When The Brain's Biological Clock Goes Haywire

[cross-posted from our Psychology Today blog]

Our perception of time varies greatly depending on our age, mood, stress level and psychological health and stability. Psychological disorders, such as Parkinson's disease, attention deficit hyperactivity disorder and schizophrenia, can mess with the brain's time keeping mechanism and warp our estimation of time. Patients suffering from these disorders are unable to properly coordinate events in time. Patients over- or underestimate time intervals ranging from several seconds to minutes.

How does this happen? How does the brain manage to keep track of time and what goes wrong in psychological disorders? Our senses (sight, hearing, smell, taste and touch) use specialized sensory systems with task-specific neurons to process sensory input. Yet there is no specific sensory system for time. So how does our sense of time come about?

The traditional explanation of how the brain keeps track of time has been that the brain has its own continuously running and self-sustaining internal clock.

The brain's internal clock is commonly compared to the function of a pacemaker that emits pulses at some mean rate, which then leads to the experience of subjective time. This happens in several stages. An accumulator accumulates the pulses, and they are then registered in working memory in the prefrontal cortex and may also be stored in long-term memory. Temporal judgments involve comparing registered times in working memory to information about the duration of tasks retrieved from long-term memory.

What exactly gets messed up in the brain's time keeping processes in psychological disorders remains a subject of great controversy.

Traditional research has indicated that the warped time perception in psychological disorders occurs because the brain misreads its own biological clock, or because the brain's time pacemaker emits pulses at an unusual rate.

Recent studies completed by Brent Parsons and his colleagues from Baylor College of Medicine and Harris County Psychiatric Center in Houston suggest an alternative explanation of why time estimation goes wrong in patients with schizophrenia. The defects in time perception in schizophrenia, the researchers suggest, occur at a very low level of perception as a result of extra energy required by sensory neurons to process incoming stimuli.

The Houston researchers were particularly interested in the observation that the duration and integration of short time intervals is lengthened in schizophrenics. Using a flicker fusion paradigm, the researchers presented participants with an array of dots forming four numbers that rapidly alternated with a corresponding negative image. The four numbers are easily reportable at a slow rate, but when the rate is increased, reporting eventually drops to chance.

The results showed that participants with schizophrenia exhibited significantly lower thresholds than controls, that is, they were able to report the four numbers at a faster rate than controls, which suggests that visual stimuli persist longer in short-term memory in schizophrenics compared to controls.

In a second proliferation task the researchers were testing for temporal integration by asking participants to determine how many stimuli appeared to be present simultaneously on a screen. In each trial, single-letter stimuli were flashed one at a time in a randomized location. Two conditions were employed: in the first, the same stimulus was presented (Repeat condition); in the second, different stimuli were presented (Random condition).

Both groups reported greater numerosity in the Random condition than in the Repeat condition. This result was expected, as novel stimuli are known to be perceived to have a longer duration than repeated stimuli.

However, schizophrenics and controls differed in the average number of stimuli reported. In both the Repeat and Random conditions, patients with schizophrenia reported more stimuli to be simultaneously present on the screen than controls. These results confirm the hypothesis that visual persistence is longer in schizophrenia.

Parsons, et al. (2012) take these results to suggest that the misreading of time duration in schizophrenia is a result of a defect in white matter integrity that leads to excessive energy expenditure in neurons processing incoming sensory input. When neurons need to use more energy to process sensory input, this can influence the subjective duration associated with the stimulus.

Defects in the brain's time keeping mechanisms need not always line up with how time is experientally perceived. It is well known that our white matter integrity decreases with age. If, indeed, neurons require more energy to process sensory input as a result of this, then we should also expect elderly folks to overestimate time intervals, and it's likely that they do do just that. For example, they might take a time interval that lasts half a second to last a full second in time-estimation tasks.

But this type of overestimation, if it occurs, does not line up with how we perceive time when we age. Most people report that time seems to be moving faster the older they get.

An interesting hypothesis that is yet to be tested is whether there is an inverse relationship between task-related overestimation of time intervals (and an increased neuron energy expenditure) and how fast we perceive time as passing by. Perhaps if every task requires more energy, we won't be able to complete as many tasks in the same amount of time as we were previously able to, and this may affect our perception of how fast time is passing.