Systematic under-reproduction of time is consistently observed in time reproduction tasks. One explanation for this bias is that it is impossible to implement the method of limits in time reproduction tasks due to anisotropy of time (Riemer, 2015). Since time, as a physical quantity, does not allow researchers to implement the method of limits or manipulations that other physical quantities enable, identifying the factors that influence the under-reproduction of time might help explain this bias and to identify the underlying mechanisms. The paper by Riemer et al., (Riemer, Rhodes, & Wolbers, 2016) investigates whether manipulating judgment certainty affects the magnitude of bias in time reproduction tasks. Judgment certainty is manipulated by applying continuous theta- burst stimulation (cTBS) to the right posterior parietal cortex (PPC) using non-invasive transcranial magnetic stimulation (TMS). In the present work, Riemer and colleagues used two different tasks, each under TMS and sham conditions to study the effect of judgment certainty on time reproduction.
A 2-AFC time discrimination task was used to verify and quantify the effect of the TMS on judgment certainty. Participants were required to compare two supra-second time intervals presented as filled acoustic stimuli and separated by a short inter-stimulus interval (ISI), and report whether the stimulus in the second interval was longer or shorter than the stimulus in the first interval. Applying cTBS over right PPC would give rise to transient inhibitory effects in the region and would result in an increase in the precision of judgments in the discrimination task, without affecting the overall mean accuracy. Behavioural data showed an average increase in the slope of psychometric function and an average decrease in the difference limen for TMS condition relative to sham condition (figure 1 below). This change in performance shows that inhibiting the right PPC indeed improves precision in time discrimination performance by increasing the judgment certainty.
Figure 1: Psychometric functions showing average discrimination performance across subjects for TMS using cTBS of right PPC and sham conditions. Inhibiting right PPC improved sensitivity of subjects without affecting the mean accuracy, indicating an increase in judgment certainty (from Riemer, Rhodes, & Wolbers, 2016).
If judgment certainty is one of the factors that influences the systematic under-reproduction of time, then a similar inhibition of right PPC as above should result in a reduction of negative errors and reduction in response variability in time reproduction task. To verify the same, a time reproduction task was used where participants were required to reproduce a standard duration played in the first interval using filled acoustic stimuli. The standard interval had variable supra-second durations across trials and is separated from the reproduction interval by a short ISI. The reproduced intervals are then quantified as exponentials of the standard durations. Even though the behavioural data showed a systematic under-reproduction of time in both conditions, there was no difference in the reproduced durations (as quantified by the power functions) between TMS and sham conditions. Interestingly, even the variability of reproduced responses remained unchanged between the conditions (figure 2 below). This shows that increasing judgment certainty by inhibiting the right PPC did not have any effect on the under-reproduction of time. The authors also found no interaction of the estimates quantifying change in behaviour (if any) across TMS and sham conditions between discrimination task and reproduction task. This leads to a hypothesis that both tasks are based on different neural mechanisms.
Figure 2: Left: Power functions quantifying reproduced durations on vertical axis as a function of standard durations on horizontal axis, averaged across subjects in both cTBS of right PPC and sham conditions. Increasing the judgment certainty by inhibiting right PPC did not have any effect on the under-reproduction of time. Right: Variability in reproduced durations for cTBS and sham conditions. Error bars represent SEM (from Riemer, Rhodes, & Wolbers, 2016).
Various factors might influence the mechanisms that give rise to negative errors in time reproduction tasks. If time discrimination and time reproduction are driven by the same neural mechanisms, then the factors that influence performance in time discrimination tasks should also influence performance in time reproduction tasks. In other words, judgment certainty which increases the precision in time discrimination judgments should also reduce the extent of under-reproduction in a time reproduction task. However, in the current study, it has been found that judgment certainty does not improve performance in a time reproduction task. This has two implications: either time reproduction tasks involve completely different mechanisms other than time discrimination or judgment certainty does not play a role in time reproduction tasks. It is difficult to tease apart these two based on the observations made in the current work. It was also indicated in the paper that negative errors in time reproduction might be caused due to adaptation to the short ISI or because of an urgency signal that pushes subjects towards giving their response due to anisotropy of time. Future studies might help verify these ideas, for example by varying the ISI and comparing the magnitude of under-reproduction for different values of the ISI. Another recent study showed that stimulus duration, modality and intensity affect time reproduction performance (Indraccolo, Spence, Vatakis, & Harrar, 2016). These results combined with the above study suggest that time perception employs multiple brain areas and depends on a number of factors that are otherwise generally thought to have no effect. This further stresses the need for more controlled studies to identify the neural mechanisms underlying time perception.
Source Article: Riemer M; Rhodes D; Wolbers T, 2016. Systematic Underreproduction of Time Is Independent of Judgment Certainty. Neural Plasticity 2016:6890674
Indraccolo, A., Spence, C., Vatakis, A., & Harrar, V. (2016). Combined effects of motor response, sensory modality, and stimulus intensity on temporal reproduction. Experimental Brain Research, 234(5), 1189-1198.
Riemer, M. (2015). Psychophysics and the anisotropy of time. Consciousness and Cognition, 38, 191-197.