Two Parallel Lines on a Starburst. They Bulge Apart. They Do Not.
Are the lines parallel or slanted?
You are looking at the Hering illusion, described by the German physiologist Ewald Hering in 1861 · just one year after Poggendorff and Zoellner published their own tilt illusions. A starburst of radial lines emerges from a single central point, spreading out across the page. Two horizontal parallel lines run across the starburst, one above the centre and one below. The parallel lines appear to bow · each one curving outward, bulging away from the centre. They are in fact perfectly straight and perfectly parallel. The apparent bow is entirely cortical.
What you are about to learn. What the Hering illusion is, why it is the “outbowing” cousin of the Wundt (inbowing) illusion, the unified V1-orientation-contrast account that covers both plus Zoellner and Poggendorff, how the illusion’s strength depends on the radial density, and why Hering is sometimes credited as one of the first to give a quantitative account of a perceptual illusion.
What the Illusion Looks Like
Draw a radiating burst · perhaps 30 to 50 thin lines emerging from a single point at the centre of the page, extending out to the edges. Now draw two long horizontal lines crossing the burst · one above the centre point, one below. Both are perfectly straight, both are perfectly parallel to each other.
The horizontal lines do not look straight. Each one appears to bow outward · that is, to curve away from the central burst point. The upper line appears to arch upward at its midpoint; the lower line appears to dip downward at its midpoint. The two lines together appear to splay apart in the middle and converge at the ends, forming a gentle lens shape between them.
The minimal recipe. A radial line pattern (also called a “sunburst” or “starburst”) with many lines radiating from a single point. Two parallel lines superimposed on the burst, passing on either side of the centre. Density matters: 20 to 50 radial lines gives a strong illusion, 5 to 10 gives a weak one. The parallel lines should be close to horizontal or vertical for the strongest effect (the oblique effect again).
Why It Works: Orientation Contrast on a Radial Field
The Hering illusion is another member of the tilt-illusion family, sharing machinery with Zoellner, Wundt, Poggendorff, and the Orbison figure. The common mechanism: orientation contrast in V1.
The radial lines provide a local orientation field. At each point on the paper, the dominant orientation of nearby radial lines is the orientation pointing toward (or away from) the central burst point. Near the centre, this orientation changes rapidly; far from the centre, it changes slowly.
The parallel lines sit in this orientation field. At any given point along a horizontal parallel line, the V1 population is being driven both by the horizontal edge of the line itself and by the nearby oblique radial lines. The two orientations interact through mutual inhibition.
Local apparent tilt varies along the parallel line’s length. Near the middle of the line (closest to the burst centre), the radial lines are most oblique relative to the horizontal · the apparent tilt is large. Near the ends of the line (farthest from the centre), the radial lines are nearly parallel to the horizontal line · the apparent tilt is small. The net result: the line appears to tilt most strongly in the middle, producing a perceived outward bow.
Bow is the cumulative integration of local tilt. Any single point along the horizontal line has only a small apparent tilt. But when your visual system integrates the tilt across the full length of the line, the small local tilts add up into a visible curvature. You do not see individual tilted segments; you see a smooth curve. This is an important lesson: your visual system aggregates local orientation signals into global shape percepts, and the aggregation is what produces the Hering bow.
Hering vs. Wundt: Inbow and Outbow
The Hering illusion has a mirror-image cousin · the Wundt illusion, described by Wilhelm Wundt in 1896. In the Wundt figure, the radial lines are replaced by an inverted pattern · lines emerging not from a single central point but rather converging toward a central point from the edges. Against this inverted background, two parallel lines appear to inbow: each curves toward the centre rather than away from it.
The Hering-Wundt polarity. Radial lines diverging from the centre (Hering): horizontal parallel lines bow outward. Radial lines converging toward the centre (Wundt): horizontal parallel lines bow inward. Same V1 orientation-contrast mechanism, opposite orientation geometry, opposite perceived bow. Knowing these two illusions together lets you predict bow direction for any radial-pattern figure: the parallel lines bow in the direction opposite to the locally dominant radial orientation relative to the parallel line.
