Visual neglect is a common neurological syndrome in which patients fail to acknowledge stimuli toward the side of space opposite to their unilateral lesion. This disability affects many aspects of their life. For example, after a right lesion, patients typically fail to eat the food located on the left side of their plate, or to shave or make up the left side of their face, and, in extreme cases, may no longer acknowledge the left side of their body as their own. In a common clinical test known as line cancellation, they fail to mark lines toward the left of the page (see figure 1A) despite often being able to detect isolated lines presented in their left visual field, demonstrating that they are not simply blind on that side.
Figure 1 A. Left items neglected in a cancellation task. B. Visual displays from the Driver et al. (1994) experiment. The cross indicates the fixation point. Left-neglect patients performed better for the bottom configuration than the top one, even though the gap to be detected was at the same retinal location. This pattern is consistent with left object-centered neglect. C. Visual receptive field of a typical monkey parietal neuron, for three different eye positions. The retinotopic position of the receptive field is invariant across eye positions but the gain of the response changes. (Adapted from Andersen, Issick, and Siegel 1985.)
This syndrome is observed primarily after unilateral lesions to the parieto-occipital junction, especially in the right hemisphere (Heilman, Watson, and Valenstein 1985; Bisiach 1996). Lesions to the right frontal cortex and to various subcortical sites can also trigger neglect-like symptoms, although with subtle differences from parietal neglect (Heilman et al. 1985; Guariglia et al. 1993). Here we concentrate on parietal neglect, as it is the most common form, and can be related to recent data on the parietal lobe from nonhuman primates.
Two major accounts have been proposed for neglect. Some theories posit a deficit in directing attention toward contralesional events (Posner et al. 1984; Kinsbourne 1987; see ATTENTION and ATTENTION IN THE HUMAN BRAIN). For instance, right parietal patients -- who suffer from left neglect -- tend to have particular difficulty in detecting stimuli in the left hemifield if their attention has previously been drawn to the right side (Posner et al. 1984). By contrast, many "preattentive" aspects of vision appear to be spared on the affected side (Driver, Baylis, and Rafal 1992; McGlinchey-Berroth et al. 1996; Mattingley, Davis, and Driver 1997).
Other accounts argue that the patient's lesion simply disrupts the neuronal coding of contralesional space, at relatively high levels of representation (Bisiach and Luzzatti 1978; Bisiach, Luzzati, and Perani 1979; Rizzolatti and Berti 1990; Halligan and Marshall 1991; Karnath, Schenkel, and Fischer 1991). This perspective has drawn support from the finding that even mental IMAGERY can be impaired in some left-neglect patients (Bisiach and Luzzatti 1978), such that they fail to report what would appear on their left when retrieving from memory the view of a familiar visual scene.
The dichotomy between attentional and representational accounts has recently been challenged by several authors using neural network models in which attentional and representational functions are interwoven (Mozer and Behrmann 1990; Cohen et al. 1994; Pouget and Sejnowski 1997a). This work suggests a compromise view, whereby neglect results from damage to cortical areas that are located at the interface between sensory and motor systems, and which are responsible for both the representation of the position of objects and the selective control of spatial action, that is to say, "attention."
Frames of reference: In principle, "left" neglect might refer to the left of the visual field, or the left of the head, or the trunk, or even of the surrounding environment. To determine the frame of reference for hemineglect, one can test patients in various postures, so that a stimulus location changes in one frame of reference while remaining constant in the others. For instance, one might test a patient looking straight ahead vs. with the gaze deviated twenty degrees to the right, while keeping all stimuli at the same position with respect to the RETINA. If neglect were purely retinotopic, these conditions should not differ, whereas if it were head- or body-centered, performance should change accordingly. Such experiments have typically revealed that neglect affects a mixture of frames of reference concurrently, rather than just one single frame. Thus, the probability that a patient will neglect a particular visual stimulus is typically a function of its position in various egocentric frames of reference, such as eye-, head- or trunk-centered, as well as showing influences from cues in the environment, for example, as regards the gravitational upright (Bisiach, Capitani, and Porta 1985; Ladavas 1987; Ladavas, Pesce, and Provincial, 1989; Calvanio, Petrone, and Levine 1987; Farah et al. 1990; Karnath et al. 1991; Behrmann and Moscovitch 1994).
