Discrimination and integration of motion direction requires the interplay of multiple brain areas. Theoretical accounts of perception suggest that stimulus-related (i.e., exogenous) and decision-related (i.e., endogenous) factors affect distributed neuronal processing at different levels of the visual hierarchy. To test these predictions, we measured brain activity of healthy participants during a motion discrimination task, using electroencephalography (EEG) and functional magnetic resonance imaging (fMRI). We independently modeled the impact of exogenous factors (task demand) and endogenous factors (perceptual decision-making) on the activity of the motion discrimination network and applied Dynamic Causal Modeling (DCM) to both modalities. DCM for event-related potentials (DCM-ERP) revealed that task demand impacted the reciprocal connections between the primary visual cortex (V1) and medial temporal areas (V5). With practice, higher visual areas were increasingly involved, as revealed by DCM-fMRI. Perceptual decision-making modulated higher levels (e.g., V5-to-Frontal Eye Fields, FEF), in a manner predictive of performance. Our data suggest that lower levels of the visual network support early, feature-based selection of responses, especially when learning strategies have not been implemented. In contrast, perceptual decision-making operates at higher levels of the visual hierarchy by integrating sensory information with the internal state of the subject.
Stroke has become one of the main causes of visual impairment, with more than 15 million incidences of first-time strokes, per year, worldwide. One-third of stroke survivors exhibit visual impairment, and most of them will not fully recover. Some recovery is possible, but this usually happens in the first few weeks after a stroke. Most of the rehabilitation options that are offered to patients are compensatory, such as optical aids or eye training. However, these techniques do not seem to provide a sufficient amount of improvement transferable to everyday life. Based on the relatively recent idea that the visual system can actually recover from a chronic lesion, visual retraining protocols have emerged, sometimes even in combination with noninvasive brain stimulation (NIBS), to further boost plastic changes in the residual visual tracts and network. The present article reviews the underlying mechanisms supporting visual retraining and describes the first clinical trials that applied NIBS combined with visual retraining. As a further perspective, it gathers the scientific evidence demonstrating the relevance of interregional functional synchronization of brain networks for visual field recovery, especially the causal role of α and γ oscillations in parieto-occipital regions. Because transcranial alternating current stimulation (tACS) can induce frequency-specific entrainment and modulate spike timing–dependent plasticity, we present a new promising interventional approach, consisting of applying physiologically motivated tACS protocols based on multifocal cross-frequency brain stimulation of the visuoattentional network for visual field recovery.
Introduction: Understanding the causal relationship between a focal lesion and brain network (re)organization is a crucial step in order to accurately predict the resulting symptoms of the lesion and implement personalized rehabilitation strategies. Transcranial magnetic stimulation (TMS) can be used to create local and transient neural perturbations, the so-called virtual lesion approach. In this study, we tested how a virtual lesion applied to the Early Visual Areas (EVA) or the extrastriate, motion-sensitive, medio-temporal area (hMT+/V5) in different contexts affects behavior and changes neuronal network activity and organisation, assessed with functional magnetic resonance imaging (fMRI). Methods: We applied short trains of 10 Hz TMS to healthy participants over the EVA or hMT+/MT. This was done both at rest and at relevant times during a motion discrimination task, while concurrent fMRI was performed. Regional BOLD activity was analysed using a general linear model (GLM) and functional brain networks were assessed using independent component analysis (ICA). Motion direction discrimination and motion awareness were related to the imaging data. Results: TMS applied over the EVA and hMT+/V5 induced transient modulation of motion perception and discrimination. Comparing resting versus active states, both TMS sites showed a common suppression of local and remote brain activity at rest while an over-activation of the stimulated areas and related networks were found during the task. More subtly, distinct dynamic and topological TMS-induced networks properties could be revealed depending on the exact visual processing stages TMS was applied at. In particular, brain networks associated with EVA stimulation showed a clear context-dependency and were spatially more restricted than for hMT+/V5 stimulation. Discussion: The present findings highlight the possibility of interfering with distinct visual processing stages and the possibility of imaging the local and remote neural correlates of the behavioral impact. They also confirm the complexity of TMS effects on BOLD activity, switching from signal suppression at rest to irrelevant neural noise addition during active visual processing, even differing throughout the time course of information processing. Moreover, the networks analyses suggest that the EVA might be more resilient to focal perturbations by means of a virtual lesion than hMT+/V5. As a critical processing center, the EVA may be important for maintaining stability in the visual network, whereas the perturbation of a more specialized region such as hMT+/V5 has greater impact on local and network activity, as well as on behavior. These findings add to the understanding of visual motion processing and especially to the impact and potential mechanism of focal lesions in this system.
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