What is the Benham's top illusion?

Answer: Benham's top is a black and white image that produces the illusory perception of color when spun.

Our ability to detect stimuli with our visual system depends on the activation of a series of neurons in the retina, the anatomical structure at the back of the eyeball. Here, we have neurons that detect various wavelengths of light, and these color sensitive receptors are called cone photoreceptors. Most humans have three different types of photoreceptors, each one is especially sensitive to a particular wavelength of light. Combinations of activity of the three cone photoreceptors together account for all the colors that we can see. 

The Benham's top illusion is produced when a black and white image is spun around the center point of the image. The illusion was developed in 1894 by amateur toymaker Charles Benham. The image itself is half black and half white with curved black lines. For some reason, rapid spinning of this black and white pattern often produces very pale streaks of color. This perception of false colors is called the Fechner color effect. This can also be called pattern-induced flicker colors. 

Interestingly, the perception of colors is not the same between people. For example, I see dark oranges and hints of green, but others may report seeing different shades of other colors.

Explanation of the Benham’s Top illusion

The main pathway of communication of visual information relies on a series of three neurons in the retina. The photoreceptors at the back of the eye detect light, and that signal is converted into a combination of electrical impulses and neurotransmitters that are passed onto bipolar cells and finally, retinal ganglion cells. At the intersection between each of these neurons, there are a network of inhibitory interneurons that modify signaling. The horizontal cells release neurotransmitter at the interface between the photoreceptors and the bipolar cells, while the amacrine cells modify communication between the bipolar cells and the retinal ganglion cells. 

Scientists have not yet figured out what causes people to perceive false colors when black and white images are spun radially. One theory suggests that the illusion happens at the level of perception. Specifically, the firing activity of the amacrine cells seems to be changed when Benham's disk stimuli are presented to the intact frog retina. (The amacrine cells as an important processing site of pattern-induced flicker colors.)

Another study discovered that the cortex is also involved in the processing of pattern-induced flicker colors. This study (Neural Correlates and Effective Connectivity of Subjective Colors during the Benham's Top Illusion: A Functional MRI Study) found that specific regions of the visual cortex (occipital lobe) change more when a person sees the spinning top. Using fMRI, the researchers discovered that the areas with the most change were V4 and V2, structures found in the ventral stream of the visual processing pathway.

Despite these experimental studies, there has not yet been definitive proof that explains why the Benham’s top illusion causes people to see colors.