Measurement of performance
The simplest measure of performance is the fraction of trials in which the animal responds correctly—that is, responds with licking to the presentation of the S+ odor and does not lick with presentation of the S− odor.
It was shown previously, however, that the detailed sampling pattern is a more sensitive measure of discrimination performance [3]. To avoid long (> 3-week) training periods, we chose not to measure discrimination times [3] but to analyze the average sampling behavior in total. Upon presentation of a rewarded odor, the animal usually continuously breaks the beam, whereas upon presentation of an unrewarded odor the head is quickly retracted. The difference in response to the rewarded and unrewarded odor is approximately sigmoidal (Figures 1D, 1E, and 2D) and yields a sensitive measure of the discrimination performance. From this difference or from a sigmoidal fit to the difference (Figure 1E), several measures of discrimination can be determined: the average difference, peak, or maximum, time of half maximum, and slope of the fitted sigmoid. Whereas for small trial numbers (< 200) the slope often is not well constrained, any of the other parameters yielded essentially the same results. The discrimination index plotted in Figures 1F, 2C, 4D, and 6C refers to the fitted maximum, generally ranging from zero to one, one indicating the best discrimination. Identical results were obtained with other measures of discrimination, such as the average sampling difference.