Biology subjects

Ischebeck, A.

Publications and source records attributed to Ischebeck, A..

6 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↗

bioRxiv2023

High-resolution 7T fMRI reveals the visual sensory zone of the human claustrum

The claustrum is a thin subcortical gray matter structure located between the insula and the putamen. It has numerous bilateral connections with the cortex and is thought to play an important role in higher-level aspects of perception and cognition, with hypotheses including multisensory integration, attention and consciousness. The claustrums thin shape makes it difficult to investigate, leaving the hypothesis regarding its function largely untested. In the current study, we used high-resolution ultra-high field (7 Tesla) functional magnetic resonance imaging (fMRI) to measure claustrum activity in human participants, while they were presented with visual, auditory or audiovisual naturalistic stimuli. We found distinct visual responses in the claustrum at a spatial location that was consistent across participants, hemispheres and scanning sessions. This is the first study to demonstrate evoked sensory responses within the human claustrum. It opens the possibility for future noninvasive investigation of the claustrums role in sensory processing.

neuroscience↗

bioRxiv2022

Resting-state functional connectivity predictors of subjective visual Gestalt experience

Subjective perceptual experience is influenced not only by bottom-up sensory information and experience-based top-down processes, but also by an individuals current brain state. Specifically, a previous study found increased prestimulus insula and intraparietal sulcus (IPS) activity before participants perceived an illusory Gestalt (global) compared to the non-illusory (local) interpretation in a bistable stimulus. This study provided only a snapshot of the prestimulus brain state that favors an illusory interpretation. In the current study, we tested the hypothesis that the neural machinery that biases perception towards the illusory interpretation immediately before the stimulus onset, is also predictive of an individuals general tendency to perceive it, which remains stable over time. We examined individual differences in task-free functional connectivity of insula and IPS and related it to differences in the individuals duration of the two interpretations. We found stronger connectivity of the IPS with areas of the default mode and visual networks to predict shorter local perceptual phases, i.e., a faster switch to an illusory percept, but no equivalent results for the insula. Our findings suggest a crucial role of an IPS interaction with nodes of key intrinsic networks in forming a perceptual tendency towards illusory Gestalt perception.

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↗

bioRxiv2022

Increased insula activity precedes the formation of subjective Gestalt

The constructive nature of human perception sometimes leads us to perceiving rather complex impressions from simple sensory input. Bistable stimuli give us a rare opportunity to study the neural mechanisms behind this process. Such stimuli can be visually interpreted as simple or as more complex on the basis of the same sensory input. Previous studies demonstrated increased activity in the superior parietal cortex when participants perceived an illusory Gestalt impression compared to a simpler interpretation of individual elements. Here we tested whether activity related to the illusory Gestalt can be detected not only during, but also prior to it, by examining the slow fluctuations of resting-state-fMRI activity before the stimulus onset. We presented 31 participants with a bistable motion stimulus, which can be perceived either as four moving dot pairs (local) or two moving illusory squares (global). This allowed us to isolate the specific neural mechanisms that accompany the experience of an illusion under matched sensory input. fMRI was used to measure brain activity in a sparse event-related design. We observed stronger IPS and putamen responses to the stimulus when participants perceived the global interpretation compared to local, confirming the previously reported role of these areas in perceptual grouping. Most importantly, we also observed that the global stimulus interpretation was preceded by an increased activity of the bilateral dorsal insula, which is known to process saliency and gate information for conscious access. Our data suggest an important role of the dorsal insula in shaping an internally generated illusory Gestalt percept.

neuroscience↗