Beyond the Brain: How Body and Environment Shape Animal and Human Minds by Louise Barrett

Author:Louise Barrett
Language: eng
Format: mobi, epub, pdf
ISBN: 9781400838349
Publisher: Princeton University Press
Published: 2011-04-03T16:00:00+00:00

Chaos Is the Key

It turns out that an eerie type of chaos can lurk just behind a facade of order—and yet, deep inside the chaos lurks an even eerier type of order.

—Douglas Hofstadter

Freeman assumes that the initial neuronal strengthening that occurs during the learning process takes place via Hebbian learning (named after the great neuroscientist Donald Hebb, who came up with the idea in the first place), 9 where those neurons that fire together in response to an event become “wired” together and form an association. Alternatively—and following on from our ecological perspective of the previous chapters—we can say that the rabbits are picking up on the carrot odor–water invariant that exists in the experimental situation, so that the rabbit learns of the association between water and odor as a higher-order stimulus.

Using a combination of his empirical studies and mathematical modeling, Freeman has used this insight to come up with the idea that, over the course of learning a particular odor-water pairing, the interconnected neurons in the olfactory bulb form what he calls a “nerve cell assembly” (NCA). Excitation of any part of the NCA by a stimulus tends to cause activation of all of it, and this activity then spreads to include the entire bulb. The reason why this is so useful is that, on any given sniff, an animal receives only a small whiff of the reinforced smell, so that not all receptors will be activated. The strengthening of connections in the NCA to the extent that simulation of any part can trigger the whole means that, in effect, the NCA is amplifying the low-grade signal received from the environment, and this, in turn, provides the crucial mechanism that allows a characteristic odor-specific pattern of activity to spread over the bulb.

Modeling studies have shown that the formation of the NCA, along with the excitation of the bulbar neurons, primes the entire olfactory bulb to form a chaotic attractor that corresponds to a particular odor-reward pairing. A new attractor is formed for each new significant odor-reward pairing so that, as a result of its experiences, the rabbit’s olfactory bulb takes on a specific configuration—an “energy landscape”—that consists of several basins of attraction, each one corresponding to a specific class of learned combinations of odor and reward. 10 Interestingly, as new attractors are formed, all the other attractors rearrange themselves over the bulb, so that there is no single fixed pattern of attractors across time: the landscape of bulb activity is constantly shifting. Each new significant experience therefore changes, to some degree, the significance of all the rabbit’s previous experiences. This, in turn, influences the nature of the rabbit’s future odor experiences. In other words, we can see the mutually determining nature of the system: new experiences lead to changes in the rabbit’s brain, which then change its experience of its environment, and how it then acts, which changes its brain, which changes its experience of the environment. How an animal responds to the world (by forming a new attractor

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