Complex Graphemes as Functional Spelling Units: Evidence from Acquired Dysgraphia

The visual word recognition literature suggests that complex graphemes (or digraphs) such as CK function as units. This proposal has also been put forward in recent spelling models (Houghton and Zorzi, 2003) and the study we report on here provides initial empirical support for the claim. We performed detailed analyses of the spelling performance of two brain-damaged individuals with graphemic buffer deficits. Results revealed that (a) FM and BWN made fewer errors on consonant digraphs (e.g., CK) than on matched controls clusters (e.g., CR) and (b) BWN produced more transposition errors on vowel digraphs than on control clusters. These results support the view that digraphs are represented as units in which the relative order of constituent letters is encoded.     

Monocular patching affects inattention but not perseveration in spatial neglect

Monocular patching might improve perceptual-attentional, not motor-intentional deficits in a patient with chronic post-stroke left spatial neglect. Performing a line-cancellation task, his omission errors were associated with a perceptual-attentional ‘where’ deficit, while perseverative errors were associated with ‘aiming’ motor-intentional bias. Contralesional patching had no effect on the omissions (p?=?.871), whereas ipsilesional patching reduced left-sided omissions compared with the unpatched condition (p?=?.016). Neither patching condition altered perseverative errors. Further research is needed to examine whether targeting treatments to spatial neglect symptoms (omissions, perseveration) results in improved outcomes.     

病觉缺失的研究进展

一、背景资料脑损伤后患者不能意识到自身的缺陷即为病觉缺失.文献对此较早已有记载,如19世纪后叶就有关于皮质盲、纯词聋、偏瘫患者否认其疾病的报道.     

Horizontal space misrepresentation in unilateral brain damage. I. Visual and proprioceptive-motor influences in left unilateral neglect

Patients with unilateral neglect can misperceive horizontal distances in the contralesional space as being shorter than equivalent ipsilesional ones. We evaluated the visual and motor-proprioceptive components of space misrepresentation through a distance reproduction task performed both with and without visual guidance. Four groups of right brain damaged patients (neglect with hemianopia (N+H+), neglect with inferior quandrantanopia (N+Q+), neglect without hemianopia (N+H-) and patients without neglect or hemianopia (N-H-)) and one group of healthy controls (C) performed the line bisection task and reproduced horizontal distances either by setting the endpoints or by doubling the length of a line in the contralesional or ipsilesional space. The doubling length task was administered in three different conditions: (a) visuomotor (the patient draws the line in free vision); (b) visual (by sight the patient guides the examiner drawing the line); (c) proprioceptive-motor (the patient is blindfolded and manually inspects and extends the horizontal distance subtended by the line). Compared to C and N-H- patients, only N+H+ patients exhibited a significant ipsilesional shift in line bisection. N+H+ patients showed the most severe contralesional-overextension/ipsilesional-underextension asymmetry in the endpoint, visuomotor and visual line extension task. In the proprioceptive-motor condition no asymmetry was found and N+H- showed greater overextension on both sides of space. In N+H-, brain damage was mainly centered in central-frontal cortex and basal ganglia. These findings re-emphasize the relevance of damage to visual retinotopically organized representations of space in the genesis of horizontal space misrepresentation of neglect patients and suggest the possible association of a non-lateralized defective processing of proprioceptive-motor information with unilateral neglect.     

Impaired perception of temporal order in auditory extinction

It has been proposed that patients with extinction show a chronic bias of spatial attention towards the ipsilesional side. In this case, the law of 'prior entry' predicts that ipsilesional events should be perceived earlier than physically synchronous contralesional stimuli. In line with this prediction, previous studies have revealed substantial delays of awareness for contralesional visual and tactile events in patients with visual and with tactile extinction. The present study provides evidence that a 'prior entry' bias also occurs in the auditory modality. Patients with auditory extinction perceived two acoustic events (one presented to the left ear, the other to the right ear) as being 'simultaneous' when the contralesional sound was leading by 270 ms. The magnitude of this asynchrony was quite similar to that measured previously in the visual modality. Thus, the pathological delay of awareness for contralesional events may be independent of the sensory modality of the stimuli.     

Disorders of visuospatial orientation in the frontal plane in patients with visual neglect following right or left parietal lesions

