To recap, flicker fusion tests are used to measure the maximum number of visual snapshots our brain can perceive each second. They are used to assess the brain’s processing speed and are good indicators of how fast we are “recording” the world, hence how fast time is experienced. The average Flicker Fusion Frequency for humans is between 10 and 40 fps. This means the time interval between flashes range from 25 to 100 milliseconds, the same range in which movie snapshots give the illusion of continuous motion. But what is it inside the brain that defines the duration of these mental snapshots? To be able to control the speed of time, we need to understand how our brain’s “video camera” produces those short-lived mental snapshots.
How Our Brains Create Mental Snapshots
As we have seen, a key aspect of the discrete theory of perception is that information received through the senses are perceived as distinct “life moments” or mental snapshots. Information cannot be processed in an equally continuous manner at each moment in time, but is captured and processed in discrete chunks, or “mental snapshots.” The “recording” process leaves a signature, at the neuron level, in the form of electrical spikes that rise and fall in intensity with every captured mental snapshot. By measuring the timing of these EEG electrical spikes (brainwaves), we can assess the speed at which our brain’s “video camera” updates our consciousness and the number of mental snapshots it records each second.
The discrete nature of this process also implies that distinct events that occur within the confines of one mental snapshot are blended to produce one single and continuous experience. Let us look at an example to clarify this.
As I sit in the park, I see a youngster riding his skateboard, moving in a continuous straight line down the track. By continuous, I mean that the skateboard does not disappear here and reappear there but successively occupies every point along its path, at every point in time. Now if I take a video camera and start filming the teenager on his skateboard, his continuous motion will be recorded in a discontinuous manner, in the form of a rapid sequence of snapshots. When I replay the film, the recorded skateboard snapshots do not really move at all; they are just replayed at such a fast rate that they give the skateboard the illusion of continuous real-life motion. The recording speed of a typical video camera is 24 snapshots per second. Consequently, the duration of each film snapshot is 42 milliseconds long. The video camera does not record every instant of time but snaps a sample of reality every 42 milliseconds. Any events happening in between those film snapshots would not be recorded as distinct events but would fuse into one of the snapshots on film. If an alien spaceship, for instance, appears out of nowhere and pulls the boy 20 meters in the air after one snapshot is taken and then puts him back to exactly where he was, and vanishes, before the next snapshot is taken, the video camera would not capture that. That single snapshot taken by the video camera before the highly unlikely event represents all positions occupied by the skateboard during the 42-millisecond window. Within that film “moment,” reality is simply not processed by the video camera and, therefore, slips by unrecorded, as if time froze.
Our brains work in a similar discrete way, and thus the event that falls between two mental snapshots cannot be detected by our brains, but would fuse and be recorded as one snapshot in our mind. To investigate that, an intriguing study was conducted in 2010 at the Brain and Cognition Research Centre in Toulouse, France, where participants were presented with very brief flashes of light on a computer screen while they were connected to an EEG scanning machine that recorded their brain’s electrical activity. After each light flash, a question mark appeared on the screen and the participant pressed a button to indicate if he or she saw the flash or not. Each participant performed 1,500 trials and the results indicated that, on average, only half of the flashes were detected. This raised the question as to what was causing the other half to be missed entirely. We saw that the brain’s electrical activity comes and goes in waves, or spikes, cycling rapidly between phases of high and low electrical activity states. Researchers suspected that mental snapshots were being captured at the high activity states, while the low activity states were those moments in between snapshots. When they compared the EEG brainwave recordings with the flash detection results, they found that the probability to detect a light flash heavily depended on the phase of the brainwave cycles that preceded the presentation of the flash. Participants who were presented with a flash that coincided with a spike in their brainwave (positive phase) were more likely to detect it, whereas a flash could not be easily perceived when it coincided with a drop in the brainwave (negative phase). 23, 24
This suggests that our perception operates in successive periodic cycles that rapidly alternate between favorable periods with high spikes in electrical activity, where we consciously detect light flashes, and unfavorable periods with drops in electrical activity, where we miss them. The reason for this is that neurons are more likely to fire and detect a light flash when they are already in the excited positive phase of the brainwave cycle and less likely to fire when they are in the inhibited negative phase. In other words, a mental snapshot is only recorded in our mind around the peak of each brainwave cycle. 25 These brainwave cycles act like a fast camera shutter that cuts the continuous visual sequence into discrete mental snapshots.
Brainwaves come in various types and speeds, the most dominant being the Alpha brainwaves that occur when the brain is resting or in quiet flowing thought. They have an average frequency of 10 Hz, i.e. one spike every 100 milliseconds. Therefore, when we are resting, our brains will perceive reality at the rate of 10 snapshots per second or one snapshot every 100 milliseconds. When the brain is more alert, such as when solving problems, or planning, brainwaves run up to 40 Hz and a higher number of mental snapshots are captured. The slowest brainwaves occur during deep meditation or dreamless sleep, when we are less alert and very few mental snapshots are processed.
