it, and it definitely makes for some very pretty pictures. But most of what fMRI can tell you is that there is a correlation between brain activity (or at least blood flow to the brain) in certain areas, and certain things, like seeing objects, strong emotions, movement, correlate with activity in those area.
But correlation is not causation. We can’t just say that this area of the brain, and this area alone, is responsible for the particular effect seen. After all, your brain is FAR more complicated than that. For each thing you do, like, say, picking up a glass, you brain will practically light up all over. You see the glass, judge how far away it is, generate motor orders to reach your hand out and grasp it, adjust madly for the right weight and size, and then a further series brings it close to gently touch your lips, rather than smacking you in the eye. This huge ballet of brain activity is one of those things that makes Sci’s mind boggle when she contemplates science.
So you can see that to say one area of the brain, and only that area, is responsible for an effect, particularly one such as drug craving or emotional response, is to be overly simplistic. But fMRI is still a very powerful tool, capable of filtering out the massive amounts of noise your brain is generating to hone in on very specific areas during very specific tasks. And in this case, this study works beautifully in the context of the literature to take what has been found with more invasive models and bring it into a human.
Oh, and also it’s got great pictures. There’s that.
Anyway, on to the paper.
There have been some really amazing electrophysiology studies done in awake, behaving monkeys for things like robotic control of an arm and other kinds of movement. These studies have gone a long way toward showing what the brain does when it makes a movement. Not only that, they have helped to show how the brain localizes the body in space. In particular, studies in monkeys have implicated the parieto-occipital cortex in the visual responses from an object within reach.
The parieto-occipital cortex is an area of the brain which lies along the divide between the parietal lobe (known for processing sensory information), and the occipital lobe (known for primary visual processing). Previous studies have implicated this area in both reaching and pointing prepartion, both things that imply knowing where your body is in space. So this study investigated the superior parieto-occipital cortex (the SPOC!) in measures of reach, to determine if this area was involved in determining near distance.
So how do you do this? Take 10 people, give or take. Stick them in an MRI, with their arm hanging out at full reach on a table.
Note that the guy above can see the table his arm is resting on . He then has a series of objects placed on the table at various distances, some within reach, and some out of reach. The participant was then asked to either look at an object within reach, look at an object out of reach, touch an object within reach, or do nothing.
And the result looks very pretty.
The arrow on those brain scans is pointing to the SPOC. You can also see it circled on the upper right panel. It turns out that the SPOC lit up when people reached for or looked at objects. Obviously it lit up the most when the subjects actually reached for objects, but the SPOC also showed activity when subjects merely looked at objects. Not only that, the SPOC showed MORE activity for objects in the near distance, or within easy reach.
So far, this all looks like previous studies done with electrophyisology in monkeys, showing neuronal activity during reaching tasks. But there was one important difference. In the monkeys, if you gave them a tool with which to extend their reach, the activation in the SPOC expanded to cover the tool. In humans, this didn’t seem to occur.
So what does this mean? It means that the SPOC is an area important for judging distance, and probably near distance in particular. This is a lot more complicated than it sounds. It involves inputs from visual cortex, as well as inputs from lower regions on where the body is oriented in space.
Why did the SPOC not expand to cover the tool? It is possible that the SPOC in particular in humans does not code for “reach”, instead merely representing “near distance”, while in monkeys “reach” and “near distance” may overlap more extensively. Also, the areas tested in monkeys were inside the parieto-occipital sulcus, while these measurements were taking in the superior part of the cortex, so it is possible that a change of brain venue may be in order to allow us to see tool expansion in the human SPOC.
This study actually has some clinical implications. This could, for instance, be a very important brain area for studies with amputees. There are lots of possibilities for things like robotic arms, which could be connected to the SPOC to help pick up things seamlessly. Not only that, judgment of near distance is impaired in some stroke victims, and these studies into the SPOC could provide the first clues to allow those people to recover.
So it looks like now we know what makes me able to judge when Sci-cat is out of reach. But how does SHE know that she’s out of MY reach? I could blame the SPOC. But in this case, I think I’ll just blame malice.
Gallivan, J., Cavina-Pratesi, C., & Culham, J. (2009). Is That within Reach? fMRI Reveals That the Human Superior Parieto-Occipital Cortex Encodes Objects Reachable by the Hand Journal of Neuroscience, 29 (14), 4381-4391 DOI: 10.1523/JNEUROSCI.0377-09.2009
