Mechanisms of unilateral spatial neglect in copying a single object

Clock drawing in spatial neglect: a comprehensive analysis of clock perimeter, placement, and accuracy

Clock drawings produced by right-brain-damaged (RBD) individuals with spatial neglect often contain an abundance of empty space on the left while numbers and hands are placed on the right. However, the clock perimeter is rarely compromised in neglect patients' drawings. By analysing clock drawings produced by 71 RBD and 40 healthy adults, this study investigated whether the geometric characteristics of the clock perimeter reveal novel insights to understanding spatial neglect. Neglect participants drew smaller clocks than either healthy or non-neglect RBD participants. While healthy participants' clock perimeter was close to circular, RBD participants drew radially extended ellipses. The mechanisms for these phenomena were investigated by examining the relation between clock-drawing characteristics and performance on six subtests of the Behavioral Inattention Test (BIT). The findings indicated that the clock shape was independent of any BIT subtest or the drawing placement on the test sheet and that the clock size was significantly predicted by one BIT subtest: the poorer the figure and shape copying, the smaller the clock perimeter. Further analyses revealed that in all participants, clocks decreased in size as they were placed farther from the centre of the paper. However, even when neglect participants placed their clocks towards the centre of the page, they were smaller than those produced by healthy or non-neglect RBD participants. These results suggest a neglect-specific reduction in the subjectively available workspace for graphic production from memory, consistent with the hypothesis that neglect patients are impaired in the ability to enlarge the attentional aperture.

Understanding the parietal lobe syndrome from a neurophysiological and evolutionary perspective

In human and nonhuman primates parietal cortex is formed by a multiplicity of areas. For those of the superior parietal lobule (SPL) there exists a certain homology between man and macaques. As a consequence, optic ataxia, a disturbed visual control of hand reaching, has similar features in man and monkeys. Establishing such correspondence has proven difficult for the areas of the inferior parietal lobule (IPL). This difficulty depends on many factors. First, no physiological information is available in man on the dynamic properties of cells in the IPL. Second, the number of IPL areas identified in the monkey is paradoxically higher than that so far described in man, although this issue will probably be reconsidered in future years, thanks to comparative imaging studies. Third, the consequences of parietal lesions in monkeys do not always match those observed in humans. This is another paradox if one considers that, in certain cases, the functional properties of neurons in the monkey's IPL would predict the presence of behavioral skills, such as construction capacity, that however do not seem to emerge in the wild. Therefore, constructional apraxia, which is well characterized in man, has never been described in monkeys and apes. Finally, only certain aspects, i.e. hand directional hypokinesia and gaze apraxia (Balint's psychic paralysis of gaze), of the multifaceted syndrome hemispatial neglect have been described in monkeys. These similarities, differences and paradoxes, among many others, make the study of the evolution and function of parietal cortex a challenging case.

Adaptation to leftward-shifting prisms reduces the global processing bias of healthy individuals

When healthy individuals are presented with peripheral figures in which small letters are arranged to form a large letter, they are faster to identify the global- than the local-level information, and have difficulty ignoring global information when identifying the local level. The global reaction time (RT) advantage and global interference effect imply preferential processing of global-level information in the normal brain. This contrasts with the local processing bias demonstrated following lesions to the right temporo-parietal junction (TPJ), such as those that lead to hemispatial neglect (neglect). Recent research from our lab demonstrated that visuo-motor adaptation to rightward-shifting prisms, which ameliorates many leftward performance deficits of neglect patients, improved the local processing bias of patients with right TPJ lesions (Bultitude, Rafal, & List, 2009). Here we demonstrate that adaptation to leftward-shifting prisms, which can induce neglect-like performance in neurologically healthy individuals, also reduces the normal global processing bias. Forty-eight healthy participants were asked to identify the global or local forms of hierarchical figures before and after adaptation to leftward- or rightward-shifting prisms. Prior to prism adaptation, both groups had greater difficulty ignoring irrelevant global information when identifying the local level (global interference) compared to their ability to ignore irrelevant local-level information when identifying the global level (local interference). Participants who adapted to leftward-shifting prisms showed a significant reduction in global interference, but there was no change in the performance of the rightward-shifting Prism Group. These results show, for the first time, that in addition to previously demonstrated effects on lateralised attention, prism adaptation can influence non-lateralised spatial attention in healthy individuals.

