Age differences in retention of fixed-interval responding: memory or habitation effects?

Long-term retention of fixed-interval curvature as an index of recall memory which is relatively free of retrieval cues was examined in younger and older rats. Previous research on free-operant behavior in younger and older rats showing within-session decrements in behavior of older rats and within-session increments in younger rats suggested that the results of recall memory tests would depend upon the duration of the test session and the point within the session where data were collected. Consistent with such expectations, recall of fixed-interval curvature was poorer in older rats when data from late in the test session or data averaged over the entire 45-min session were used, but the opposite finding, numerically better retention in the older rats, occurred when data from the earliest part of the test session were used. The results suggest that age-related performance differences from which age-related memory differences are inferred may be confounded by other age-related effects upon performance which have nothing to do with memory.