Biology subjects

Kohler, P. J.

Publications and source records attributed to Kohler, P. J..

11 recordsLinked to original sources

bioRxiv2026

Reach plans for grasping vs. placing objects evoke different neural dynamics within a shared cortico-cerebellar mechanism.

Many reach studies treat grasping as the endpoint of a reach, but grasping often initiates a sequence of actions, ending with placing the object. Grasping and placing both require object-specific encoding, but placing has different computational demands, introduced by the intent, sensory feedback, and tolerance for spatial error when alignment is required. Recent neuroimaging work suggests that object placement shares similar cortico-cerebellar networks with grasp, with modest increases in network segregation and cerebellar activity. Here, we ask whether Grasp and Place are better distinguished by their spatiotemporal dynamics in both behaviour and scalp-recorded EEG activity. Using high-density EEG and motion tracking, 31 participants performed alternating Grasp/Place movements of a 3D object from/toward an oriented target/template. Place movements were initiated faster but executed more slowly and with greater endpoint-orientation precision than Grasp, indicating distinct control strategies. ERP and microstate analyses revealed that Grasp and Place shared a peri-movement polarity switch, from posterior negativity/frontal positivity during preparation to posterior positivity/frontal negativity during execution. This shared pattern was amplified during Place preparation and prolonged during Place execution, with stronger expression of preparatory medial frontal/cerebellar and execution-related posterior/cerebellar microstate dynamics. Finally, SVM decoding indicated that task-specific information was primarily encoded in low-frequency EEG activity (below 6 Hz). These results confirm that grasping and placing recruit similar cortico-cerebellar mechanisms, yet different spatiotemporal dynamics are employed to support the distinct computational demands of these behaviours.

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bioRxiv2026

Seeing at Will: Shared Neural Representations of Motion Perception and Intention

Volitional intention can bias perception in cases where two or more interpretations of a stimulus are available to us. The neural mechanisms whereby such an intention influences perception are poorly understood. Here we investigated whether intending to see horizontal versus vertical motion in a subsequently presented instantaneous position shift of a quartet apparent motion stimulus establishes decodable sensory representations prior to both the position shift and the perception of motion. Twelve participants underwent fMRI scanning under three conditions: (1) while passively viewing either continuously or (2) discretely moving quartet stimuli, or (3) while actively intending to see a subsequent single-shot apparent motion as either a vertical or horizontal motion. Multivariate decoding analyses revealed that activity patterns during the intention period of (3) generalized to patterns evoked by both (1) physical and (2) ambiguous motion perception. Cross-decoding was strongest within dorsal/lateral visual regions, including hMT+, V3AB, and the intraparietal sulcus (IPS), but was largely absent from ventral visual cortex. Widespread overlap was also observed between intention-related and perceptual motion representations throughout the dorsal/lateral visual cortex. Our findings suggest that volitional intention establishes prospective sensory representations before perceptual experience emerges and that these representations closely resemble those associated with illusory motion perception. The predominance of intention-related representations within dorsal/lateral visual regions is consistent with top-down influences from attentional control systems. More broadly, the results demonstrate that internally generated cognitive states can shape sensory representations, constraining subsequent perceptual experience.

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bioRxiv2026

Shared symmetry detection with divergent temporal dynamics in marmosets and humans

Symmetry is a fundamental organizing principle of mid-level vision, yet which aspects of human symmetry processing are shared across primates remains unclear. Using steady-state visual evoked potentials during passive viewing, we compared responses to two well-matched wallpaper groups, double reflection (PMM) and four-fold rotation (P4), in common marmosets and humans, using identical stimuli. Marmosets showed robust reflection-symmetry responses comparable in relative magnitude, temporal dynamics and scalp distribution to those of humans. Rotation responses were also reliable but transient, lacking the sustained late component prominent in humans. This dissociation suggests that feedforward computations underlying symmetry detection are conserved across primates, whereas sustained processing that elaborates symmetry representations (likely recurrent or feedback in origin) is reduced in marmosets, particularly for rotation. These findings establish marmosets as a tractable model for symmetry processing and open a path to intracortical recording in accessible extrastriate cortex.

