Biology subjects

Zaidi, Q.

Publications and source records attributed to Zaidi, Q..

10 recordsLinked to original sources

bioRxiv2026

Dynamism in paintings and photographs: how the power of diagonals in art overcomes the oblique effect in visual perception

Artists and photographers use diagonals to convey dynamism, tension and instability in naturalistic and abstract images. However, the oblique effect in visual perception refers to a reduced sensitivity to oblique compared to cardinal (vertical/horizontal) orientations, linked to anisotropy in the distribution of preferred orientations of primary visual cortex neurons, which reflect the statistical distribution of edge orientations in natural images. Distal diagonals generically project onto oblique orientations on the retina, creating a conundrum between their perceptual salience in art and their reduced sensitivity in vision. We conjectured that global spatial configurations could shape perceived dynamism overriding local orientations. Inspired by van Doesburgs 1929 Arithmetical Composition, we created 60 distinct linear configurations of four identically-shaped rhombi each, in which global and local-edge orientations were independently manipulated to be cardinal or oblique, and measured their perceived dynamism with behavioral, psychophysical, and psychophysiological methods. First, 250 observers rated oblique configurations as more dynamic, tridimensional, and unstable than cardinal configurations, with wedge shapes and element spacing enhancing dynamism. Second, pairwise comparisons (50 observers) yielded a perceptual dynamism scale that confirmed the dominant role of global orientations. Third, pupillometry (35 observers) showed that pupil dilation correlates systematically with subjective dynamism ratings. Perceived dynamism in static images is thus driven primarily by the orientation and shape of spatial configurations, indicating the importance of estimating orientations and shapes of multi-element objects beyond decoding local orientations. By linking visual structure to emotional perceptual experience, this investigation provides empirically grounded principles for creating dynamic percepts in visual art. SIGNIFICANCE STATEMENTDiagonals have been used since Michelangelo to create dynamic compositions in representative and abstract art and photography, but paradoxically humans have been shown to be less sensitive to oblique orientations like those projected on the retina by diagonal edges. We resolve this conflict by distinguishing between global configurations versus local edges and showing through behavioral, psychophysical, and psychophysiological experiments that the global orientation and shape of a multi-element configuration has a much larger effect on perceived dynamism than do the edge orientations of its elements. Neural estimation of orientations and shapes of multi-element configurations is thus essential for image understanding. Our results provide perceptually grounded bases for using diagonal configurations with wedge shapes to create dynamic percepts in visual art.

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bioRxiv2025

Generative Phenomenology of form perception: Perceptograms and cortical models for amblyopic form distortions

What we see is generated by our cortical neurons processing sensory input. Amblyopia provides a perfect case for studying the neural generation of percepts because abnormal neural development of cortex leads to many amblyopes seeing more complex phantom forms than the viewed stimulus through their amblyopic eye (AE) but not through the fellow eye (FE). Using computer guided dichoptic displays, we showed that 92.6% of phantom percepts generated by single gratings shown to AE were matched exactly by plaids (sums of two gratings) shown to FE. A formal equation between the cortical signals generated by the gratings seen through AE and the signals generated by their matched patterns seen through FE, provided the derivation of AE cortical receptive fields as linear transforms of FE receptive fields. The transformed AE cortical model accurately generated the phantoms, was validated via reverse-correlation, and explained deficits in form detection and spatial localization. This study elucidates the close link between percepts and neuronal receptive fields. TEASERChanges in cortical neurons receptive fields can lead to amblyopes seeing phantom features that are not present in viewed images.

