1. Cross-task Prediction of Working Memory Performance: Working Memory Capacity as Source Activation Larry Z. Daily, Marsha C. Lovett, and Lynne M. Reder Carnegie Mellon University Thanks to Scott Filipino for assistance in collecting the data. This work supported grant F49620-97-1-0455 from the Air Force Office of Scientific Research and grant N00014-95-1-0223 from the Office of Naval Research.

2. Working Memory in ACT-R Provides the resources needed to retrieve and maintain information during cognitive processing (Baddeley, 1986) Working memory capacity is limited –W is limited (Anderson, Reder, & Lebiere, 1996) Working memory limits vary across individuals –W varies over individuals (Lovett, Reder, & Lebiere, 1999)

3. Prior Work: The MODS Task MOdified Digit Span Read strings of digits and letters aloud Remember the numbers Memory set size varied from 3 to 6

4. MODS Task Aggregate Results

5. MODS Task Individual Accuracy

6. MODS Task Individual Serial Position

7. CAM Inductive Reasoning Subtest

8. Cross-task Correlations Estimates of W strongly correlated with scores on CAM inductive reasoning subtest –r = 0.50r 2 =.25 Correlations are a weak test of W’s predictive ability No model of the CAM

9. New Work: The n-back task Subjects view a long sequence of letters For each, indicate whether it is a target or non-target Targets defined by condition –0-back - target is a letter given at the start –1-back - letter is a target if it matches the previous letter –2-back - letter is a target if it matches the letter before the previous one –3-back - letter is a target if it matches the letter 3 before the current one

10. N-back Strategies Familiarity-based –Subjects use familiarity to decide whether an item is a target –Doesn’t depend on W Update –Subjects actively try to maintain a list of prior items –Does depend on W –We model this

11. The n-back Model Encodes the item currently in vision –Memory chunk encodes item and position Attempts to match the item to a stored memory –Looks for a memory chunk with a position that matches the n-back condition and the correct item –Sets a flag to indicate match or not

12. The n-back Model Responds –Target or non-target based on flag Updates memory chunks –Changes the position of each chunk up through the current n-back condition –If this process fails, several responses required to get back on track

13. Aggregate Results

14. Individual Results: MODS Task

16. Individual Results: N-back Task

18. Conclusions Varying W captures differences in individuals’ working memory performance. W can be used to predict performance across tasks. W is a workable measure of working memory capacity. ACT-R can be fruitfully applied to the study of individual differences.