A key step in the neuronal analysis of a behavior is to identify its sensorimotor transformation, i.e. the function that relates the amplitude and waveform of the stimulus to the amplitude and waveform of the response. To study the this transformation, we devised a concentration clamp that delivers rapid changes in attractant concentration to unrestrained worms crawling on a typical substrate. This was done by placing worms individually on a thin porous membrane supported over a yoked pair of miniature, inverted showerheads. Each showerhead emitted an attractive salt solution with a different salt concentration. Step-wise changes in salt concentration were delivered by sliding the showerhead assembly relative to the worm. Using this new device, we found that the step response is consistent with chemotaxis toward regions of high salt concentration, exhibits distinct early and late phases, varies nonlinearly with the sign and amplitude of the step, and is altered by experience and neuronal ablation. These results offer some of the first insights into how the sensorimotor transformation might be computed at the level of individual neurons.

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