Bayesian Inference Underlies the Contraction Bias in Delayed Comparison Tasks
Figure 1
The delayed comparison task and subjects' performance.
A, The standard task. Subjects viewed a horizontal bar (L1) on a computer screen for 1 sec and memorized its length. After a delay period of 1 sec, during which the screen remained blank, the subjects viewed a second bar (L2) and were instructed to report which of the two bars was longer. The second bar, L2 remained visible until subjects made a response. The difference in length between L1 and L2 varied between −30% and +30%. Unbeknownst to the subjects, on roughly 50% of the trials, the lengths of the first and second bars were equal (L1 = L2). B, The average psychometric curve of 9 subjects. The abscissa corresponds to the difference between the two bar lengths, and the ordinate corresponds to the fraction of trials in which subjects chose L1 as longer than L2. Error bars depict standard error of the mean (SEM). Line is a least-square fit of an error function:
where
and
. C, Average response curve of 9 subjects. Fraction of times in which subjects reported ‘L1>L2’ on the impossible trials are plotted as a function of bar length. Subjects overestimated the magnitude of the memorized L1 bar when it was relatively small and underestimated L1 when it was relatively long, consistent with the contraction bias. Each data point corresponds to 21 impossible trials per subject. Error bars depict SEM.