Which part of the brain is damaged in left side neglect

VI. HEMISPHERIC ASYMMETRY

A characteristic feature of hemispatial neglect is its asymmetry. Contralesional left neglect is common, whereas contralesional right neglect is rare, even in lefthanders. This asymmetry has led to the hypothesis that the left hemisphere directs attention predominantly to the contralateral right side, whereas the right hemisphere directs attention to both sides of space. According to this model, unilateral left hemisphere lesions are not expected to cause much contralesional neglect as the ipsiversive attentional functions of the right hemisphere are likely to take over. Right hemisphere lesions, however, are expected to trigger severe left neglect because the left hemisphere has no substantial ipsiversive attentional functions. Experiments based on evoked potentials, transcranial magnetic stimulation, and functional imaging have supported this model (see Mesulam, 1999, for review). Another prediction of this model, that naturalistic settings where attention is symmetrically distributed to both sides of space would cause a greater engagement of the right hemisphere, has received preliminary support (Gitelman et al., 2002).

One of the most unexpected implications to emerge from functional imaging experiments is the possibility that different aspects of spatial attention may be associated with different patterns of asymmetry. For example, rightward asymmetry appears more pronounced in the parietal lobe for exogenously triggered covert shifts of attention and in the FEFs for visual search (Gitelman et al., 2002). Spatial attention and hemispatial neglect offer model systems for investigating the distinctively human phenomenon of functional hemispheric asymmetry. Understanding the principles of hemispheric specialization may have extensive implications for exploring factors that facilitate (or hinder) compensatory cerebral reorganization following focal brain injury.

Neglect Dyslexia

In this type of dyslexia the patient misreads the initial (left neglect) or terminal (right neglect) parts of words. The errors are not simply “deletions” of the letter string, but typically consist of the production of an alternative real word of approximately the same length as the target. The clearest examples come from patients with left neglect (see Table 10.2). These errors can occur in reading single words in isolation, in reading columns of words, or in reading prose passages. In some cases the disorder may be so severe that normal reading is completely vitiated.

Table 10.2. Examples of Left-Neglect Dyslexia Errors Made by Three Patientsa

The first suggestion that there was a subtype of reading impairment which differentially affected one side of a word was reported in a brief single case report. Warrington & Zangwill (1957) noted that their patient (T.S.) had a marked “dyslexia without dysgraphia” which had different characteristics from spelling dyslexia. He had a tendency to misread the final parts of words, producing alternative real words (e.g., breaking → breakfast; registrar → registration; tongue → together).

A series of six patients who made reading errors on the beginning of words were reported by Kinsbourne & Warrington (1962a). They frequently misread individual words, irrespective of whether they were presented in isolation, in columns, or in a connected passage. The majority of the patients errors implicated the beginnings of words and consisted of substitutions of letters or groups of letters. The last few letters were almost invariably incorporated into the patients’ response. In virtually every error, these letter substitutions changed the stimulus into an alternative real word. However, it was not related in meaning to the target.

Subsequent quantitative accounts of neglect dyslexia have confirmed and extended these clinical case reports. In a large corpus of reading responses reported in a single case (J.A.F.) it was observed that real-word neglect errors occurred on approximately one-quarter of the items which she attempted (Baxter & Warrington, 1983). There was a remarkable correspondence between the number of letters in the target word and the “length” of the response over a wide range of word lengths (see Table 10.2).

Ellis, Flude, & Young (1987) described a further single case in considerable detail (V.B.). They also showed that her errors were mostly substitutions of alternative letters rather than additions of letters or simple letter deletions (see Table 10.2). Thus the patient read elate as plate and not late and peach as beach, not each. They analysed their corpus of over 300 errors using a systematic method for classifying their patient's mistakes as “neglect” or due to other visual factors.

They defined as “neglect errors” those responses in which the word was identical to the target to one side of a “neglect point” but diverged completely from the target to the right of that point. Even using these very conservative criteria they found that over two-thirds of the patient's errors could be classified as neglect errors. The majority of non-neglect responses were also real-words that were based on “general visual similarity between target and error” (slave → shave; screech → speech; abhor → labour; weather → sweater).

A similar analysis using Ellis et al.'s definition of a neglect point was applied in the investigation of a further case of left-neglect dyslexia (J.O.H.) (Costello & Warrington, 1987). Although the incidence of nonword responses was by no means negligible in this patient, a detailed examination of the pool of real-word neglect errors showed that his errors closely matched the target words in length (with items ranging from two to seven letters). Indeed nearly half of his neglect errors were identical in length to the target (see Table 10.2). On those errors which did not conserve word length exactly, he was just as likely to increase the length of a word as to decrease it. Finally, his reading of short words was less accurate than his reading of long words.