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bioRxiv2024

Visual search efficiency is modulated by symmetry type and texture regularity

More than a century of vision research has identified symmetry as a fundamental cue, which aids the visual system in making inferences about objects and surfaces in natural scenes. Most studies have focused on one type of symmetry, reflection, presented at a single image location. However, the visual system responds strongly to other types of symmetries, and to symmetries that are repeated across the image plane to form textures. Here we use a visual search paradigm with arrays of repeating lattices that contained either reflection or rotation symmetries but were otherwise matched. Participants were asked to report the presence of a target tile without symmetry. When lattices tile the plane without gaps, they form regular textures. We manipulated texture regularity by introducing jittered gaps between lattices. This paradigm lets us investigate the effect of symmetry type and texture regularity on visual search efficiency. Based on previous findings suggesting an advantage for reflection in visual processing, we hypothesized that search would be more efficient for reflection than rotation. We further hypothesized that regular textures would be processed more efficiently. We found independent effects of symmetry type and regularity on search efficiency that confirmed both hypotheses: visual search was more efficient for textures with reflection symmetry and more efficient for regular textures. This provides additional support for the perceptual advantage of reflection in the context of visual search and provides important new evidence in favor of visual mechanisms specialized for processing symmetries in regular textures.

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bioRxiv2021

Symmetry processing in the macaque visual cortex

Symmetry is a highly salient feature of the natural world that is perceived by many species of the animal kingdom and that impacts a large array of behaviours such as partner selection or food choice. In humans, the cerebral areas processing symmetry are now well identified from neuroimaging measurements. However, we currently lack an animal model to explore the underlying neural mechanisms. Macaque is a potentially good candidate, but a previous comparative study (1) found that functional magnetic resonance imaging (fMRI) responses to mirror symmetry in this species were substantially weaker than those observed in humans under similar experimental conditions. Here, we re-examined symmetry processing in macaques from a broader perspective, using both rotation (experiment 1) and reflection (experiment 2) symmetry. Our experimental design was directly derived from that of a previous human fMRI study (2), in order to facilitate the comparison between the two primate species. Highly consistent responses to symmetry were found in a large network of areas (notably V3, V3A, V4, V4A and PITd), in line with what has been observed in humans. Within this network, response properties in areas V3 and V4 (notably their dependency on the rotation symmetry order) were strikingly similar to those observed in their human counterparts. Our results suggest that the cortical networks that process symmetry in humans and macaques are much more similar than previously reported and point toward macaque as a relevant model for understanding symmetry processing. Significance statementSymmetry processing is an important aspect of human visual perception. We currently lack an animal model for characterizing the neural mechanisms that underlie it at the microscopic scale. Here, we use fMRI measurements in macaques to demonstrate that the cortical responses to symmetry in this species are comparable to those observed in humans under similar experimental conditions to a much higher extent than previously documented. Our results call for a re-examination of the relevance of the macaque model for symmetry processing in humans and open the door to an exploration of the underlying neural mechanisms at the single-cell level, notably in V3, an area often neglected in most current models of visual processing.

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bioRxiv2021

The human visual system preserves the hierarchy of 2-dimensional pattern regularity

Symmetries are present at many scales in images of natural scenes. A large body of literature has demonstrated contributions of symmetry to numerous domains of visual perception. The four fundamental symmetries, reflection, rotation, translation and glide reflection, can be combined in exactly 17 distinct ways. These wallpaper groups represent the complete set of symmetries in 2D images and have recently found use in the vision science community as an ideal stimulus set for studying the perception of symmetries in textures. The goal of the current study is to provide a more comprehensive description of responses to symmetry in the human visual system, by collecting both brain imaging (Steady-State Visual Evoked Potentials measured using high-density EEG) and behavioral (symmetry detection thresholds) data using the entire set of wallpaper groups. This allows us to probe the hierarchy of complexity among wallpaper groups, in which simpler groups are subgroups of more complex ones. We find that this hierarchy is preserved almost perfectly in both behavior and brain activity: A multi-level Bayesian GLM indicates that for most of the 63 subgroup relationships, subgroups produce lower amplitude responses in visual cortex (posterior probability: > 0.95 for 56 of 63) and require longer presentation durations to be reliably detected (posterior probability: > 0.95 for 49 of 63). This systematic pattern is seen only in visual cortex and only in components of the brain response known to be symmetric-specific. Our results show that representations of symmetries in the human brain are precise and rich in detail, and that this precision is reflected in behavior. These findings expand our understanding of symmetry perception, and open up new avenues for research on how fine-grained representations of regular textures contribute to natural vision.

