The eyes are the main organs that receive light and allow us to see. When light enters our eyes, it is converted into electrical signals. These signals then travel through different parts of our brain, and eventually reach the primary visual cortex (an area in the brain), which helps us make sense of what we see.
Under normal conditions, our vision has certain limitations that can change depending on different conditions and circumstances. To understand these limitations, researchers conduct experiments under controlled conditions, often involving flickering lights.
When a light flickers very quickly, there is a minimum speed at which we no longer perceive the flickering and the light looks steady instead. This specific minimum flickering speed is called the critical flicker fusion frequency (cFFF) or Flicker Fusion Threshold (FFT). At this point, our eye receptors can't detect the individual changes in what we see anymore. Instead, they merge together, creating a steady signal that our brain interprets as continuous light.
What Does Flickering Lights Have To Do With Perceiving Motion?
People with higher Flicker Fusion thresholds (the ability to no longer see high speed flickering in flickering lights but instead see them as steady light) tend to have better accuracy in what they perceive in general, and it's also linked to increased alertness and improved visual processing in the brain. In other words, people who can see fast “flickering lights” at a higher speed than others, can see fast moving objects at a higher speed than others. [
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This concept is also applied to TVs, cinemas, and other similar forms of media. That’s why frame-by-frame viewing needs to happen at frequencies close to the limit at which we can perceive “flickering lights”. Animals that have better vision than humans need to “see fast flickering lights” better to survive in the wild. According to this study, Birds that have a great ability to see “flicker lights” can see fast-moving objects better than humans, especially when these objects could potentially collide with them in the air. [
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Let’s look at the eye and the brain.
We need light to see. Light bounces off things and goes into our eyes. The retina in the back of our eye collects these things. (The Fovea is a part of the retina that collects colors.)
All that light that’s bouncing off of the moving/stationary things we are looking at is turned into electrical signals that go to the back of our brain. That place in the back of our brains is called the visual cortex. The first neurons in that part of the brain that collect the information is called V1 neurons. Their only job is to react to the light. Just light. Anything involving light. Which means flickering light or changes of light in the environment (Changes of light can happen when something moves, so if a red light moves to the left, we would know it moves to the left when it collects that info. In other words, it knows the direction of light bouncing off moving things. Don’t forget that all we see is light bouncing off of things). Then all that light goes from the V1 neurons to all the other neurons (MT/V5, MST, 7a, all that stuff) that all specialize in motion (is this moving? Is this stationary? Where exactly is it moving? What is it doing? What does it look like when it’s moving?), identifying what we are looking at (Wtf is this?), and so on. All of this does not matter because they all depend on the very first neurons to work well (the light detecting and light changing neurons, also known as “flickering lights” neurons). That’s why scientists use experiments that involve flickering lights, because they want to see the limits of this first set of neurons, NOT the following sets of neurons that deal with motion and all that stuff. Because all those other neurons rely on the first ones for perception. [
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Things can flicker or move at such high speeds that the primary visual cortex (V1) becomes unable to detect the rapid changes in light. Instead, the V1 simply presents the most recent, fully processed information as more light enters the eye. This phenomenon explains why afterimages and other fast-motion-vision-related illusions occur. As the visual system struggles to keep up with the rapid changes, it relies on the last processed image, creating the illusion of persistence and the blending of moving objects into the background, appearing invisible.
In summary, as things move, light changes; as things flicker, light changes. Our brain can only know what is moving if it can detect changing light. That’s why scientists use flickering lights to determine this, cuz flickering lights and moving objects are essentially just changing lights to first areas where our brains collect these changing lights. Without detecting the changes itself, the rest of our motion sensitive brains are completely useless.
What is Flicker Fusion Threshold?
Scientists determine this by figuring out the frequency our brain gives ups on detecting rapidly changing light. They do this through experiments that involve showing people flickering lights and speeding it up till the flickering fuses to become one single image of light. At this speed, the brain literally can’t detect changes in light. That’s why when characters move at this speed, the brain won’t detect the changing positions of light bouncing off them at different points in space.
So in other words, the minimum speeds that surpass the limits of our vision is called Critical Fusion Threshold, which is also called Flicker Fusion Threshold. That speed is measured in Frequency Hertz. [
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There are many experiments to find this speed. The most widely accepted being between 50 to 90 Hz like you see in the OP. [
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Characters who are FTE to the point they appear invisible or cause visual illusions depending on how they move have to be moving faster than the eye's visual processing. In other words the character or thing is so fast that the brain just gives up detecting the object altogether. This means said character must be moving between 1/50th of a second to 1/90th of a second. Therefore, I propose 1/70s be used as the timeframe for FTE calcs at this level. If accepted I will continue to fine tune the guidelines to factor in less typical conditions for better use of this method.
TL;DR: Flicker fusion refers to the point at which a flickering light appears as a continuous stream to our eyes. It represents the limit at which our visual system can detect changes in light. People with higher flicker fusion thresholds can perceive fast-moving objects better. Scientists study this phenomenon because it reveals the limit of the initial light-detecting neurons (V1 neurons) in our brain. When light changes too rapidly, these neurons can't keep up, and our brain perceives a steady image in the form of afterimages or other visual illusions. The critical flicker fusion frequency (cFFF) or Flicker Fusion Threshold is the minimum speed at which things move beyond the limits of visual perception. Characters who can move faster than the ability the observer's visual system can detect them should be moving at least 1/50th to 1/90th of a second to appearing invisible by blending into the background or cause visual illusions.
