Hering Illusion
The two purple lines are perfectly straight and parallel, but the radiating background makes them appear to bow away from each other.
Look at the purple lines below. Do they seem to curve outward near the middle, as though the space between them is expanding?
They do not bend at all. Each line runs vertically from top to bottom, and the distance between them remains constant. The apparent curvature comes entirely from the black lines radiating behind them.
This is the Hering illusion, a geometrical-optical illusion in which a radial background distorts the apparent shape and orientation of straight lines.
The effect is highly organized. The lines do not appear randomly uneven. They seem to bend outward in a symmetrical arc, following the changing angles created by the background.
Why the Straight Lines Look Curved
Every radial spoke crosses a purple line at a slightly different angle.
Near the horizontal center of the figure, the spokes cross the vertical purple lines at angles close to 90 degrees. Moving toward the top and bottom, the intersections become progressively more acute.
Your visual system must estimate the direction of each purple line while simultaneously processing dozens of nearby orientations. Those surrounding directions influence the apparent orientation of small sections of the target line.
A traditional explanation is that acute angles are perceptually expanded. As the intersections become narrower toward the ends of the target lines, local sections are biased away from their true orientation. The changing pattern of these local distortions is then integrated into the appearance of one continuous line bowing outward.
The purple line is still experienced as one coherent object, so a series of small orientation errors becomes an apparent curve rather than a collection of disconnected tilts.
How to Prove the Lines Are Straight
Cover most of the radial background with two sheets of paper while leaving one purple line visible. Without the surrounding spokes, its straight shape becomes much easier to see.
You can also place a ruler against either line. The edge will match from top to bottom, with no gap opening near the center.
Another method is to compare the distance between the lines at several heights. The gap is exactly the same near the top, middle, and bottom.
Once the radiating background is uncovered, the apparent bow usually returns. Knowing that the lines are straight does not stop the surrounding orientations from influencing how they look.
The Intersecting Angles Matter
The Hering illusion is not equally strong with every radial pattern.
In research comparing Hering and Wundt figures, investigators changed the angles at which the background lines intersected the target lines.
The results supported the idea that apparent distortion depends strongly on those intersecting angles. The illusion was not simply produced by having a busy background. The direction and progression of the surrounding lines mattered.
If the background lines meet the targets at nearly identical angles everywhere, there is less reason for different sections of the target to appear differently oriented.
The classic radial design is effective because its intersection angles change continuously from near-right angles at the horizontal center to increasingly acute angles toward the top and bottom.
Does the Background Warp Perceived Space?
The display can feel as though the whole visual field has expanded around the center. That raises a broader question: does the background distort the perceived positions of objects, or does it mainly distort their apparent directions?
Jeroen Smeets and Eli Brenner studied this question using movement across a background of radiating lines.
Participants judged the path of a moving dot and also made movements across the display. The researchers found systematic curvature related to the Hering pattern.
Their modeling suggested that a misjudgment of direction could explain the curved paths without requiring the entire visual field to be represented as one globally warped map.
The study also found that sensitivity to the moving-dot version was related to curvature in participants’ movements, while sensitivity when judging a stationary line’s straightness showed a different relationship.
This suggests that similar radial backgrounds can affect different visual tasks through partly different judgments.
Try a Related Visual Scanning Test
The Hering illusion shows how surrounding lines can interfere with judging a target. Cognitive Train’s Symbol Search Test asks you to scan a target group and determine whether a matching symbol appears among several distractors:
What Might Be Happening in the Visual System?
The exact mechanism is still debated, and the illusion may not come from one process alone.
One influential account focuses on neurons that respond selectively to line orientation. Nearby orientation signals can interact, shifting the apparent direction or position assigned to a target edge.
In experiments on spatial warping and the Hering illusion, David Vaughn and David Eagleman found results consistent with a role for networks of orientation-tuned visual neurons.
The researchers also explored the proposal that some spatial distortions could reflect a visual heuristic related to neural processing delays. Because perception is not instantaneous, the brain may use orientation and motion information to anticipate how a scene is changing.
They did not establish that prediction as the single purpose of the Hering illusion. Their results showed that relatively simple orientation-based mechanisms could produce spatial warping under both static and moving conditions.
Where the Hering Illusion Came From
The illusion is named after German physiologist Ewald Hering, who published the figure in 1861.
Hering studied several aspects of vision, including spatial perception, eye movements, binocular vision, and color.
His radial figure became one of the best-known geometrical-optical illusions because it transforms two extremely simple straight lines into an apparently curved pair.
The display is also useful because the physical geometry can be verified immediately. The illusion does not depend on an unclear photograph, unusual lighting, or a hidden measurement.
How It Differs From the Zöllner Illusion
The Hering and Zöllner illusions both distort straight parallel lines through surrounding orientation cues.
In the Zöllner illusion, short diagonal strokes cross several long lines. Alternating stroke directions make neighboring lines appear to lean in opposite directions.
In the Hering illusion, many spokes radiate from one central point. Their progressively changing intersections make the target lines appear smoothly curved rather than simply tilted.
The Café Wall illusion also changes apparent line orientation, but it uses staggered black-and-white tiles and brightness transitions rather than radial spokes.
Is It a Perspective Illusion?
The radiating pattern resembles lines converging toward a vanishing point. It can therefore suggest depth or forward movement through space.
That resemblance may contribute to some versions of the illusion, especially when the spokes strongly resemble roads, tunnels, or perspective grids.
However, perspective alone is not a complete explanation. The effect depends on local intersections and can be studied through orientation interactions without assuming that every viewer consciously interprets the image as a three-dimensional scene.
This separates the Hering illusion from the Ponzo illusion, where converging lines more directly suggest that one object lies farther away than another.
Why Knowing the Answer Does Not Remove the Curve
You can confirm that both lines are straight and still experience them as bowed.
Your conscious knowledge and your automatic visual processing are giving different answers. Measurement tells you that the lines are parallel, while the surrounding orientation signals continue to bias their appearance.
This is one reason geometric illusions remain useful in perception research. The stimulus can stay fixed while the experienced shape differs from its physical shape.
What It All Comes Down To
The Hering illusion places two straight parallel lines over a field of radiating spokes.
The spokes cross the targets at nearly right angles near the horizontal center and at increasingly acute angles toward the top and bottom. Those changing intersections bias the apparent orientation of local sections, and the distortions combine into the appearance of two outward-bowing lines.
For more examples of distorted lines, misleading depth, changing size, and constructed contours, browse the full Optical Illusions guide. Cognitive Train’s brain tests explore perception, attention, processing speed, and reasoning, while its free brain training tools offer more ways to challenge visual judgment.