a: Schematic depicting the hypothesis that replay content reflects delayed experience. Although unlikely given the reported rapid effects of experience on replay content (see main text for discussion), this scenario correctly predicts no change between pre- and post-task rest, and an overall replay bias opposite the preferred outcome. However, it further predicts that the behavioral bias (preference for one side or the other, defined as max(p left , 1 − p left )) on day n predicts SWR content bias the next day: note the relatively small swing in SWR content following a relatively unbiased session (for example session 2 with 63% food (left) arm experience) and comparatively large swing following a strongly biased session (session 4 with 95%, top right). The two bottom panels depict the three example session data points shown (dark gray symbols), which form the predicted positive correlation; the data, depicted here schematically as light gray circles, do not exhibit such a relationship. This figure also illustrates why it is informative to compute bias scores (bottom right panel; ranging from 0.5 to 1 by using the max operation above) rather than using raw values (bottom left panel). This is an instance of Simpson’s Paradox, where using raw values would always show a positive correlation between day n behavior and day n + 1 pre-task replay content (lower left panel): the structure of the task combined with the overall opposite bias in replay content confines the data points to the lower left and upper right quadrants. Geometrically, the bias scores align the food and water sessions to a common axis (note the reflection of the yellow and green quadrants in the lower right panel, made visible by the notch in one corner), enabling the testing of the more specific predictions shown here. b: If motivational state determines replay content, replay content bias during the pre-task rest period as well as other epochs should predict that session’s behavioral bias (after all, a hungrier animal would show a stronger preference for food). This prediction, illustrated in the lower two panels, is confirmed in the data. Note that again, bias scores are important in avoiding spurious results (Simpson’s Paradox). Because the comparison is within-session rather than across-session (as in a), the raw data are now confined to the upper left and lower right quadrants (lower left panel).