Neglect and Extinction

In this essay we will be looking at what is Neglect and Extinction and why it is said to cause impairments of the selective attention? We will also look at different aspects of the research done in this area. First of all we would like to define what is unilateral and selective attention before we go any further.

Unilateral neglect is a disorder associated primarily with right-brain damage; it causes individuals to behave as if the contralesional half of their world has become unimportant or has simply ceased to exist.

Extinction is

On the other hand Selective attention is defined as the capacity for or process of reacting to certain stimuli selectively when several occur simultaneously. We will look at how both of these inter-relate and evaluate studies done in this area of neuropsychology.

Neglect is thought to be the result of parietal lobe lesions, but it is now known that lesions outside the parietal cortex and even purely subcortical lesions correlated with neglect symptoms. However, lesions in the right inferior-posterior parietal cortex are common in neglect. This area is often damaged in neglect patients, and spared in unaffected patients. Patients with lesions in superior parietal cortex usually do not show full neglect, but may show problems in visually guided reaching.

Extinction is an attentional disorder following unilateral damage (mostly in the right parietal lesions). Extinction patients may well be able to report single stimuli presented in the left or the right hemifield, but when stimuli are presented simultaneously in both hemifields, they will report only the ipsilesional stimulus. This phenomenon of extinction has been related to a competition for attention between simultaneously delivered stimuli, which is biased against stimuli in the contralesional field. Extinction can be reduced by instructing patients to actively ignore ipsilesional stimuli Di Pellegrino & De Renzi, 1995, thereby presumably biasing attentional competition in favour of contralesional stimuli. It is yet undecided whether extinction has to be regarded as a mild form of neglect or as an entirely different attentional disorder (see Driver, 1998).

Visual neglect is seen in patients with unilateral brain damage, often in the right parietal cortex. These patients may fail to respond and act upon stimuli on the side of space contralateral to the lesion. A patient with a right-hemisphere lesion may fail to eat food on the left side of the plate, draw only the right side of an object, address only people standing to the right of them, etc. When asked to bisect horizontal lines or cancel all lines on a page, they will likely put their marks to the right of the midpoint. Such measures of neglect may however show dissociations - patients may show neglect on line cancellation, but perform almost normally for line bisection (Halligan & Marshall, 1992, Cortex). Several lines of evidence show that neglect is not a defect of basic visual processing. Patients will losses in half of their visual field (hemianopia) often do not show neglect, and neglect patients do not necessarily show hemianopia (that is, they may be able to detect visual flashes presented in the affected hemifield). A famous study by Bisiach & Luzzati (1978, Cortex 26) also suggested that neglect was attentional rather than sensory in nature. They asked right-hemisphere damaged patients to describe the Piazza del Duomo in Milan, first while imagining to look towards the cathedral, and then while imagining standing on the cathedral steps. These patients always left out the buildings to the left of their imagined perspective. They omitted different sides of the Piazza depending on their imagined viewpoint! Obviously, impaired sensory functions cannot explain this result. Bisiach & Luzzati argued that neglect is an attentional failure to construct internal representation of one side of space.

Spatial cueing studies of neglect shows additional evidence supporting the hypothesis that neglect is an attentional disorder which comes from spatial cueing experiments. Posner et al. 1984 tested six patients with right-hemispheric damage that showed neglect symptoms (extinction or complete inattention to stimuli in the left hemifield). They had to detect targets appearing in a box at the left or right side. A brightening of one of these boxes served as precue, and the cue was valid on 80% of all trials. Cue-target interval was varied (0, 150, 550, or 1000 ms). When the cue was valid, RTs were very similar for targets presented in the ipsilesional and contralesional visual field. In invalid trials, however, RTs were dramatically slowed when the target appeared in the neglected (contralesional) hemifield. With short cue-target intervals, many of the contralesional uncued targets were not detected at all (see Posner et al., 1982, PhilTransRSocLond, 298, for similar results)! How can this pattern of results be explained? Posner et al. distinguish three components of visual-spatial attention: (i) the ability to engage attention to a location; (ii) the ability to disengage it; and (iii) the ability to shift attention between locations. Since there were only small RT differences between validly cued targets at ipsilesional and contralesional sites, they concluded that the engagement of attention at contralesional locations was not affected in neglect. Since effects of cue-target interval on RTs were similar for validly cued ipsi- and contralesional targets, moving attention to the contralesional side in response to the cue seemed also unaffected. Performance deficits with contralesional targets only became obvious when patients were previously cued to the opposite side. Therefore Posner et al. concluded that neglect patients have difficulty in disengaging attention from an ipsilesional cued location in order to detect targets contralaterally.

Is this 'disengagement problem'