Orienting of attention in left unilateral neglect
Introduction
Left unilateral neglect is a severe disabling condition resulting from right-hemisphere damage, usually centered on the inferior parietal lobule [1] or on the superior temporal lobe [2]. Neglect patients ignore events occurring on their left (Fig. 1), sometimes to the dramatic extent of ‘forgetting’ to eat from the left part of their dish or of bumping into obstacles situated on their left.
Unilateral neglect negatively affects patients’ motor recovery [3] and social rehabilitation. Deficits at different levels of impairment may be at work in different patients; however, the frequency and severity of attentional problems in neglect patients have been repeatedly underlined [4]. A precise characterization of these deficits can shed light on how brain mechanisms process spatial information, and is necessary in order to devise rational strategies of rehabilitation. Recent advances in the knowledge of the mechanisms of spatial attention in normals may help to characterize these deficits. The present review focuses on studies which explore several aspects of attentional processing in unilateral neglect, with particular reference to the dichotomy between ‘exogenous’, or stimulus-related, and ‘endogenous’, or strategy-driven, orienting of attention. With reference to this distinction, Gainotti suggested that a basic mechanism leading to left neglect behavior might be a relatively selective deficit of exogenous orienting towards left-sided targets [5], [6]. A large amount of neuropsychological evidence is now available which is consistent with this proposal. Moreover, a number of studies have shown that endogenous processes are relatively preserved, if slowed, in left unilateral neglect. The present review aims at discussing these results and at exploring the implications of such a model of left neglect for hemispheric specialization for attentional orienting and for the mechanisms of visual consciousness.
Section snippets
Varieties of attention
The concept of attention refers to a heterogeneous set of processes that aim to maintain coherent behavior in the face of irrelevant distractions. James [7] observed that “my experience is what I agree to attend to… Without selective interest, experience is an utter chaos” (p. 402). In a recent review, Parasuraman [8] identified at least three independent but interacting components of attention: (1) selection, that is, mechanisms determining more extensive processing of some input rather than
Accounts of left unilateral neglect
Even a superficial consideration of left neglect behavior immediately suggests that these patients have problems in orienting their attention toward the left side. However, a number of different hypotheses have been proposed to explain left neglect and there is no consensus about its causal mechanisms [50]. In a putative ‘sensation-to-action’ sequence [4], accounts of left neglect can be schematically summarized as a visual sensory deficit [51], an amputation [52] or distortion [53] of a mental
A rightward attentional bias in left neglect
A well-articulated account of neglect based on orienting of attention is the opponent processor model [57], [76], [77], [78]. This model draws upon the very general biological evidence that reciprocally inhibiting opponent systems are an evolutionary advantageous way of solving the problem of deciding whether to turn right or left. The dominant system would achieve its goal of turning the organism by progressively inhibiting its contralateral counterpart. A first assumption of the opponent
Impaired exogenous orienting in unilateral neglect: implications for the neural implementation of attentional mechanisms
The proposal that left unilateral neglect primarily results from an asymmetrical exogenous orienting has implications for both the interhemispheric and the intrahemispheric implementation of attentional mechanisms. The importance for attentional orienting of brain regions in the parietal [144], [145] and frontal [80], [146] lobes has long been known. Gottlieb et al. [147] have recently identified neurons in the macaque lateral intraparietal area which selectively respond to visual stimuli with
Impaired exogenous orienting in unilateral neglect: implications for phenomenal consciousness
As mentioned in Section 2.5, attention is thought to be crucial for binding together the different features (color, location, etc.) of an object [44]; in particular, exogenous orienting influences feature detection [49] and binding [35]. Failure to exogenously orient one's attention toward an object may thus result in incomplete feature processing, which in turn may cause either a complete lack of awareness of the object (whose perception is too inconsistent to be attributed to a specific
References (174)
Lateralization of brain mechanisms underlying automatic and controlled forms of spatial orienting of attention
Neurosci Biobehav Rev
(1996)- et al.
Sustained and transient components of focal visual attention
Vision Res
(1989) - et al.
Preparatory attention: experiment and theory
Conscious Cogn
(2000) Inhibition of return
Trends Cogn Sci
(2000)- et al.
The effect of visual attention on peripheral discrimination thresholds in single and multiple element displays
Acta Psychol
(1988) - et al.
Preattentive object files: shapeless bundles of basic features
Vision Res
(1997) - et al.
A feature-integration theory of attention
Cogn Psychol
(1980) The binding problem
Curr Opin Neurobiol
(1996)- et al.
The reviewing of object files: object-specific integration of information
Cogn Psychol
(1992) - et al.
Perceiving visually presented objects: recognition, awareness, and modularity
Curr Opin Neurobiol
(1998)