biology zoology blog benno meyer rochow the united colours of benham_s disc

The United Colours of Benham’s Disc

We see them, but do animals see united colours of Benham’s disc too?

I heard of a man who worked as a magician after World War II and in his performances used a disc with a strange pattern of black and white lines. However, when turned around and spun at high speed, the disc’s black and white pattern created the impression of faint reddish or orange, even greenish or yellowish colours in human observers.

Yet, it’s not a magic trick, but something “physiological”, known ever since a German by the name of Gustav T. Fechner (well known for the “Weber-Fechner law” in psychophysics) discovered the phenomenon, which even to date has no explanation that is universally accepted by all sensory physiologists.

What is happening? Charles Benham upon learning of Fechner’s discovery, constructed the disc and to his own and his observers’ amazement produced the sensation of colours with the help of black and white stripes whenever they were moved around, actually spun on a turntable. The Benham disc was created. I wrote about colour perception before and pointed out that in the human eye’s retina we’d find so-called rods (photoreceptive cells used for what is generally termed black and white vision) and cones (of which there are three kinds with sensitivity peaks in the blue, green and red range of the spectrum) for colour vision.

Animals active at night usually possess mostly rods, while those more active during the day may possess a majority of cones with (depending on the species) all three kinds or just two of the colour-sensitive spectral cell types present). Most insects in addition to photoreceptive cells sensitive to green wavelengths also possess UV-receptors and some like bees also have blue or like butterflies even have red and purple receptors. But would these animals be fooled by the Benham disc and see the “United colours of Benham”? I don’t know the answer, but it would be interesting and help to shed light on the physiological nature of the phenomenon.
The notion that the sensation in humans of colour created by the black and white stripes on the spun Benham disc was simply an ‘imagination’ like a hypnotic state (since not everyone apparently can see the colours) is clearly wrong: it is a real phenomenon. But can it be photographed (like the enigmatic and equally controversial “Green Flash” of the setting sun) and do the photographs show a colour pattern?

Apparently not. So, it must have something to do with the repeated stimulation of the retina’s photoreceptors and their unequal latencies (i.e., small delays) in responding to the repetitive signals. The impression of white in the brain occurs only when all colour sensitive receptors are responding ‘in concert’, that is together, united, with no differences in latencies and transmission rates. Flickers like those created by the black-and-white pattern on Benham’s disc apparently disrupt the ‘united responses’ and thus the sensation of black and white in the brain. This has been one of the possible explanations advanced to explain what happens in humans with their three spectrally different receptors. But what about people with red/green colour blindness or animals which do not possess the same spectrally sensitive visual cells as humans or have receptors with latencies and transmission times different from those of humans? Do they, too, perceive colours and if so which colours? And finally, how about flower-seeking bees and butterflies? Can they be ‘tricked’ to accept a spinning Benham disc as a flower? As a scientist, I find such questions wonderfully challenging, but my head begins to spin at the thought of all this and I begin to see colours where there aren’t any. I wonder who’ll be the first to examine what kinds of response animals show when presented with a spinning Benham disc!

© Dr V.B. Meyer-Rochow and, 2019.
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