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bioRxiv2024

Anisotropy of object non-rigidity: High-level perceptual consequences of cortical anisotropy

We demonstrate an unexpected anisotropy in perceived object non-rigidity, a higher-level perceptual phenomenon, and explain it by the population distribution of low-level neuronal properties in primary visual cortex. We measured the visual interpretation of two rigidly connected rotating circular rings. In videos where observers predominantly perceived rigidly-connected horizontally rotating rings, they predominantly perceived non-rigid independently wobbling rings if the video was rotated by 90{degrees}. Additionally, vertically rotating rings appeared narrower and longer than horizontally rotating counterparts. We decoded these perceived shape changes from V1 outputs incorporating documented cortical anisotropies in orientation selectivity: more cells and narrower tuning for the horizontal orientation than for vertical. Even when shapes were matched, the non-rigidity anisotropy persisted, suggesting uneven distributions of motion-direction mechanisms. When cortical anisotropies were incorporated into optic flow computations, the kinematic gradients (Divergence, Curl, Deformation) for vertical rotations aligned more with derived gradients for physical non-rigidity, while those for horizontal rotations aligned closer to rigidity. Our results reveal how high-level non-rigidity percepts can be shaped by hardwired cortical anisotropies. Cortical anisotropies are claimed to promote efficient encoding of statistical properties of natural images, but their surprising contribution to failures of shape constancy and object rigidity raise questions about their evolutionary function.

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bioRxiv2024

Primate V2 Receptive Fields Derived from Anatomically Identified Large-Scale V1 Inputs

In the primate visual system, visual object recognition involves a series of cortical areas arranged hierarchically along the ventral visual pathway. As information flows through this hierarchy, neurons become progressively tuned to more complex image features. The circuit mechanisms and computations underlying the increasing complexity of these receptive fields (RFs) remain unidentified. To understand how this complexity emerges in the secondary visual area (V2), we investigated the functional organization of inputs from the primary visual cortex (V1) to V2 by combining retrograde anatomical tracing of these inputs with functional imaging of feature maps in macaque monkey V1 and V2. We found that V1 neurons sending inputs to single V2 orientation columns have a broad range of preferred orientations, but are strongly biased towards the orientation represented at the injected V2 site. For each V2 site, we then constructed a feedforward model based on the linear combination of its anatomically-identified large-scale V1 inputs, and studied the response proprieties of the generated V2 RFs. We found that V2 RFs derived from the linear feedforward model were either elongated versions of V1 filters or had spatially complex structures. These modeled RFs predicted V2 neuron responses to oriented grating stimuli with high accuracy. Remarkably, this simple model also explained the greater selectivity to naturalistic textures of V2 cells compared to their V1 input cells. Our results demonstrate that simple linear combinations of feedforward inputs can account for the orientation selectivity and texture sensitivity of V2 RFs.

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bioRxiv2023

Object Rigidity: Competition and cooperation between motion-energy and feature- tracking mechanisms and shape-based priors

Why do moving objects appear rigid when projected retinal images are deformed non-rigidly? We used rotating rigid objects that can appear rigid or non-rigid to test whether shape features contribute to rigidity perception. When two circular rings were rigidly linked at an angle and jointly rotated at moderate speeds, observers reported that the rings wobbled and were not linked rigidly but rigid rotation was reported at slow speeds. When gaps, paint or vertices were added, the rings appeared rigidly rotating even at moderate speeds. At high speeds, all configurations appeared non-rigid. Salient features thus contribute to rigidity at slow and moderate speeds, but not at high speeds. Simulated responses of arrays of motion-energy cells showed that motion flow vectors are predominantly orthogonal to the contours of the rings, not parallel to the rotation direction. A convolutional neural network trained to distinguish flow patterns for wobbling versus rotation, gave a high probability of wobbling for the motion-energy flows. However, the CNN gave high probabilities of rotation for motion flows generated by tracking features with arrays of MT pattern-motion cells and corner detectors. In addition, circular rings can appear to spin and roll despite the absence of any sensory evidence, and this illusion is prevented by vertices, gaps, and painted segments, showing the effects of rotational symmetry and shape. Combining CNN outputs that give greater weight to motion energy at fast speeds and to feature tracking at slow, with the shape-based priors for wobbling and rolling, explained rigid and nonrigid percepts across shapes and speeds (R2=0.95). The results demonstrate how cooperation and competition between different neuronal classes leads to specific states of visual perception and to transitions between the states.

