Biology subjects

Olman, C. A.

Publications and source records attributed to Olman, C. A..

6 recordsLinked to original sources

bioRxiv2025

Orientation-tuned surround suppression exhibits a unique laminar signature in human primary visual cortex

Spatial context modifies visual perception by enhancing novel and salient features over spatially redundant features in the underlying neural code of primary visual cortex (V1). Although multiple intracortical pathways contribute to contextual modulation, their specific contributions to different types of contextual modulation are not fully understood. Leveraging the distinct laminar connectivity patterns of feedforward, feedback, and lateral pathways, we used ultra-high-resolution fMRI (7T T2*-weighted, 0.6 mm isotropic resolution) to infer their relative contributions to contextual modulation in V1 by analyzing blood-oxygenation-level-dependent (BOLD) signal across cortical depth. Participants viewed sine-wave grating disks embedded in large surround gratings. Segmentation cues were introduced or removed by manipulating the relative phase and orientation of the surround gratings, yielding three contextual conditions and a surround-only condition to measure the effects of context in the absence of feedforward input. Our analysis isolated the effects of orientation-tuned surround suppression (OTSS) from orientation-independent border-induced modulation (BIM). The results show that BOLD laminar profiles differ by modulation type: OTSS was absent from deep layers, whereas BIM was more broadly distributed. We also find that voxels at all depths are driven by spatial context in the absence of feedforward input, which accords with the discovery of contextually-driven neural responses in mammalian V1. These laminar differences likely reflect different proportional contributions of feedback from higher-order visual areas and long-range lateral connections within V1. Our findings help to explicate the contributions of recurrent processing to visual contextual modulation and its impacts on laminar-dependent BOLD fMRI. Significance StatementOur sensory experience requires interpretation: the brain uses both hyper-local and large-scale scene cues to process sensory inputs. In primary visual cortex (V1), this contextual modulation arises from a mixture of intra- and inter-regional neural connections. Because these computations occur at different depths in the cortical gray matter, sub-millimeter resolution fMRI offers an opportunity to quantify the separate contributions of these neural pathways. Using a visual surround suppression paradigm, we measured a distinct depth profile for intra-regional contextual modulation, separating this orientation-tuned signal from other forms of border-induced modulation. This work validates an important new tool for understanding both typical and atypical neural network architectures in the human brain.

neuroscience↗

bioRxiv2024

Surround Suppression of Broadband Images

Visual perception is profoundly sensitive to context. Surround suppression is a well-known visual context effect in which the firing rate of a neuron is suppressed by stimulation of its extra-classical receptive field. The majority of contrast surround suppression studies exclusively use narrowband, sinusoidal grating stimuli; however, it is unclear whether the results produced by such artificial stimuli generalize to real-world, naturalistic visual experiences. To address this issue, we developed a contrast discrimination paradigm that includes both naturalistic broadband textures and narrowband grating textures. All textures were matched for first order image statistics and overall perceptual salience. We observed surround suppression across broadband textures (F(1,6)=19.01, p=.005); however, effect sizes were largest for narrowband, sinusoidal gratings (Cohens d=1.83). Among the three broadband texture types, we observed strongest suppression for the texture with a clear dominant orientation (stratified: Cohens d=1.29), while the textures with a more even distribution of orientation information produced weaker suppression (fibrous: Cohens d=0.63; braided: Cohens d=0.65). We also observed an effect of texture identity on the slope of psychometric functions (F(1.98,11.9)=7.29, p=0.01), primarily driven by smaller slopes for the texture with the most uniform distribution of orientations. Our results suggest that well-known contextual modulation effects only partially generalize to more ecologically valid stimuli.

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bioRxiv2020

Assessing methods for geometric distortion compensation in 7T gradient echo fMRI data

Echo planar imaging (EPI) is widely used in functional and diffusion-weighted MRI, but suffers from significant geometric distortions in the phase encoding direction caused by inhomogeneities in the static magnetic field (B0). This is a particular challenge for EPI at very high field ([≥] 7T), as distortion increases with higher field strength. A number of techniques for distortion correction exist, including those based on B0 field mapping and acquiring EPI scans with opposite phase encoding directions. However, few quantitative comparisons of distortion compensation methods have been performed using human EPI data, especially at very high field. Here, we compared distortion compensation using B0 field maps and opposite phase encoding scans in two different software packages (FSL and AFNI) applied to 7T gradient echo (GE) EPI data from 31 human participants. We assessed distortion compensation quality by quantifying alignment to anatomical reference scans using Dice coefficients and mutual information. Performance between FSL and AFNI was equivalent. In our whole-brain analyses, we found superior distortion compensation using GE scans with opposite phase encoding directions, versus B0 field maps or spin echo (SE) opposite phase encoding scans. However, SE performed better when analyses were limited to ventromedial prefrontal cortex, a region with substantial dropout. Matching the type of opposite phase encoding scans to the EPI data being corrected (e.g., SE-to-SE) also yielded better distortion correction. While the ideal distortion compensation approach likely varies depending on methodological differences across experiments, this study provides a framework for quantitative comparison of different distortion compensation methods.

