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Geometrical-optical illusions

Geometrical-optical illusions are visual phenomena in which the brain misinterprets the geometrical properties of a two-dimensional figure, such as lines, angles, or shapes, causing systematic errors in perceived size, length, orientation, or curvature, independent of actual viewing distance or angle.^[1] These illusions arise from the visual system's processing of static images composed of straight or curved lines, leading to distortions that differ from the objective reality of the stimulus.^[2] The term "geometrical-optical illusions" was coined in 1855 by Johann Joseph Oppel, a German physicist and physiologist, based on observations made while teaching geometry, where he noted phenomena like the apparent enlargement of vertical dimensions compared to equal horizontal ones and distortions in angles or line straightness near curves.^[3] Oppel's work distinguished these illusions from broader sensory deceptions, emphasizing their basis in spatial perception rather than optical factors like refraction.^[4] Subsequent research in the late 19th century, including studies by Franz Carl Müller-Lyer in 1889, expanded on these early findings, establishing geometrical-optical illusions as a key area in perceptual psychology.^[5] Prominent examples include the Müller-Lyer illusion, where two lines of equal length appear unequal due to opposing arrowheads at their ends, often explained by the visual system's misapplication of depth cues from three-dimensional interpretations; the Ponzo illusion, in which identical horizontal lines seem different in length when placed between converging lines suggesting perspective depth, akin to railroad tracks; and the Ebbinghaus illusion, where a central circle's size is misjudged based on the surrounding circles' relative largeness or smallness, reflecting contrast effects in size perception.^[6] These illusions demonstrate errors up to 25% in length judgments for simple line drawings like the Poggendorff or Judd configurations.^[7] Theoretical explanations for geometrical-optical illusions generally fall into categories such as perspective and depth cue misapplication, where the visual system inappropriately scales two-dimensional images as if they represent three-dimensional scenes; size-contrast mechanisms, involving local comparisons that bias global perception; and neural processing models, including uncertainty in edge detection or attentional biases that amplify distortions.^[8] For instance, the Müller-Lyer effect has been linked to compensatory eye movements or cortical representations of space, while broader theories invoke natural scene statistics where the brain favors interpretations aligned with everyday visual environments.^[9] Ongoing research explores these illusions' implications for understanding visual neuroscience, including how they persist across cultures and species, and their reduced susceptibility in certain neurodevelopmental conditions.^[10]

Definition and Properties

Core Definition

Geometrical-optical illusions refer to visual misperceptions in which the perceived geometrical attributes—such as length, angle, or curvature—of static two-dimensional images deviate from their physical measurements due to interactions between optical projections and perceptual processing in the human visual system.^[11] This term was first coined by Johann Joseph Oppel in 1855 to describe distortions involving spatial properties like size and shape, distinguishing them from illusions related to color or brightness.^[3] These illusions arise from the mismatch between the objective geometry of a stimulus and its subjective interpretation, often quantified as judgmental errors in relative linear extents or plane angles.^[12] Unlike physiological illusions, which stem from overstimulation of sensory receptors (e.g., afterimages from bright lights), or cognitive illusions that involve higher-level interpretations influenced by memory or expectations (e.g., misjudging sizes based on prior knowledge), geometrical-optical illusions engage visual processing of simple line-based patterns, involving interactions from low- to high-level mechanisms, without requiring motion or complex contextual narratives.^[13] They are characterized by their reliance on static, achromatic geometric configurations that exploit inherent properties of retinal projection and early cortical analysis, leading to consistent perceptual biases across observers.^[7] This foundational concept underpins the study of visual perception, serving as a basis for exploring specific geometrical properties, measurement techniques, and categories of such illusions in subsequent analyses.^[14]

