Figure 7. Confidence Ratings for Error Trials at Near-Threshold Tilts.

The top histogram plots the proportion of responses of error trials at each confidence level, averaged across four observers (all naive of the goals of the experiment). The green bars show responses for set size 16, and the red patterned bars show responses for set size 1, with bars showing ± 1 SEM. In the lowest confidence level, the proportion of errors is higher at set size 1 than at set size 16, while at the three higher confidence levels, the reverse holds. The differences at confidence levels 1 and 3 were statistically significant (Studentt test,p < 0.01). The difference was insignificant at confidence level 2 (where the proportions were similar), and also at level 4 (by binomial test), as there were only five responses in this bin (observers tended to shy away from the response extremes). The bottom bar graphs plot the mean confidence averaged across the same four observers in the set size 1 (patterned red bar) and set size 16 (green bar) condition. The error bars show ± 1 SEM, revealing that our subjects were more confident about their erroneous responses in with a cluttered display than in a single stimulus (Studentt test,p < 0.001).