Biology subjects

Arsenovic, A.

Publications and source records attributed to Arsenovic, A..

3 recordsLinked to original sources

bioRxiv2023

Distributed illusory figure-ground segmentation signatures across the dorsal and ventral streams

Visual illusions give rise to percepts that are more complex than the incoming sensory information, thereby exposing the inferential nature of perception. Early visual cortex (EVC) can represent not only physical input, but also illusory content of perception. Specifically, much like in real shape perception, during perception of an illusory shape the topographic representation of the shapes surface is enhanced, while the regions surrounding it are suppressed, reflecting the figure-ground segmentation mechanisms on the neural level. It remains unclear whether such topographically specific figure-ground segmentation signatures are present in higher topographic maps along the visual hierarchy. To test this, we measured brain activity of 30 healthy human participants using functional magnetic resonance imaging (fMRI). The participants performed a central fixation task while passively observing a surrounding configuration of four inducers, which either formed or did not form an illusory shape. Using an additional functional localizer scan we could identify voxels uniquely representing the illusory surface and the background in the EVC, posterior parietal topographic region IPS0/V7 and several other maps along the dorsal and ventral streams. Apart from the EVC, we found signatures of figure-ground segmentation in several topographic maps. Illusory surface enhancement was observed in IPS0/V7, but without background suppression. Ventral topographic maps VO1 and VO2 showed background suppression only. Finally, other extrastriate areas, V3a, V3b and hV4, displayed both surface enhancement and background suppression. Our results demonstrate that topographically specific illusory shape responses are distributed along multiple topographic maps beyond the EVC. Furthermore, they point to the difference in neural signatures of figure-ground segmentation at various hierarchical levels and along different processing streams, stressing the independence of the surface enhancement and background suppression mechanisms.

neuroscience↗

bioRxiv2022

Representation of illusory shapes within the topographic areas of the posterior parietal cortex

The human visual system consists of multiple topographic maps that extend from the early visual cortex along the dorsal and ventral processing streams. Responses to illusory shapes within these maps have been demonstrated in the ventral stream areas, in particular the lateral occipital complex. Recently, the intraparietal sulcus of the dorsal stream has been linked to the processing of illusory shapes defined by motion. It therefore remains unclear whether the topographically organized parietal areas also respond to static illusory shapes, which would suggest their generic role in representing illusory content. Here we measured brain responses using fMRI while human participants observed flickering inducers around the fixation task. The inducers either formed an illusory diamond in the center, a triangle in the left or in the right hemifield, or were inverted such that no illusory figure was formed. We compared responses of parietal regions IPS0-IPS5 and SPL1 to each illusory figure with the non-illusory condition. To determine the role of attention in illusory shape responses we manipulated the difficulty of the fixation task. Our results show that all IPS areas responded to illusory shapes. The more posterior areas IPS0-IPS3 additionally displayed a preference towards the contralateral shapes, while the more anterior areas IPS4 and IPS5 showed response attenuation with increased task difficulty. We suggest that the IPS can represent illusory content irrespective of the perceptual mechanism that generated it. These responses may serve as a potential feedback signal that drives illusory shape responses in early and ventral visual areas. Significance statementThe traditional view of the ventral visual pathway being solely responsible for representation of objects has recently been challenged by demonstrating illusory shape representation within the dorsal visual pathway with moving bistable stimuli. Our results provide evidence for the dorsal stream contribution to representing not only moving, but also static illusory shapes. Our results also show a functional subdivision along the topographic maps, with spatially specific shape responses in the more posterior, and attention-dependent responses in the more anterior areas. IPS areas of the dorsal stream should thus be considered in the theoretical accounts and neural models of how subjective content is generated in the brain.

neuroscience↗

bioRxiv2022

Fast and functionally specific cortical thickness changes induced by visual stimulation

Structural characteristics of the human brain serve as important markers of brain development, aging, disease progression and neural plasticity. They are considered stable properties, changing slowly over time. Multiple recent studies reported that structural brain changes measured with MRI may occur much faster than previously thought, within hours or even minutes. The mechanisms behind such fast changes remain unclear, with hemodynamics as one possible explanation. Here we investigated the functional specificity of cortical thickness changes induced by a flickering checkerboard and compared the them to BOLD fMRI activity. We found that checkerboard stimulation led to a significant thickness increase, which was driven by an expansion at the gray-white matter boundary, functionally specific to V1, confined to the retinotopic representation of the checkerboard stimulus, and amounted to 1.3 % or 0.022 mm. Although functional specificity and the effect size of these changes were comparable to those of the BOLD signal in V1, thickness effects were substantially weaker in V3. Furthermore, a comparison of predicted and measured thickness changes for different stimulus timings suggested a slow increase of thickness over time, speaking against a hemodynamic explanation. Altogether, our findings suggest that visual stimulation can induce structural gray matter enlargement measurable with MRI.

neuroscience↗