Key Geometrical Properties

Geometrical-optical illusions primarily involve distortions within the framework of 2D Euclidean geometry, where the perceived attributes of visual elements deviate from their objective measurements. These distortions target fundamental components such as points, lines, angles, and shapes, often involving the interpretation of two-dimensional images through three-dimensional depth cues or other contextual factors. By isolating these 2D elements, researchers can examine how contextual arrangements alter spatial relations, such as the apparent position of points relative to surrounding lines or the perceived length of a line influenced by flanking angles.^[4] Points in these illusions often appear displaced from their true positions due to interactions with adjacent linear elements. For instance, a point at the intersection of lines may seem shifted when embedded in a pattern of converging or diverging segments, highlighting how positional accuracy is compromised in Euclidean space. Lines exhibit multiple distortions: their lengths can appear elongated or shortened based on orientation or contextual brackets, as seen in configurations where horizontal lines seem shorter than vertical ones of equal measure; orientation may cause lines to tilt perceptually; and straight lines can manifest illusory curvature when intersected by oblique patterns. Angles are frequently overestimated or underestimated at intersections, with acute angles expanding and obtuse ones contracting, thereby altering the overall geometric configuration. These line and angle effects interact dynamically—for example, adjacent angles can amplify length misjudgments in simple line drawings, demonstrating the interdependence of Euclidean properties.^[15] Shapes, encompassing closed figures like polygons or circles, show distortions in area and parallelism. Perceived area may expand or contract depending on boundary orientations, such as when a square rotated 45 degrees appears larger than its true extent. Parallelism is particularly susceptible, with parallel lines seeming to converge or diverge in the presence of interrupting transversals, violating Euclidean postulates of equal spacing. These shape distortions arise from cumulative effects on constituent lines and angles, preserving overall topological integrity while altering metric properties. The study of such 2D geometrical properties originated with Johann Joseph Oppel's 1855 identification of length illusions, where he demonstrated systematic errors in judging line extents within basic figures like rectangles and triangles, laying the foundation for focusing on these isolated geometric elements.^[15]^[4]

Perception and Measurement

Illusions in Visual Space

Visual space refers to the subjective, projective representation of the environment constructed by the visual system, distinct from the objective physical space. This perceptual space arises from a non-veridical mapping of retinal input to conscious experience, where the brain interprets two-dimensional projections on the retina into a three-dimensional layout, often introducing distortions that do not align with Euclidean geometry.^[16] In geometrical-optical illusions, this mapping leads to systematic errors, such as misperceived lengths or angles, as the visual system applies assumptions about spatial structure that mismatch the actual stimulus.^[11] Unlike physical space, which is measurable and independent of the observer, visual space is a private construct that can deviate markedly from reality, even when individuals possess explicit knowledge of the true configuration. For instance, in illusions like the Müller-Lyer figure, observers report unequal line lengths despite recognizing the physical equality, demonstrating the persistence of perceptual distortions rooted in neural processing rather than optical artifacts.^[16] These subjective experiences highlight visual space's role as an inferred representation, where top-down influences from prior knowledge interact with bottom-up sensory signals, yet fail to override illusory effects. Such differences are verifiable through subjective reports, underscoring the subjective nature of perception. Context plays a pivotal role in shaping illusions within visual space, as surrounding elements can alter perceived geometry without modifying the retinal image. Adjoining lines or patterns, for example, induce expansions or contractions in perceived size and orientation by influencing how the visual system parses local cues into a coherent global structure, revealing hierarchical processing priorities like the dominance of cardinal directions over oblique ones.^[11] This contextual modulation exemplifies how visual space emerges as an adaptive, but imperfect, projection that prioritizes ecological validity over photometric accuracy, leading to robust illusory experiences across diverse stimuli.^[17]

