Crosscorrelation to compute data points for phase-response curves. Using locomotor results from the two groups of flies in Figure 9, the data sets were cross-correlated starting from a time just after the light-pulse (Figure 9a) until the end of the locomotor records. Whereas autocorrelation evaluates the relationship between a data set with itself over time (see Figures 2, 3), crosscorrelation evaluates the relationship between two different data sets. The plot shows correlation coefficients plotted with respect to the lag (in hours) between the two (averaged) locomotor timecourses (see text). This comparison was used to determine whether there is a difference in phase between the untreated vs. light-pulsed flies. The lag is read as the phase-offset from 0 on the abscissa. This particular crosscorrelation analyses resulted in a "first" peak at -3.5, indicating a phase delay of that number of hours. This result agrees with that obtained in Figure 9, albeit less formally (i.e., by inspection of the plots in 9c and 9d).