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Maastricht, Netherlands

Sack A.T.,Maastricht University | Sack A.T.,Maastricht Brain Imaging Center
Restorative Neurology and Neuroscience

Visuospatial processing refers to the spatial perception, recognition and analysis of visual input. Human functional brain imaging studies have consistently revealed the involvement of fronto-parietal brain areas during the execution of visuospatial tasks. Just as the execution of these tasks activates fronto-parietal regions in the healthy brain, lesions to those structures, e.g. after stroke or brain injury, cause specific spatial deficits. The most prominent of these is known as spatial neglect. There are several competing theories on the neural mechanisms underlying spatial neglect. Although each of these theories postulates different underlying physiological mechanisms, they all account in their own way for the fact that the prevalence of neglect is much higher following right hemisphere lesions. This makes it difficult to distinguish between the different models at a behavioural level. Until today, it was impossible to empirically address these matters and to provide direct and conclusive empirical evidence in favour of one of the competing theories of spatial neglect. This review article describes the neural correlates of intact visuospatial processing as revealed by non-invasive functional brain imaging studies. It subsequently focuses on the approach of using the non-invasive brain inference technique of transcranial magnetic brain stimulation (TMS) to transiently and reversibly disrupt neural activity in these visuospatial processing-related brain regions. Using this approach, we can now imitate specific spatial deficits and neglect-like symptoms in healthy volunteers. Mimicking and manipulating the spatial deficits following unilateral brain lesions, under controlled experimental conditions, may allow for the development of new therapeutic interventions for parietal stroke patients suffering from real spatial neglect. The perspective is to use non-invasive brain interference to guide and promote functional recovery on a brain-system level in stroke and neglect patients, based on knowledge directly derived from fundamental brain research in healthy volunteers. © 2010 IOS Press and the authors. All rights reserved. Source

Luckmann H.C.,Maastricht University | Jacobs H.I.L.,Julich Research Center | Sack A.T.,Maastricht University | Sack A.T.,Maastricht Brain Imaging Center
Progress in Neurobiology

Neuroimaging studies have repeatedly reported findings of activation in frontoparietal regions that largely overlap across various cognitive functions. Part of this frontoparietal activation has been interpreted as reflecting attentional mechanisms that can adaptively be directed towards external stimulation as well as internal representations (internal attention), thereby generating the experience of distinct cognitive functions. Nevertheless, findings of material- and task-specific activation in frontal and parietal regions challenge this internal attention hypothesis and have been used to support more modular hypotheses of cognitive function. The aim of this review is twofold: First, it discusses evidence in support of the concept of internal attention and the so-called dorsal attention network (DAN) as its neural source with respect to three cognitive functions (working memory, episodic retrieval, and mental imagery). While DAN activation in all three functions has been separately linked to internal attention, a comprehensive and integrative review has so far been lacking. Second, the review examines findings of material- and process-specific activation within frontoparietal regions, arguing that these results are well compatible with the internal attention account of frontoparietal activation. A new model of cognition is presented, proposing that supposedly different cognitive concepts actually rely on similar attentional network dynamics to maintain, reactivate and newly create internal representations of stimuli in various modalities. Attentional as well as representational mechanisms are assigned to frontal and parietal regions, positing that some regions are implicated in the allocation of attentional resources to perceptual or internal representations, but others are involved in the representational processes themselves. © 2014 Elsevier Ltd. Source

Schiltz C.,University of Luxembourg | Dricot L.,Catholic University of Louvain | Goebel R.,Maastricht Brain Imaging Center | Goebel R.,Fc Donders Center For Cognitive Neuroimaging | Rossion B.,Catholic University of Louvain
Journal of Vision