Psychophysical Measurement

Psychophysical measurement of geometrical-optical illusions involves empirical techniques to quantify the discrepancy between physical stimuli and perceived attributes, such as length, size, or orientation, allowing researchers to assess illusion magnitude and perceptual variability. These methods draw from classical psychophysics to establish reliable metrics for how distortions manifest in human vision. For instance, the point of subjective equality (PSE), where a stimulus is perceived as equal to a reference despite physical differences, serves as a core estimate of illusion strength, often expressed as a percentage distortion relative to the true physical measure.^[18] In length illusions like the Müller-Lyer, this can yield distortions of 5-25%, highlighting the scale of perceptual error.^[7] A primary approach is the nulling technique, a variant of the method of adjustment, where participants actively modify a stimulus parameter—such as varying the length of one line in a pair—until the illusory distortion is canceled and the elements appear equal. This method provides direct measures of illusion magnitude by identifying the adjustment required to achieve perceptual neutrality, and it has been widely used due to its simplicity and sensitivity to individual perceptual thresholds.^[19] Complementary techniques include the method of constant stimuli, in which observers judge a series of fixed stimulus pairs to derive the psychophysical function and compute the PSE, offering robust data on perceptual sensitivity without active adjustment.^[20] Adjustment tasks more broadly, including ascending and descending trials to bracket the PSE, help mitigate biases like expectation effects, while reaction time measures capture the speed of perceptual judgments under illusion conditions, revealing how distortions influence processing efficiency—for example, longer response times correlate with stronger illusory interference in tasks involving misaligned lines.^[21]^[22] These methods reveal significant variability in illusion susceptibility across individuals, with stable differences persisting over time, across eyes, and between measurement paradigms, suggesting underlying trait-like perceptual styles.^[23] Reliability can be influenced by contextual factors.^[8] Fatigue also impacts measurements, as prolonged visual tasks lead to neural adaptation that biases perceptions away from the stimulus configuration, increasing error variability and reducing precision in quantifying illusion strength.^[24] Such factors underscore the need for controlled conditions to ensure consistent metrics, with applications varying slightly between illusion categories, like length versus orientation distortions.^[25]

Classification and History

Categories of Illusions

Geometrical-optical illusions are classified primarily according to the specific perceptual property that is distorted, providing a taxonomic framework that emphasizes spatial misjudgments in static, two-dimensional patterns.^[11] This approach distinguishes them from non-geometrical illusions, such as those involving color contrasts or motion perception, by focusing exclusively on distortions of form and extent without reliance on chromatic or dynamic cues.^[26] The core categories include length illusions, where segments appear longer or shorter than their actual size, often due to flanking elements like fins; orientation and parallelism illusions, which induce perceived tilts or non-parallelism in aligned lines, as seen in tilt aftereffects; curvature illusions, involving the apparent bending of straight lines or exaggeration of curves; and position illusions, characterized by misperceived alignments or displacements of elements.^[27]^[26] Early classifications, such as those by Oppel in 1855 and Wundt in the late 19th century, grouped illusions into broad types like extent (length and size) and direction (orientation and position), laying the groundwork for later refinements.^[3]^[28] Modern taxonomies, exemplified by empirical analyses, have expanded this into more nuanced clusters, such as shape/direction distortions and linear extent over- or underestimations, based on intercorrelations in perceptual responses across diverse configurations.^[27] These refinements highlight the multidimensional nature of geometrical-optical illusions while maintaining the focus on verifiable spatial discrepancies.^[26]

Historical Development

The study of geometrical-optical illusions originated in the mid-19th century, marking a transition from casual observations to more structured investigations into visual perception. In 1855, German high-school teacher Johann Joseph Oppel published the first scientific paper on the topic, coining the term "geometrical-optical illusions" to describe distortions in perceived size and orientation of geometric figures. Oppel's work included quantitative observations of length illusions, such as vertical lines appearing longer than horizontal ones of equal length in rectangles, and influenced subsequent researchers through supplementary publications in 1857 and 1861 that introduced variants like the Oppel-Kundt illusion. This emerging field was advanced by Ewald Hering in 1861, who described the Hering illusion, a radial pattern where straight lines appear curved due to intersecting oblique lines, exemplifying early systematic exploration of orientation distortions. The shift from anecdotal reports to empirical study was profoundly shaped by Gustav Fechner's psychophysics, introduced in his 1860 work Elements of Psychophysics, which provided methods to quantify the relationship between physical stimuli and perceptual responses, enabling researchers to categorize and measure illusions more rigorously. Fechner's framework influenced 19th-century figures like Hering and later Wilhelm Wundt, facilitating the application of experimental techniques to perceptual phenomena.^[29]^[30] In the 1870s, Wilhelm Wundt, founding his Leipzig laboratory in 1879, incorporated psychophysical methods to investigate geometrical-optical illusions, emphasizing controlled experiments on spatial perception and contributing to the field's establishment as a cornerstone of experimental psychology. By the 1920s, Gestalt psychologists Max Wertheimer, Wolfgang Köhler, and Kurt Koffka built on these foundations, shifting focus to holistic perceptual organization; Wertheimer's 1923 studies on grouping principles like proximity and good continuation explained illusions as emergent properties of whole configurations rather than isolated sensations, while Köhler's psychophysical isomorphism linked them to brain processes.^[31]^[32] Post-1950 developments saw a surge in empirical research, consolidated in the 1970s by Stanley Coren's syntheses, particularly his 1978 book Seeing is Deceiving: The Psychology of Visual Illusions co-authored with Joan Girgus, which integrated mid-20th-century findings to propose multi-level mechanisms underlying geometrical-optical illusions, evaluating theories and highlighting their role in broader visual processing.^[33]

