Left Unilateral Neglect as a Disconnection Syndrome

Abstract

Unilateral spatial neglect is a disabling neurological condition that typically results from right hemisphere damage. Neglect patients are unable to take into account information coming from the left side of space. The study of neglect is important for understanding the brain mechanisms of spatial cognition, but its anatomical correlates are currently the object of intense debate. We propose a reappraisal of the contribution of disconnection factors to the pathophysiology of neglect based on a review of animal and patient studies. These indicate that damage to the long-range white matter pathways connecting parietal and frontal areas within the right hemisphere may constitute a crucial antecedent of neglect. Thus, neglect would not result from the dysfunction of a single cortical region but from the disruption of large networks made up of distant cortical regions. In this perspective, we also reexamined the possible contribution to neglect of interhemispheric disconnection. The reviewed evidence, often present in previous studies but frequently overlooked, is consistent with the existence of distributed cortical networks for orienting of attention in the normal brain, has implications for theories of neglect and normal spatial processing, opens perspectives for research on brain-behavior relationships, and suggests new possibilities for patient diagnosis and rehabilitation.

Causal effect of disconnection lesions on interhemispheric functional connectivity in rhesus monkeys

“…Data at each vertex were normalized, and the mean activity over the entire surface was regressed out at each vertex. Data were then summarized by one time series of activity for each of 56 Brodmann areas (28 in each hemisphere), defined as the mean of all voxels' time series within that region. The Brodmann regions were defined using the Brodmann macaque atlas (36), transcribed into the space of the F99 brain template (37), and implemented in CARET (Computerized Anatomical Reconstruction and Editing Toolkit) (38).…”

“…The concept of disconnection has long been important in neurology (50)(51)(52), and several of the major neurological syndromes, including aphasia (50,53), amnesia (54), and neglect (55,56), can result from white matter damage rather than damage to the cortical gray matter. In that context, restingstate functional connectivity could provide a useful tool for determining the nature of the functional network retained after white matter damage, for example, in stroke, and targeting therapies accordingly (57,58).…”

Normal and Impaired Reflexive Orienting of Attention after Central Nonpredictive Cues

“…Note that our study did not aim at investigating the neuroanatomy of neglect, which is still a highly controversial issue (e.g., Bartolomeo, Thiebaut de Schotten, & Doricchi, 2007;Mort et al, 2003;Karnath, Ferber, & Himmelbach, 2001). Lesions were reconstructed for each patient (see Methods section of Experiment 2) on axial brain slices into a standard T1 MRI template (Rorden & Brett, 2000).…”

Structural Variability within Frontoparietal Networks and Individual Differences in Attentional Functions: An Approach Using the Theory of Visual Attention

“…Functional and structural neuroimaging studies show that visual attention is contingent upon large and distributed neuronal networks consisting of several cortical areas subdivided into functionally distinct dorsal and ventral components (Mesulam, 1990;Corbetta and Shulman, 2002;Szczepanski et al, 2013) interconnected by long-association frontoparietal pathways, including the superior longitudinal fasciculus (SLF) and the inferior fronto-occipital fasciculus (IFOF) (Bartolomeo et al, 2007;Schmahmann et al, 2007;Doricchi et al, 2008;Thiebaut de Schotten et al, 2011). A recent study used diffusion imaging and tractography to reconstruct three separate branches of the SLF (SLF I subserving the dorsal attention network, SLF II interconnecting the dorsal and ventral networks, and SLF III subserving the ventral attention network) and showed that variations in SLF II correlated with greater biases in both line bisection and in target detection (Thiebaut de Schotten et al, 2011).…”