The Wagon Wheel Illusion
Further evidence for the discrete perception of reality comes from the so-called wagon wheel illusion. You might have noticed that in fast action movies, the wheels of a car seem to be rotating backwards when the vehicle is clearly moving forward. This is obvious in racing scenes or speeding stagecoaches in Western movies. The illusion occurs because the recording speed at which the movie is filmed, usually 24 frames per second, is slower than the speed at which the wheels are turning. The result is that the video camera captures one distinct snapshot of the rotating wheel at a position that moves slightly behind with every turn. When these positions are replayed in film, the wheel appears to be rotating backwards, giving rise to the illusion. In engineering terms, this process is called “aliasing.” Surprisingly, this wagon wheel illusion is not just limited to movie scenes but can also be observed in real life, under continuous light. You can actually notice it when looking intently at the wheels of a car traveling on the highway lane beside you, in jet fan engines, cooling fan blades, or in airplane propellers rotating in broad daylight. This implies that internal “aliasing” is also taking place within the visual system itself and that our brains must be working like a video camera. Our brain’s visual system appears to periodically be sampling the motion of the turning wheel, in a series of discrete snapshots, that causes the wheel to be perceived as rotating backwards. 26
The first to observe the wagon-wheel effect under truly continuous light was J.F Schouten, a Dutch physicist, in 1967. He noticed that the spokes on a wheel rotating at a rate of 8–12 cycles per second (Hz) can be seen as standing still under focused attention and would appear in reverse rotation when the wheel turns at a faster speed of 30–35 cycles per second. He interpreted these observations by suggesting that the rotating wheel is processed not only by visual detectors sensitive to the true rotation but also by detectors sensitive to the reverse rotation. The illusion appears in a form of rivalry between the reverse direction detectors that become sufficiently active to dominate perception of the true rotation detectors. 27
This interpretation opposes the discrete perception theory and there has been debate in the scientific community over which of the two interpretations is closer to the truth. There is strong evidence that supports each side but recently the balance seems to be tipping towards the discrete perception theory.
Further research confirmed that the continuous wagon wheel illusion is most prominent when the rotation speed ranges between 10 to 16 Hz and is greatest around 13 Hz (rotating 13 times per second). Interestingly, if you observe the illusion while connected to an EEG machine, there is only one frequency band in your EEG brainwaves that changes significantly when you see the illusion, and that is the 13 Hz brainwave, the same frequency of the rotating wheel. This coincides with the range of alpha brainwaves (8 to 13 Hz) and implies that the visual part of the brain “records” around 13 snapshots per second, or one snapshot every 77 milliseconds when the illusion is observed. 28, 29 This provides further evidence that the speed of the neurons’ cyclic firing pattern is why we observe reverse motion in the wagon wheel illusion and why the brain absorbs reality in a discontinuous manner.
The wagon wheel illusion depends on the degree of focused attention one devotes to observing the rotating wheel. 30 When two rotating wheels are viewed simultaneously, the reverse motion is only observed in one wheel at a time, as if attention flips successively between the two wheels. This raises the question as to whether the way we pay attention is a discrete cyclic process or whether it is a continuous one.
Our Attention: Blinking or Continuous?
For the past 30 years the consensus was that attention works like the scanning beam of a searchlight, lighting up parts of a vision field and analyzing successive targets that come into focus. However, there is now increasing evidence that suggests that our attention can concurrently select multiple locations or targets. But what is uncertain is whether that ability relies on a continuous allocation of attention to all targets at the same time, known as the “parallel strategy,” or whether attention switches rapidly between targets in a discrete and sequential manner, known as the “sampling strategy.”
Imagine you are meeting a friend in a crowded coffee shop. You arrive late and scan the room in search of your friend. Do you move your eyes from face to face like a roaming spotlight, or do you take in the whole scene hoping that your friend's face pops out at you? Researchers at MIT found that people tend to scan the room, switching from face to face, in a discrete sequential manner. But what determines how fast you can scan the room? The timing of each switch in attention appears to be regulated by the level of electrical activity in the brain, which acts like an internal built-in clock, providing the framework for shifting attention from one target to the next in a sequential manner. 31, 32 Several studies have confirmed that attention seems to work like a nightclub’s flashing stroboscope or a blinking spotlight that scans targets at an intrinsic rate of seven items per second. That is one switch every 140 milliseconds. This again coincides with the range of our mental snapshot durations that we saw earlier. 33
Attention is considered to be the gateway to our consciousness. Thus, if the attention gateway of our mind opens and closes in a fast periodic manner, at the rate of seven times per second, then the contents of our awareness will be rapidly updated at the same rate and in a discrete periodic manner. Our brain’s electrical fluctuations and brainwaves define the speed of that periodic updating and define the duration of the successive mental snapshots of our awareness. Faster information processing, means more mental snapshots are being captured within a certain time interval, causing that interval to stretch in our mind, as if time ran slowly, and vice versa.
