This paper presents a quantitative description of the variability and the adaptive properties of information processing in the femur-tibia feedback system of the stick insect. The gain of this proprioceptive feedback system is determined by external stimuli changing the behavioural state of the animal and by internal properties that make it dependent on different parameters of the stimulus programme, e.g. stimulus frequency and amplitude, repetition rate and resting pauses.

The gain of the feedback loop in the inactive animal was investigated under open-loop conditions by applying mechanical sine-wave stimuli to the femoral chordotonal organ (fCO). The resistance movement of the tibia caused by these stimuli was measured with a new optoelectronic device.

A large increase or decrease in gain (up to a factor of 50) can be induced by stimulation, but also occurs spontaneously. The system shows habituation and sensitization. The initial gain can be decreased by repetitive sine-wave stimulation of the fCO. Disturbance of the animal (e.g. by tactile stimuli) increases the gain. The gain of the system decreases with increasing stimulus amplitude.

The described nonlinearities form a system which adjusts gain to a value that permits effective feedback and prevents instability. This was verified by closedloop experiments.

Note: Present address: Institut fur Zoologie II, Friedrich-Alexander-Universitat, Staudtstrasse 5, D-8520 Erlangen, Germany.

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