Hemispatial Neglect | Why Do Patients Ignore Half of Their World?

Hemispatial neglect, also known as unilateral neglect, is an attentional disorder, not a visual one. It arises from damage to one of the brain's hemispheres, most commonly the right parietal lobe, often following a stroke. The parietal lobe is critical for integrating sensory information from various parts of the body and for constructing a spatial representation of the external world. An individual's sense of their body's position in space, as well as the location of objects around them, is managed by this region. When the right parietal lobe is damaged, the brain's ability to process and attend to stimuli on the left side of the body and space is severely impaired. This occurs because the right hemisphere directs attention to both the left and right sides of space, whereas the left hemisphere predominantly focuses only on the right side. Consequently, damage to the left hemisphere is often compensated for by the right, but right-hemisphere damage leads to a profound and noticeable neglect of the left side, as there is no corresponding compensatory mechanism. The patient’s eyes may be perfectly functional, but the brain fails to allocate the necessary attentional resources to consciously perceive the left half of their universe.

The behavioral consequences of hemispatial neglect are striking and can profoundly impact a person's independence. Patients may consistently fail to eat the food on the left side of their plate, apply makeup or shave only the right side of their face, and read only the right-hand pages of a book. When asked to draw a clock, they might cram all the numbers onto the right side or omit the left side entirely. In conversation, they may ignore people standing on their left. A defining and challenging aspect of this condition is anosognosia, a lack of awareness or denial of the deficit itself. The patient does not feel as though they are missing anything; from their perspective, the left side of the world simply does not exist in a meaningful way. This lack of insight complicates rehabilitation, as the patient may not understand the need for therapy or for compensatory strategies in their daily activities.

Diagnosis is established through specific neuropsychological tests that expose the attentional bias. The 'Line Bisection Test' requires a patient to mark the midpoint of a horizontal line; a person with neglect will place the mark significantly to the right of the true center. In 'Cancellation Tests,' the patient is presented with a page full of various symbols and instructed to cross out all instances of a specific target. They will consistently fail to cancel targets on the left side of the page. Another common diagnostic tool is the 'Copying Task,' where the patient is asked to replicate a drawing of a symmetrical object, such as a house, flower, or clock. The resulting drawing will characteristically be missing its entire left side. These assessments are crucial for differentiating neglect from pure visual field loss.

Compelling evidence indicates that sensory information from the neglected space is not entirely lost but rather fails to reach conscious awareness. The sensory organs, such as the eyes and ears, are still detecting stimuli. Classic experiments have demonstrated this phenomenon. For instance, in the "burning house" study, patients were shown two identical pictures of a house, one of which had flames coming from the left window. While they reported seeing no difference, when asked which house they would prefer to live in, they consistently chose the non-burning one. This suggests that the threatening information (the fire) was processed at an unconscious level and influenced their decision-making. This dissociation between automatic, unconscious processing and conscious perception is a core feature of the disorder.

Recovery from hemispatial neglect is variable. Some degree of spontaneous recovery often occurs within the first weeks to months following the brain injury. However, persistent neglect requires targeted rehabilitation. Therapeutic strategies are designed to force the brain to redirect attention to the neglected side. 'Visual Scanning Training' involves structured tasks that actively encourage patients to turn their head and eyes toward the left to find objects. A more technologically advanced treatment is 'Prism Adaptation.' Patients wear specialized prism goggles that shift their visual world to the right. When they initially try to point to an object, they will miss it, pointing too far to the right. With practice, their brain adapts to this distortion and recalibrates its spatial mapping. When the prisms are removed, this recalibration results in an aftereffect where the patient's attention is shifted more to the left, temporarily alleviating neglect symptoms. The long-term efficacy of these treatments depends on the severity of the initial injury and the consistency of the therapy.