Unilateral neglect is a phenomenon which may occur after damage (whether from stroke, trauma or other causes) to either hemisphere of the brain, although it is more common in right hemisphere damage. For the purposes of this writeup, right sided damage will be assumed.

Unilateral neglect (aka hemispatial neglect) is the failure to perceive stimuli coming from the left side. These stimuli may be auditory, visual, or tactile. In other words, the patient acts as if the left side of his world had ceased to exist. There is also the phenomenon of extinction, where if two stimuli are presented simultaneously, only one will be perceived (i.e. touching both hands at the same time, only the touch on the right will be felt). This phenomenon can be tested by, for example, asking the patient to draw a symmetrical object (clock, flower, face..), or to bisect a straight line. A patient with unilateral neglect will draw only the right half of the object, or will mark the line 3/4 of the way to the right (bisecting the 1/2 that he perceives).

As a consequence of this, the patient acts as if nothing to their left existed - they eat only the food on the right side of their plate, shave only the right side of their face, only see things to the right of them etc. This phenomenon may extend to the patient's own body. For example, patients with this problem have been found lying next to the bed on the floor, and when asked what happened, related that they awoke and there was a severed leg (or arm) in the bed with them, so they threw it out - and somehow got thrown out along with it! These patients are truly unable to perceive that the arm or leg is, in fact, attached to them and a part of their body.

This makes rehabilitation difficult - it is hard to work on deficits in a body part the patient does not acknowledge as their own. Sometimes patients can be taught to turn in a circle so that eventually the entire room goes through their right visual field, thus allowing them to see whatever they are looking for. This is one of the deficits that patients are truly completely unaware of.

Unilateral neglect is a rather bizarre condition where a sufferer will tend to ignore anything that is located in one particular half, or spatial hemisphere. The immediately apparent cause is a lesion to a particular brain hemisphere. Due to the contralateral relationship between the brain and the body (the left hemisphere controls the right half, and vice versa), the affected hemisphere of neglect is contralesional. Very often neglect will be caused by a lesion to the right hemisphere and therefore produce left hemineglect.

Effects of neglect:

Symptoms can include ignoring visual stimuli in a particular half of the visual field, or at least, not appearing to process them. Also there tends to be a poverty of gaze to the contralesional side, with a distinct bias to gazing ipsilesionally. Neglect is a condition that can manifest itself in numerous ways with not every sufferer displaying the same symptoms. If a person addresses a neglect sufferer form their affected side, some patients will ignore the addresser, or sometimes will look and search in the opposite direction for them. Sufferers have been know to only eat the food on one side of their plate, or complete only the squares on one side of a crossword puzzle. When walking about (should motor functions not be affected by hemiparesis) a sufferer may walk in a disoriented fashion due to not processing visual cues in the contralesional hemifield, and may also find that they bump into objects on their impaired hemisphere. Such neglect is known as "extrapersonal neglect".

"Personal neglect" manifests itself in equally strange ways. A sufferer may neglect to dress, shave, comb their hair, or wash on one side of their body. They may even deny that their limbs on their neglected side are theirs. Sufferers have been observed to attribute sensation in their contralesional limbs to someone else, such as the patient in the next bed.

Before discussing neglect further, it is important to note that it can take effect on different levels. Sometimes the whole contralesional hemisphere will be neglected, and sometimes the neglect is localised to a particular side of an individual object no matter in which hemisphere it resides. This effect probably reflects different levels and types of processing being affected by a lesion.

Often the easiest ways to test for hemineglect are confined to the medium of vision as these effects tend to be most obvious in tests. There are many ways to test for visual hemineglect. One method is the visuo-motor scanning and cancellation task, in which patients are presented with a sheet of stimuli and asked to "cross out" all of a certain kind of stimulus with their ipsilesional hand. Such experiments find that neglect sufferers will not only fail to cross out the majority of stimuli on their neglected side, they will also tend to confine their search ipsilesionally.

Line bisection paradigms, such as those used by Bisiach, Bulgarelli, Sterz, and Vallar (1983) and Columbo, De Renzi, and Faglioni (1971), involve a patient being bilaterally presented with a line stimulus and asked to mark the midpoint. It has been found time and time again that patients tend to bisect lines with a profound bias towards the ipsilesional side.