Current models of spatial neglect focus on deficits in the patients’ horizontal or midsagittal plane. However, other evidence suggests that patients with temporoparietal lesions centered on the parieto-insular-vestibular cortex show disturbed spatial perception of the subjective visual vertical and oblique orientation discrimination in another spatial plane, the frontal plane. As the relationship between neglect and spatial orientation deficits is unclear, we examined how patients with and without visual neglect perform visuospatial tasks in the roll plane and how their performance is related to neglect. Thirteen patients with predominantly right parietal lesions and left-sided neglect, 14 control patients without neglect after right-hemispheric cerebral lesions (RBD-controls), 11 patients without neglect after left-hemispheric lesions (LBD-controls), 3 patients with right-sided neglect after left parietal lesions, and 12 normal subjects were investigated. Constant errors and difference thresholds were measured with a PC-based system when the subjects had to adjust a luminous line to their subjective visual vertical, subjective visual horizontal, and in relation to an obliquely oriented reference line. Subjects were oriented with their head and body earth-vertical while sitting in a chair in total darkness. Patients with left-sided as well as those with right-sided neglect showed a significant, in most cases contraversive, tilt of the three spatial orientations (about 5° counterclockwise in the left neglect group and 5.5°–8.5° clockwise in the right neglect group). In contrast, the two patient groups without neglect as well as the normal subjects showed nearly perfect visuospatial judgements with constant errors of less than 0.8°. Difference thresholds were significantly elevated in patients with left neglect and in two of three patients with right-sided neglect, whereas normal control subjects and both control patient groups without neglect performed indistinguishably, having thresholds of one-tenth of those of the neglect patients. Tilt of all three spatial axes was significantly related to the severity of neglect (mean r for unsigned errors, 0.74; for difference thresholds, 0.40), indicating a significant contribution to the symptomatology of left and right spatial neglect. These results indicate a close although not necessarily causal link between spatial orientation deficits in the frontal plane and hemispatial neglect in patients with left or right parietal lesions, surpassing the well-documented impairments of these patients in the horizontal plane. Received: 1 August 1997 / Accepted: 1 April 1998     

Simulating and testing visual exploration in spatial neglect based on a new model for cortical coordinate transformation

Most studies of object and space perception have focused on neural representations of either object-centered or egocentric coordinate systems. But daily life requires interactions of both kinds of coordinates. We have recently proposed an 'integrated space-object (ISO-) map' combining both coordinate systems in one representation. Based on a lesioned version of this model, here we present results from visual search simulations demonstrating that the model accounts for contralesional neglect during space-centered and object-centered exploration tasks. Interestingly, the model simulations also predicted an amelioration of neglect symptoms during exploration with more ipsilesional object positions. By measuring the eye movements of neglect patients during different exploratory tasks, we confirmed all model predictions. These results corroborate the view that the brain might combine coordinates for object and space perception in an integrated coordinate system as suggested by the ISO-map model.     

The effect of ipsilesional cues on line-bisection errors: the importance of predictive value

Patients with unilateral neglect were tested on a line-bisection task in which vertical lines were added to the ipsilesional or contralesional end. During some blocks, these vertical lines accurately predicted the horizontal length of the line, while on other blocks the vertical lines had little predictive value. We found that much of the improvement for ipsilesional vertical bars can be explained by their ability to predict the extent of the horizontal line. These results suggest that the predictive value of a ipsilesional cue is responsible for the reduction in line-bisection errors noted by Halligan and Marshall [Neuropsychologia 32 (1994) 13]. This effect of the predictive ipsilesional cue may work in combination with the expanded attentional effects posited by the ipsilesional contraction model that they proposed.     

Strain differences in recovery from sensorimotor neglect after unilateral globus pallidus lesions in mice

Recovery of somatosensory orientation after unilateral electrolytic lesions of globus pallidus was studied in two inbred strains of mice (C57BL/6, Sec 1/ReJ). During the first four post-lesion days mice of both strains did not react to tactile stimuli (von Frey hair of 2 g intensity) applied to the thorax area contralateral to the lesion. Successively, there was a gradual replacement by non-oriented activation and incomplete oriented responses, followed by normal precise orientation. Recovery of orientation proceeded at a significantly higher rate in C57 mice. After a period of four months C57 mice reached a recovery level of 91% in comparison to 56% of Sec mice. The genotype dependent differences of recovery mechanisms are discussed in terms of plasticity and dopaminergic systems.     

Influence of the radial and vertical dimensions on lateral neglect

The influence of radial (near-far) and vertical (upper-lower) dimensions on lateral visuo-spatial neglect was studied using two horizontal line-bisection tasks (one motor and one perceptual). A group of 15 patients with neglect and a group of 14 right-brain damaged patients without neglect were examined. This latter group was used to define the range of variability in line-bisection performance that was independent of neglect. For the radial dimension, some neglect patients showed greater errors in far space than in near space (for both stimuli presented in the upper and lower space). Fewer patients showed the opposite pattern (i.e., greater errors for near-space stimuli). These near-far asymmetries were present for both the motor and perceptual conditions and showed a good degree of intra-individual consistency. This finding contradicts the hypothesis that the motor component is critical for yielding such asymmetry. For the vertical dimension, the results indicated that neglect patients make more bisection errors for lower-space stimuli than for upper-space stimuli. This vertical asymmetry was nearly always confined to stimuli in near space. Asymmetries along the vertical dimension were present for both perceptual and motor conditions, although intra-individual consistency was low. When perceptual and motor conditions were directly compared, several neglect patients showed greater errors in the perceptual than in the motor task.