Peter W. Halligan

VISUOSPATIAL NEGLECT: UNDERLYING FACTORS AND TEST SENSITIVITY

Visuospatial neglect, a frequent consequence of unilateral (usually right-hemisphere) stroke, is associated with poor functional recovery and in many patients is resistant to remedial treatment. Studies of the nature and prevalence of the disorder have been hindered by problems of definition and assessment. In this study 80 unselected stroke patients were assessed for the presence and severity of neglect on the behavioural inattention test. The six subtests of this battery all intercorrelated highly, and a subsequent factor analysis showed that all tests loaded significantly on one underlying factor. The construct of neglect as defined by performance on the battery is therefore robust. Nonetheless, the individual tests differed substantially in their sensitivity. Star cancellation was the most sensitive measure of neglect and correctly diagnosed all patients whose aggregate score on the full battery fell below that of the control population.

Individual variation in line bisection: A study of normal subjects with application to the interpretation of visual neglect

Investigations of left visuo-spatial neglect are reviewed with special reference to line bisection performance. Attention is then drawn to inconsistencies in the direction and magnitude of transection displacements in group studies of normal controls. We argue that the lack of reliable information about normal mechanisms for line bisection makes it impossible to interpret pathological performance in neglect. Accordingly, we report a case-series of 22 normal young adults, each of whom bisects 10 lines of differing lengths 10 times each. There is very substantial between-subject variation in both the magnitude and direction of the linear regression of transection displacement on line length; there are likewise considerable differences in the magnitude of the linear regression of standard deviation on line length. These two sources of individual variation are uncorrelated. We propose a psychophysical theory of line bisection, and suggest that the basic mechanisms responsible for task-performance are qualitatively intact in visuo-spatial neglect, albeit quantitatively impaired.

Hemispheric activation vs spatio-motor cueing in visual neglect: A case study

We report a case of mild visuo-spatial neglect consequent upon right-hemisphere stroke. At the time of testing, the patient had a complete left visual field deficit but only a very slight left hemiparesis. Under conventional testing conditions, line bisection performed with the right hand showed more severe left neglect than when performed with the left hand. This pattern of performance could, however, be modified, both quantitatively and qualitatively, by changing the starting position of the patients hand when bisecting horizontal lines. The results suggest that spatio-motor cueing has a more profound effect upon task performance than does differential hemispheric activation per se. We also provide a demonstration that, in a normal subject, the starting position of the hand is likewise a crucial determinant of task performance. In this case, however, there is also an interaction between the hand (and hence hemisphere) deployed and the position of that hand in space.

Hemispheric asymmetries in global/local processing are modulated by perceptual salience

It has been claimed that a left hemisphere bias toward local and right hemisphere bias toward global visual processing can be explained in terms of specialization for relatively high and low spatial frequencies, respectively. Using non-representational figures, we tested this hypothesis in experiment 1 using positron emission tomography (PET) measures of cerebral activity in 10 normal volunteers. In each block of trials subjects viewed either a relatively high or a relatively low spatial frequency grating. The orientation (vertical or horizontal) of the grating changed from trial to trial. In a directed attention task, subjects reported the orientation of either the whole stimulus (globally directed attention) or the orientation of the component parts thereof (locally directed attention). A significant interaction between hierarchical processing level (global or local) and stimulus level (relatively high or relatively low spatial frequency within the absolute low spatial frequency range) was found. Globally directed attention led to significantly increased cerebral activity in the right hemisphere when relatively high spatial frequency stimuli were used but not when relatively low spatial frequency stimuli were used. Likewise, locally directed attention increased cerebral activity when low but not high spatial frequency stimuli were used. On the basis of these results we suggest that perceptual salience of the global or local form modulates hemispheric processing asymmetries in early visual cortex. In experiment 2, the perceptual salience of global form in relatively high spatial frequency stimuli and of local form in relatively low spatial frequency stimuli was confirmed in a reaction time (RT) study. In combination, the results of the two experiments suggest that perceptual salience takes precedence over spatial frequency (within the range studied here) in determining the cerebral organization of global/local processing.

