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The core device of the setup is the yaw torque compensator (Fig. 6). Originally devised by Götz (1964) and repeatedly improved by Heisenberg and Wolf (1984), it measures a fly's angular momentum around its vertical body axis. The fly, glued to a small hook of silver wire as described above (2.1), is attached to the torque meter via a clamp to accomplish stationary flight in the center of a cylindrical panorama (arena, diameter 58mm), homogeneously illuminated from behind. Closing the feedback loop to make the rotational speed of the arena proportional to the fly's yaw torque (coupling factor 11°/s*10-10Nm) enables the fly to stabilize the rotational movements of the panorama (Flight Simulator mode). The position of an arbitrarily chosen point of reference on the arena azimuth delineates a flight direction of 0-360°. Arena position (i.e. flight direction) is recorded continuously via a circular potentiometer and stored in the computer memory together with yaw torque (sampling frequency 20Hz).
Fig. 6: Block diagram of mode of operation of the torque compensator for recording yaw torque of Drosophila in stationary flight. HF - high frequency; LF low frequency. (redrawn from Götz, 1964, where a detailed description of the mode of operation is given)
Four black, T-shaped patterns of alternating orientation are evenly spaced on the arena wall (width psi=40°, height theta=40°, barwidth=14°). Reinforcement, where applied, is made to be contiguous with the appearance of one of the two pattern orientations in the frontal quadrant of the fly's visual field.
The reinforcer is a light beam (diameter 4mm at the position of the fly), generated by a 6V, 15W Zeiss microscope lamp, filtered by an infrared filter (Schott RG1000, 3mm thick) and focused from above on the fly. The strength of the reinforcer was determined empirically by adjusting the voltage to attain maximum learning. In all experiments the heat was life threatening for the flies: more than 30s of continuous reinforcement were fatal for the animal. The heat is applied by a computer-controlled shutter intercepting the beam (Fig. 7).
Fig. 7: Simplified diagram of the flight simulator setup. Yaw torque is continuously transduced into d.c. voltage by the torque meter. The computer couples this signal to the pattern drum by calculating the angular image deviations that the measured flight maneuvers would have caused on the fly's eyes in free flight. Thus the fly can control the angular rotation of the drum with its yaw torque: intended right turns of the fly rotate the arena counterclockwise, intended left turns rotate it clockwise - similar to a human pilot controlling the panorama in a flight simulator program with its joystick. The same computer controls the electric shutter intercepting the infrared light beam used as reinforcer.
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