Dozens of flickering candles are shown, each with a small flame.

Visual Flicker

Psychology runs hot and cold as we sample its topics. Verbal learning, or mastering paired sets of nonsense syllables and words, was hot in the 1960s. Today few people care about it. Hypnotism was once a big deal, but not any more.

The reverse can also happen. When I was an undergraduate student back in the 1960s, the most boring topic in the curriculum was flicker and the critical flicker fusion frequency. No one quite knew how a series of lights turning on and off became an unbroken light stream when the flicker rate rose, but no one at my level cared, either.

Now it’s different. The rate at which flicker disappears may predict a person’s or insect’s time perception, or one kind of dyslexia, or Alzheimer’s disease. Flicker is responsible for a pain that lighting manufacturers are concerned about.

The critical flicker fusion rate, or CFF, is often given as 60 interruptions of light per second, but it depends on many things: the brightness of the light, the contrast between light and dark, and what you’re testing, whether it’s photopic vision that relies on cone photoreceptors, scotopic vision that uses rod photoreceptors, or mesopic vision, which uses both.

Of course, such changes in the “hotness” of a topic is typical of many disciplines. When I was a graduate student, endothelial cells were as dull as dishwater. Now dishwater has disappeared, but endothelial cells have risen in importance with the discovery that inflammation is a gateway to many disorders, including some mental illness.

WARNING: People who are photosensitive may experience seizures triggered by flickering light. If that includes you, please avoid viewing any videos that present flickering light.

A flickering wood fire is shown.

BIO: The maximal CFF is higher for cones than for rods because cones process light faster than rods do, perhaps five to ten times faster. Rod photoreceptors are much more sensitive to light than are cones, but their response to light decays more slowly.

The response rate is relevant because the rate at which a cell’s response to light decays determines how long it will be until the cell can respond again.

Cones in the peripheral retina respond faster than do cones in the fovea. That may explain why the CFF is higher for the peripheral retina than for the foveal retina, although the density of cones drops off sharply with distance from the fovea, which contains only cones.

The CFF is determined finally not in the retina but in the visual cortex.

PSYCHO: All we’re talking about is seeing a light go on and off. It’s just that we can’t do it if the light turns on and off too fast. Home hobbyists can breadboard a test apparatus fairly easily, or (WARNING: Photosensitive people should avoid the next two linked videos.) an appropriate testing instrument may be purchased for the classroom. The phenomenon may be viewed on YouTube as well. In addition, the U.S. Department of Energy has published a report on handheld flicker meters entitled “Characterizing Photometric Flicker”.

The testing procedure is classical psychophysics. In the field, it has been suggested, animals that are potential prey dart away from a predator to cause flicker at a high rate that fuses into continuous brightness in the predator’s eyes, as a defensive maneuver.

Flicker, like flashing and blinking, is processed at a high level by our systems of attention. Like our sensory receptors, the cerebral cortex is organized to detect change. So when something flickers, it catches our attention—unless it flickers too fast. Is that why a wood fire catches our attention, and a nonflickering gas range does not?

What about polished gemstones? I wonder what we might find to be the flicker rate—we call it sparkle—of jewelry? Has gem cutting taken the CFF into account?

Headaches and seizures are among the hazards of flickering lighting in today’s buildings, especially with the rising dominance of LED lighting. At home, poor lighting maintenance can trigger a variety of flicker problems. In commercial buildings, low quality fixtures and bad wiring [WARNING: flicker video) can be responsible.

SOCIAL: There was a time when movies flickered, and we experienced it in groups and called the movies “flicks”. Faster shutters and improvements in lights and film have eliminated the problem, but you can still see it in old films.

As films have become a cultural institution, they have improved. We retain our old sensitivity to flicker, but the causes are diminishing. In these technical efforts there are psychological lessons. First is that flicker fusion and motion in movies are not a phenomenon called “persistence of vision”. That is, dying activity in photoreceptors is not the cause of fusion and the illusion of movement.

So flicker perception ends up embedded in a larger context of visual processing at the movies.

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