neuroscience↗

bioRxiv2019

Reduced Influence of Perceptual Context in Schizophrenia: Behavioral and Neurophysiological Evidence

BackgroundAccurate perception of visual contours is essential for seeing and differentiating objects in the environment. Both the ability to detect visual contours and the influence of perceptual context created by surrounding stimuli are diminished in people with schizophrenia. The central aim of the present study was to better understand the biological underpinnings of impaired contour integration and weakened effects of perceptual context. Additionally, we sought to determine whether visual perceptual abnormalities reflect genetic factors in schizophrenia and are present in other severe mental disorders. MethodsWe examined behavioral data and event-related potentials (ERPs) collected during the perception of simple linear contours embedded in similar background stimuli in 27 patients with schizophrenia (SCZ), 23 patients with bipolar disorder, 23 first-degree relatives of SCZ and 37 controls. ResultsSCZ exhibited impaired visual contour detection while patients with bipolar disorder exhibited intermediate performance. The orientation of neighboring stimuli (i.e., flankers) relative to the contour modulated perception across all groups, but SCZ exhibited weakened suppression by the perceptual context created by flankers. Late visual (occipital P2) and cognitive (centroparietal P3) neural responses showed group differences and flanker orientation effects, unlike earlier ERPs (occipital P1 and N1). Moreover, behavioral effects of flanker context on contour perception were correlated with modulation in P2/P3 amplitudes. ConclusionIn addition to replicating and extending findings of abnormal contour integration and visual context modulation in SCZ, we provide novel evidence that abnormal use of perceptual context is associated with higher-order sensory and cognitive processes.

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bioRxiv2019

Defining region-specific masks for reliable depth-dependent analysis of fMRI data

In high-field fMRI research, anatomical reference information (e.g., gray matter (GM) segmentation, cortical depth delineation) is often defined in volumes acquired with pulse sequences subject to different distortions than those in functional volumes. In these cases, reliable interpretation of ultra-high resolution fMRI data depends on excellent cross-modal registration of functional volumes to reference anatomical volumes. In this paper, we describe a two-step approach to automating assessments of cross-modal registration quality for the purpose of guiding depth-dependent analysis. First, each functional/anatomical registration was scored by the ratio of the number of GM voxels in the functional data overlapping anatomical GM to the number of GM voxels in the functional data overlapping anatomical white matter (WM). This GM:WM overlap ratio provided an objective metric for determining whether an alignment algorithm had converged on a solution that would pass visual inspection. Second, surface-based maps indicating the consistency of overlap between functional and anatomical GM throughout the GM depth were generated for the entire region of cortex covered by the experiment. These maps served as a mask for the purpose of excluding regions where registration between functional and anatomical data was inadequate and thus unable to support depth-dependent analyses. We found, for both real and simulated data, that functional response profiles that were less biased toward superficial responses in regions where these metrics indicated satisfactory registration.

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bioRxiv2018

Human brain activity during mental imagery exhibits signatures of inference in a hierarchical generative model

Humans have long wondered about the function of mental imagery and its relationship to vision. Although visual representations are utilized during imagery, the computations they subserve are unclear. Building on a theory that treats vision as inference about the causes of sensory stimulation in an internal generative model, we propose that mental imagery is inference about the sensory consequences of predicted or remembered causes. The relation between these complementary inferences yields a relation between the brain activity patterns associated with imagery and vision. We show that this relation has the formal structure of an echo that makes encoding of imagined stimuli in low-level visual areas resemble the encoding of seen stimuli in higher areas. To test for evidence of this echo effect we developed imagery encoding models--a new tool for revealing how imagined stimuli are encoded in brain activity. We estimated imagery encoding models from brain activity measured with fMRI while human subjects imagined complex visual stimuli, and then compared these to visual encoding models estimated from a matched viewing experiment. Consistent with an echo effect, imagery encoding models in low-level visual areas exhibited decreased spatial frequency preference and larger, more foveal receptive fields, thus resembling visual encoding models in high-level visual areas where imagery and vision appeared to be almost interchangeable. Our findings support an interpretation of mental imagery as a predictive inference that is conditioned on activity in high-level visual cortex, and is related to vision through shared dependence on an internal model of the visual world.

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