A black-and-green grid image that has resurfaced on social media is prompting a fresh debate over whether its straight lines appear curved, with viewers calling the effect "irritating" and "brain-breaking." The picture, an eight-by-eight array of grey squares separated by thin green lines, produces an apparent curling or motion of the green lines until a viewer focuses on any single line, at which point it looks straight.
The image was created by Florida-based illustrator Lesha Porche and first shared on Facebook in December 2021. It later circulated on Reddit, where users described the effect as "neat," "fantastic" and, in some cases, maddening. One commenter wrote, "This is genuinely irritating. Well done." Another posted, "Brain now broken thanks."
Jolyon Troscianko, a visual ecologist at the University of Exeter who studies perception across species, described the image as "a very interesting illusion" and said it primarily affects peripheral vision—what the eye sees outside direct fixation. "The bits I'm looking straight at have straight lines, but the lines further from where I'm directly fixated go all curved," he told the Daily Mail.
Troscianko and other researchers point to small patterns inside each grey square as the source of the effect. Close inspection reveals tiny shapes and stripes of varying grey tones inside the 64 squares. Those internal marks, he said, form subtle curved cues that the visual system registers more strongly in peripheral vision than in the high-acuity central region of the retina, called the fovea.
To test and illustrate how those hidden patterns influence perception, Troscianko ran the image through a computer model developed for a 2023 study that simulates how human vision detects contrast. The filtered output makes the curved alignments inside the boxes more apparent, supporting the idea that local contrasts, not movement, produce the illusion.
Experts at the University of Queensland's Brain Institute described the mechanism in similar terms: optical illusions exploit the fact that the eyes and brain communicate using simplified signals and that the brain often "fills in the blanks" from limited visual clues. "Mostly those guesses are right... sometimes, however, the brain guesses wrong," the institute said in an explainer.
The distinction between foveal and peripheral vision is central to understanding the illusion. The fovea occupies a small portion of the retina and provides high-resolution, detail-rich vision for whatever the eye is directly fixated on. Peripheral vision covers a much larger area of the visual field but with lower spatial acuity and greater sensitivity to contrast and motion, which can accentuate patterns that go unnoticed under direct scrutiny.
Researchers note that differences in eye structure across species change how such stimuli are perceived. Humans typically have three types of cone photoreceptors for color vision, while some birds and other animals possess four, a condition called tetrachromacy that enables sensitivity to ultraviolet wavelengths. Photoreceptors—rods and cones—convert light into electrical signals relayed to the brain via the optic nerve. Signals from both eyes are compared in the brain to build a sense of depth and spatial layout, processes that can also be exploited by carefully designed images.
The image joins a long tradition of visual puzzles and illusions that resurface online as platforms amplify rapid sharing and commentary. In this instance, the debate has centered not on whether the image is moving but on why the brain reports a warped grid when the green lines are geometrically straight. Researchers say the answer lies in how local contrasts and angled elements within the squares interact with the differing sensitivities of central and peripheral vision.
Porche’s grid has continued to prompt conversation about visual perception and the limits of human sight, as well as renewed interest in the laboratory methods used to study how brains interpret complex visual scenes. The image serves as a reminder that perception is an active, interpretive process—and that what people see can change dramatically depending on where they look.