Abstract
The assessment of task-independent functional connectivity (FC) after a lesion causing hemianopia remains an uncovered topic and represents a crucial point to better understand the neural basis of blindsight (i.e. unconscious visually triggered behavior) and visual awareness. In this light, we evaluated functional connectivity (FC) in 10 hemianopic patients and 10 healthy controls in a resting state paradigm. The main aim of this study is twofold: first of all we focused on the description and assessment of density and intensity of functional connectivity and network topology with and without a lesion affecting the visual pathway, and then we extracted and statistically compared network metrics, focusing on functional segregation, integration and specialization. Moreover, a study of 3-cycle triangles with prominent connectivity was conducted to analyze functional segregation calculated as the area of each triangle created connecting three neighboring nodes. To achieve these purposes we applied a graph theory-based approach, starting from Pearson correlation coefficients extracted from pairs of regions of interest. In these analyses we focused on the FC extracted by the whole brain as well as by four resting state networks: The Visual (VN), Salience (SN), Attention (AN) and Default Mode Network (DMN), to assess brain functional reorganization following the injury. The results showed a general decrease in density and intensity of functional connections, that leads to a less compact structure characterized by decrease in functional integration, segregation and in the number of interconnected hubs in both the Visual Network and the whole brain, despite an increase in long-range inter-modules connections (occipito-frontal connections). Indeed, the VN was the most affected network, characterized by a decrease in intra- and inter-network connections and by a less compact topology, with less interconnected nodes. Surprisingly, we observed a higher functional integration in the DMN and in the AN regardless of the lesion extent, that may indicate a functional reorganization of the brain following the injury, trying to compensate for the general reduced connectivity. Finally we observed an increase in functional specialization (lower between-network connectivity) and in inter-networks functional segregation, which is reflected in a less compact network topology, highly organized in functional clusters. These descriptive findings provide new insight on the spontaneous brain activity in hemianopic patients by showing an alteration in the intrinsic architecture of a large-scale brain system that goes beyond the impairment of a single RSN.
Highlights
Hemianopia is a visual field defect resulting in loss of vision in half of the visual field of both eyes, as consequence of a contralateral post-chiasmatic lesion of the visual pathways and occipital cortex
According to the specific lesion location and patients’ symptoms, we focused on the evaluation of intra- and inter-networks functional connectivity considering four main Resting State Networks (RSNs): the Visual Network (VN), directly damaged by the lesion and that determines the phenotype of our clinical population; the Salience (SN) and the Attentional network (AN) [42], as the injury
The Brain Connectivity Toolbox of Matlab was used to extract graph summary measures from adjacent unweighted undirected matrices created for each group, considering the whole brain as well as each RSN separately
Summary
Hemianopia is a visual field defect resulting in loss of vision in half of the visual field of both eyes, as consequence of a contralateral post-chiasmatic lesion of the visual pathways and occipital cortex. Some patients retain residual visual sensitivity without conscious perception in their otherwise blind hemifield This phenomenon has been termed “blindsight” that is, the ability to detect, localize or discriminate visual stimuli without being aware of them [3]. Over the past 30 years, task-related brain activity [4] and electrophysiological signals [5] have been recorded during visual stimulus presentation to the blind hemifield of these patients with the aim of exploring the neural mechanisms of unconscious vision. In these experiments, patients are asked to give a response even if they cannot consciously perceive anything in the blind visual field. Moving stimuli have often been used to study blindsight as visual motion has been shown to be a perceptual feature that can increase the probability of finding this phenomenon
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