Current practice and clinical relevance of somatosensory assessment after stroke

Objective: To examine the perceived clinical relevance, current practice and knowledge of somatosensory testing in three professional groups involved in the management and rehabilitation of stroke. Design: Structured postal questionnaire sent to therapists and doctors. Subjects: One hundred and eighty occupational therapists from the National Association of Neurological Occupational Therapists (NANOT), 180 physiotherapists from the Association of Chartered Physiotherapists with an Interest in Neurology (ACPIN) and 360 doctors from the Association of British Neurologists (ABN) and the British Geriatrics Society (BSG). Setting: Hospitals and rehabilitation centres in the UK. Results: Replies were obtained from 84 occupational therapists (47%), 98 physiotherapists (55%) and 78 doctors (22%). Sixty-four occupational therapists (77%), 82 physiotherapists (84%) and 66 doctors (87%) indicated that they routinely performed somatosensory assessment as part of their clinical assessment. The two most commonly used measures were proprioception and light touch. Seventy-eight occupational therapists (93%), 88 physiotherapists (90%) and 67 doctors (91%) regarded sensory assessment as clinically significant in determining prognosis after stroke. Conclusions: Despite published reservations concerning its usefulness and reliability, most therapists and doctors routinely assess somatosensory loss after stroke. All three professions agreed that somatosensory testing presented useful information for prognosis. Furthermore, all professional groups believe somatosensory assessment to be clinically relevant.

Individual variation in line bisection: A study of four patients with right hemisphere damage and normal controls

In a previous study of line bisection, [Manning et al., Neuropsychologia 28, 647-655, 1990] we have shown that normal young adults show significant linear regressions of both transection displacement (TD) and standard deviation (SD) on line length. Furthermore, there was considerable individual variation in the magnitude of SDs and the direction and magnitude of TDs. These two sources of variance were uncorrelated. In the current study we provide a constructive replication of these findings on an older population that constitutes a more appropriate control sample for patients with stroke. The performance of four patients with right hemisphere stroke is then interpreted in terms of the constraints observed in normal subjects. All the patients showed qualitatively intact line bisection, but with quantitative impairments of varied severity. When analysed in two pairs, the patients also showed a double-dissociation between the magnitude of their mean transection displacements and the SDs of those means. This result is in conformity with the prediction based upon a lack of correlation between these two measures in normal subjects.

Line bisection in a case of visual neglect: Psychophysical studies with implications for theory

Abstract We report a study of line bisection in a patient with left hemiplegia and a dense left homonymous hemianopia. The patient, PS, sustained a subarachnoid haemorrhage at the bifurcation of the basilar artery; right fronto-temporal craniotomy was performed with clipping of the neck of the basilar aneurysm. The only neuropsychological sequela of note was florid left neglect. In a series of six experiments, we show that the magnitude of PSs transection displacements is linearly related to the length of the stimulus line, and that rightwards displacements reliably cross over to leftwards displacements at lengths shorter than 25mm. This crossover is due neither to visuo-motor inco-ordination nor to range effects from the presence of longer lines within the experimental set. We also stress the increased standard deviations that characterise PSs performance on the line bisection task, and show that these measures of intra-subject variability are likewise strongly correlated with stimulus length. A new th...

Laterality of motor response in visuo-spatial neglect: a case study

We report a case of severe visuo-spatial neglect consequent upon right hemisphere stroke. At time of testing, the patient had a left visual field deficit and a very mild left hemiparesis. Star cancellation and line bisection performed with the right hand revealed profound left neglect; tested with the left hand, performance was only slightly impaired. The results are compatible with recent accounts of differential hemispheric activation provoked by lateralized pre-motor programming.

Within- and Between-Task Dissociations in Visuo-Spatial Neglect: A Case Study

We describe a patient, J.B., who suffered an infarction of the right internal capsule and thalamus. There was a pronounced left hemiplegia but no visual field deficit. Investigated at four months and four years post-stroke, there were no signs of left visuo-spatial neglect other than a severe impairment of line bisection. The results on a test of perceptual judgement of the left and right extents of pretransected lines showed mild neglect of the left. The magnitude of this deficit did not predict quantitatively the severity of the patients rightward deviation on a manual bisection task. Additional data are also reported on perceptual judgements and (manual) bisections of radially-oriented lines.

Perceptual cueing and perceptuo-motor compatibility in visuo-spatial neglect: A single case study

Abstract A patient with severe left neglect consequent upon a large right temporoparietal infarct is described. We report her performance on line bisection with computerised visual display unit (VDU) presentation. In this format, the patient makes her transections by moving a “mouse”-controlled cursor arrow. The technique allows cueing (that is, the initial positioning of the cursor at either the left or right end of the stimulus line) to be locked to the task under analysis (line bisection). The effects of starting position and of perceptuo-motor compatibility upon the magnitude and direction of displacements from the true mid-point are compared and contrasted. Left-start cueing effects a significant reduction in neglect; stimulus-response incompatibility precipitates only minimal qualitative or quantitative changes.