Reversing Time: Temporal Illusions

Everyone's familiar with a variety of visual illusions, but what about temporal illusions? Consider the following example, from a recent poster by Stetson et al.: you are wandering through the forest and you hear a twig crack ... but did it crack when you set your foot down, or just before? For much of our evolutionary history, questions such as these were of dire importance; a tiny temporal offset can be the difference between life, and death by a large predator. Unfortunately, these survival-based mechanisms can sometimes lead us astray.

To see how minimizing temporal offset can lead to a temporal illusion, first remember that every sense has its own processing latency - for example, as Stetson et al. note, visual processing is actually slowed in low light conditions. Other times there is variable latency within a single sense, such as touch (because it takes longer for a signal to reach the brain from your toe than from your arm). Given these small temporal discrepancies in each of our senses, how do humans reliably perceive simultaneous multi-sensory events as actually being simultaneous?

The answer is surprisingly simple: we literally live in the past. In order to correctly perceive the temporal order of events in the world, our brain is constantly recalibrating the temporal relationship between the motor system and our perceptual systems. It does this by implementing a variable delay in the perceived onset of our own motor actions, so that we are able to dynamically adapt to changing environmental and sensory conditions.

In other words, if the twigs consistently cracked some number of miliseconds after you put your foot down on that walk through the forest, by the end of the day you would perceive them as cracking in complete synchrony with your footsteps. Thus, our brains appear to have a relatively simple - and usually safe - assumption built-in: miniscule and yet highly consistent temporal offsets between motor output and sensory response are more likely due to delays in sensory processing than to some strange doppelganger, capable of copying your every move with 50 ms precision. Therefore, this recalibration mechanism simply adds some "delay compensation" to the perception of motor outputs, and thereby synchronizes the motor and sensory systems.

If such a "recalibration mechanism" does exist, Stetson et al. conjectured, then using modern technology, we should be able to force subjects to recalibrate to an artificially long delay. This would then cause them to perceive their own motor actions as happening later in time than they actually did. Then, if we abruptly expose subjects to the real world, in which delays are not artificially lengthened, it might seem to them that the world reacts to things they have not yet done! (Imagine hearing your own footsteps before you think you've put your foot down.)

By repeatedly pairing a keypress with a flash of light, in which the light flash lagged behind the keypress by 100 ms 75% of the time, and then recording subjects' perception of whether the light flashed before or after their keypress, Stetson et al. were able to cause a reversal of perceived temporal order - at the end of the experiment, the light flashed immediately after the keypress, and subjects reliably thought the light flashed before they pressed the key!

In fact, their analysis showed that this recalibration process can occur in as little as 20 trials - just 20 exposures to an artificially lengthened delay is enough to kick the recalibration mechanisms into action. Comparison of the neural activity belonging to the flash-lag group with a baseline group (in which the experimenters did not insert an artificial lag between flash and keypress) showed selective activity in the anterior cingulate - often considered the "error detector" or "conflict monitor" of the cortex. The same activation patterns are seen when comparing the "illusory simultaneity" and "illusory temporal reversal" trials within the flash-lag group; this is also compatible with a view of anterior cingulate as involved in error or conflict detection. However, this does not suggest that the anterior cingulate is part of the recalibration mechanism itself - these comparisons show only that it is activated during illusory reversals of temporal order.

Currently, Stetson et al. have an article in press at Neuron, titled "Motor-sensory recalibration leads to an illusory reversal of action and sensation," which I'm guessing will cover much of the same topics as this poster. Other experiments from the Eagleman lab look equally fascinating - such as this experiment, in which subjects free fall from a 80 foot tower, in order to test possible time distortion effects. The same lab has also weighed in heavily on the illusory motion reversal debate, covered here previously.