Prism adaptation reverses the local processing bias in patients with right temporo-parietal junction lesions

Lesions to the right temporo-parietal cortex commonly result in hemispatial neglect. Lesions to the same area are also associated with hyperattention to local details of a scene and difficulty perceiving the global structure. This local processing bias is an important factor contributing to neglect and may contribute to the higher prevalence of the disorder following right compared with left hemisphere strokes. In recent years, visuomotor adaptation to rightward-shifting prisms has been introduced as a promising treatment for hemispatial neglect. Explanations for these improvements have generally described a leftward realignment of attention, however, the present investigation provides evidence that prism adaptation reduces the local processing bias. Five patients with right temporal-parietal junction lesions were asked to identify the global or local levels of hierarchical figures before and after visuomotor adaptation to rightward-shifting prisms. Prior to prism adaptation the patients had difficulty ignoring the local elements when identifying the global component. Following prism adaptation, however, this pattern was reversed, with greater global interference during local level identification. The results suggest that prism adaptation may improve non-spatially lateralized deficits that contribute to the neglect syndrome.

Visual neglect and cognitive impairment in elderly patients late after stroke

OBJECTIVES

To investigate the prevalence of visual neglect and its relationship to cognitive impairment and dementia in elderly stroke patients 20 months after stroke.

METHODS

Of 243 stroke patients aged 70-91 years, 149 underwent neuropsychiatric testing, 138 with the Star Cancellation Test for visual neglect.

RESULTS

Fifteen per cent (n = 21) had visual neglect, 9% (n = 12) severe and 9% (n = 12) lateralized. Cognitive impairments were twice as common in patients with neglect as in patients without it and threefold more common when the neglect was severe. Neglect was related to apraxia, impaired visual field, and a previous stroke. Severe neglect was related to dementia, amnesia and impaired visual field.

CONCLUSIONS

Chronic neglect is common after stroke in elderly patients. So are cognitive impairments, especially in those with neglect. This calls for high alertness to impairments in spatial attention when treating patients with dementia and other cognitive impairments.

The contribution of spatial remapping impairments to unilateral visual neglect

Left visual neglect following right hemisphere damage is a heterogeneous phenomenon, in which several underlying impairments have been identified. Despite recent advances in understanding the neural and cognitive bases of these impairments, current theories of neglect, particularly those that emphasise attentional deficits, do not explain a number of phenomena, including: 'Ipsilesional' neglect after left orienting. Positive or 'productive' manifestations. Spatial transposition errors. Mislocalisations. Revisiting behaviour during visual search. Lack of awareness for objects toward the contralesional side of space. We propose that these manifestations of neglect can be accounted for by an additional underlying disorder of spatial remapping due to parietal dysfunction. In primary visual areas, retinotopic maps are renewed and thus overwritten at each new ocular fixation. Remapping processes operating in higher-level oculocentric visual maps of the parietal cortex ensure visual integration of these successive retinal images over time and space, by creating a constantly updated representation of stimulus locations in terms of distance and direction from the fovea. They consist in the storage, refreshment and re-localization of the different components of the visual scene that are successively attended during its exploration, and provide spatial constancy of visual perception and a spatial buffer for working memory [Cereb Cortex 5 (1995) 470; Visual Cogn 7 (2000) 17]. We begin this article by reviewing theoretical and experimental arguments that have highlighted the importance of parietal remapping processes in maintaining an accurate representation of space across saccadic shifts. We then focus on findings from the double-step saccade task, [Ann Neurol 38 (1995) 739] as a basis for our model of the role of remapping impairments in many of the symptoms of neglect. From these results, remapping impairments would be demonstrated when a saccade has to be guided across the midline after having fixated an object in either the left or right visual field for patients with either left- or right-side parietal lesions. In addition, patients with right-side lesions will have remapping impairments within the left visual field following a saccade to a left-side target (see Fig. 5). In a large part of the article, we seek to build our hypothesis based on this basic model and more speculative assumptions supported with extensive evidence from the literature.