Contralesional neglect has been found in reading tasks. Patients may totally disregard one side of the text, while some will neglect just one side of each word. In incidences of the latter, patients have been found to even add letters at the beginning of words to compensate (Ellis, Flude, & Young, 1987). Contralesional neglect has also been observed by Valenstein and Heilman (1979) in patients using a typewriter keyboard who only used keys on their ipsilesional side.

There are some interesting effects in visual neglect. Warrington (1962) found that patients report seeing complete forms when a form stimulus is presented to them with one half in their neglected visual hemifield, and the other in the normal field. This could point to there being significant visual processing remaining in neglect patients.

But a striking example of visual neglect is in the numerous drawing and copying tasks that have been carried out. It has been found that if a patient copies a simple drawing they may fail to copy anything on one side. Sometimes they will repeat this mistake from drawing from memory, and other times not. Sometimes a patient will displace all, or most, of the contralesional side's features to the ipsilesional side thus producing a distorted picture. Other displacement errors occur. An example is the findings of Benton, Levin, and Van Allen (1974) when they identified an ipsilateral displacement error when neglect patients tried to locate major cities on a map. Hécaen and Marcie (1974) found that when writing patients tended to displace text to one side of the page. Displacement appears to be somewhat separate than pure neglect however, as shown by Bisiach, Capitani, Luzzatti, and Perani (1981) in the double dissociation in the different performances of two patients asked to copy a clock face. One patient drew only the right hand features, while the other crammed all the features into the right half of the clock face, squeezing all the numbers into the one half whereas the other patient ignored them.

Neglect can be just as present in domains outside visual processing. Such effects can be related to the visual ones in their causes so it may be prudent to briefly touch on them. We find that similar effects are shown in the tactile sensory modality. Patients fail to explore the contralesional side of space in the absence of visual cues when seaching for a marble in a laterally symmetrical maze (De Renzi, 1970), typing on a keyboard (Chedru, 1976), taking pegs out of holes (Bisiach et al., 1985), and collecting cubes lying in a horizontal plane before them (Bisiach et al., 1983). Altman, Balonov, and Deglin (1979) also demonstrated significant errors in the localisation of auditory stimuli. Bisiach and Luzatti (1978) showed neglect in "the mind's eye" when asking their Milanese neglect patients to describe the Piazza del Duomo from a certain viewpoint. The buildings on the contralesional side were omitted, however when asked to describe what they would see from the opposite position in the Piazza, the previously omitted buildings were described, while those previously mentioned were not. Motor neglect manifests itself as a poverty of movement in the contralesional limbs, often in response to stimuli such as gripping in response to an examiner's touch.

We find that there is not an archetypal neglect sufferer. Most display different symptoms and we also, importantly, find many double dissociations suggesting that there are multiple brain areas implicated in neglect.

Neurophysiology of neglect:

To explain how neglect may arise, it is probably best to look at whereabouts brain damage is localised. Firstly, looking from a grand scale, we find that a significant majority of neglect patients have right hemisphere damage. Estimates from studies suggest that around 40% of patients with right hemisphere damage suffer some form of neglect, whereas only about 12.5% of left hemisphere damaged patients do. But there is a problem here in that many patients with left hemisphere damage are aphasiacs and therefore very often untestable for neglect. It has been suggested that if these patients were included in the statistics then there could be an incidence of around 38% of neglect in left hemisphere damaged patients. But there is a great weight of evidence, some of which will be mentioned later, that suggests neglect is largely a problem caused by some kind of right hemispheric damage.

So concentrating on the right hemisphere, can more specific areas of damage be found? Initial studies were slightly conflicting. W.R. Brain (1941) and Critchley (1953) identified the parietal lobe as key, while Heilmann and Valenstein (1972b) considered extrapersonal neglect may be caused by lesions confined to the frontal lobe. Battersby et al. (1956) found that there was a 25% incidence of unilateral neglect in patients with damage to anterior areas of the brain, and a 54% incidence with posterior damage. A study by Hécaen (1972) can dispute the claims of Battersby partially, and Heilmann and Valenstein primarily, as he found that out of 179 patients, there was no association between neglect and frontal brain damage. Vallar and Perani (1986) lend support to neglect causing damage being spread in an anterio-posterior fashion, as they found that 80% of patients with such widespread lesions displayed extrapersonal neglect. As might be expected due to the more occipital location of the main visual processing areas, 63% of patients were found to have visual neglect with posterior lesions while only 8% displayed such signs with anterior lesions. Lending weight to this finding is the 1998 SPET (Single Photon Emission Tomography) study of Leibovitch, Black, Caldwell et al. They found a definite association between left hemispheric visual neglect and damage to the right posterior parietal cortex and also the posterior white matter fibre bundles. However, it cannot be discerned for sure whether this is an effect or cause.

