Flicker Fusion is 'More Than Meets the Eye'

Our retina’s sensory experience of reflected light is almost instantaneously transformed into a perception we call our vision. Almost is the critical word here because what our retina experiences and what we see requires processing, which takes time. That time will vary, but there is a  physiological limit to how quickly our “visual system can discriminate visual signals.” That flicker fusion threshold is the basis for motion pictures – images displayed at a “frame rate” of 30 per second change too quickly for our visual system to see them as individual images; they fuse together, and we perceive that fusion as motion.

We alter our perception if we change the rate at which a camera captures a still image and play it back at 30 frames per second. Time-lapse images have lower capture rates, which we perceive as speeded-up action; our perception of slow motion comes about by very fast capture rates. The flicker fusion threshold (FFT) is measured in Hz, cycles per second – our FFT is around 60.

Umwelt, a German word for surroundings, was first used in a theory seeking to understand the “pre-linguistic” origins of our perceptions, the biological underpinnings of how we perceive the world.

“All sensory processing is constrained by the maximum rate at which the system can parse information. This processing rate is different among species and is known as their temporal resolution.”

Our FFT defines our human umwelt. However, the FFT for other creatures in the world differ, often substantially, creating their umwelt in our shared world. The peregrine falcon can dive after its prey at speeds of 180 mph, and its FFT, at least double ours, allows it to maintain a sharp image of its prey. The fly has an FFT four-fold greater than ours, which is why swatting one is near impossible; they see your hand coming. Slower FFTs are more advantageous in detecting slower-moving objects, especially in low light.

The point at which images fuse, the limit of our physiological processing speed, termed the critical flicker fusion threshold or CFF, can be measured by showing us a light and changing the rate at which it turns on and off. The point at which we perceive the light becoming steady or flickering defines our CFF. We know from prior studies that CFF can vary in areas of the retina, with stressors including exercise and exhaustion, impaired cerebral function, and, to an extent, age.

A study reported in PLOS One considered the hypothesis that CFF naturally varied significantly between individuals but was a “stable trait within individuals.” 88 volunteers aged 18-35 were recruited and tested.

Participants viewed a continuously lit LED light and turned a dial to make it flash, increasing the rate until they couldn’t perceive flashes. The LED was then reset to 65 Hz, “above the typical perception threshold,” and lowered the flashing rate until it was perceived. The highest values from these two tests were used to create a randomized series of flicker rates above and below those highest values – the CFF was the highest frequency at which the participants perceived flashing light 80% of the time. A subset of participants repeated the test three times over a week.

“…we have shown that there are considerable individual differences in CFF thresholds and that CFF is relatively stable over time. We found no significant difference in mean CFF thresholds between males and females, … However, the results from our post-hoc analysis do show a clear difference between sexes in the test-retest repeatability of CFF. This could be an indication that CFF might be more stable in males than in females…”

80% of that large variance was between participants, a residual of roughly 10% was due to within-individual difference,” i.e., those “stressors.” And those within-individual differences were more pronounced for women.

“… an Umwelt is specifically the part of those surroundings that an animal can sense and experience—its perceptual world. Like the occupants of our imaginary room, a multitude of creatures could be standing in the same physical space and have completely different Umwelten.”

– Ed Yong Immense World  [1]

While identifying where in the path of perception the significant variation in CFF between individuals arises is unclear, there is little doubt that it exists. As a consequence, our visual perception of the world differs. Based on the research, it varies over an interval of roughly 21 Hz. The range is so great that it may contribute to how well we function “in real-world scenarios, especially those requiring high-speed perception and action.”

“ ’What happened on that play?’ And he’d go through eight things that happened. ‘The tackle flashed in front of me. This guy slipped. I saw the linebacker drop wide. The safety was a little deeper than I thought he would be. Then this guy stepped in front, and I kind of put it a little bit behind him because I saw this other guy closing.’ And then you would go back and look at the film, and every one of those things happened in the exact sequence that he explained it to you on the field coming off. I’m like, ‘This guy sees everything.’”

- Bill Belichick describing Tom Brady

Could Tom Brady, among other athletes, be blessed with a higher CFF? His ability to “read the field” built from his experience and unique physiology, creating an umwelt with just a bit of a visual edge. We already recognize more easily measurable advantages, for example, Michael Phelps, with “longer-than-average arms and a proportionally longer torso than legs, assets for swimming but not track. Or Secretariat, the fastest Triple Crown winner, whose heart was three times larger than the average horse's.

While we may think we see the world in the same seamless flow, the truth is far more complex. Our perception is not just about what meets the eye but how quickly our brains can process the visual buffet before us. Our CFF defines the boundaries of our perceptual world and contributes in its own way to our unique vision.

[1] New York Times Science Writer Ed Yong’s book, Immense World, explores the umwelts of the creatures surrounding us. It is an easy read and a wonderful gateway to understanding our world.

Source: The speed of sight: Individual variation in critical flicker fusion thresholds PLOS One DOI: 10.1371/journal.pone.0298007