The operant and the classical in conditioned orientation of Drosophila melanogaster at the flight simulator

Abstract

Ever since learning and memory have been studied experimentally, the relationship between operant and classical conditioning has been controversial. Operant conditioning is any form of conditioning that essentially depends on the animal's behavior. It relies on operant behavior. A motor output is called operant if it controls a sensory variable. The Drosophila flight simulator, in which the relevant behavior is a single motor variable (yaw torque), fully separates the operant and classical components of a complex conditioning task. In this paradigm a tethered fly learns, operantly or classically, to prefer and avoid certain flight orientations in relation to the surrounding panorama. Yaw torque is recorded and, in the operant mode, controls the panorama. Using a yoked control, we show that classical pattern learning necessitates more extensive training than operant pattern learning. We compare in detail the microstructure of yaw torque after classical and operant training but find no evidence for acquired behavioral traits after operant conditioning that might explain this difference. We therefore conclude that the operant behavior has a facilitating effect on the classical training. In addition, we show that an operantly learned stimulus is successfully transferred from the behavior of the training to a different behavior. This result unequivocally demonstrates that during operant conditioning classical associations can be formed.

Figure 1

Flight simulator setup. The fly is flying stationarily in a cylindrical arena homogeneously illuminated from behind. The fly's tendency to perform left or right turns (yaw torque) is measured continuously and fed into the computer. The computer controls pattern position, shutter closure, and color of illumination according to the conditioning rules.

Figure 2

Comparison of the mean (±s.e.m.) learning scores of operant master flies and classical replay flies. N = 30. (Open bars) Training; (hatched bars) test.

Figure 3

Comparing performance indices and spike amplitude of operantly and classically trained flies. For each fly, the mean of the 4-min preference test has been subtracted prior to averaging. (Open bars) Training; (hatched bars) test. (a) Mean (±s.e.m.) performance indices of classically and operantly conditioned flies. N = 100. (b) Mean (±s.e.m.) spike amplitude (SA) indices of classically and operantly conditioned flies. SA indices were calculated as (a₁ − a₂)/(a₁ + a₂) where a₁ denotes the mean SA in the quadrants containing the pattern orientation associated with heat and a₂ the mean SA in the other quadrants. N_operant = 97; N_classical = 94.

Figure 4

Spike polarity and cold pattern fixation of operantly and classically trained flies. For each fly, the mean of the 4-min preference test has been subtracted prior to averaging. (Open bars) Training; (hatched bars) test. (a) Mean (±s.e.m.) polarity indices in the cold sectors. The polarity of a spike is defined as towards pattern if it leads to a rotation of the arena that brings the center of the nearest pattern closer to the very front, which is delineated by the longitudinal axis of the fly. Accordingly, the spike polarity from pattern brings the nearest quadrant border closer to the most frontal position. The polarity index yields the fraction of spikes towards the pattern. It is defined as (s_t − s_f)/(s_t + s_f) with s_t being the number of spikes towards the pattern and s_f the number of spikes away from the pattern. N_operant = 92; N_classical = 83. (b) Mean (±s.e.m.) pattern fixation in the cold sectors. N = 100.

Figure 5

Learning performance and pattern fixation of flies trained either operantly or classically with slowly rotating patterns. (a) Mean (±s.e.m.) performance indices of classically (rotating patterns) and operantly trained flies. (b) Mean (±s.e.m.) fixation indices in the cold sectors. For each fly, the mean of the four-min preference test has been subtracted prior to averaging. N_operant = 18; N_classical = 23. (Open bars) Training; (hatched bars) test.

Figure 6

Mean (±s.e.m.) performance indices in a representative sw mode experiment, using color as visual cue. (Open bars) Training; (hatched bars) test.

Figure 7

Summary table presenting the results of all transfer experiments. (A) Patterns as visual cues; (B) colors as visual cues. Experimental design is schematized by the nine squares above each performance index. All experiments are divided in 2-min test or training periods, except in A, IV-VI, where 1-min periods are used. Reminder training is always 60 sec. Statistics were performed as a Wilcoxon matched pairs test against zero. (*) P < 0.05; (**) P < 0.01.

Figure 8

(a) Typical 30-sec yaw torque flight trace (left) with a frequency distribution of the torque maxima and minima (right; rotated by 90°). Broken lines denote the detection thresholds. Detected spikes are indicated by arrowheads. (b) Enlarged stretch of the yaw torque flight trace with 33 data points (i.e., 1.6 sec) showing two spikes. Data points are connected by lines for better illustration. Broken lines indicate detection thresholds. (See Materials and Methods for details.)