Abstract
Human decision-making often involves sensory integration over a sequence of stimulus samples. Typically stimulus samples are not equally weighted in this integration process. Rather subjects show a tendency to emphasize either earlier (primacy) or more recent (recency) samples. However, little is known how the temporal continuity of the sample sequence contributes to the subjective weighting in the integration process. Here we tested how a short temporal break in an otherwise regular sample sequence affects temporal integration. Seven human subjects performed a visual estimation task where they had to estimate the angular position of an unknown generative mean based on 8 stimulus samples drawn from a Gaussian with fixed variance. Subjects centrally fixated (controlled with eye-tracker) while the stimulus samples were presented in quick succession (SOA = 150 ms) at 5 deg visual eccentricity. After presentation, subjects reported their position estimate using a joystick, after which they received visual feedback (true position of generative mean). Subjects performed the task in three blocks that only differed in the timing of the sample sequences. In the first block, samples were presented without interruption (no-break condition). In the second block, the sequence was interrupted for 1.75s after the 4th sample (break condition). In the third block no-break and break trials were randomly interleaved. We found that interrupting the sample sequence had no significant effect on subjects’ estimation performance. However, a regression analysis revealed a clear effect of temporal regularity on the weighting profiles. For the no-break condition, the weights consistently decrease across all samples (primacy). By contrast, in the break condition weights show a second peak after the break, followed by another decay (‘double-primacy’). This difference holds even when break and no-break trials are randomly interleaved. Our results demonstrate that interrupting the temporal regularity of a stimulus sequence influences how subjects integrate sensory information.