Notable Examples

Length and Size Illusions

Length and size illusions are a prominent category within geometrical-optical illusions, where contextual elements distort the perceived dimensions of lines or shapes despite their objective equality. These illusions exploit the visual system's reliance on surrounding cues to infer extent, leading to systematic misjudgments that can exceed 20% in magnitude for classic configurations. As a subtype of length illusions, they highlight how local features like endpoints or enclosures bias global size estimation, independent of depth cues or motion. The Müller-Lyer illusion, first described in 1889, exemplifies length distortion through arrowhead attachments at line endpoints. In the standard form, a line terminated by inward-pointing arrowheads (fins) appears shorter than an equally long line with outward-pointing fins, with the former often underestimated by up to 20-30% in perceived length. This misjudgment arises from the arrowheads' orientation, which modulates the apparent shaft length via conflicting assimilation (where the total figure length influences the shaft) and contrast effects (where fins oppose the shaft direction). The illusion's strength varies with fin angle: narrower angles (e.g., 20°) produce greater distortion (point of subjective equality shifts of approximately 41 pixels in model simulations aligning with human data), while wider angles (e.g., 60°) diminish it nearly to zero.^[34]^[35] The Delboeuf and Ebbinghaus illusions demonstrate size distortion in circular targets via concentric or surrounding contexts. In the Delboeuf illusion, two identical central circles are each encircled by a larger ring; the circle within a slightly larger ring appears enlarged due to contour attraction pulling the perceived boundary outward, while one in a much larger ring seems diminished by contrast, with effect magnitudes around 0.19 degrees of visual angle for circular targets. The Ebbinghaus illusion extends this with multiple surrounding circles: a central circle amid smaller inducers appears larger (up to 10% overestimation), whereas larger inducers induce shrinkage, driven by the relative size and proximity of the context, which alters grouping and boundary integration. These effects underscore the role of enclosure in size constancy, persisting across shapes and distances.^[36]^[37]^[38]