Spatial cognition: evidence from visual neglect

Recent work on human attention and representational systems has benefited from a growing interplay between research on normal attention and neuropsychological disorders such as visual neglect. Research over the past 30 years has convincingly shown that, far from being a unitary condition, neglect is a protean disorder whose symptoms can selectively affect different sensory modalities, cognitive processes, spatial domains and coordinate systems. These clinical findings, together with those of functional neuroimaging, have increased knowledge about the anatomical and functional architecture of normal subsystems involved in spatial cognition. We provide a selective overview of how recent investigations of visual neglect are beginning to elucidate the underlying structure of spatial processes and mental representations.

Significance of paradoxical leftward error of line bisection in left unilateral spatial neglect

We intended to determine if leftward error of line bisection means "right" or "left" neglect in a patient with left unilateral spatial neglect. The patient placed the subjective midpoint to the left of the true center when bisecting lines without cueing. By contrast, when cued to the left endpoint, he showed typical rightward errors that became greater as longer lines were presented. Cueing to the right endpoint increased leftward errors compared with the bisections without cueing. The results suggest that paradoxical leftward error of line bisection is a form of "left" unilateral spatial neglect in that the shortness of the left extent is ignored.

Why are some patients with severe neglect able to copy a cube? The significance of verbal intelligence

Cube-copying is often used to assess constructional ability of brain-damaged patients and the influence of unilateral spatial neglect is often pointed out in patients with right hemisphere lesions. However, some patients with severe neglect perform cube-copying satisfactorily. The aim of the present study is to identify the factors that affect the performance of cube-copying in patients with left unilateral spatial neglect. Constructional performance was investigated in 100 patients with unilateral spatial neglect using a task to copy the Necker cube. The relationship of the patients' cube-copying performance to the severity of their neglect, as well as other factors (verbal intelligence, age, duration after onset of the disease, educational level, lesion site, piecemeal approach, and side of starting to copy) was analyzed. Twenty-two normal subjects also participated in this study as controls. Among many factors adopted for analysis, neglect severity and verbal intelligence were found to be primary factors affecting the cube-copying performance of the patients with unilateral spatial neglect. The effect of neglect severity on cube-copying performance was apparent in the patients whose verbal intelligence was deteriorated, but was not observed in the patients with preserved verbal intelligence. Similarly, the effect of verbal intelligence on cube-copying performance was apparent in the patients with severe neglect, but not in the patients with mild neglect. We conclude that constructional ability in the copying of a cube is determined by verbal intelligence, as well as by the severity of unilateral spatial neglect.

Neglect-associated constructional disorders: a paradoxical phenomenon?

Five neglect patients without diffuse cognitive impairment or overt constructional disabilities were asked to bisect lines and rectangles and to copy rectangles bisected in their midplane. As a group, patients showed the usual rightward bias in bisecting lines and a milder deviation in bisecting horizontally-aligned rectangles, but showed a leftward deviation of the subjective midline in the copying task. This was due to drawing the left half shorter with respect to normal controls but three patients also drew the right half longer (the total length was the same as that of controls). A possible interpretation of rectangle copying results in these three patients is that they could create a representation of the stimulus to be copied accurately enough to reproduce its total length correctly but the subjective distribution of right and left space within that representation was unbalanced. However, specific experimental work is needed to verify why our patients with mild to moderate unilateral spatial neglect overrepresented the left side in a line bisection task and underrepresented it in a copying task.

Disappearance of unilateral spatial neglect following a simple instruction

OBJECTIVES

To clarify the reason why patients with left unilateral spatial neglect fail to copy the left side of a daisy like flower, not continuing to draw petals all around.

METHODS

A flower was simplified and a figure was made that consisted of a large central circle and small circles surrounding it. Four patients with typical left unilateral spatial neglect performed copying and arrangement tasks to make this figure. In the arrangement task, they were instructed to arrange small circles all around the printed central circle.

RESULTS

The patients' identification of the composition seemed flawless. In the copying task, they showed neglect, leaving a space on the left side. They seemed to adhere to their plan to place the same number of small circles as those of the model figure. By contrast, neglect disappeared in the arrangement task.

CONCLUSION

Patients with neglect can draw the figure satisfactorily if they use a spatial strategy to arrange small circles all around. This strategy seems to improve motivation for drawing and awareness for the left space. It is considered that in the copying of figures such as a daisy, failure to use a spatial strategy plays an important part in the appearance of left unilateral spatial neglect.