The perception of a facial feature (e.g., the eyes) is influenced by the position and identity of other features (e.g., the mouth) supporting an integrated, or holistic, representation of individual faces in the human brain. Here we used an event-related adaptation paradigm in functional magnetic resonance imaging (fMRI) to clarify the regions representing faces holistically across the whole brain. In each trial, observers performed the same/different task on top halves (aligned or misaligned) of two faces presented sequentially. For each face pair, the identity of top and bottom parts could be both identical, both different, or different only for the bottom half. The latter manipulation resulted in a composite face illusion, i.e., the erroneous perception of identical top parts as being different, only for aligned faces. Release from adaptation in this condition was found in two sub-areas of the right middle fusiform gyrus responding preferentially to faces, including the "fusiform face area" ("FFA"). There were no significant effects in homologous regions of the left hemisphere or in the inferior occipital cortex. Altogether, these observations indicate that face-sensitive populations of neurons in the right middle fusiform gyrus are optimally tuned to represent individual exemplars of faces holistically. © ARVO. Source

de Graaf T.A.,Maastricht University | de Graaf T.A.,Maastricht Brain Imaging Center | Gross J.,University of Glasgow | Paterson G.,University of Glasgow | And 5 more authors.

Oscillations are an important aspect of neuronal activity. Interestingly, oscillatory patterns are also observed in behaviour, such as in visual performance measures after the presentation of a brief sensory event in the visual or another modality. These oscillations in visual performance cycle at the typical frequencies of brain rhythms, suggesting that perception may be closely linked to brain oscillations. We here investigated this link for a prominent rhythm of the visual system (the alpha-rhythm, 8-12 Hz) by applying rhythmic visual stimulation at alpha-frequency (10.6 Hz), known to lead to a resonance response in visual areas, and testing its effects on subsequent visual target discrimination. Our data show that rhythmic visual stimulation at 10.6 Hz: 1) has specific behavioral consequences, relative to stimulation at control frequencies (3.9 Hz, 7.1 Hz, 14.2 Hz), and 2) leads to alpha-band oscillations in visual performance measures, that 3) correlate in precise frequency across individuals with resting alpha-rhythms recorded over parieto-occipital areas. The most parsimonious explanation for these three findings is entrainment (phase-locking) of ongoing perceptually relevant alpha-band brain oscillations by rhythmic sensory events. These findings are in line with occipital alpha-oscillations underlying periodicity in visual performance, and suggest that rhythmic stimulation at frequencies of intrinsic brain-rhythms can be used to reveal influences of these rhythms on task performance to study their functional roles. © 2013 de Graaf et al. Source

Jacobs C.,University of Westminster | Jacobs C.,Maastricht University | Jacobs C.,Maastricht Brain Imaging Center | de Graaf T.A.,Maastricht University | And 3 more authors.

Neuroscience research has conventionally focused on how the brain processes sensory information, after the information has been received. Recently, increased interest focuses on how the state of the brain upon receiving inputs determines and biases their subsequent processing and interpretation. Here, we investigated such 'pre-stimulus' brain mechanisms and their relevance for objective and subjective visual processing. Using non-invasive focal brain stimulation [transcranial magnetic stimulation(TMS)] we disrupted spontaneous brain state activity within early visual cortex (EVC) before onset of visual stimulation, at two different pre-stimulus-onset-asynchronies (pSOAs). We found that TMS pulses applied to EVC at either 20msec or 50msec before onset of a simple orientation stimulus both prevented this stimulus from reaching visual awareness. Interestingly, only the TMS-induced visual suppression following TMS at a pSOA of -20msec was retinotopically specific, while TMS at a pSOA of -50msec was not. In a second experiment, we used more complex symbolic arrow stimuli, and found TMS-induced suppression only when disrupting EVC at a pSOA of~-60msec, which, in line with Experiment 1, was not retinotopically specific. Despite this topographic unspecificity of the -50msec effect, the additional control measurements as well as tracking and removal of eye blinks, suggested that also this effect was not the result of an unspecific artifact, and thus neural in origin. We therefore obtained evidence of two distinct neural mechanisms taking place in EVC, both determining whether or not subsequent visual inputs are successfully processed by the human visual system. © 2014 Elsevier Ltd. Source

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