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bioRxiv2020

PARIETAL CONTRIBUTIONS TO ABSTRACT NUMEROSITY MEASURED WITH STEADY STATE VISUAL EVOKED POTENTIALS

Non-symbolic number changes produce transient Event Related Potentials over parietal electrodes, while numerosity effects measured with Steady-State Visual Evoked Potentials (SSVEPs) appear to originate in occipital cortex. We hypothesized that the stimulation rates used in previous SSVEP studies may be too rapid to drive parietal numerosity mechanisms. Here we recorded SSVEPs and behavioral reports over a slower range of temporal frequencies than previously used. Isoluminant dot stimuli updated at a consistent "carrier" frequency (3-6 Hz) while periodic changes in numerosity (e.g. 8[->]5) formed an even slower "oddball" frequency (0.5-1 Hz). Each numerosity oddball condition had a matched control condition where the number of dots did not change. Carrier frequencies induced SSVEPs with midline occipital topographies that did not differentiate the presence or absence of numerosity oddballs. By contrast, SSVEPs at oddball frequencies had parietal topographies and responded more strongly when oddballs were present. Consistent with our hypothesis, numerosity effects were stronger at slower stimulation rates. In a second study, the numerosity change was either supra-threshold (e.g. 8[->]5 dots) or near the threshold required for detecting numerosity changes (e.g. 8[->]9 dots). We found robust parietal responses for the supra-threshold case only, indicating a numerical distance effect. A third study replicated the parietal oddball SSVEP effect across four distinct suprathreshold numerosity-change conditions and showed that number change direction does not influence the effect. These findings show that SSVEP oddball paradigms can probe parietal computations of abstract numerosity, and may provide a rapid, portable approach to quantifying number sense within educational settings.

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bioRxiv2019

The neural signature of numerosity: Separating numerical and continuous magnitude extraction in early visual cortex with frequency-tagged EEG

The ability to handle approximate quantities, or number sense, has been recurrently linked to mathematical skills, though the nature of the mechanism allowing to extract numerical information (i.e., numerosity) from environmental stimuli is still debated. A set of objects is indeed not only characterized by its numerosity but also by other features, such as the summed area occupied by the elements, which often covary with numerosity. These intrinsic relations between numerosity and non-numerical magnitudes led some authors to argue that numerosity is not independently processed but extracted through a weighting of continuous magnitudes. This view cannot be properly tested through classic behavioral and neuroimaging approaches due to these intrinsic correlations. The current study used a frequency-tagging EEG approach to separately measure responses to numerosity as well as to continuous magnitudes. We recorded occipital responses to numerosity, total area, and convex hull changes but not to density and dot size. We additionally applied a model predicting primary visual cortex responses to the set of stimuli. The model output was closely aligned with our electrophysiological data, since it predicted discrimination only for numerosity, total area, and convex hull. Our findings thus demonstrate that numerosity can be independently processed at an early stage in the visual cortex, even when completely isolated from other magnitude changes. The similar implicit discrimination for numerosity as for some continuous magnitudes, which correspond to basic visual percepts, shows that both can be extracted independently, hence substantiating the nature of numerosity as a primary feature of the visual scene.

