Hemispatial neglect, also known as unilateral or spatial neglect, is a neurological syndrome that profoundly affects a person’s perception of the world. This condition is not caused by blindness or a primary sensory defect, but rather by a failure of the brain’s attentional system. Individuals with this disorder have difficulty attending to stimuli on the side of space opposite to their brain injury. The underlying cause is damage to specific regions that govern how the brain allocates attention across the environment.
Defining Hemispatial Neglect and Its Manifestations
Hemispatial neglect is characterized by a failure to report, respond to, or orient toward stimuli presented on the contralesional side of space. The most common presentation is a neglect of the left side of space following a right-sided brain injury. This inattention can manifest across various senses, including visual, auditory, and tactile domains.
Patients exhibit behaviors such as eating only the food on the right half of their plate or shaving only the right side of their face. When asked to draw a clock, patients often crowd all the numbers onto the right side of the circle, leaving the left side blank. They may also bump into objects on their left side or ignore people speaking to them from that direction.
It is important to understand that hemispatial neglect is fundamentally different from a visual field cut, known as hemianopia. Hemianopia is a visual processing problem where damage to the visual pathway causes actual partial blindness in one half of the visual field. A patient with hemianopia knows they cannot see that side and will compensate by moving their head and eyes to scan the blind area.
In contrast, a patient with hemispatial neglect often remains unaware of their deficit, a condition called anosognosia, and does not spontaneously look toward the neglected side. The failure is not in seeing the information, but in allocating attention to it, highlighting the disorder’s attentional nature.
Identifying the Core Damaged Region
The brain region most commonly associated with causing severe and persistent hemispatial neglect is the right cerebral hemisphere. Specifically, the primary anatomical structures involved are the Posterior Parietal Cortex and the adjacent Temporoparietal Junction (TPJ). Damage to these areas is typically the result of a stroke, often involving the middle cerebral artery, which supplies a large portion of the hemisphere.
The posterior parietal cortex integrates sensory information and spatial awareness. Lesions here disrupt the brain’s ability to map surrounding space and direct attention. While the parietal lobe is the most frequent site of injury, neglect can also occur with damage to other connected areas, including the frontal lobe and subcortical structures like the basal ganglia and thalamus.
The most profound and long-lasting cases of neglect are overwhelmingly linked to injury in the right hemisphere, resulting in neglect of the left side of space. This striking asymmetry points to a unique functional specialization in the right hemisphere’s role in spatial attention. The TPJ, located where the temporal and parietal lobes meet, appears to be a particularly significant area for the full manifestation of the neglect syndrome.
The Role of the Right Hemisphere in Spatial Attention
The reason neglect is so much more common and severe after right-sided damage lies in the functional organization of the brain’s attention networks. The accepted model of spatial attention posits a fundamental asymmetry between the two hemispheres. The left hemisphere is specialized for controlling attention only to the right side of space, which is contralateral to it.
In contrast, the right hemisphere possesses a broader role, controlling attention to both the left and the right sides of space. The left hemisphere cannot compensate for the loss of the right hemisphere’s control over the left visual field, resulting in the full neglect syndrome.
This functional asymmetry is supported by the organization of the brain’s attention circuits, specifically the Ventral Attention Network, which is heavily right-hemisphere dominant. This network, which includes the TPJ, is responsible for detecting novel or salient stimuli and reorienting attention in a bottom-up, stimulus-driven manner. When the right-sided ventral network is damaged, it fails to detect stimuli on the left. This creates an imbalance where the intact left hemisphere’s attention bias toward the right side is unopposed, pulling attention away from the left and causing severe neglect.