A Quantitative Illusion
Hering was one of the first to attempt a quantitative account of a geometric illusion. He measured the apparent bow at different parameter settings and proposed a formula relating the perceived curvature to the radial line density and the distance from the burst centre. Though his specific formula did not survive, the style of his investigation · treat an illusion as a measurable psychophysical quantity · became the template for all subsequent illusion research.
Hering and perceptual mathematics. Hering was a physiologist by training and a quantitative thinker. He proposed the opponent-process theory of colour vision (red vs. green, blue vs. yellow, black vs. white), which later became the dominant account of post-retinal colour processing. He also contributed to the study of binocular depth perception and eye movements. The Hering illusion was almost a side project, but it is one of his most recognisable contributions.
A Harder Variant
Below is a Hering figure at difficulty 3 · more radial lines and a more prominent central burst. The apparent bow is large and hard to ignore.
Are the lines parallel or slanted?
Common misconception: “this is a 3D perspective illusion.” It is not. The Hering figure does superficially resemble a 3D perspective view · radial lines look like roads receding toward a vanishing point · and some early theorists argued for a 3D interpretation. But the illusion works equally well when the radial lines are obviously not a perspective scene (for example, when they are coloured differently or have different thicknesses that break the 3D reading). The core mechanism is 2D V1 orientation contrast. Perspective interpretation may modulate the illusion’s strength, but it is not the cause.
The Tilt Illusion Family, Unified
The Hering, Wundt, Zoellner, Poggendorff, and Orbison illusions all arise from the same V1 orientation-contrast machinery, applied to different geometries.
The shared mechanism. V1 has orientation-selective neurons tuned to every angle, with nearest-neighbour mutual inhibition. When one orientation is strongly represented at a location (say, a 45-degree radial line), it suppresses nearby orientations (say, the horizontal orientation of a test line at that location), which shifts the perceived orientation of the test line away from the inducer orientation. Every “tilt illusion” in the classical corpus is a different geometric arrangement of this same repulsion effect. Hering bows outward because the radial inducer orientation at each point pushes the local apparent direction of the horizontal line outward. Every tilt illusion reduces to this principle.
Where the Hering Illusion Appears
- Logo design. Radial-burst logos (sunburst medallions, star motifs, ray patterns) produce mild Hering effects on any nearby horizontal or vertical lines · typography near a sunburst motif often looks faintly curved.
- Architectural photography. Wide-angle lens distortion produces radial-pattern-like effects. Combining that with horizontal building elements (eaves, cornices, window rows) can introduce a Hering-style apparent bow in the rectilinear parts of the image.
- Circular stadium seating and concert halls. The radial pattern of seats emanating from a stage interacts with horizontal railings and balcony edges to produce faint Hering curvatures that architects accommodate.
- Road-sign design. Road signs warning of radial or converging features (“converging roads,” “rays indicating a hazard”) can produce Hering-style distortions on adjacent text. Sign designers use plain backgrounds where possible.
- Optical toys and flip-books. Spinning radial patterns (Benham’s top, phenakistoscopes) often include horizontal markings that bow visibly when the pattern spins · a dynamic version of the Hering illusion.
Test Yourself on 50 More Illusions
The Hering illusion is one of more than 50 classical illusions on PlayMemorize. Each round draws a deterministic SVG scene and asks one grounded question: which is larger, which is brighter, which is actually parallel. The reveal overlay shows the true geometry plus a one-line “why it works” caption.
- Keep playing Hering → · the standalone game, pinned to this one figure with fresh seeds each round
- Play Illusions → · spot the tricks across size, colour, orientation, and impossible figures
- Play Spatial → · train mental rotation and area estimation
- Play Matrix → · abstract pattern reasoning under time pressure
The takeaway. The Hering illusion is a demonstration that your visual system processes orientation locally and bow globally. Parallel lines over a radial starburst experience slightly different orientation contrasts at every point along their length · tilts that push the apparent direction outward near the centre and inward at the ends · and the cumulative effect is an outward bow. Ewald Hering saw it in 1861 and described it with physiological precision. One hundred and sixty-five years later, we still use his figure as the benchmark demonstration of orientation-contrast effects. The horizontal lines are straight. Your V1 orientation population says otherwise.
Illusions
Your eyes lie - the math knows the truth. Spot equal lengths, identical greys, and truly parallel lines across 57 classic optical illusions
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