| Animals use sensory information to navigate through their environments. Two outstanding questions in behavioral neuroscience are what behavioral algorithms animals employ and how sensory input is processed to produce those behaviors. C. elegans offers an ideal system for studying such questions because of its small nervous system, genetic tractability, and relatively complex behaviors. C. elegans react to small changes in temperature, and their thermotactic behaviors are robust. When navigating a temperature gradient above their cultivation temperature, C. elegans migrate towards colder temperatures by using a biased random walk. They move in relatively straight paths interspersed with random reorientation events. They bias those events so that runs directed up the gradient are curtailed and runs directed down the gradient are extended. Below their cultivation temperature, their random walk is unbiased.;In this thesis, I show that C. elegans bias their random walks by using a short term adaptive filter as a temporal derivative detector. The time scale of the filter is ideal for detecting changes between the Poisson distributed reorientation events. I also develop a theory of history-dependent biasing of random walks that shows how the shape of the biasing filter affects the short-term movement and steady state distribution of an organism employing such a strategy. In order to produce favorable short and long term behavior, the ideal filter is adaptive and on the time scale of the random walk.;In the second portion of this thesis, I measure the calcium activity in the thermosensory neuron AFD to examine how temperature sensation is encoded in the first stage of neural processing. In an immobilized worm, AFD stores the worm's cultivation temperature, and its activity represents a short term temporal filter of temperature, which is most sensitive to temperature derivatives. It appears to perform most of the computations necessary to produce cryophilic behavior. By measuring AFD's activity in freely moving worms, I show that the adaptive filter of temperature appears to be the same as in immobilized worms. The calcium signal alone in AFD provides sufficient information to produce the biasing in worms executing thermotactic behavior. |