Orientation and Position Illusions

Orientation and position illusions involve distortions in the perceived alignment, angle, or tilt of geometric lines and figures, often due to contextual elements that disrupt apparent parallelism or collinearity. These illusions highlight how the visual system interprets spatial relationships in two-dimensional patterns, leading to misperceptions of straight lines as angled or offset. Key examples include configurations where interrupting or intersecting lines induce shifts in perceived position or orientation. The Poggendorff illusion features an obliquely oriented straight line interrupted by a rectangular occluder, such as vertical or horizontal bars, causing the line segments to appear non-collinear despite their physical alignment. The misalignment is most pronounced when the interrupting rectangle is wider or when the oblique line is more vertical, with the outer segment often seeming displaced downward or laterally. This effect was first identified by German physicist Johann Christian Poggendorff in 1860 while reviewing illustrations for a scientific journal. Explanations link the distortion to statistical regularities in natural scenes, where interrupted lines are more likely to represent non-collinear structures due to occlusions. In the Zöllner illusion, a set of parallel lines is crossed by shorter oblique lines at acute angles, making the parallels appear to converge or diverge, as if tilted relative to each other. The perceived tilt increases with the acuteness of the intersecting angles and the density of the short lines. German astrophysicist Johann Karl Friedrich Zöllner discovered this pattern in 1860, inspired by textile designs observed in his father's factory, and published it in the journal Annalen der Physik. The illusion arises from local angular distortions at intersections, propagating to alter the global perception of parallelism. The Hering illusion consists of two straight parallel lines placed against a background of radially arranged spokes, causing the lines to appear curved or bowed outward near the center, as if diverging angularly. This radial context mimics motion or depth cues, inducing a sense of expansion that warps the lines' perceived straightness. German physiologist Ewald Hering first described the illusion in 1861, contributing significantly to early studies of geometrical distortions in visual perception. The magnitude of the distortion varies with viewing distance, generally diminishing as the observer moves farther away, which reduces the angular subtense of the radial elements and weakens the contextual influence. Related to the Hering illusion, the café wall illusion displays rows of alternating black and white squares, offset horizontally and separated by thin horizontal "mortar" lines, making the rows' outlining parallels appear tilted into wedges. The effect depends on luminance gradients at the borders, where the mortar lines' intermediate brightness—between the dark and light tiles—creates asymmetric contrast that shifts perceived contours. British psychologist Richard Gregory and colleague Priscilla Heard documented this illusion in 1979, noting its observation on café tiling in Bristol, England. Distortion strength is maximal with narrow mortar lines (around 1 mm at typical viewing distances) and high tile contrast, but diminishes with wider lines or equalized luminances, and it varies with viewing distance due to changes in the relative scale of these gradients.

Explanations and Theories

Physiological Mechanisms

Geometrical-optical illusions frequently originate from low-level physiological processes in the retina, where lateral inhibition among neurons sharpens edges and enhances contrast. This mechanism involves excited retinal ganglion cells suppressing activity in adjacent cells, creating exaggerated brightness and darkness at boundaries, as seen in Mach bands—illusory light and dark stripes that appear alongside abrupt luminance changes.^[39] In illusions like the Müller-Lyer, where lines with arrowheads appear unequal in length despite being identical, retinal lateral inhibition contributes to these distortions by amplifying contrast at the endpoints, leading to misperceived lengths.^[40] Further processing in the primary visual cortex (V1) builds on retinal inputs through specialized structures such as hypercolumns, which organize neurons into functional units tuned to specific orientations, enabling precise detection of line angles and directions. Surround modulation in V1, where contextual stimuli influence central neuron responses, underlies apparent tilts in orientation-based illusions like the Zöllner, in which parallel lines seem to converge or diverge due to intersecting oblique lines that bias orientation selectivity and enhance perceived contrast between similar angles. The Poggendorff illusion, involving misaligned lines interrupted by a parallelogram, similarly reflects these V1-driven orientation modulations. Neuroimaging evidence from functional magnetic resonance imaging (fMRI) confirms early activation in V1 and adjacent areas during exposure to geometrical-optical illusions, with cortical responses aligning more closely to the illusory percept than the physical stimulus, as observed in the Müller-Lyer effect.^[41] These findings indicate that physiological mechanisms at the sensory periphery and early cortex generate the core distortions in such illusions.

