I can’t believe it took me so long to find this. The nub of my approach turns on seeing the crazy things we report on this side of experience in terms of our inability to see that there is a far side, let alone what it consists in. Flicker fusion provides a wonderful illustration of the way continuity leaps out of neglect: as soon as the frequency of the oscillation exceeds our retina’s ability to detect, we see only light. While watching this short video, you are vividly experiencing the fundamental premise informing pretty much everything here on Three Pound Brain: whatever cognition and consciousness turn out to be, insensitivity to distinctions reports as the absence of distinctions. Identity.
Human vision possesses what psychophysicists, scientists investigating the metrics of perception, call a ‘flicker fusion threshold,’ a statistical range mapping the temporal resolving power of our photoreceptors, and so our ability to detect intermittent intensities in light. Like technological video systems, our biological visual systems possesses discriminatory limits: push a flickering light beyond a certain frequency and, from our perspective at least, that light will suddenly appear to be continuous. By and large, commentators peg our ability to consciously report flickering lights at around 60Hz (about ten times faster than the rotor speed of most commercial helicopters), but in fact, the threshold varies considerably between individuals, lighting conditions, across different regions of the retina, and even between different systems of the brain.
Apart from native differences between individuals, our fusion threshold decreases not only as we fatigue, but as we grow older. The degree of modulation and the intensity of the light obviously have an effect, but so does the colour of the light, as well as the initial and background lighting conditions. Since rod photoreceptor cells, which predominate in our periphery, have much higher temporal resolution than cone cells, the fusion threshold differs depending on where the light strikes the retina. This is why a source of light can appear stable when viewed focally, yet flicker when glimpsed peripherally. One of the more surprising discoveries involves the impact of non–visible flicker from fluorescent lighting on office workers. With some kinds of fluorescent light, certain individuals exhibit flicker-related physiological effects even when no flicker can be seen.
Given the dependence of so much display technology on static frames, these complexities pose a number of technical challenges. For manufacturers, the goal is to overcome the ‘critical flicker fusion threshold,’ the point where modulated and stable imagery cannot be distinguished. And given the complications cited above, this can be far more complicated than you might think.
With movie projectors and Cathode Ray Tubes (CRTs), for instance, engineering pioneers realized that repeating, or ‘refreshing,’ frames before displaying subsequent frames, masked the perception of flicker. This was what allowed the movie theatre industry to adopt the cost-saving 24 frames per second standard in 1926, far short the critical flicker fusion threshold required to conjure the illusion of a stable visual field. Shuttering the image once or twice a second doubles or triples the flicker frequency, pushing 24Hz to 48Hz or 72Hz, well within the comfort zone of human vision.
Chop one image into two, or even better, into three, and our experience becomes more continuous, not less. The way to erase the perception of flicker, in other words, is to introduce more flickers.
But how could this be possible? How does the objective addition of flickers amount to their subjective subtraction? How can complicating a stimuli erase the experience of complexity?
The short answer is simply that human cognition, visual or otherwise, takes time and energy. All cognitive sensitivities are sensitivities to very select physical events. Light striking photoreceptive proteins in rod and cone cells, changing their shape and causing the cell to fire. Sound waves striking hair bundles on the organ of Corti, triggering the release of signal-inducing neurotransmitters. The list goes on. In each case, physical contact triggers cascades of astronomically complicated physical events, each taking a pinch of time and energy. Physical limits become discriminatory limits, rendering high-frequency repetitions of a signal indistinguishable from a continuous one. Sensory fusion thresholds dramatically illustrate a fundamental fact of cognitive systems: insensitivity to difference reports as business as usual. If potential difference-making differences are not consumed by a cognitive system, then they make no difference to that system. Our flicker frequency threshold simply marks the point where our visual system trundles on as if no flicker existed.
The capacities of our cognitive systems are, of course, the product of evolution. As a result, we only discriminate our environments so far as our ancestors required on the path to becoming us. 6oHz was all we got, and so this, given certain technical and economic constraints, became the finish line for early display technologies such as film and CRTs. Surpass 60Hz, and you can fool most of the people most of the time.
Dogs, on the other hand, possess a critical flicker fusion threshold of around 75Hz. In overcoming our fusion threshold, industry left a great many other species behind. As far as we know, the Golden Age of Television was little more than a protracted ocular migraine for man’s best friend.
Imagine a flickering world, one where millions of dogs in millions of homes endured countless stroboscopic nights, while the families cherishing them bathed in (apparent) continuous light. Given the high frame per second rates characteristic of modern displays, this is no longer the case, of course. Enterprises like DogTV are just beginning to explore the commercial potential of these new technical ecologies. But the moral remains no less dramatic. The limits of cognition are far more peculiar and complicated than a great many people realize. As this blog attempts to show, they are a place of surprise, systematic error and confounding illusion. Not only can they be technologically exploited, they already have been engineered to a remarkable extent. And now they are about to be hacked in ways we could have scarce imagined at the end of the 20th century.