Identification of Auditory Evoked Potentials (aERP). Standard ERPs were thoroughly identified to ensure good data quality (as shown in Fig. 1). We observed classic fronto-central N1 and P2 components in response to the sounds, with activity centered around Fz, Cz, and FCz electrodes 27–31. Source reconstruction of the N1 component revealed generators in bilateral auditory cortices, the pars opercularis of the left inferior frontal gyrus, and the right postcentral gyrus. These generators of the N1 component are in line with previous studies and are typical of the aERP 32–34. The P2 component is generally more distributed than the N1 component with generators in the bilateral frontal lobes, the bilateral parietal lobes, and the right auditory cortex, which is also in accordance with the literature 31.
Distinct Waveforms for Below and Above 1.2s Processing. When comparing the ERPs of short and long deviants to their respective standards, two distinct significant waveforms were observed. The first emerged early after the deviance (negativity around 100 ms) and was significant only for the long condition (see Fig. 1A). The second component emerged later (positivity around 200 ms) and was only significant for the short condition (see Fig. 1A).
These two distinct components related to deviance detection are coherent with components identified during human error monitoring 35. The processing of error-related information can be dissected into two different components: error-related negativity (ERN/Ne) and error-positivity (Pe). The former is a fronto-central negative component peaking around 20–130 ms after an error detection, and the latter is a positive fronto-central component peaking around 200 to 500 ms 36–38. The ERN/Ne appears during error detection 39, correction, and compensation 40,41. The latter, the Pe component, is elicited for conscious and easier error processing 42,43. Observing a Pe component only for the shorter condition in the present study is in line with previous work suggesting that shorter intervals may be easier to estimate accurately than longer intervals, as the Weber fraction in timing perception is increasing when intervals are longer than circa 1.2 to 1.5 sec 11,13.
Delayed Deviance Allows Proof of Error Accumulation. As shown in Fig. 3A, only the deviances occurring after the expected standard (delayed deviants) were significantly detected. No effect was found for the early deviants. This finding is in line with the pacemaker-counter model of time perception 8, where an accumulation of pulses tracks the time that has passed. This suggests that in the delayed deviant condition, participants accumulate more pulses in their counter than they usually accumulate with a standard presentation, which translates into further evidence that an error has occurred. In contrast, in the eD condition, the accumulation of pulses does not exceed the standard’s pulse count, which may not allow error detection. This might explain why only the ERP amplitude was significant, in both the below and above 1.2s conditions when longer rather than shorter deviants were presented.
Structures Underlying Below and Above 1.2s Processes. In the literature, there seem to be recurrent structures that are typical of time perception and independent of interval length. One of these structures is the cingulate cortex, which can be observed in both long and short interval conditions.
Parts of the cingulate cortex, such as the anterior cingulate cortex, have previously been identified as the generators of the ERN/Ne component and Pe in error monitoring 37,44,45 and thus might be independent of the length of intervals to be timed; along these lines, our results indicate the presence of activation in the anterior cingulate cortex in both timing conditions.
Structures Underlying Above 1.2s Processes. As shown in Fig. 4A, the ERN/Ne component observed in the delayed long condition (but not the eD condition) originates from the left parietal cortex and the right motor cortex. In this delayed long condition, the parietal sources overlay with two parietal regions, including the intraparietal sulcus (IPS) and the superior parietal lobe. All these structures are active in the 135–150 ms post-stimuli time range. Lewis and Miall 16 have suggested that the left inferior parietal lobe (including the IPS) plays a key role in time perception in the longer ranges. The IPS and the inferior parietal lobes are known to play a role in orientation of attention and working memory 46–48. Thus, it is not surprising to observe that these structures are involved in the processing of long intervals, a fortiori considering that processing such intervals is known to be cognitively oriented. Finally, the motor cortex is known to play a role in beat perception and time perception 3,49,50. The motor cortex coupled with a parietal activation has been identified as a network used in the mental simulation of action 51–53. Our results also show this parietal-motor cortex network in longer interval timing that may require motor processes to maintain a rhythm with longer intervals. It can be argued that, in previous studies, the motor cortex may have been falsely identified in shorter timing ranges because of the necessity to use a motor response in the timing task.
Concerning the activity observed in the parietal cortex, similar findings have been reported by Hayashi, et al. 54 who showed that the parietal inferior lobule is associated with duration-tuning. When a specific duration is repeated, the activity of the inferior parietal lobule is reduced but is higher when the difference between durations is increased, and thus might explain its activation in the process of long intervals. The left parietal cortex has been identified to play a key role in motor attention 55. This is consistent with the noticeable concomitant activation in the motor cortex we observe in the long (above 1.2s) condition, even though there is no motor response required in the present experimentation. This finding is also in line with the cognitive model of timing in the longer duration ranges 19,20 which underlies a contribution of the central executive network.
Structures Underlying Below 1.2s Processes. As shown in Fig. 4A, at around 200–245 ms, the Pe component observed only in the delayed short condition is localized in the left auditory cortex and the SMA. The critical role of the SMA in time perception is well known 23,56−58. In our study, the SMA operates bilaterally in the short timing condition. Coull et al. 59 have identified the SMA’s implication in perceptual timing and in error monitoring. Indeed, Spieser, et al. 60 applied anodal transcranial Direct Current Stimulation (tDCS) to the preSMA, during a conflict task in which participants were impulsively selecting an incorrect response. Stimulation of the (pre)SMA reduced the number of overt errors. This suggests that the (pre)SMA plays an active role in selectively suppressing unwanted responses or stimulations. This may be the case here only with short intervals, where the SMA selectively identifies deviance in the short timing range.
Dedicated and intrinsic time perception models. Debates about what type of model to use to explain time perception have been around for a long time (see 2 for a review). Here, we hypothesize that the dedicated time model such as the pacemaker-counter type model is valid regardless of the range of durations to be processed; this model works for both short and long intervals timing systems (at least, when the deviant is delayed). Our data do not allow us to draw a clear conclusion about the use of intrinsic models of time perception. However, it is reasonable to think that intrinsic time perception patterns may not be valid for the shorter range given the specific sensory structures that make up our short interval perception network (SMA and auditory cortex). The shorter interval time perception mechanism may not be modality-specific, as described in the literature supporting intrinsic models 61,62.
Conclusion. This study provides direct evidence for the existence of distinct time perception mechanisms for processing intervals below and above than 1.2s. An important contribution of the present study is the difference in processing latency of short and long-time deviance. Intervals longer than 1.2s are processed rapidly (135–150 ms post-stimulus) and intervals shorter than 1.2s time are processed later (200–245 ms post-stimulus). Mostly consistent with the literature, a passive time perception paradigm involving short intervals exclusively recruits the left auditory cortex and the bilateral pre-SMA. This result is in line with the idea that processes behind the perception of short intervals are mostly automatic 16. On the other hand, longer intervals recruit the left parietal cortex and the right motor cortex. This result is in agreement with the literature describing the need for cognitive support during the processing of longer intervals. The cingulate cortex is recruited in both interval conditions in the present study. This suggests that the cingulate cortex contributes to the processing of time intervals whatever the duration range. In both duration conditions, deviants occurring after the expected standard always elicit a higher amplitude cortical response, translating in more accumulated proof of deviance occurrence, which is in line with the pacemaker-counter model of time perception 8. This suggests there is a common pacemaker-counter mechanism for processing intervals shorter and longer than 1.2s. However, differences in the mechanisms of above and below 1.2s timing lie in the cortical responses’ components, latency of process and in their localizations.