Cognitive Interpretations

Cognitive interpretations of geometrical-optical illusions emphasize higher-level perceptual processes where the brain organizes and interprets visual stimuli based on learned principles and expectations, rather than solely on low-level sensory input. Gestalt psychology posits that the mind tends to structure ambiguous visual information into coherent wholes through laws such as proximity, continuity, and closure, which can lead to misgrouping of elements and erroneous perceptions of shape, size, or position. For instance, in the Ponzo illusion, converging lines are perceptually grouped via the law of continuity as representing perspective depth, causing parallel lines to appear unequal in length due to implied size differences in a three-dimensional scene.^[11] These principles reveal how the visual system prioritizes smooth contours and closed forms, overriding objective geometry to form stable percepts.^[32] Building on this, constructive theories view perception as an active process of hypothesis-testing, where the brain generates predictions about the environment using prior knowledge and contextual cues, often resulting in illusions when hypotheses mismatch the actual stimulus. Richard Gregory's framework describes geometrical-optical illusions as outcomes of such inferences, where incomplete retinal images are supplemented by top-down assumptions about object properties like depth and scale.^[42] In the Müller-Lyer illusion, for example, arrowhead orientations trigger depth-scaling hypotheses derived from everyday experiences with corners, making lines appear longer or shorter despite equal physical lengths.^[42] This approach underscores that illusions arise from the brain's adaptive strategies to resolve ambiguity, integrating sensory data with stored representations.^[42] Cultural factors further modulate these cognitive interpretations, with susceptibility to geometrical-optical illusions varying across societies due to differences in spatial assumptions and visual experiences. Studies across 15 diverse cultures demonstrate that individuals from non-Western or non-industrialized groups exhibit reduced vulnerability to illusions like the Müller-Lyer and Sander parallelogram, attributed to ecological adaptations that foster alternative perceptual inferences, such as less reliance on linear perspective in carpentered environments.^[43] These variations highlight how cultural learning shapes the hypotheses underlying geometric perception, leading to context-biased interpretations that align with habitual visual environments.^[43]

Mathematical Transformations

Mathematical approaches to geometrical-optical illusions often model perceptual distortions as transformations of geometric space, departing from Euclidean assumptions to capture how the visual system interprets contextual cues. Non-Euclidean mappings, particularly those derived from projective geometry, explain illusions where parallel lines appear to converge, as in the Ponzo illusion. In this setup, the converging lines mimic perspective cues in three-dimensional space, leading the visual system to apply a projective transformation that scales perceived distances non-uniformly; distant objects are inferred to be larger, distorting the apparent length of embedded segments.^[11] Similarly, affine transformations account for shear effects in the Poggendorff illusion, where an interrupting rectangle induces a misalignment between collinear segments by preserving parallelism but altering relative positions through linear shearing, as the visual system compensates for perceived depth or orientation shifts.^[11] Despite these advances, mathematical transformations face limitations due to individual variability in perceptual responses, which prevents the development of universal models applicable across observers. Historical attempts, such as Jan Koenderink's application of differential geometry in the 1980s to describe curved visual spaces and invariant features under affine projections, achieved partial success by predicting average distortions but struggled with inter-subject differences and non-linear cortical mappings. These frameworks occasionally formalize Gestalt predictions from cognitive theories, yet remain deterministic tools focused on spatial metrics rather than psychological processes.^[44]

Ambiguous and Impossible Figures

Ambiguous figures are two-dimensional images that support multiple stable perceptual interpretations, often leading to spontaneous alternations between them due to insufficient cues for a unique three-dimensional structure. A seminal example is the Necker cube, a line drawing of a cube introduced by Swiss crystallographer Louis Albert Necker in 1832, which can be perceived with either the lower-left square as the front face or the upper-right square in that position, reversing depth without altering the stimulus. This bistability arises from the figure's wireframe nature, providing equal validity to competing depth assignments and resulting in perceptual flips that occur every few seconds on average during prolonged viewing.^[45] Impossible figures, also known as impossible objects, extend this ambiguity by depicting configurations that cannot be realized in three-dimensional Euclidean space, as their two-dimensional projections violate consistency in spatial relations and projective geometry. The Penrose triangle, created by psychiatrist Lionel Penrose and mathematician Roger Penrose in 1958, consists of three right-angled bars forming a triangular loop where each bar appears to connect orthogonally to the next, but no single viewpoint reconciles the orientations into a coherent 3D form. Similarly, the Penrose staircase from the same work illustrates a closed loop of steps that perpetually ascends or descends, with risers and treads misaligning in height across the cycle, rendering physical construction impossible. These figures exploit incomplete or contradictory perspective cues in 2D renderings to produce perceptual paradoxes.^[46] While sharing a foundation in the geometrical projection of 3D forms onto 2D surfaces, ambiguous and impossible figures differ from core geometrical-optical illusions by prioritizing perceptual reversibility and structural impossibility over fixed distortions of metric properties like length or angle. Instead of consistent misjudgments, they highlight the visual system's reliance on interpretive assumptions that falter under ambiguity, often as extensions of position-based cues in illusion categories.^[47]