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bioRxiv2018

Functional organization of motion and disparity sensitivity in human visual cortex

Vision with two eyes makes perception of weak visual contrast signals easier and, due to the lateral separation of the eyes, allows for the triangulation of depth relationships. While binocular summation of contrast signals affords the observer increased sensitivity, binocular summation of spatial cues related to changes in depth is associated with decreased sensitivity to the corresponding retinal image displacements. Perceptual models of contrast and motion-in-depth sensitivity have explained this divergence in sensitivity by proposing that probabilistic neural noise limits summing and differencing operations on small signals. Because these models do not scale well for highly suprathreshold visual signals typical of the natural environment, we approached the question of how dynamic binocular image differences are coded using direct neural measurements. Here we use Steady-State Visual Evoked Potentials in human participants to show that inter-ocular differences in retinal image motion that produce elevated perceptual thresholds generate strongly suppressed evoked response amplitudes compared to motion that is matched between the two eyes. This suppression is strongly dependent on the availability of well-defined spatial references in the image and is highly immature in 5-month-old infants. Because the suppression is of equal strength for horizontal and vertical directions of motion, it is not specific to the perception of motion in depth. Relational image cues play a critical role in early to intermediate perceptual processing stages, and these results suggest that a succession of spatial and inter-ocular differencing operations condition the visual signal representation, prior to the extraction of motion-in-depth.\n\nSignificance StatementThe present work underscores the importance of relational spatial cues in both the motion and disparity domains for binocular visual coding. Relative motion and relative disparity cues not only support fine-grain displacement sensitivity but strongly influence suprathreshold responsiveness. Extraction of these cues supports a powerful binocular interaction within the motion pathway that is suppressive in nature and poorly developed in infants. This suppressive interaction is present for both horizontal and vertical directions of motion and is thus not specific to motion-in-depth, as previously believed, but is rather hypothesized to be a pre-processing step, with motion-in-depth being computed at a later or separate stage.

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bioRxiv2017

From image formation to image representation: the human visual system preserves the hierarchy of 2-dimensional pattern regularity

Symmetries are present at many scales in images of natural scenes, due to a complex interplay of physical forces that govern pattern formation in nature. The importance of symmetry for visual perception has been known at least since the gestalt movement of the early 20th century. Since then, symmetry has been shown to contribute to the perception of shapes[1, 2] scenes[3] and surface properties[4], as well as the social process of mate selection[5]. In the two spatial dimensions relevant for images, the four fundamental symmetries, reflection, rotation, translation and glide reflection, can be combined in 17 distinct ways, the \"wallpaper\" groups[6-8]. The 17 wallpaper obey a hierarchy of complexity, determined by mathematical group theory, where simpler groups are subgroups of more complex ones[9]. Here we use Steady-State Visual Evoked Potentials (SSVEPs) to measure responses to the complete set of wallpaper groups, and present a complete description of the neural basis of symmetry. We find that activity in human visual cortex is remarkably consistent with the hierarchical relationships between the wallpaper groups. Specifically, the amplitudes of symmetry-specific responses in individual participants (n=25) preserve these relationships at an above-chance level in 88.3% (53 out of 60) of cases. Visual cortex thus encodes all the fundamental symmetries using a representational structure that closely approximates the subgroup relationships from group theory. Given that most participants had no knowledge of group theory, the ordered structure of visual responses to wallpaper groups is likely learned implicitly from regularities in the visual environment.

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bioRxiv2017

Image segmentation based on relative motion and relative disparity cues in topographically organized areas of human visual cortex

The borders between objects and their backgrounds create discontinuities in image feature maps that can be used to recover object shape. Here we used functional magnetic resonance imaging (fMRI) to study the sensitivity of visual cortex to two of the most important image segmentation cues: relative motion and relative disparity. Relative motion and disparity cues were isolated using random-dot kinematograms and stereograms, respectively. For motion-defined boundaries, we found a strong retinotopically organized representation of a 2-degree radius motion-defined disk, starting in V1 and extending though V2 and V3. In the surrounding region, we observed phase-inverted activations indicative of suppression, extending out to at least 6 degrees of retinal eccentricity. For relative disparity, figure responses were only robust in V3, while suppression was observed in all early visual areas. When attention was captured at fixation, figure responses persisted while suppression did not, suggesting that suppression is generated by attentional feedback from higher-order visual areas. Outside of the early visual areas, several areas were sensitive to both types of cues, most notably hV4, LO1 and V3B, making them additional candidate areas for motion- and disparity-cue combination. The overall pattern of extra-striate activations is consistent with recent three-stream models of cortical organization.

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