Subjective cognitive decline (SCD) is a preclinical stage of Alzheimer’s disease (AD), during which patients notice a change in cognitive performance. Nonetheless, this modification can’t be identified in neuropsychological tests.

A recent Scientific Reports study assesses time perception in AD and whether timing alterations occur in SCD.

Study: Multidimensional assessment of time perception along the continuum of Alzheimer’s Disease and evidence of alterations in subjective cognitive decline. Image Credit: Sergey Nivens / Shutterstock.com

Background 

Time perception is considered based on an internal clock or pacemaker-accumulator that emits pulses. The experience of time has been assessed through various methods, including questionnaires and tasks to estimate the date of public events.

The healthy aging process is related to dissociations between temporal prediction tasks, tasks that require explicit duration judgments and assessing the representation of time implicitly.

Nonetheless, through the early stages of AD, like mild cognitive impairment (MCI), changes in time perception occur. Patients are unaware of time and orientation and, because of this, are unable to locate events in the right chronological order. In MCI, few studies have analyzed time perception, with the findings from these studies often inconclusive about changes in time perception in MCI and AD.

In regards to the study

The researchers hypothesized the existence of precise alterations within the preclinical and prodromal stages of AD. To this end, the researchers assumed that internally based learning (IBL) and externally cued learning (ECL) could differentially impact the progression of AD. Alterations in retrospective time processing could occur in individuals with SCD, which is connected with a discount of their memory skills.

There may be a possibility that each explicit and implicit timing could differentially influence the progression of healthy aging and AD. The present study assessed explicit prospective timing based on production and reproduction paradigms and estimation of duration.

A complete of 16 patients with SCD, 17 with MCI, 13 with AD, and 17 healthy controls (HC) participated within the study. The diagnosis of every participant was based on clinical guidelines from the National Institute on Aging-Alzheimer’s Association workgroups (NIA-AA). These individuals performed an array of tasks testing various dimensions of processing time and duration.

All study participants were between 55 and 86 years of age. Individuals with a history of psychiatric comorbidities, cerebrovascular disease, repeated head trauma, or severe central nervous system infections inside the last five years were excluded. Moreover, patients under antipsychotic drugs and benzodiazepines weren’t considered for the study.

Study findings

The present study utilized the Bouncy ball task, a novel method to evaluate IBL and ECL conditions. A correlation between time-processing tasks and neuropsychological test scores was established.

Similar performance was observed on the 2 conditions of the Bouncy ball task for the HC and MCI groups. Nonetheless, the performance of AD patients was worse in ECL as in comparison with IBL, which might be resulting from distinct temporal lobes atrophy present in these patients.

The experimental results indicated that cognitive decline doesn’t influence implicit time processing in tasks that require internally-based timing (IBT). This will be attributed to IBT being supported by a striato-thalamo-cortical network, which is linked with motor and premotor, prefrontal, and insula regions.

Performance in ECL was positively correlated with long-term memory and executive functioning test scores. Since this correlation was more distinguished for ECL, IBL may impose low cognitive demands. Consistent with previous research, the present study observed that implicit time processing might be differentially impacted by dementia, because it has specific task demands.

In comparison with other test groups, the performance of AD patients was related to higher absolute errors in duration estimation, production, and reproduction. Interestingly, no altered performance was observed in MCI patients.

Central tendency measures in MCI were between HC-SCD and AD patients. Nonetheless, a larger-scale study is required to find out the precise impact of various clinical features of MCI patients that impact their performance in prospective timing tasks.

In comparison with the control group, all test groups exhibited higher errors in retrospective duration estimation. Previous studies have shown that point estimations depend on the retrieval of contextual changes linked with event sequences and that these events are encoded in medial temporal lobes.

Consistent with previous reports, the present study observed that hippocampal atrophy is a necessary pathological hallmark related to alterations in retrospective duration processing with AD progressions.

Importantly, all test groups overestimated durations. Public events that occurred within the last yr or through the last five years were higher collocated in time as compared to those who occurred greater than five years ago. In comparison with other groups, AD patients exhibited greater difficulty ascertaining events with time that occurred greater than five years ago.

Conclusions

The present study presents recent insights into alterations in temporal processing along the continuum of AD. In the longer term, a scientific assessment is required to find out the neuropsychological variables on different dimensions of temporal processing during AD progression and in patients with MCI.

The study findings provide necessary insights into the particular changes that occur within the temporal processing of SCD, which might be an early marker of future cognitive decline.