Recent Neuroscience and Computational Insights

Recent advances in neuroimaging have illuminated the neural underpinnings of geometrical-optical illusions, particularly through functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) studies conducted in the 2010s and beyond. fMRI research has demonstrated significant activation in primary visual cortex areas V1 and V2, alongside the lateral occipital complex (LOC), during tasks involving illusions such as the Müller-Lyer and Ponzo, where contextual elements distort perceived length and size. These regions facilitate the integration of local features with global context, with effective connectivity from the pulvinar to V1 underscoring the role of the extrageniculate pathway in higher-order perceptual processing. EEG studies have captured the temporal dynamics of illusion perception; for instance, the Fraser spiral illusion elicits stronger posterior scalp components between 220 and 280 ms, reflecting early differential processing of twisted versus circular configurations.^[48]^[49]^[50] Predictive coding frameworks have emerged as a key theoretical lens for understanding illusions as arising from prediction errors between expected and actual sensory inputs. In these models, the visual cortex minimizes discrepancies through hierarchical inference, leading to perceptual biases in geometrical-optical figures. A 2024 psychophysically tuned model of V1, incorporating Gabor filters for orientation selectivity and long-range connectivity, replicates illusions like the Hering, Zöllner, and Poggendorff by adjusting receptive field parameters to match human psychophysical data from 30 participants. Similarly, a 2020 predictive coding approach unifies brightness-related illusions—such as those involving contextual luminance—under error-driven updates. These models build on earlier Gestalt principles by quantifying how top-down expectations propagate through cortical layers.^[51]^[52] Computational simulations using artificial intelligence have further advanced insights, with convolutional neural networks (CNNs) trained for object recognition exhibiting human-like vulnerabilities to geometrical-optical illusions. For example, a 2024 study across 12 deep neural networks analyzed five illusions, including the Müller-Lyer, using representational dissimilarity matrices and class activation mapping; models like VGG16 and DenseNet169 showed high fidelity in length misperception, with contextual arrows biasing feature extraction layers akin to V2/V4 integration in primates. These findings underscore how illusions probe the limits of feedforward versus recurrent computation in both biological and artificial vision systems.^[53] The involvement of attention networks has been highlighted in recent work, where differential spatial attention to illusion elements deforms neuronal receptive fields, systematically biasing perceptions of extent, angle, and position. A 2024 attentional model explains distortions in the Ebbinghaus and Ponzo illusions through uneven resource allocation, supported by behavioral manipulations that modulate effect strength. Neuroimaging corroborates this via a dual frontoparietal network architecture, with dorsal and ventral streams modulating visuospatial attention to contextual cues during illusion viewing. Emerging applications extend to clinical contexts; illusions reveal altered susceptibility in populations with Parkinson's disease or congenital visual impairments, where reduced Ponzo effects signal depth processing deficits, informing potential visual rehabilitation strategies by targeting attentional and contextual integration deficits. Cross-cultural behavioral data further confirm cognitive modulations, with rural participants showing weaker effects than urban ones, suggesting experiential tuning that future neuroimaging could validate.^[54]^[55]^[12]^[56] In 2025, neuroscience research identified "illusory contour-encoder" neurons in the mouse visual cortex that respond to optical illusions involving incomplete edges, providing insights into early neural mechanisms of geometric distortions. Computational studies have also compared human and AI perception through illusions, revealing differences in how artificial systems construct visual representations compared to biological ones.^[57]^[58]

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Observer errors of judged length when viewing the Müller-Lyer illusion arise partially from distortions caused by lateral neural interactions in the retina.
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Exploring Deep Neural Networks in Simulating Human Vision ...
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An attentional approach to geometrical illusions - PMC
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Neuroimaging studies conducted in the last three decades have distinguished two frontoparietal networks responsible for the control of visuospatial attention.
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Jun 17, 2025 · Visual illusions show profound cross-cultural differences; rural Namibian participants often fail to see percepts obvious to UK/US participants ...