Time is nothing but the speed at which we capture and process sensory information from the ever-changing world around us. The perceived speed of time is dependent on the speed at which our brain updates the content of our consciousness, i.e. the speed at which we experience the world. Now, let us look at how brain scientists use that knowledge to explain the various time distortions we encounter daily.
Chapter 4
Time Distortions
The Fallibility of Our Internal Clock
The Moving Finger writes; and, having writ,
Moves on: nor all thy Piety nor Wit
Shall lure it back to cancel half a Line,
Nor all thy Tears wash out a Word of it
― Omar Al-Khayyam
An Accidental Discovery
In the winter of 1930, the wife of American physiologist, Hudson Hoagland, became sick with a severe flu and high fever. Dr. Hoagland noticed that whenever he left his wife’s room for a short while, she complained that he had been gone for a long time. In the interest of scientific investigation, and briefly putting aside his nursing duties, he asked his wife to count to 60, with each count corresponding to what she felt was one second, while he kept a record of her temperature. It is not hard to imagine the irritated look on her face at this suggestion, but his wife accepted and he quickly noticed that the hotter she was, the faster she counted. When her temperature was 38 degrees Celsius, for instance, she counted to 60 in 45 seconds. He repeated the experiment a few more times, presumably in total disregard to his wife’s feverish objections, so that when her temperature reached 39.5 degrees Celsius, she counted one minute in just 37 seconds. The doctor thought that his wife must have some kind of “internal clock” inside her brain that ran faster as the fever went up. This, in turn, would have made it seem as though he had been out of the room for a long time because her faster internal clock would have made her feel that more time had elapsed. To confirm his findings, Dr. Hoagland went back to the University of Massachusetts where he resided, and subjected his students to high temperatures of up to 65 degrees Celsius, by making them wear heated helmets, while asking them to guess certain time durations. The results, which were later reproduced in countless other studies, confirmed that raising the brain’s temperature can increase the brain’s “internal clock” speed by as much as 20 per cent, causing time durations to stretch subjectively by the same proportion. 34
The Brain’s Ticking Clock
Psychologists spent the next few decades trying to understand what made the brain’s clock tick. They carried out experiments on mice, pigeons, turtles, cats, monkeys, children, and adults, shocking them with electricity, irritating them with unrelenting sound clicks, and strapping them to heated chairs, all in the hope this would slow down or speed up their internal clocks. It was thought that the brain’s internal clock counted time in the range of milliseconds, seconds, and minutes, and was at the basis of all skills requiring a sense of timing. Over the years, several areas in the brain have been suggested as a possible location for that internal clock, and with good arguments for each. However, doctors who performed brain CT scans and surgical dissections were unable to detect a single specific structure for it. There was considerable uncertainty about where in the brain time is processed and it took a huge amount of research for brain scientists to finally begin to understand.
We now know that the brain’s internal clock is not based on just one specific area in the brain but on a combination of several interconnected brain regions. These regions are linked by an intricate network of neurons that communicate by firing neurotransmitters. As we saw in the previous chapter, this complex neuron network produces synchronized electrical activity, or brainwaves, that fluctuate in cyclic phases and define how fast the brain processes sensory information i.e. how fast it absorbs the world around us. It is this information processing speed that defines how fast we experience the “flow” of time. The brain’s internal clock is merely an oversimplified metaphor for how fast our brains “record” the world around us.
With this in mind, the relationship that Dr. Hoagland discovered between brain temperature and the subjective speed of time can be explained. We know that chemical reactions occur more rapidly when heated and the speed of biological processes doubles when the temperature increases by 10 degrees Celsius. Therefore, raising the brain’
s temperature produces changes in the brain’s chemical processes that boost the brain’s electrical activity, resulting in faster brainwaves and faster processing of reality. This, in turn, causes time intervals to expand and the flow of time to slow down, which explains why Hoagland’s wife felt he was gone for too long when her fever was up. This is probably why animals in hibernation can get through that period fairly quickly, as time subjectively runs fast in their brain, as body temperatures and metabolic rates decline, though as you might imagine, this is difficult to verify with a squirrel and practically impossible with a hibernating bear!
It is helpful to think of the brain’s information processing speed as forming the basis of an “internal clock” that ticks faster as the speed of processing sensory information increases, and slower as the processing speed decreases. Our brainwaves define the duration of our mental snapshots and information processing speed, which in turn affects how fast or how slow our internal clock is ticking. The brain’s internal clock is similar to the “clock speed” on a computer microchip processor that determines how many calculations a computer can perform per second (GHz). In theory, the higher the processor’s “clock speed,” the faster the computer is at processing information. However, this computer vs. brain comparison is not entirely accurate, as there are major differences between the types of information processing of a human brain and the computations performed by a computer. A calculator, for instance, can perform mathematical calculations much faster than a human brain because our brains are not optimized for calculating square roots, division, or compounded interest. Our brains evolved to track motion and can record a stream of images, figure out that they belong to a ball moving in the air, predict its trajectory, and catch it, while simultaneously processing other visual signals needed to avoid obstacles on the way.
The Power of Time Perception Page 6