Neural representation of temporal duration: coherent findings obtained with the "lossy integration" model

Lower loss rate of serotonergically modulated neuronal accumulator for time in patients with major depressive disorder

Major depressive disorder (MDD) is characterized by dramatic and persistent worsening of mood, as well as a subjective feeling of time slowing. However, experimental data on time perception are inconsistent. As serotonergic dysfunction implicated in MDD etiology, we aim to examine time perception in MDD through the framework of lossy temporal integration model, previously also related to serotonergic transmission. Thirty-one patients with recurrent depressive disorder in partial remission and thirty control participants, without a history of psychiatric and neurological disorders, performed duration discrimination of visual stimuli (duration ranges from 3.2 to 6.4 s) and subjective minute production tasks. To infer about central serotonergic transmission, an electroencephalogram in response to the 1000 Hz tone of different intensity (50, 60, 70 and 80 dB SPL) was recorded. Patients with MDD shorten the past durations in the duration discrimination task significantly less than controls, thus being more objective. No difference in the subjective minute production was recorded. Patients with MDD have also exhibited larger auditory evoked potentials in response to the tones of high intensity (70 and 80 dB SPL) when compared with the controls. This resulted in a steeper slope of auditory evoked potentials by intensity function. These converging findings suggest a lower loss rate of neuronal temporal accumulator modulated by serotonergic transmission in patients with MDD.

How much time has passed? Ask your heart

Internal signals like one's heartbeats are centrally processed via specific pathways and both their neural representations as well as their conscious perception (interoception) provide key information for many cognitive processes. Recent empirical findings propose that neural processes in the insular cortex, which are related to bodily signals, might constitute a neurophysiological mechanism for the encoding of duration. Nevertheless, the exact nature of such a proposed relationship remains unclear. We aimed to address this question by searching for the effects of cardiac rhythm on time perception by the use of a duration reproduction paradigm. Time intervals used were of 0.5, 2, 3, 7, 10, 14, 25, and 40 s length. In a framework of synchronization hypothesis, measures of phase locking between the cardiac cycle and start/stop signals of the reproduction task were calculated to quantify this relationship. The main result is that marginally significant synchronization indices (SIs) between the heart cycle and the time reproduction responses for the time intervals of 2, 3, 10, 14, and 25 s length were obtained, while results were not significant for durations of 0.5, 7, and 40 s length. On the single participant level, several subjects exhibited some synchrony between the heart cycle and the time reproduction responses, most pronounced for the time interval of 25 s (8 out of 23 participants for 20% quantile). Better time reproduction accuracy was not related with larger degree of phase locking, but with greater vagal control of the heart. A higher interoceptive sensitivity (IS) was associated with a higher synchronization index (SI) for the 2 s time interval only. We conclude that information obtained from the cardiac cycle is relevant for the encoding and reproduction of time in the time span of 2-25 s. Sympathovagal tone as well as interoceptive processes mediate the accuracy of time estimation.

Perception of acoustically presented time series with varied intervals

Data from three experiments on serial perception of temporal intervals in the supra-second domain are reported. Sequences of short acoustic signals ("pips") separated by periods of silence were presented to the observers. Two types of time series, geometric or alternating, were used, where the modulus 1+δ of the inter-pip series and the base duration Tb (range from 1.1 to 6s) were varied as independent parameters. The observers had to judge whether the series were accelerating, decelerating, or uniform (3 paradigm), or to distinguish regular from irregular sequences (2 paradigm). "Intervals of subjective uniformity" (isus) were obtained by fitting Gaussian psychometric functions to individual subjects' responses. Progression towards longer base durations (Tb=4.4 or 6s) shifts the isus towards negative δs, i.e., accelerating series. This finding is compatible with the phenomenon of "subjective shortening" of past temporal intervals, which is naturally accounted for by the lossy integration model of internal time representation. The opposite effect observed for short durations (Tb=1.1 or 1.5s) remains unexplained by the lossy integration model, and presents a challenge for further research.

Effects of emotional valence and arousal on acoustic duration reproduction assessed via the "dual klepsydra model"

We report results of an acoustic duration reproduction task with stimulus duration of 2, 4, and 6 s, using 45 emotionally negative, positive, and neutral sounds from the International Affective Digitized Sounds System, in a sample of 31 young healthy participants. To investigate the influence of induced emotions on perceived duration, the effects of emotional modulation were quantified in two ways: (1) via model-free indices (aggregated ratios of reproduced times), and (2) via dual klepsydra model (dkm)-based estimates of parameters of internal time representation. Both data-analytic approaches reveal an effect of emotional valence/arousal, namely, a significantly longer reproduction response for emotional stimuli than for the neutral stimuli. The advantage of the dkm-based approach is its ability to disentangle stimulus-related effects, which are represented by "flow intensities," from general effects which are due to the lossy character of temporal integration. We explain the rationale of the dkm-based strategy and interpret the observed effect within the dkm-framework as transient increase of internal "flows." This interpretation is in line with recent conceptualizations of an "embodiment" of time where the model-posited flows correspond to the ongoing stream of interoceptive (bodily) neural signals. Neurophysiological findings on correlations between the processing of body signals and the perception of time provide cumulative evidence for this working hypothesis.

The inner sense of time: how the brain creates a representation of duration

A large number of competing models exist for how the brain creates a representation of time. However, several human and animal studies point to 'climbing neural activation' as a potential neural mechanism for the representation of duration. Neurophysiological recordings in animals have revealed how climbing neural activation that peaks at the end of a timed interval underlies the processing of duration, and, in humans, climbing neural activity in the insular cortex, which is associated with feeling states of the body and emotions, may be related to the cumulative representation of time.

Duration reproduction: lossy integration and effects of sensory modalities, cognitive functioning, age, and sex

The "dual klepsydra model" (DKM) of internal time representation successfully models duration reproduction data, but relations between the DKM-based parameter kappa ("loss rate") and procedural variables (presentation modality) or individual characteristics (cognitive indices, age, sex) remained as yet unexplored. For that purpose, were-analyzed data from an earlier time reproduction study (N = 100), using visually or acoustically presented intervals of 1-5 sec. duration. Typical values of parameter kappa were approximately 0.03-0.04 sec.(-1), corresponding to relaxation times of internal "lossy integrators" of approximately 30 sec. Significant effects of presentation modality (smaller kappa values for the visual reproduction task) and of age (greater kappa in acoustic reproduction with increasing age) were observed. Cognitive variables (working memory, general fluid reasoning, attention) and sex of participants were not associated with kappa. Cognitive functions seem to play only a minor, if any, role at the level of time representation addressed by the DKM.