Publisher Summary

This chapter discusses several aspects of right-hemisphere disorders. A variety of terms have been used to describe related but different hemi-inattention syndromes after right-hemisphere injury. Related to the syndrome of unilateral spatial neglect are more gross forms of left neglect in which the right-hemisphere-damaged patient behaves as if he/she has completely lost the left side of space and the left side of his/her body. Anosognosia for hemiplegia may take a variety of forms. Some patients will insist that nothing is wrong at all and that their hospitalization is a mistake. Aphasia makes it difficult to assess anosognosia accurately in patients with left-hemisphere damage. Anosognosia is generally associated with marked sensory disturbance. The lesion producing anosognosia is usually large. Another possible explanation for anosognosia is that mechanisms essential for the recognition of hemiplegia or other body defects are in fact localized to the right hemisphere. Injury to the right hemisphere, thus, could produce a true agnosia for illness by disrupting the structures essential for the recognition of illness.

VI Unawareness of Neglected/Extinguished Stimuli

When extrapersonal or personal hemispatial neglect are severe, unawareness seems general, as if the contralateral sides of the world or of the body do not exist. In less severe cases, the picture is usually mixed: The unawareness is more variable. The patient might be aware of certain stimuli but not others. The picture is even more surprising in extinction since the general behavior of the patient (if having only extinction) does not include any signs of unawareness except during double simultaneous stimulation. Although most patients insist on seeing only a single stimulus, some observations suggest that in some circumstances the “extinguished” stimulus is perceived. The unawareness of neglect/extinction syndrome should be differentiated from other disorders associated with unawareness, including blindsight and simultanagnosia. Blindsight refers to the ability to perform some visual (e.g., detecting movements) and visuomotor tasks (e.g., pointing or grasping a target) in the areas of visual scotoma that resulted from damage to the primary visual cortex. The patient does not have a visual experience of the stimulus on which he or she acted. In simultanagnosia, the patient has difficulty simultaneously perceiving more than a single object.

Another type of deficit of awareness in some patients with neglect might be described as “hyperawareness,” in the form of completion phenomena. The patient copies or draws from memory only the ipsilesional half of a drawing (e.g., a butterfly with only the left wing) but believes that what he or she drew is the complete picture (e.g., a butterfly with two wings). Directing his or her attention to the neglected side or to the missing parts might not help. This phenomenon might provide an indication of the veridicality of the patient's visual experience despite the deficit of awareness.

III.B.2 Lateralization of Object and Spatial Attention

The second finding from neglect patients relevant to object attention is an apparent hemispheric difference between object-based attention and spatially based attention. Patients with damage to the right parietal lobe most often present with hemispatial neglect in which the left side of space is ignored. (Patients who demonstrate object-centered neglect also exhibit spatial neglect.) Left neglect can also be demonstrated in these patients by using object-based attention tasks. For example, in the task shown in Fig. 5, left neglect patients are slower to detect targets in the contralesional visual field than in the ipsilesional visual field. However, left neglect patients show preserved object-based attention: They detect targets in the cued rectangle faster than targets in the uncued rectangle, and this object-based effect is found in both the contralesional and ipsilesional visual fields. In contrast to patients with left neglect, patients with damage to the left parietal lobe (right neglect) appear to have deficits in object-based attention. Right neglect patients also show slower target detection in the contralesional visual field than in the ipsilesional field. However, these patients exhibit larger object attention effects in the contralesional field; it is more difficult for these patients to switch attention from the cued rectangle to the uncued rectangle in the contralesional field than in the ipsilesional field. These hemispheric differences between object-based and spatially based attention have been supported by research with a split-brain patient. In performing the cued detection task shown in Fig. 5, this patient had greater difficulty switching attention from the cued rectangle to the uncued rectangle when stimuli were presented in the right visual field (left hemisphere) than when stimuli were presented in the left visual field (right hemisphere).

