Our sense of time spans multiple scales; from seconds to minutes and days to years. Judgements for different temporal intervals however do not rely on a single unitary timing system, but instead rely on separate neural networks. For example, judgements within the sub- and supra-second range rely on networks involved in motor control (basal ganglia and cerebellum), whereas brain regions involved in long term memory and spatial navigation (prefrontal cortex and hippocampus) are involved in judgements spanning weeks and years.
A new functional magnetic resonance imaging study, led by researchers from Princeton, fills the gap in our understanding of how we judge time across intermediate timescales. The study examined time perception in the range of minutes by testing how accurately participants could estimate the amount of time that had elapsed between short excerpts taken from a 25-minute, science-fiction radio story.
The study investigated the idea that temporal estimation for intermediate intervals is related to the degree to which events are associated with similar contextual cues. This idea is based on theories of memory which posit that the recency of events can be ascertained by retrieving slowly varying contextual representations associated with global mental states. These representations include external environmental features (i.e., spatial location) and internal states such as goals and emotions. According to this theory, contextual cues bias temporal judgements such that the interval between events containing similar contextual features should be underestimated, whereas the the interval between events containing few contextual features should be overestimated.
To test this theory, participants listened to the radio story while brain activity was measured with functional magnetic resonance imaging. After the scanning session, participants completed a surprise temporal judgement test. Participants were presented two short clips that were either 2 or 6 minutes apart and were asked to estimate the time that elapsed between each excerpt. The degree to which these target clips were associated with similar mental context was estimated by examining brain activity recorded while participants initially listened to each clip. The authors used multi-voxel pattern analysis, a method that exploits distributed patterns of brain activity within a region of interest to measure the neural representations formed by different perceptual and cognitive states. MVPA was carried out by correlating the pattern of neural responses (across voxels) evoked by each clip with the idea that clips that share similar content, should also evoke highly correlated patterns of brain activity.
A region of interest analysis showed that pattern similarity in the right entorhinal cortex was correlated with temporal estimates. That is, clips that evoked similar patterns of brain activity within this region were associated with shorter duration estimates in the temporal judgement test. This result was also found when the correlations between judgements and pattern similarity were calculated for each clip pair across participants, indicating that variations in temporal judgements were not solely due to clips sharing perceptual features (i.e., if clips shared similar music).
Evidence that temporal judgements are based on representations formed in the entorhinal cortex is consistent with this region’s’ role in binding event content (i.e., objects, people, actions) within a broader spatial and temporal context. Indeed, a follow up analysis that examined the auto-correlation of evoked patterns within the entorhinal cortex showed that pattern similarity in this region fluctuated more slowly throughout the story than in neighbouring lateral temporal lobes. Together these results confirm the major predictions of the mental context theory of temporal estimation: temporal judgements are based on slowly varying representations that bind event content within broader contextual cues.
It remains to be seen whether the entorhinal cortex plays a general role in retrospective duration estimates for different tasks, contexts and timeframes. One possibility is that this region is particularly attuned to the temporal relations between events in spoken narratives. Depending on the nature of the story, the temporal relationships between events in a spoken narrative may be somewhat compressed compared to everyday experiences. In this case, it might be expected that the entorhinal cortex usually supports temporal judgements over hours and days in more naturalistic contexts.
Research published by the same group has shown that brain regions appear to be attuned to different temporal frequencies, a finding that most likely reflects the kind of representations formed within a region. It might be possible that temporal judgements for other timeframes (i.e., tens of seconds or hours) or different event content (daily activities, details of a conversation) may rely on other brain regions that are better suited for retrieving key information about different experiences.
The MARCS Institute, Western Sydney University