A study by Vallar and Perani (1987) may give more insight into the aetiology of neglect. They found that eight out of nine patients with unilateral neglect showed damage to the right parietal region. Importantly, the damage was caused by fast growing glioblastomas. The significance of this becomes apparent in the light of the 1985 study of Heilman, Watson, and Valenstein, which showed that damage caused by slow growing meningiomas were seldom associated with neglect. This suggests that with the slow growing tumours other brain regions may have managed to compensate for the damage, whereas the fast growing ones allow no such opportunity for neuronal plasticity. This would imply that neglect is a direct consequence of such damage.

As the studies increased in number, several key regions have been identified though convergence of their data. Prominent amongst them is supramarginal gyrus of the right inferior parietal lobule. Leibovich et al. (1998) and Samuelson, Jensen, Ekholm et al. (1997) have shown that the underlying temporal-parietal-occipital white matter is also important. Subcortical damage such as this would disrupt posterior and antero-posterior connections - these areas have been implicated above. Samuelson et al. also showed that lesions to the right middle temporal gyrus might produce or contribute to unilateral neglect.

Still, damage to the frontal cortex cannot be ruled out. Heilman and Valenstein (1972b) have implicated Brodmann's areas 6, 8, and 44, located in the right dorsolateral premotor cortex and the medial frontal regions. However, much of the literature seems to be focussed on the main cause of neglect being damage to the parietal areas. It seems that inferior parietal damage results in unilateral neglect, whereas superior parietal damage results in optic ataxia - as shown by the reaching difficulties of patients of Ratcliff and Davies-Jones (1972).

There is much less known about left hemisphere damage and neglect as the condition is so uncommon and when it does appear, seems much less pronounced. The general consensus seems to be that parietal areas are involved here also.

In more recent years, CT (computer tomography) techniques have allowed experimenters to see how subcortical areas are involved in neglect. There are three regions that have been heavily implicated; the thalamus (Schott et al., 1981), the basal ganglia (Damasio, Damasio, and Chang Chui, 1980), and the subcortical white matter (Cambier, Graveleau, Decriox et al. 1983). Vallar and Perani (1996) found that there was a 100% incidence between subcortical white matter lesions and neglect, a 50% incidence with thalamus lesions, 28% with basal ganglia lesions, and 5% with lesions of white matter but no grey matter. fMRI scans have shown that areas far away from lesions, but are still connected, suffer from hypometabolism. This is just further evidence of how widespread the problems are in identifying areas of implication in neglect. Perhaps, then, an explanation of unilateral neglect needs to be found by theorising rather than just spotting damage.

Psychological and cognitive theories of neglect:

Firstly, explanations can be made based on a "lower level of integration", that is, relatively early on in information processing. Denny-Brown, Meyer, and Horenstein (1952) proposed their "amorphosynthesis hypothesis", which states that "failure to attend to the left side of [the] body or of space can be viewed as a disturbance of synthesis of multiple sensory data". However, appeals from the position of reduced or somehow scrambled processing of sensory input have been countered by De Renzi (1982), who claims that the problem of visual neglect arises due to the poverty of ocular movements that neglect patients display, particularly towards the neglected contralesional side. However this view has been rejected on the basis that it is a mistake to assert that a patient neglects space on the basis that the perception of it has been lost. Instead, an explanation based on a disorder of orienting response to stimuli was proposed. But even this explanation does not suffice. Bisiach et al. (1979) showed that eye movement disorders are not always associated with neglect. Also, it does not explain how oculomotor problems can explain neglect phenomena outside of visual processing. Still, as pointed out by Rubens (1985), this explanation may be important insofar as gaze directing tasks and situations are concerned. This may still account for some aspects of visual neglect. However, there is no reason to believe that this is a cause rather than just an effect of incidental damage.