3.2 Right Hemisphere Lesions and Nonlanguage Spatial Processing

These results from language testing contrast sharply with results on tests of nonlanguage spatial cognition. RHD signers are significantly more impaired on a wide range of spatial cognitive tasks than LHD signers, who show little impairment. Drawings of many of the RHD signers (but not those with LHD) show severe spatial distortions, neglect of the left side of space, and lack of perspective. RHD deaf signers show lack of perspective, left neglect, and spatial disorganization on an array of spatial cognitive nonlanguage tests (block design, drawing, hierarchical processing), compared with LHD deaf signers. Yet, astonishingly, these severe spatial deficits among RHD signers do not affect their competence in a spatially nested language, ASL. The case of a signer with a right parietal lesion leading to severe left neglect is of special interest: Whereas his drawings show characteristic omissions on the left side of space, his signing (including the spatially organized syntax) is impeccable, with signs and spatially organized syntax perfectly maintained.

The finding that sign aphasia follows left hemisphere lesions but not right hemisphere lesions provides a strong case for a modality-independent linguistic basis for the left hemisphere specialization for language. These data suggest that the left hemisphere is predisposed biologically for language, independent of language modality. Thus, hearing and speech are not necessary for the development of hemisphere specialization—sound is not crucial. Furthermore, the finding of a dissociation between competence in a spatial language and competence in nonlinguistic spatial cognition demonstrates that it is the type of information that is encoded in a signal (i.e., linguistic vs. spatial information) rather than the nature of the signal itself (i.e., spatial vs. temporal) that determines the organization of the brain for higher cognitive functions.

Unconscious or Implicit Processing in Neglect

Many students of neuroscience consider neglect to be a classic disorder of visual awareness where awareness is equated with the psychological construct of attention as previously discussed. However, given the remarkable findings from blindsight, where patients show some degree of information processing when tested indirectly, one of the fascinating questions is the level or extent to which covert cognitive processing continues despite overt failure on the part of the patient to consciously report or attend to left-sided space or objects. In this way, studies of visual neglect have made a useful contribution to the classic debate regarding the processing levels of attentional selection. According to the late selection view, preattentive analysis of the whole scene takes place up to and including initial recognition of the object whereas early selection accounts argue that only basic perceptual processing takes place preattentively and focal attention is subsequently necessary for object recognition. The key question for neglect research hinges on the cognitive level to which stimuli are preattentively processed.

Studies of visual neglect have shown that dependent on the relevance of the stimuli, preattentive processing up to the level of meaning can take place in the ‘neglected’ field without conscious awareness. Using cross-field matching and priming experiments, patients with neglect show significantly shorter reaction times to right field stimulus content for congruent rather than noncongruent stimuli despite denying seeing anything in the left visual field. Even on line bisection, patients with severe left neglect show implicit sensitivity to many figural characteristics of the stimulus display, confirming that preattentive visual capacities can influence explicit visuomotor performance. When requested to bisect a line located in the center of a page, most patients with neglect show a displacement of absolute magnitude that is linearly related to line length. This consistent linear performance may be explained in terms of implicit processing of the visual information on the neglected side. Thus, the neglected end of the stimulus line is covertly influencing the patient's performance in deciding the subjective center of the line.

Marshall and Halligan showed evidence of a further type of dissociation between two forms of conscious perceptual awareness – with hierarchical figures on a free vision task. In a series of experiments, they showed that some neglect patients, when shown hierarchical drawings of shapes or alphabetic letters composed of smaller (local) features or letters in free vision, gave an accurate verbal description of both the global and local subfigures despite subsequently (and within seconds) showing neglect when required to cancel the smaller (local) components on the left side of the global figure and despite being able to describe the global figures accurately both before and after cancellation (Figure 10(a)–10(e) left). This ‘within subject disconnection’ in communication between local and global processing was found even when patients produced an accurate copy of the global figure (Figure 10(c)-right-sided figures) – a circle comprised of localized dots. Requested to cancel the same set of dots comprising the circle just drawn, patients again neglected to mark much of the left circumference of the circle. In such cases, global processing can no longer be used to direct automatic focal attention to spatial locals that require further focal analysis. Without this ability, local attention which is usually biased to the right will always represent too little of the visual world. However, once attention has been focused, the panoramic or global view appears lost to conscious awareness unless exogenously cued. When their focal attention is biased to the right as in the case of left neglect, patients are in a position to observe the absence of left-sided input. Even if selective attention can be voluntarily moved leftward, the necessary guiding framework provided by the global scale is no longer available. Consequently, patients no longer have any reason to continue to explore leftward. In these and other examples where performance within an individual patient appears normal on one aspect and grossly impaired on another involving the same stimulus seconds later, left neglect may be regarded as a partial disconnection of conscious visual awareness where residual processes of the impaired right hemisphere cannot be used to constrain the performance of the intact left hemisphere in performing the designated task. Collectively, these findings highlight the danger of equating phenomenological conscious experience with the operation of the perceptual mechanisms involved.

