The choices we make define who we are, began Dr. William Kristan at the start of the 2006-07 Grey Matters Lecture Series, held monthly at the Natural History Museum in Balboa Park through April 2007. “You can be exciting or boring, loving or dangerous, trustworthy or flaky. All those things depend how you respond to the circumstances at hand and initiate behaviors.”
Kristan’s lecture, entitled “Decisions: How do we animals decide what to do?” was the inaugural event of the series, which began on Oct. 4.
During the talk, Kristan explained to the audience his work investigating the mechanics of decision making by researching the medicinal leech. At first take, the leech might seem like an unlikely subject for the study, but the field of neuroscience has a long history of success with this “comparative approach” using model organisms. For example, the research that first discovered how neurons are able to electrically communicate with each other was carried out in a squid because one particular neuron, the giant axon, of the squid is bigger in diameter than the axons of almost any other animal, which made it possible to study the electrical properties of the neuron. The basic principles underlying the nervous systems of all multi-cellular organisms are fundamentally very similar. Important discoveries about the nervous system have been made in several unlikely animals, including sea slugs, frogs and horseshoe crabs.
The leech, in turn, is an ideal model for studying decisions because its nervous system is much simpler than a human’s (13,000 neurons instead of trillions) but still complex enough to make decisions, such as whether to swim or crawl after it is poked. In addition, using a leech allows researchers to do experiments that are impossible to do with humans for technical or ethical reasons. For example, researchers can remove the nervous system from a leech and it still functions normally, allowing them to study how individual neurons act during behaviors like swimming or crawling.
Kristan’s work over the last 30 years is starting to uncover how the leech makes decisions. The decision to swim or crawl appears to happen as the result of a hierarchy of smaller decisions in a series of neurons. The first poke activates a command cell, which tells the leech to “do something.” The command cell then activates other cells “” known as group discriminating cells because they act in a group “” that narrow down the decision. Also during this time, sensory information influences the decision, such as the depth of the water. If the water is deep enough, the leech will swim; if the water is too shallow, it will crawl. Then the signal progresses to single-cell discriminators that tell the muscles to assume the proper posture to swim or crawl. Finally, motor neurons actually direct the muscles to go through the motions of swimming or crawling. The final behavior occurs about 3 seconds after the first poke, but the actual decision was made much earlier by the group discriminating cells after about 0.4 seconds.
Swimming occurs only when all of the cells in the group act in a particular pattern. If one cell does something different, a different behavior happens, such as crawling or shortening. Each behavior results from a unique combination of cells firing in a distinct pattern. Since some behaviors use the same cells in different combinations, this allows a smooth transition from behavior to behavior.
Kristan believes that people probably also make decisions in a hierarchical series. While a conscious decision such as “I will eat at a Chinese restaurant tonight” seems like a simple conscious choice, it may actually result from the culmination of a series of smaller decisions. For example, the series might progress as: “Do I want to eat? Yes. Do I want to cook or go out? Go out. Do I want to drive or walk? Walk. What restaurants can I walk to? An Irish pub, an Italian restaurant or a Chinese restaurant “” which of those sounds most appealing? Chinese.”
All along the way, additional information about how hungry you are, the weather outside, the price of gas and how recently you had Chinese contribute to those smaller decisions. A conscious decision most likely results from dozens or hundreds of subconscious decisions that we aren’t fully aware of.
While little work has been done directly on humans about how our nervous systems make decisions, it is very likely that the general principles Kristan derives from studying leeches will be applicable to humans.
The next lecture in the series, “New Drug Treatments and the Future of Stem Cells for the Aging Brain,” will be presented by Stuart Lipton of The Burnham Institute on Jan. 16, 2007 (there is no lecture in December). For more information on the Grey Matters Lecture Series, visit http://greymatters.ucsd.edu.