DISEÑO DEL PROCESO DE GESTIÓN DE INCIDENTES

Unilateral spatial neglect is a quite common and disabling neurological syndrome following unilateral brain damage. Even though lesions to different areas may cause neglect, it is most often associated with right inferior parietal damage (Vallar et al., 1994). Patients suffering from neglect may lack

awareness for sensory events in the contralesional side o f space (i.e. towards the left after right damage) and often fail to orient their attention towards that side, even when no other deficits in the primary sensory pathways are present. In daily life their behaviour can be striking. They may orient and talk only to people standing on their right (i.e. the ipsilesional side) and ignore everybody and everything else on their left side. Their daily routine actions can also be affected, as patients with neglect may eat from only the right side of the plate, dress only the right side of their body, make up or shave only the right side of the face, even read from only the right side of a newspaper page.

On clinical examination, their spatial bias towards the ipsilesional side of space is apparent in several tests. On paper-and-pencil cancellation tests, where the patient is asked to search for and mark targets among distractors evenly distributed on a sheet of paper, they typically cancel only targets in the ipsilesional side of the space. Similarly, when required to mark the centre o f a line they tend to shift the bisection point towards the ipsilesional side. Even

when drawing from memory or copying pictures, neglect patients usually miss details from the contralesional side.

As mentioned earlier, spatial neglect can emerge after various unilateral lesions, but is most common and long-lasting after lesions involving the right inferior parietal lobe, in the angular and supramarginal gyri (i.e. Brodmann areas 39 and 40). Certain subcortical and frontal areas (Vallar, 1993; Vallar & Perani, 1986; Ratal & Posner, 1987; Husain and Kennard, 1997) can also be associated with neglect. Damage to the white-matter beneath the parieto- temporo-occipital junction (Leibovicth et al., 1998) may also be important. In sum, damage within an extended network of brain areas involving subcortical, frontal, cingulate and superior temporal structures, but most crucially the inferior parietal lobe, anatomically underpins the spatial bias of neglect patients.

Interestingly, a very similar neural system involving the right anterior

cingulate gyrus (Brodmann area 24), the intraparietal sulcus of right posterior parietal cortex (Brodmann areas 39 and 40), and the mesial and lateral

premotor cortices (Brodmann area 6) has been described by Nobre et al. (1997) as underlying visuospatial attention in normals based on functional imaging, providing further evidence for an attentional component in neglect. In fact, it has been suggested that one reason why neglect patients ignore the left-side information is because their attention is pathologically focused on the right-side of space, or cannot be disengaged from that side (e.g. Humphreys & Riddoch, 1993; Posner et al., 1984). The view of neglect as involving an inattentional deficit is not universally agreed (e.g. see Bisiach & Berti, 1987;

Bisiach, Luzzatti & Perani, 1979), but does provide a functional account for some aspects of the syndrome. For example, “extinction” during double stimulation can be observed in neglect patients, especially those with focal parietal lesions (see Driver et al., 1997). It is thought to be a pathological exaggeration of a phenomenon found also in healthy neurological people, namely a difficulty in distributing attention to multiple targets concurrently (Duncan, 1980). Patients with extinction can detect a single event in isolation even when it happens in the “bad” side of the space (i.e. left), but they will miss an event on this side when presented together with another concurrent one on the right side, showing that their spatial deficit emerges mainly in competitive situations. Neglect patients also have difficulties in switching attention from one location to another and tend to lock their attention onto local details of a configuration (e.g. Marshall and Halligan, 1995).

The spatial bias in neglect is ascribed not only to locations, but also to

segmented objects and, particularly relevant to the aim of this chapter, to faces (and face components). Neglect patients can fail to report or recognise the left­ side of a face or an object made by two different halves (i.e. chimeric stimuli), even when accurately traced. Nevertheless, some residual processing o f the neglected side can still take place and may influence the conscious processing of the perceived one (e.g. see Young et al., 1992; Vallar et al., 1995; Peru et a l, 1997).

A wide category of perceptual processing, most of which have been described as “preattentive” in healthy neurologically healthy people, can still take place

for stimuli on the affected side in many neglect patients, but unconsciously. These include preservation of certain perceptual illusions (Mattingley et ah, 1997; Ro & Ratal, 1996), figure-ground segregation (Driver et al., 1992), perception of symmetry (see Driver, 1999), and even semantic priming from the identity of a neglected or extinguished object (Berti & Rizzolatti, 1992; Berti et al., 1994; McGlinchery-Beroth et al., 1993). Recent patient studies have also shown that perceptual grouping can affect extinction, again suggesting that preattentive segmentation processes can be preserved (see Driver, 1996). Despite escaping awareness, contralesional neglected or extinguished stimuli can thus apparently still be processed by early vision, with some preserved unconscious processing of feature analysis, such as extraction of colour and shape, and even some activation of semantic

responses. It is established that in neglect some residual processing may take place unconsciously.

The present chapter addresses such issues specifically for gaze perception,

which is of clear importance for social and attentional function in daily life (see previous chapters), yet has been little studied in neglect patients to date. Recently, it has been put forward that the “ventral” visual pathway (e.i. occipital projection to temporal areas) may be responsible for the preserved unconscious perception in neglect patients, accounting for the implicit processing described above. Those areas, normally spared in neglect and extinction patients with their more “dorsal” lesions (Driver & Vuilleumier, in press) are also thought to be involved in gaze perception and face processing based on recent evidence (e.g. Campbell et al, 1990; Hoffman and Haxby,

2000). Since natural gaze stimuli comprise of two halves (i.e. left and right eyes), they can easily be transformed into a chimeric stimulus (see Chapter 6, Experiment 14) and thus employed to study processing of gaze stimuli in neglect. In addition as a social salient stimulus, gaze is particularly suitable for investigating whether or not social attention is impaired in neglect patients, along with their general deficit in spatial attention.

Previous work shows that normal perception of gaze direction is best when

two eyes are seen, rather than one (Ehrlich & Field, 1993; see also Chapter 6). But it is unknown how the deficit in patients with neglect and extinction might affect gaze perception. For instance, does gaze perception benefit from

preserved “grouping” together of the two eyes to convey gaze direction, despite the two eyes being concurrent stimuli and, thus, potential competitors for attention? Is the advantage in perceiving gaze direction that has been shown for two eyes (i.e. the “Ehrlich” effect; see Chapter 6, Experiment 14) still present in neglect? If the leftmost of the two eyes is extinguished from awareness in the patient, is there any unconscious residual processing still taking place implicitly and possibly influencing the perception o f the right eye?

The present chapter examined whether a right-hemisphere damaged patient with left neglect and extinction might be unaware o f the leftmost of two presented eyes, and yet still show some implicit effects from that eye upon gaze perception (see Fig. 7.1).

’'f'-t '

Fig. 7.1. Examples o f “incongruent” bilateral gaze stimuli (a,b) plus bilaterally deviated gaze

(e,d). N ote that the right eye in (a) looks straighter than the same right eye in (b), at least for

normal observers (see Chapter 6, Experiment 14).