Fig. 1. Experimental set-up. (A) Arrangement of the telemetry and video-recording systems. The crayfish was placed in an experimental aquarium filled with water to a depth of 15 cm. The floor was tilted by 10°. Four PIN-photodiodes (PINPDs) as receivers were placed at the corners of a square of 30 cmx30 cm. An LED was driven by synchronous signals that were simultaneously fed, together with nerve and EMG signals, to a DAT recorder. Due to the optical signal decay along the distance from the transmitter, the recording was reliable only in a limited area. The area illustrated in dark blue indicates the area in which the telemetric transmission was secure. The light blue indicates water in the aquarium. (B) The angular coordinate adopted in the present study to describe the direction of animal body orientation. The head direction and behavior of crayfish were video-recorded from above. (C) The optical transmitter mounted on the animal. Wire electrodes for electromyographic (EMG) recording from the mero-carpopodite flexor muscle and the chronic electrode for extracellular recording from the circumesophageal commissure were connected to the dual-channel transmitter. (D) Schematic drawing of the chronic electrode for extracellular recording from the circumesophageal commissure. Wire connection from the electrode to the transmitter is shown by broken lines. The commissure was hooked up lightly by the electrode to secure the recording during free movements. Two broken ovals are lithium cells for the transmitter. They were omitted from the illustration in C for the sake of clarity. IRLED; infrared LED.