I joined the faculty at Cedar Crest College in August 2009, so the class of 2013 is, in some ways, also my class as they are the students who were new to Cedar Crest, and spent four years learning all about it alongside me.  I don’t teach classes that freshmen take, so I didn’t actually see many of them until they were sophomores or beyond.  One student who came in that year, Lauren Della, is someone I ended up working on research projects two years in a row with, during her junior and senior years.  One of the great privileges of teaching is to watch students grow as they learn.  Grow in knowledge, grow in confidence, grow in leadership.  And all of those were definitely true of Laurian.

brain

A lovely gift from a wonderful student.

When she graduated this past spring, Laurien gave me a wonderful gift, a ceramic brain with a figure of a man with a cane walking along the parietal lobe.  I love it.  It’s sitting on my desk, though a piece of it fell off (from the inferior temporal lobe) and people coming into my office all had to comment on my broken brain.  It was part of her Art Therapy show, and I love that she integrated brain and behavior so beautifully and creatively.  One of the many things the parietal lobe is responsible for is helping to understand and navigate the space around us, so the figure walking across the surface is especially fitting.

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The parietal lobe.

It seems appropriate, then, to make the parietal lobe the theme of this series of blogs.  The parietal lobe is located on top of the brain, slightly toward the back and center.  We began the course by looking at the role the parietal lobe plays in consciousness.  People who suffer from unilateral neglect fail to be aware of information coming in from the opposite side.  So if someone has suffered a stroke that damages their right parietal lobe they will ignore things on their left side.  They will write on half a piece of paper, draw half a house, or eat half a pancake.  It’s not that they are numb or blind on the left side;  their sensory capabilities are not diminished.  They simply are not conscious of things on the left, and act as if any objects to their left simply do not exist.

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An individual with unilateral neglect drew the flower on the right.

As I mentioned earlier, the parietal lobe helps us to organize and navigate space, not only in terms of what’s out there, but also how we orient relative to objects in the spatial environment.  Also, the parietal lobe also provides feedback for how our body is oriented in space.  So it’s easy to see why damage to this part of the brain manifests itself in an inability to attend to spatial information to the left side of the body.

Most of the sensory information that comes into the brain goes through the back half first to be processed, then is sent forward to the front half to be acted upon.  In addition to kinesthetic feedback from the muscles and joints, the back portion of the brain, including the parietal lobe, also processes information from some of the other senses, particularly touch and vision.  For many of our senses, information from the sensory organs is topographically mapped onto the brain.  The best example of this is the somatosensory cortex, which is an area in the parietal lobe, just before the central fissure and running down from the center to the side of the brain along the surface.  This is the part of the brain that receives touch information from various places around your body.  The entire surface of your body is mapped onto this area of the brain.  It is a distorted map, because some parts of your body, such as your fingertips, are more sensitive and thus require more space in the brain to process the information coming from them, than other areas, such as your back or your thighs.

somatosensory cortex

The parts of our body are topographically mapped in our brains on the primary motor cortex (in red) and the somatosensory cortex (in green). The somatosensory cortex is located in the parietal lobe, just before the central fissure.

Recently, researchers have discovered that the parietal lobe also plays an important role in numerosity or “number sense” seen in many species. Numerosity doesn’t refer to counting because that involves associating specific quantities with a symbolic representation in the form of a number, which is a pretty advanced cognitive capability.  Instead, what our number sense allows us to do is to estimate quantity in a general way, such as when we’re offered a choice between two slices of pie and know that even though we might want the bigger piece, for the sake of our diets we need to take the smaller piece.  It’s the sense that allows us to make “greater than” or “less than” distinctions.

Harvey, Klein, Petridou, and Dumoulin (2013) recently studied the number sense in humans specifically to determine whether or not there was a similar topographic organization in the brain for numerosity despite the fact that there were no specific sense organs associated with a number sense.  They presented stimuli containing different numbers of dots and measured brain activity with a high-field fMRI.  Different areas of the parietal lobe responded to different numbers of dots in the stimuli.  When the number of dots in the stimulus was small, they found the greatest level of activity in the most forward part of the parietal lobe.  As the number of dots increased, not only did the focus of neural activity move further and further back along the parietal lobe, but the total number of neurons activated by the stimuli decreased.  These findings help to explain  why we tend to be good at estimating small numbers of objects, but become less and less accurate as the total number of objects in the stimuli increases.  The authors suggest that our parietal lobe acts as a sort of internal abacus, which is an ancient calculator that represents numbers spatially (Lewis, 2013).

Though it may not seem so, this is an important sense.  For most species, their day is spent searching for and acquiring resources such as food.  The ability to estimate quantity, or even to judge one potential source of food as better in terms of the amount it can provide over another site helps to make the foraging process more efficient.  The less energy an organism has to expend in order to acquire resources to survive, the better, and even something as simple as being able to quickly judge the richness of a resource compared to another one can make all the difference.

References

Harvey, B.M., Klein, B.P., Petridou, N., & Dumoulin, S.O. (2013).  Topographic representation of numerosity in the human perietal cortex. Science, 341(6150), 1123-1126 doi: 10.1126/science.1239052

Lewis, T. (2013). Is “Numerosity” Humans’ Sixth Sense? 

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