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bioRxiv2022

A theory of cortical map formation in the visual brain

The cerebral cortex receives multiple afferents from the thalamus that segregate by stimulus modality forming cortical maps for each sense. In vision, the primary visual cortex maps the multiple dimensions of the visual stimulus in patterns that vary across species for reasons unknown. Here we introduce a general theory of cortical map formation, which proposes that map diversity emerges from species variations in the thalamic afferent density sampling sensory space. In the theory, increasing afferent sampling density enlarges the cortical domains representing the same visual point, allowing the segregation of afferents and cortical targets by multiple stimulus dimensions. We illustrate the theory with an afferent-density model that accurately replicates the maps of different species through afferent segregation followed by thalamocortical convergence pruned by visual experience. Because thalamocortical pathways use similar mechanisms for axon segregation and pruning, the theory may extend to other sensory areas of the mammalian brain.

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bioRxiv2020

Mental geometry of perceiving 3D size in pictures

We show that the classical problem of 3D size perception in obliquely viewed pictures can be understood by comparing human performance to the optimal geometric solution. A photograph seen from the camera position, forms the same retinal projection as the physical 3D scene, but retinal projections of sizes and shapes are distorted in oblique viewing. For real scenes, we previously showed that size and shape inconstancy result despite observers using the correct geometric back-transform, because some retinal images evoke misestimates of viewing elevation. Now, we examine how observers estimate 3D sizes in oblique views of pictures of objects in different poses on the ground. Compared to the camera position estimates, sizes in oblique views were seriously underestimated for objects at fronto-parallel poses, but there was almost no change for objects perceived as pointing towards the viewer. The inverse of the function relating projected length to pose, camera elevation and viewing azimuth, gives the optimal correction factor for inferring correct 3D lengths if the elevation and azimuth are estimated accurately. Empirical correction functions had similar shapes to optimal, but lower amplitude. Measurements revealed that observers systematically underestimated viewing azimuth, similar to the fronto-parallel bias for object pose perception. A model that adds underestimation of viewing azimuth to the geometrical back-transform, provided good fits to estimated 3D lengths from oblique views. These results add to accumulating evidence that observers use internalized projective geometry to perceive sizes, shapes and poses in 3D scenes and their pictures.

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bioRxiv2019

Mental geometry of 3D size and shape perception

Judging poses, sizes and shapes of objects accurately is necessary for organisms and machines to operate successfully in the world. Retinal images of 3D objects are mapped by the rules of projective geometry, and preserve the invariants of that geometry. Since Plato, it has been debated whether geometry is innate to the human brain, and Poincare and Einstein thought it worth examining whether formal geometry arises from experience with the world. We examine if humans have learned to exploit projective geometry to estimate sizes and shapes of objects in 3D scenes.\n\nNumerous studies have examined size invariance as a function of physical distance, which changes scale on the retina, but surprisingly, possible constancy or inconstancy of relative size seems not to have been investigated for object pose, which changes retinal image size differently along different axes. We show systematic underestimation of length for extents pointing towards or away from the observer, both for static objects and dynamically rotating objects. Observers do correct for projected shortening according to the optimal back-transform, obtained by inverting the projection function, but the correction is inadequate by a multiplicative factor. The clue is provided by the greater underestimation for longer objects, and the observation that they appear more slanted towards the observer. Adding a multiplicative factor for perceived slant in the back-transform model provides good fits to the corrections used by observers. We quantify the slant illusion with relative slant measurements, and use a dynamic demonstration to show the power of the slant illusion.\n\nIn biological and mechanical objects, distortions of shape are manifold, and changes in aspect ratio and relative limb sizes are functionally important. Our model shows that observers try to retain invariance of these aspects of shape to 3D rotation by correcting retinal image distortions due to perspective projection, but the corrections can fall short. We discuss how these results imply that humans have internalized particular aspects of projective geometry through evolution or learning, and how assuming that images are preserving the continuity, collinearity, and convergence invariances of projective geometry, supplements the Generic Viewpoint assumption, and simply explains other illusions, such as Ames Chair.

