First, observe the alignment of the red circles as they move in a straight vertical path, up and down. Then keep your gaze on one of the three Xs in the middle. What do you notice?
The red circles seem to drift away from their true physical trajectories, as if they were following the curves of the static lines. This perceptual shift is known as the “Mainz-Linez Illusion“.
When you keep your gaze on the central X, the moving dots shift into peripheral vision, where spatial resolution is limited and detail is reduced. The visual system compensates by interpolating missing information based on contextual cues and prior experience. As a result, the dots become perceptually “bound” to the nearby curved lines, as if threaded on them, and their straight vertical motion is misread as oscillating.
The Mainz-Linez phenomenon reflects a broader principle: peripheral vision is largely constructive. Under certain conditions, this predictive filling-in can also distort motion judgments in real-world tasks—such as driving—where events in the periphery may be misperceived.
In English, the expression to smell the color 9 describes something completely impossible…
And yet, some people have the unusual ability to mentally visualize colors or spatial patterns when thinking about units of time—or more broadly, numbers. This phenomenon, known as synesthesia (from the Greek syn, “together”, and aisthēsis, “sensation”), occurs when stimulation of one sense involuntarily triggers sensations in another. It’s not a figure of speech—these perceptions feel very real to those who experience them.
The first documented case in medical literature appeared in 1710. Dr. John Thomas Woolhouse (1650–1734), an ophthalmologist to King James II of England, reported a blind young man who claimed he could perceive colors induced by sounds.
Neuroscientist Vilayanur S. Ramachandran and his team at the University of California, San Diego, observed that the most common form of synesthesia links “graphemes“—letters or numbers—to specific colors. Since my work bridges art and mathematics, I’ll focus here on number-based synesthesia.
People who experience synesthesia in its pure form are relatively few. However, many report strikingly similar associations between numbers and colors or spatial layouts, suggesting these perceptions aren’t just products of imagination or attention-seeking. For example, number–form synesthesia may result from cross-activation between brain regions in the parietal lobe that handle numerical and spatial processing. In contrast, number–color synesthesia likely stems from an overabundance of connections between adjacent areas that interfere with each other when triggered (see fig. 1 below).
Figures 2 and 3 illustrate common synesthetic patterns—either as color associations (fig. 2) or spatial arrangements (fig. 3, based on observations by Sir Francis Galton). Statistically, people often associate the digits 0 or 1—and sometimes 8 or 9—with black or white. Yellow, red, and blue are typically linked to smaller digits like 2, 3, or 4, while brown, purple, and gray tend to be tied to larger ones like 6, 7, or 8. Curiously, it’s not the idea of the number but the visual form of the digit that seems to trigger the sensation. For instance, when the number 5 is shown as the Roman numeral V, many synesthetes report no color at all.
And you—do you see numbers in color or arranged in space? Feel free to share your synesthetic experiences with me.
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