Hemispatial neglect

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Clinic

  • After damage to one hemisphere of the brain, a deficit in attention and awareness towards the side of space opposite brain damage (contralesional space) is observed. It is defined by the inability of a person to process and perceive stimuli towards the contralesional side of the body or environment.
  • Hemispatial neglect is very commonly contralateral to the damaged hemisphere, but instances of ipsilesional neglect have been reported.


Presentation

  • Neglect for one side of the visual field, causing a patient with neglect to behave as if that side of sensory space is nonexistent. In an extreme case, a patient with neglect might fail to eat the food on the one half of their plate, even though they complain of being hungry.
  • Neglect may also present as a delusional form, where the patient denies ownership of a limb or an entire side of the body. Since this delusion often occurs alone, without the accompaniment of other delusions, it is often labeled as a monothematic delusion.[citation needed]
  • Neglect not only affects present sensation but memory and recall perception as well. A patient suffering from neglect may also, when asked to recall a memory of a certain object and then draw said object, draw only half of the object. It is unclear, however, if this is due to a perceptive deficit of the memory (to the patient having lost pieces of spatial information of the memory) or whether the information within the memory is whole and intact but simply being ignored, the same way portions of a physical object in the patient's presence would be ignored.[citation needed]


Causes

  • Brain areas in the parietal and frontal lobes are associated with the deployment of attention (internally, or through eye movements, head turns or limb reaches) into contralateral space. Neglect is most closely related to damage to the temporo-parietal junction and posterior parietal cortex.
  • Lack of attention to the left side of space can manifest in the visual, auditory, proprioceptive, and olfactory domains.

Although hemispatial neglect has been identified following left hemisphere damage (resulting in the neglect of the right side of space), it is most common after damage to the right hemisphere. This disparity is thought to reflect the fact that the right hemisphere of the brain is specialized for spatial perception and memory, whereas the left hemisphere is specialized for language - there is redundant processing of the right visual fields by both hemispheres. Hence the right hemisphere is able to compensate for the loss of left hemisphere function, but not vice versa. Neglect is not to be confused with hemianopsia. Hemianopsia arises from damage to the primary visual pathways cutting off the input to the cerebral hemispheres from the retinas. Neglect is damage to the processing areas. The cerebral hemispheres receive the input, but there is an error in the processing that is not well understood.[citation needed]

Theories of mechanism[edit]

Researchers have debated about whether neglect is a disorder of spatial attention or spatial representation, or even non-spatial deficits of attention combined with a directional bias that results from unilateral brain injury.

Spatial attention[edit]

Spatial attention is the process where objects in one location are chosen for processing over objects in another location. This would imply that neglect is more intentional. The patient has an affinity to direct attention to the unaffected side. Neglect is caused by a decrease in stimuli in the contralesional side because of a lack of ipsilesional stimulation of the visual cortex and an increased inhibition of the contralesional side.

In this theory, neglect is seen as disorder of attention and orientation caused by disruption of the visual cortex. Patients with this disorder will direct attention and movements to the ipsilesional side and neglect stimuli in the contralesional side despite having preserved visual fields. The result of all of this is an increased sensitivity of visual performance in the unaffected side. The patient shows an affinity to the ipsilesional side being unable to disengage attention from that side.

Spatial representation[edit]

Spatial representation is the way space is represented in the brain. In this theory, it is believed that the underlying cause of neglect is the inability to form contralateral representations of space. In this theory, neglect patients demonstrate a failure to describe the contralesional side of a familiar scene, from a given point, from memory.[citation needed]

To support this theory, evidence from Bisiach and Luzzatti's study of Piazza del Duomo can be considered. For the study, patients with hemispatial neglect, that were also familiar with the layout of the Piazza del Duomo square, were observed. The patients were asked to imagine themselves at various vantage points in the square, without physically being in the square. They were then asked to describe different landmarks around the square, such as stores. At each separate vantage point, patients consistently only described landmarks on the right side, ignoring the left side of the representation. However, the results of their multiple descriptions at the different vantage points showed that they knew information around the entire square, but could only identify the right side of the represented field at any given vantage point. When asked to switch vantage points so that the scene that was on the contralesional side is now on the ipsilesional side the patient was able to describe with details the scene they had earlier neglected.

The same patterns can be found with comparing actual visual stimuli to imaging in the brain (Rossetti et al., 2010). A neglect patient who was very familiar with the map of France was asked to name French towns on a map of the country, both by a mental image of the map and by a physical image of the map. The image was then rotated 180 degrees, both mentally and physically. With the mental image, the neglect stayed consistent with the image; that is, when the map was in its original orientation, the patient named towns mostly on the East side of France, and when they mentally rotated the map they named towns mostly on the West side of France because the West coast was now on the right side of the represented field. However, with the physical copy of the map, the patient's focus was on the East side of France with either orientation. This leads researchers to believe that neglect for images in memory may be disassociated from the neglect of stimuli in extrapersonal space. In this case patients have no loss of memory making their neglect a disorder of spatial representation which is the ability to reconstruct spatial frames in which the spatial relationship of objects, that may be perceived, imagined or remembered, with respect to the subject and each other are organized to be correctly acted on.

This theory can also be supported by neglect in dreams (Figliozzi et al., 2007). The study was run on a neglect patient by tracking his eye movements while he slept, during the REM cycle. Results showed that the majority of the eye movements were aimed to his right side, indicating that the images represented in his dreams were also affected by hemispatial neglect.[citation needed]

Another example would be a left neglect patient failing to describe left turns while describing a familiar route. This shows that the failure to describe things in the contralesional side can also affect verbal items. These findings show that space representation is more topological than symbolic. Patients show a contralesional loss of space representation with a deviation of spatial reference to the ipsilesional side. In these cases we see a left-right dissimilarity of representation rather than a decline of representational competence.

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