Drosophila Conditioned Courtship: Two Ways of Testing Memory
Abstract
In Drosophila, courtship reduction in male flies that have previous experience of courting a mated female is a result of the counterconditioning of an attractive unconditioned stimulus (US)—the aphrodisiac—which becomes an aversive conditioned stimulus (CS) after being paired with an aversive US—the antiaphrodisiac. In a retention test with a virgin female lacking the antiaphrodisiac, males retain a lower level of courtship for 3 hr after training. However, a measure of courtship suppression, the learning index (LI), decreases significantly after only 1 hr. In contrast, in the retraining test with a mated female, the LI shows no decrease for 8 hr but falls below significance 16 hr after training. These results are discussed in terms of the transfer of training. Nonspecific transfer and nonassociative behavioral modifications play little, if any, role in the transfer of training. The retraining test is recommended as a new protocol for studying conditioned courtship. According to the model proposed here, in tests with a virgin female, the duration of memory retention is limited by the retention of the direct association between the CS and the aversive motivational system or by the retention of an internal representation of the US. In retraining tests, the CS–US association seems to be the only factor involved in transfer 3 or more hours after training.
Footnotes
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↵4 Corresponding author.
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↵1 Let A be a level of sexual arousal of a male in a population. Let us assume that in the absence of learning,A is constant for each male but shows interindividual variability. Let us further assume that A determines the level of courtship under various conditions (courting mated or virgin females, etc.), such that under any condition the greater the value ofA, the greater the mathematical expectation of CI. Excluding males with a low initial level of courtship from behavioral tests should obviously truncate the left tail of the sample distribution ofA. If the same procedure is not performed in the control group of naive males, this would result in a bias in the courtship level between experimental and control groups in the absence of learning. This bias has a direction, which is opposite to the changes in courtship level owing to conditioning, and hence may hide weak aftereffects of training (e.g., in the delayed tests). Such problems are present even when learning is measured in one individual. The CI observed in any given male results, in fact, from time sampling. If all observations are truncated below some minimum level at the first time interval, but this is not performed at the next time interval, artifactual changes in CI would be observed, coinciding in direction with changes expected from learning.
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↵2 This can easily be shown to be true by some simple algebraic calculations. Suppose, for the sake of simplicity, that in some hypothetical sample before training all n males have the same level of courtship b (the same level of courtship is expected for the independent sample of naive males, because both samples are derived from the same population) and after training this level is decreased to a. The mean difference between CIna and CItr, then, will bed = n(b − a)/n = b − aand LI = 1 − a/b. Let in another sample m among n males show no courtship at all, whereas others have the same level of courtship before and after training, as in the first sample. Thend = (n − m) (b − a)/n = (1 − m/n) (b − a) and LI = [(1 − m/n) (b − a)]/ [(1 − m/n)b] = 1 − a/b. Although the difference score depends on the presence of inactive males, those cases are automatically excluded when the LI is calculated. Thus, the only negative effect of the presence of noncourting males in a sample is a decrease in the precision of estimating the LI.
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↵3 Temporal contiguity of the CS and the US (in the case of Pavlovian conditioning) or of response and reinforcement (in the case of instrumental conditioning) is not sufficient for conditioning to occur. There must also be a differential contigency between them. This means that the probability of the paired presentation of the US and the CS should be different from the probability of presenting the US in the absence of the CS. In other words, organisms are sensitve to the extent to which one stimulus provides information about the other. Similarly, they can distinguish response-dependent from response-independent reinforcement. See Schartz and Reisberg (1991) for a full explanation.
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- Received March 10, 1997.
- Accepted November 4, 1998.
- Cold Spring Harbor Laboratory Press
