Read Online It's a Jungle in There: How Competition and Cooperation in the Brain Shape the Mind by David A. Rosenbaum - Free Book Online
Authors: David A. Rosenbaum
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search for “grandmother” cells has so far failed, presumably because grandmothers’ faces appear in infinitely many poses. Even if a grandmother cell were found, you’d be left wondering whether there would be a specialized cell for the sight of grandma sitting at the piano with a red tulip in the green vase atop the keyboard, a different specialized cell for the sight of grandma sitting at the piano with a
blue
tulip in the green vase atop the keyboard, and so on.
The problem with high-level detectors is that they can’t keep up with the endless combinations of experience. For any assortment of features, some other assortment can be imagined. It’s hard to believe that every possible scene can be recognized only through activation of some particular specialized cell whose
raison d’être
is recognition of just that environment. All possible stimuli can’t be anticipated, so all possible detectors can’t be pre-formed. By having relatively low-level features that can combine in novel ways, however, you have the basis for perceiving endlessly varied inputs. Low-level feature detectors have a good chance of surviving because they’re called on regularly. High-level features—or ensembles of features that correspond to more complex, varied arrangements—can survive as well but take longer to form and may have a more tenuous future.
Neural Plasticity
Another aspect of neurophysiology that points to the plausibility of the inner jungle principle is
neural plasticity
. To understand the idea of neural plasticity, it’s useful to think of a convenient but now outdated fiction about the brain. The fiction is that every region of the brain serves a fixed, hard-wired function. According to this view, cells for feeling are faithful to touch all their lives, cells for seeing are loyal to looking from cradle to grave, and so on. At a more microscopic level, the particular sensory features that individual cells are tuned to—lines of some orientation for seeing, sounds of some frequency for hearing—remain the same forever. Like sailors whose girlfriends’ names remain tattooed to their arms forever, neurons stay faithful to their first functional flames. That, anyway, was the belief associated with the hard-wired view.
It turns out that the functional properties of different brain regions are not engraved in stone but instead are malleable, or
plastic
. The term “plastic” conjures up images of squeeze bottles. More generally, though, plastic means reshapable. The brain is plastic in the sense that its functions are not hard-wired, but instead can be recast through experience. 16
Brain plasticity is central to the theme of this book. In fact, it was when I was teaching about neural plasticity that I first uttered the phrase, “It’s a jungle in there.” Neural plasticity illustrates the jungle principle vividly—perhaps more vividly than any other phenomenon in neuroscience.
What exactly is neural plasticity and how was it discovered? I’ll begin with the second question.
In the 1980s, a young neuroscientist named Michael Merzenich was puzzled by observations he made in his neurophysiological recordings of squirrel monkey brains. 17 From his training, Merzenich had a pretty good idea of the functions served by the brain structures he was studying. He knew that vision is served by one region, that hearing is served by another, that touch is served by still another, and so on. 18 He also knew that adjacent regions within each of these brain regions tend to have similar functions. Within the somatosensory cortex, for example, some neurons fire in response to touch on the left index finger, adjacent neurons fire in response to touch on the left middle finger, and so on. 19
Another principle that Merzenich was aware of was that some neural enclaves are bigger than others. Neurons turned on by tongue touches are more plentiful than neurons turned on by foot feels, neurons activated by digit dabs are more numerous
blue
tulip in the green vase atop the keyboard, and so on.
The problem with high-level detectors is that they can’t keep up with the endless combinations of experience. For any assortment of features, some other assortment can be imagined. It’s hard to believe that every possible scene can be recognized only through activation of some particular specialized cell whose
raison d’être
is recognition of just that environment. All possible stimuli can’t be anticipated, so all possible detectors can’t be pre-formed. By having relatively low-level features that can combine in novel ways, however, you have the basis for perceiving endlessly varied inputs. Low-level feature detectors have a good chance of surviving because they’re called on regularly. High-level features—or ensembles of features that correspond to more complex, varied arrangements—can survive as well but take longer to form and may have a more tenuous future.
Neural Plasticity
Another aspect of neurophysiology that points to the plausibility of the inner jungle principle is
neural plasticity
. To understand the idea of neural plasticity, it’s useful to think of a convenient but now outdated fiction about the brain. The fiction is that every region of the brain serves a fixed, hard-wired function. According to this view, cells for feeling are faithful to touch all their lives, cells for seeing are loyal to looking from cradle to grave, and so on. At a more microscopic level, the particular sensory features that individual cells are tuned to—lines of some orientation for seeing, sounds of some frequency for hearing—remain the same forever. Like sailors whose girlfriends’ names remain tattooed to their arms forever, neurons stay faithful to their first functional flames. That, anyway, was the belief associated with the hard-wired view.
It turns out that the functional properties of different brain regions are not engraved in stone but instead are malleable, or
plastic
. The term “plastic” conjures up images of squeeze bottles. More generally, though, plastic means reshapable. The brain is plastic in the sense that its functions are not hard-wired, but instead can be recast through experience. 16
Brain plasticity is central to the theme of this book. In fact, it was when I was teaching about neural plasticity that I first uttered the phrase, “It’s a jungle in there.” Neural plasticity illustrates the jungle principle vividly—perhaps more vividly than any other phenomenon in neuroscience.
What exactly is neural plasticity and how was it discovered? I’ll begin with the second question.
In the 1980s, a young neuroscientist named Michael Merzenich was puzzled by observations he made in his neurophysiological recordings of squirrel monkey brains. 17 From his training, Merzenich had a pretty good idea of the functions served by the brain structures he was studying. He knew that vision is served by one region, that hearing is served by another, that touch is served by still another, and so on. 18 He also knew that adjacent regions within each of these brain regions tend to have similar functions. Within the somatosensory cortex, for example, some neurons fire in response to touch on the left index finger, adjacent neurons fire in response to touch on the left middle finger, and so on. 19
Another principle that Merzenich was aware of was that some neural enclaves are bigger than others. Neurons turned on by tongue touches are more plentiful than neurons turned on by foot feels, neurons activated by digit dabs are more numerous
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