Age does not affect the rate or extent of procedural learning. A, Trial structure and timing of aversive conditioning task. Animals were required to detect when a safe stimulus (unmodulated broadband noise) smoothly transitioned to a warn stimulus (0 dB at 100% depth AM noise; 5 Hz rate). Warn trials were immediately followed by an aversive shock (300 ms, orange bars), which could be avoided by withdrawing from the spout. Warn trials were separated by three to five safe trials, such that the onset of each warn stimulus was unpredictable. B, Animals were trained to drink continuously from a spout of water while in the presence of the safe stimulus. Maintaining spout contact during safe trials was therefore scored as a correct reject, whereas withdrawing from the spout was considered an incorrect false alarm. Withdrawing from the spout upon presentation of the AM warn stimulus was scored as a correct hit. Maintaining contact with the spout during the warn stimulus was considered an incorrect miss and punished with a shock. C, Experimental timeline of procedural training. All animals were weaned at P30. Subjects had 3 d of procedural training before progressing to perceptual training (Fig. 2). Some adults required additional sessions due to poor initial motivation to participate in the task, likely due to a robust (mature) ability to retain water. Ages depicted here are exact for young animals, and approximate for adolescent and adult animals (for details, see Materials and Methods). D, Behavioral performance (d′) was calculated as a function of trial number collapsed across all procedural training sessions using a 15-trial-wide sliding window. Data are depicted as the mean ± SEM. For this and subsequent panels in this figure, the number of subjects in each age group are as follows: young, n = 19; adolescent, n = 17; adult, n = 16. E, The number of trials to reach training criterion (d′ ≥ 1.5) is plotted for each animal as a function of age group. Horizontal lines represent median values, boxes represent 25th and 75th percentiles, and whiskers represent 1.5× the interquartile range. F, The maximum d′ value achieved during procedural training is plotted for each animal as a function of age group. Data are depicted as in E. Note that boxes and whiskers are present, but may be too small to see. Hit rates and false alarm rates were adjusted to avoid d′ values that approach Inf, such that the maximum d′ possible on this task was 3.29.
