The model provided a good fit to the color recall data (). It accounted for 99%, 100%, and 98% of the variance (adjusted r2) for the 1-, 4-, and 10-s delay conditions, respectively. The mixture model could not be rejected for any delay at the group level or for any of the individual observers at any delay (Kolmogorov-Smirnov test, p > .10 for all cases). A gradual decay model, in which SD can vary but p(failure) is held constant at the value estimated from the 1-s delay, was also fit to the data. Although this model fit the data extremely well at the 1-s and 4-s delays, it could be rejected for the 10-s delay at the group level (p<.001) and for 11 of the 12 individual observers (p<.05).
shows the mean p(failure) and SD values. SD increased very slightly between the 1- and 4-s delay conditions and then remained constant between the 4- and 10-s delay conditions. A one-way analysis of variance (ANOVA) indicated that these differences were not significant (F<1). Thus, there was little or no evidence of memory decay over a 10-s period.
Figure 3 Estimates of memory precision (SD, solid line, right y-axis) and probability of memory failure (p(failure), dashed line, left y-axis) for each retention interval from the color (a) and shape (b) experiments. Error bars show within-subjects 95% confidence (more ...)
P(failure) was essentially constant at the 1- and 4-s delays but increased sharply at the 10-s delay (F(2,22)=5.10, p<0.02). This can also be seen in as an increase in probability of extreme errors in the 10-s delay condition. Follow-up tests showed no difference between the 1- and 4-s delays (F<1) and a significant increase between the 4- and 10-s delays (F(1,11)=7.71, p<0.02), with a 50% increase in memory failure between 4 and 10 seconds. Memory performance was somewhat lower in this procedure than in the more typical change-detection procedure, but these two procedures yield highly correlated estimates of the number of items stored in memory (Zhang & Luck, 2008
Similar results were obtained when observers were asked to remember shape ( & ). The model accounted for 99%, 96%, and 99% of the variance (adjusted r2) for the 1-,4-, and 10-s delays, respectively. The model could not be rejected for any delay at the group level or for any of the individual observers at any delay (Kolmogorov-Smirnov test, p > .05 for all cases).
SD exhibited a slight increase across delays, but this effect was not statistically significant (F(2,22)=2.64, p>.10). In contrast, p( failure) increased significantly across delays (F(2,22)=3.76, p<0.05). The increase in p(failure) between 1 and 4 seconds was small and statistically insignificant (F<1), but the increase between 4 and 10 seconds was large and significant (F(1,11)=4.68, P<0.05). Thus, just as for color, we found little or no decline in memory precision as the delay interval increased, but we found a substantial increase in the probability that the cued item was lost from working memory between 4 and 10 seconds.