Partial Visual Occlusion

Interventions aimed at partially occluding visual input via eye patching are also showing promise in the literature in terms of demonstrating improved functional skill in those with unilateral spatial neglect. Early work in this area by Butter and Kirsch23 examined 13 stroke survivors with left-sided neglect and documented improvement in 11 of the 13 subjects from monocular patching of the right eye in at least one (of five) impairment tests of neglect. They also noted that the beneficial effect was mostly limited to the period when the patch was worn. In a second study of another group of stroke survivors with left neglect (N = 18), performance on a line-bisection task with monocular patching and/or lateralized visual stimulation was examined. Although each intervention had positive results, the two interventions combined resulted in significantly larger benefits than either alone. The authors concluded that monocular patching, in conjunction with lateralized visual stimulation, may significantly reduce neglect in daily activities.

A study by Arai and associates7 examined 10 subjects with left neglect and investigated whether using glasses shaded on the non-neglected side would lead to a decrease in the severity of unilateral neglect during pen and paper activities. The authors found that improvement was mixed for each of the three outcome measures (deviation from marking the middle of a 20-cm horizontal line, number of lines left unmarked on the left-hand portion of a page of 40 randomly oriented lines, and degree of failure to copy a representation of a cube). One subject was noted to have substantial and lasting improvement in functional activities by wearing the hemispatial sunglasses. Although somewhat promising results were obtained, the aforementioned studies only used impairment tests of neglect.

In a randomized study, Beis and colleagues15 examined 22 subjects with left unilateral neglect. Interventions included the use of right half-field patches (n = 7), a right monocular patch (n = 7), and a control group (n = 8). Patches were worn throughout the day during inpatient rehabilitation (Figure 6-10). Results of paired comparison tests showed significant differences between the control group and the group with the half-eye patches for the total FIM score and objective measures of displacements of the right eye in the left field. No significant differences were found between the control group and the group with the right monocular patch. The authors concluded that patching the right half-field helped subjects initially regain voluntary control over the neglect impairment, thereby improving daily function. The authors hypothesized that their intervention was successful because wearing a patch over the two right hemifields causes a right homonymous hemianopsia and activation of the right hemisphere in isolation, therefore causing an increase in the level of attention. In addition, the authors hypothesized that covering the right half-field helped establish a balance between the activation of the two hemispheres as well as improvement in the mechanisms that control voluntary redirection of the gaze.

Productive manifestations

Right-brain-damaged patients with left spatial neglect may show different kinds of productive symptoms and signs (Vallar, 1998). Productive manifestations in extrapersonal space include the following.

Perseveration and other gratuitous productions in drawing and cancellation tasks, which are the most frequent and investigated productive disorder shown by neglect patients (Na et al., 1999; Rusconi et al., 2002; Nys et al., 2006; Ronchi et al., 2009). Perseveration occurs mainly in the ipsilateral side of extrapersonal space. Perseveration may manifest as the multiple marking of the same target located in the right-hand side of the sheet in a target cancellation task, with addition of gratuitous details (Fig. 14.5A and B), as the retracing of lines already drawn in the right-hand side of a copy (Fig. 14.5C), and as the addition of unnecessary details in the right-hand side of a copied complex drawing (Fig. 14.5D). Substitution and addition errors in left neglect dyslexia, which are associated with perseveration, are also considered productive manifestations of neglect (Ronchi et al., 2016).

Magnetic attraction towards ipsilateral stimuli (De Renzi et al., 1989; Làdavas et al., 1990). Right-brain-damaged patients with left spatial neglect may show a disproportionate deployment of visuospatial attention towards events occurring in the ipsilateral side of space, as indexed by response latencies.

Active avoidance of contralateral stimuli. This may be regarded as the contralateral counterpart of magnetic attraction. This functional distinction is reminiscent of Denny-Brown and Chambers’ (1958) suggestion that the parietal lobes provide approach tendencies, and orientation in space, antagonized by avoidance-oriented areas in the frontal and limbic lobes (see Chapter 1). One right-brain-damaged patient with left neglect, requested to collect a number of cubes aligned in front of her in a left-to-right order, picked up with the right hand those lying on the right, and then pushed the remaining ones leftward, saying: “There are no more of them” (Bisiach et al., 1983). Avoidance of the contralateral left side may also account for the greater rightward bias shown by one patient in line bisection, when the working space was seen mirror-reversed, as compared with baseline bisection. This pattern indicates not only leftward directional hypokinesia, since the rightward bias did not decrease when the left neglected side was shown in the right unaffected side, but also avoidance, since the rightward bias actually increased (Kodsi and Heilman, 2002).