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bioRxiv2018

Short-wavelength cone signals contribute to sparse, high-dimensional color tuning in primate OFF midget ganglion cells

In the trichromatic primate retina, the midget retinal ganglion cell is the classical substrate for red-green color signaling, with a circuitry that enables antagonistic responses between long (L)- and medium (M)-wavelength sensitive cone inputs. Previous physiological studies show that some OFF midget ganglion cells may receive sparse input from short (S)-wavelength sensitive cones, but the effect of S-cone inputs on the chromatic tuning properties of such cells has been unexplored. Moreover, anatomical evidence for a synaptic pathway from S cones to OFF midget ganglion cells through OFF-midget bipolar cells remains ambiguous. In this study we address both questions for the macaque monkey retina. First, we used serial block-face electron microscopy (SBEM) to show that every S-cone in the parafoveal retina synapses principally with a single OFF-midget bipolar cell which in turn forms a private-line connection with an OFF midget ganglion cell. Second, we used patch electrophysiology to characterize the chromatic tuning of OFF midget ganglion cells in the near peripheral retina that receive combined input from L, M and S cones. These S-OFF midget cells have a characteristic S-cone spatial signature, but demonstrate heterogeneous color properties due to variable strength of L, M, and S cone input across the receptive field. Together these findings strongly support the hypothesis that the OFF midget pathway is the major conduit for S-OFF signals in primate retina, and redefines the pathway as a chromatically complex substrate that encodes color signals beyond the classically recognized L vs. M and S vs. L+M cardinal mechanisms.\n\nSignificance statementThe first step of color processing in the visual pathway of primates occurs when signals from short- (S), middle- (M) and long- (L) wavelength sensitive cone types interact antagonistically within the retinal circuitry to create color-opponent pathways. The midget (L vs. M or red-green) and small bistratified (S vs. L+M, or blue-yellow) appear to provide the physiological origin of the cardinal axes of human color vision. Here we confirm the presence of an additional S-OFF midget circuit in the macaque monkey fovea with scanning block-face electron microscopy (SBEM) and show physiologically that a subpopulation of S-OFF midget cells combine S, L and M cone inputs along non-cardinal directions of color space, expanding the retinal role in color coding.

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bioRxiv2018

Geometrical structure of perceptual color space: mental representations and adaptation invariance

A central issue in neuroscience is to understand how the brain builds structured representations of percepts that facilitate useful inferences about the world. Similarity between percepts is used to accomplish many everyday tasks, e.g. object identification, so is widely used to construct geometrical spaces that represent stimulus qualities, but the intrinsic validity of the geometry is not tested critically. We introduce an experimental approach to equating relative similarities by setting perceived midpoints between pairs of stimuli. Midpoint settings are used with Varignons Theorem to test the intrinsic geometry of a representation space, and its mapping to a physical space of stimuli. For perceptual color space, we demonstrate that geometrical structure depends on the mental representation used in judging similarity: an affine geometry is valid only when observers use an opponent-color mental representation. An affine geometry implies that similarity can be judged within straight lines and across parallel lines, and its neural coding could involve ratios of responses. We show that this perceptual space is invariant to changes in illumination color, providing a formal justification to generalize to all of color space, color constancy results measured for color categories. Our midpoint measurements deviate significantly from midpoints in the \"uniform\" color spaces CIELAB and CIELUV, used extensively in industry and research, so these spaces do not provide adequate metric representation of perceived colors. Our paradigm can thus test for intrinsic geometrical assumptions underlying the representation space for many perceptual modalities, and for the extrinsic perceptual geometry of the space of physical stimuli.\n\nSignificanceMathematical spaces based on similarity judgments are widely used to represent stimulus qualities in perception, cognition and neuroscience. We introduce a perceptual approach to equate relative similarities, and use them to test the geometry of a perceptual space and its mapping to a physical space of stimuli. For color perception, our results show that perceptual geometry depends on the mental representation used in judging similarity, and it has an affine structure when observers use an opponent-color representation. An affine geometry implies that neural coding of similarity could involve simple ratios of responses. Our measurements also reveal that the uniform color spaces CIELAB and CIELUV, used extensively in industrial applications, do not provide adequate representation of similarity between moderately spaced colors.

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