Our perception of time is something we take for granted. It drags. It goes too fast. It’s always there in the background, ticking away.
But the means by which we measure, interpret and remember the flow of events in time is a difficult problem for neuroscientists.
While we understand much of the ways in which the brain is involved in the perception of objects in space, our understanding of the mechanisms of temporal awareness remain crude, to say the least.
While time may sometimes seem to stall or disappear, most of us instinctively feel we’re living in the moment.
But sometimes people can feel dislocated from time, a symptom known as temporal dissociation.
My current PhD studies are exploring how we can better understand why this dissociation happens. In doing so, the research draws upon the basic neuro-sciences as well as how we experience our sensory worlds.
Attempts in the 1960s and 70s to understand how we perceive time followed on from extensive work on pigeons, who are surprisingly adept at learning to respond at particular intervals, even two overlapping intervals at once.
It was found that accuracy in estimating an interval of time decreased as the length of the interval increased, the so-called Weber’s Law. While this observation has survived the test of time, there have also been many observations in which timing doesn’t follow the rules.
With Weber’s Law not always holding true, it was found people could more accurately time short periods if they paid close attention the the timing task. While not surprising in itself, this observation opened the path to including attention and memory in a multitude of different timing theories.
As things stand, there are at least 13 different theories of how humans measure and reproduce temporal intervals – and none has succeeded in providing a complete description of the phenomenon.
While they appear diverse on the surface, the theories broadly fit into two different categories: timing with a timer and timing without a timer.
Timer theories
In its simplest context the timer is a neural clock, some type of neural structure producing ticks and a method to store (or accumulate) them.
Generally the neural substrate (set of brain structures) for the clock is not specified in the theories. This is a deficiency a new theory of how groups of neurons may be involved in timing attempts to address.
It has been shown that some clusters of neurons respond preferentially to particular stimuli – for example, some cells in the visual area V1 respond strongly to a horizontal line while adjacent cells respond to a vertical line. These clusters of neurons are known as channel based networks.
It has been suggested that a similar system may be involved in perception of temporal intervals, with different neuronal clusters tuned to respond to different lengths of time.
To test this hypothesis, in 2007 Mark Becker and Ian Rasmussen from Lewis and Clark College in Portland devised an adaptation experiment, in which subjects were adapted (changed over time in the responsiveness of their sensory system) to a fast auditory rhythm and then perceived a similar rhythm to be slower than it actually was.
The authors also suggested the effect was specific to a particular sense: hearing rather than vision. Recent work by James Heron and colleagues at the University of Bradford in the UK builds upon this by showing adaptation effects can cross between the senses of hearing and vision.
This is an important development because any candidate for a timing mechanism, or more loosely a “sense of time” needs to be integrated enough to act to synchronise our perceptions whether they be of sight or sound or smell.
With these discoveries there’s hope of getting closer to explaining which areas of the brain may be involved in timing. But there are no definitive measurements of these sites yet. In fact, there’s some evidence to suggest our ability to perceive time may not be contained in any one area of the brain at all.
Timing without a timer
How could a sense of time arise without the involvement of a neural clock? In 2007, Dean Buonomano and colleagues from the University of California, modelled how timing functions could arise from the changes in activity of cortical networks (groups of cells in the grey matter of the brain that act together functionally) over time.
They liken their model to telling time by the “ripples on the surface of a lake” rather than counting discrete events. There are advantages to this approach, particularly when trying to understand some of the more perplexing illusions of time.
One such illusion involves the apparent reversal of the order in time of two events. In 2005, Concetta Morrone, then at the Universita Vita-Salute San Raffaele in Milan, and her colleagues from the University Western Australia and the University of Pisa noted something interesting: if the observation of two flashes of light is made close to the time of making a fast eye movement, subjects often reported the second event happening before the first.
As yet, no theory of human timing has been able to clearly explain how our sense of time can be so completely fooled. It’s this observation my colleagues and I at Melbourne University and the University of Cambridge are attempting to explain.
With an integrated approach to how we perceive time as well as a better understanding of the underlying neuroscience, we may be soon be able to reduce the multitude of theories to a more coherent view of what may be our most fundamental sense of all – time.