In personal space, the productive manifestation is somatoparaphrenia, a diverse set of delusional bodily disorders involving the contralateral limbs. Patients’ spectrum of delusions includes: a sense of unfamiliarity with respect to contralateral body parts, of separation from the patient's body, of disownership, ranging from beliefs that the affected body parts do not belong to the patient to beliefs that they belong to another person; more complex delusional misidentifications of affected body parts (e.g., the disowned hand is a “reptile”; a number of associated disorders, comprising supernumerary limbs, misoplegia (verbal, motor aggressive behavior toward the contralateral limbs), and personification of the contralateral limbs (Vallar and Ronchi, 2009; Feinberg and Venneri, 2014; see Chapter 15).

E Is Neglect a Disorder of Attention or Representation?

Neglect patients exhibit gross limitations in where they attend. They do not look over to the contralesional side or act or react in that direction. Moreover, the tendency of the neglect patient to seek out and fixate the right extreme of a display is an obvious attentional bias.

Other findings are less obviously attentional. Automatic response to left-sided stimulation in left neglect is also deficient. Indeed, volition can to some extent overcome the rightward bias, while the bias remains profound at the automatic level. Premotor covert attentional shifts are also biased (Posner, Walker, Friedrich, & Rafal, 1984).

Neglect occurs even with displays that are so brief as to preclude an attentional shift (Kinsbourne & Warrington, 1962; Warrington, 1962). When forms or words are briefly exposed to intact areas of neglected patients' visual fields, they respond only to right-sided features (and confabulate or “fill-in” the left in disregard of what is actually there). The same occurs in visual imagery as in visual stimulation; the patient images and reports only right-sided features (Bisiach & Luzzatti, 1978; Bisiach, Luzzatti, & Perani, 1979). Even hallucinations are limited to the intact visual field in neglect patients. To accommodate such findings within an attentional framework one must locate the attentional problem at the level of representation, not only at the level of overt behavior.

A modularity theorist might suppose that each hemisphere houses a module for contralateral hemispatial attention. In neglect, one module attending within opposite hemispace is disabled. The preponderance of left neglect would be due to disconnection of input that impinges on the right hemisphere from the left-sided language centers (Geschwind, 1965). But neglect is demonstrable within both hemispaces, the left side of the stimulus being neglected regardless of its location relative to the midline. Each module therefore would have to be responsible for the awareness of one side of any display, regardless of where it is located within the visual field, a curiously post hoc idea.

If one discards paired noninteractive modules but retains hierarchical brain organization, one could posit a bias in the sweep of inner “eye” that scans the passively represented input for attentional purposes (Kosslyn, 1980). The input itself, or the image, is fully represented, but the inner monitor sweeps only to one side, just as receptor orientation to external objects is laterally biased. But such a monitor must itself be guided by a model, and how is that model monitored? An infinite regress is avoided if one discards the distinction between the attending monitor and the attended representation, using the alternative parallel brain model and the concept of a self-organizing network.

If selective attending is implemented by enhancing the activation of the chosen representation, then neglect teaches us that the two half-brains activate the cell assemblies that represent stimuli disposed along the lateral plane in a complementary fashion. Each hemisphere contributes activation to that part of the representation that is induced by contralateral segments of the stimulus. By graduated change in this dynamic interaction, the normal individual can attend selectively to either side or any intermediate part of a thing at will. In neglect the component of the cell assembly that represents the contralesional section of the stimulus is underactivated. Contralesional contents is therefore not experienced and does not trigger action.

When a section of a familiar shape is missing at its contralesional extreme, there is nothing in awareness that signals its absence. In the absence of contradictory information, the shape is perceived as an intact whole, the objectively missing portion being “imaginatively completed” (e.g., Kinsbourne & Warrington, 1962). The contralesional extremity of the input is the loser when representations compete for the control of behavior. The percept is formed by interaction between ipsilesional information and expectancy. There is indeed an abnormality of representation in unilateral neglect, namely one that interferes with the means by which we selectively attend.