The Journal of Neuroscience, February 15, 1998, 18(4):1571-1582
Quantitative Analysis of a Directed Behavior in the Medicinal
Leech: Implications for Organizing Motor Output
John E.
Lewis and
William B.
Kristan Jr
Department of Biology, University of California, San Diego, La
Jolla, California 92093-0357
The local bend is a directed behavior produced by the leech,
Hirudo medicinalis, in response to a light touch.
Contraction of longitudinal muscles near the touched location results
in a bend directed away from the stimulus. We quantify the relationship between the location of touch around the body perimeter and the behavioral output by using video analysis, muscle tension measurements, and electromyography. On average, the direction of the behavioral output differed from the touch location by <8% of the total body perimeter. We discuss our results in the context of two contrasting behavioral strategies: a Continuous strategy, in which
the local bend is directed exactly opposite to stimulus location, and a Categorical strategy, in which there are four distinct
bend directions, each elicited by stimuli given in a single quadrant of
the body perimeter. To distinguish between these strategies, we
delivered two competing stimuli simultaneously. The resulting
behavioral output is best described by an average of the effects of
each stimulus given alone and thus provides support for the Continuous strategy. We also use a simple model, based on anatomical and physiological data, to predict the responses of the known motor neurons
to different stimulus locations. The model shows that the activation of
two of the motor neurons (D and V) is inconsistent with a Categorical
strategy. However, these neurons are known to be active during the
local bend behavior. This result, along with our experimental
observations, suggests that the local bend network uses a Continuous
strategy to encode stimulus location and produce directed behavioral
output.
Key words:
behavioral accuracy; Hirudo medicinalis; local
bend; population coding; sensorimotor transformation; touch
Copyright © 1998 Society for Neuroscience 0270-6474/98/1841571-12$05.00/0
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