There are proposed explanations at a higher level of integration. Kinsbourne (1970, 1987, 1993) suggested that we have two horizontal antagonistic vectors directing attention and that in damage associated with neglect destroys one of these vector thus distorting the attention to the ipsilesional side. This can account for the general rightward attentional bias that is found in humans due to that vector being more "focussed". Rizzolatti, Gentilucci, and Matelli (1985) have formulated a slightly related hypothesis. They found that in the postarcuate cortex of monkeys - an area in which egocentric space is likely to be processed - 29% of neurons were exclusively contralateral, while only 8% were ipsilateral. The remaining 68% were bilateral. They suggested that in the brain there are two attentional gradients, the right hemisphere one highest in neural representation in the left periphery and lowest in the right, the left hemisphere one highest in the right and lowest in the left. The left hemisphere gradient is steeper than the right. The resultant gradient is a gentle slope showing the asymmetrical bias of attention to the right. According to this hypothesis, neglect can be explained by the removal of a component gradient. If the left hemisphere component is eliminated due to left hemisphere damage, there is little effect as the right hemisphere gradient is shallow. However, if the right hemisphere component is eliminated, then neglect is produced as the resultant gradient is now distorted and will produce misrepresentation phenomena in the left hemisphere.

Other suggestions include that of Posner and colleagues, who suggested that neglect might be explained by difficulty in disengaging attention from the ipsilesional side and focussing it on something new contralesionally. But Hornak (1992) and Karneth (1997) show that this has no real role in visual neglect as patents still display an ipsilesional attention bias in the dark. Watson, Miller, and Heilman (1978) suggest that there might also be some intentional as well as an attentional aspect to consider due to directional hypokinisia, where a disruption of premotor processing (planning movements) on the contralesional side prevents reaching for visual stimuli.

Bender (1952) was among the first to suggest that visual neglect was a problem caused because of a dearth of visual processing rather than visual information not being received at all. But many studies have suggested that there is much more processing of visual information than Bender claims. Critchley (1953) showed that neglect patients can explore their contralesional half of body and have an awareness that it is there, except they deny that it belongs to them. Halligan and Marshall (1991) showed that there is at least some underlying processing of visual information as they found that they could increase reaction time to a target stimulus by presenting priming stimuli in the neglected visual field.

Having residual visual processing in visual neglect would make sense physiologically since the main visual areas of the brain in the occipital and temporal lobes are relatively spared. Ungerleider and Mishkin (1982) contribute an interesting idea to the debate, related to the dorsal and ventral visual processing streams. The dorsal stream leads to parietal areas and is responsible for spatial analysis of visual information. This general area is damaged in visual neglect, but the inferotemporal lobe, in which ventral stream processing for form perception occurs, appears relatively undamaged. This could indicate that damage to parietal areas can cause such a deficit in vision as visual neglect without the lesion damage having any significant effect on the main striate cortex, and most of the extrastriate cortex.

How can this finding relate to the neglect of visual attention? There are two possibilities. Firstly, there is the idea that attention somehow mediates visual responses to stimuli although it is not necessary for their processing. A more attractive proposition is that the conscious perception of visual stimuli requires that they can be attended. With no spatial coding of visual stimuli, no perception of action towards them can be attained. This is how visual hemineglect may arise. The deficit of direction of attention can be explained also by the type of damage suffered in neglect. Stein (1992) identifies the post parietal cortex as being important in the representation of egocentric space, and also is key to being able to redirect attention. The post parietal cortex is among the areas damaged in visual neglect.

Conclusions:

The cause of neglect is definitely brain lesions, primarily in the right parietal areas. However, an explanation of visual neglect (and neglect generally) is very hard to pin down to anything specific. But perhaps that is not necessarily a problem. The effects of neglect cause problems in many aspects of behaviour so it is reasonable to assume that damaged areas might be equally as far reaching, as supported by the many double dissociations found in patients.


Sources:

Bisiach, E., Vallar, G. (1988) Hemineglect in humans. In Boller and Gaffmann (eds) Handbook of Neuropsychology, vol. 1, 195-222

Berti, A. (1992) "Visual procession without awareness: Evidence from unilateral neglect" Journal of Cognitive Neuroscience; 4(4) pp 345-351

Gaffan, D. and Hornak, J. (1997) Amnesia neglect, beyond the Delay-Brion system and the Hebb synapse. Philosophical Transactions of the Royal Society, London, B, 352, 1481-1488

Halligan, P.W. and Marshall, J.C. (1988) "How long is a piece of string? A study of line bisection in a case of visual neglect" Cortex, 24 pp 321-328

Halligan, P.W. and Marshall, J.C. (1991) "Figural modulation of visuo-spatial neglect: a case study" Neuropsychologia; 29(3) pp 619-28

Stein, J. (1992) "The represpentation of egocentric space in the posterior parietal cortex" Behavioural and Brain Sciences; 15

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