A few experiments suggest that visual neglect can also be "object-centered," that is, patients tend to neglect the left side of an object regardless of its position or orientation (Driver et al. 1994; Tipper and Behrmann 1996). For example, Driver et al. (1994) devised a situation in which left-neglect patients could detect a gap in part of a triangle when this gap was perceived to be on the right side of an object, but missed the same gap when it was seen as belonging to the left side, even though it still fell at the same location relative to the patient (figure 1B). Such results seem consistent with the existence of object-centered representations in the parietal cortex.
Many other studies claim to have found evidence for object-centered neglect (Driver and Halligan 1991; Arguin and Bub 1993; Halligan and Marshall 1994), but as pointed out by Driver et al. (1994), their results could be explained instead by what we will call relative neglect in strictly egocentric coordinates (see also Kinsbourne 1987; Mozer and Behrmann 1990; Desimone and Duncan 1995; and Pouget and Sejnowski 1997a for variations on this idea). When confronted with two competing objects, patients may neglect the one farther to the left even if both fall in the right hemispace egocentrically, and likewise for the subparts of a single object (Driver and Halligan 1991; Driver et al. 1992; Driver et al. 1994; Halligan and Marshall 1994). Thus, it appears that the relative position of objects or their subparts is just as important as their absolute position with respect to the patient. This phenomenon can be explained if the lesion induces a gradient of neglect with increasing severity in the egocentric contralesional direction (Kinsbourne 1987; Driver et al. 1994; Pouget and Sejnowski 1997a).
Neural basis: There have been several attempts to relate neglect to what is known of the response properties of parietal neurons from single-cell recordings in monkeys (Mozer and Behrmann 1990; Duhamel et al. 1992; Anderson 1996; Mozer, Halligan, and Marshall 1997; Pouget and Sejnowski 1997a; see also MODELING NEUROPSYCHOLOGICAL DEFICITS and SPATIAL PERCEPTION). Such models generally rely on cells in the parietal cortex having retinotopic receptive fields, with each hemisphere tending to overrepresent the contralateral visual field (see, however, Duhamel et al. 1992 for a different approach). Consequently, a right lesion leads to a neuronal gradient in which the left side of the retina is less strongly represented than the right side, producing left neglect. In such models, there is no particular dividing midline such that any stimulus to the left of it is invariably neglected. Instead, neglect depends only on the relative position of competing stimuli, as discussed above, with objects or object parts that are farther toward the retinal left than their competitors being neglected. These models readily capture the behavior of patients in tasks such as line bisection, line cancellation, and in some of the paradigms discussed above that have revealed relative neglect.
Parietal neurons, however, do not simply respond to visual stimulation, but also integrate sensory responses with posture signals such as eye and head position. Andersen and colleagues have shown that the retinotopic receptive fields of parietal cells are gain-modulated by such posture signals (Andersen, Essick, and Siegel 1985; Andersen et al. 1997; see figure 1C for an example in which the visual receptive field of a cell is modulated by eye position). These response properties can be modeled as basis functions of the inputs, a type of function which is particularly well-suited to the computational demand of sensorimotor transformations (Pouget and Sejnowski 1997b).
A simulated unilateral lesion in such a basis-function representation produces an impairment that resembles clinical neglect, in that the deficit affects a mixture of egocentric frames of reference as found in patients (Pouget and Sejnowski 1997a). This approach can also be generalized to encompass object-centered neglect, as in the Driver et al. (1994) experiment depicted in figure 1B, by considering the perceived orientation of the object as providing a signal analogous to the posture signals integrated by the basis functions (Deneve and Pouget in press). This basis-function framework can explain why neglect may be influenced by stimulus position relative to the retina, head, body, other objects, and other parts of the same object, all at the same time, without requiring cells in the parietal cortex to have visual receptive fields explicitly defined in any single one of these frames of reference.
Neglect remains a fascinating but disabling disorder, which still poses a major challenge to rehabilitation. Its further study will hopefully lead to more effective treatments, as well as reveal more about how the brain represents space, and allows for selective spatial attention.
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