Hemispatial neglect is a condition where a person ignores stimuli on one side of space, usually the left, after damage to the right parietal lobe. It is caused by failure to pay attention to sensory input on the contralesional side rather than sensory loss. Damage to the right parietal cortex, especially the inferior parietal lobe or temporoparietal junction, is most closely associated with hemispatial neglect. The parietal cortex is involved in spatial perception, attention, and sensorimotor functions, so damage disrupts these abilities and causes neglect of the left side of space.

1. Hemispatial Neglect
Understanding the Brain: The Neurobiology of Everyday Life
By Elizabet Simeonova
Introduction
Hemispatial neglect is a neuropsychological condition in which, after
a brain injury (a tumor or a stroke) to one of the hemispheres, particularly
the right one, the awareness of stimuli on the contralateral side, the left
one, is reduced, even though there may be no sensory loss.
Parts of the nervous system associated with the condition
Classically, hemispatial neglect is associated with a lesion to the
right parietal lobe and it is most closely related to damage to the posterior
parietal cortex, particularly the inferior parietal lobe (IPL) or
temporoparietal junction (TPJ).
Some authors propose a critical association with damage to the right
superior temporal gyrus, but this view remains a subject of much debate
and controversy. Subcortical lesions that do not encroach directly on the
cortex can lead to neglect as well, via indirect effects on overlying cortical
regions. In addition, isolated lesions of the right frontal lobe may be
associated with neglect too.

2. Why the left space?
In most left-dominant brains the left hemisphere is specialized for
language, whereas the right hemisphere is specialized for spatial
perception and memory. However, areas in the parietal and frontal lobes
in both hemispheres are associated with the deployment of attention into
contralateral space and damage to the right ones results as a neglect of
the left space.
The reason that the right-sided spatial neglect is rare is because
stimuli from the right space are processed by both hemispheres, whereas
the left space is processed only by the right hemisphere.
The role of the parietal cortex
The parietal cortex is a subset of the parietal lobe and is involved in
visual, auditory, somatosensory, and movement-related activity. It does
not only receive input from the three sensory systems that play roles in
the localization of the body and external objects in space (visual, auditory
and somatosensory system), it is also related to the transformation of
information from one reference frame to another.
Much of the output of the posterior parietal cortex goes to areas of
frontal motor cortex: the dorsolateral prefrontal cortex, various areas of
the secondary motor cortex, and the frontal eye field. Damage to the area
of the posterior parietal cortex can produce a variety of sensorimotor
deficits, including deficits in the perception and memory of spatial
relationships, in accurate reaching and grasping, in the control of eye
movement, and in attention.
What is important about hemispatial neglect is that although it looks
like a sensory loss, it is actually failure to pay sufficient attention to
sensory input. For patients having this condition, the left part of the world
just doesn’t exist.

3. Individuals with a lesion to right parietal cortex:
 Ignore contralesional, left sights, sounds, smells, or tactile stimuli
(inattention neglect);
 Ignore the left side of memories, dreams, and hallucinations
(representational neglect);
 May not use contralesional, left limb despite the neuromuscular
ability to do so;
 Neglect the left side of their peri-personal or extrapersonal space
(allocentric neglect);
 Neglect the opposite to the lesion side of their own body (egocentric
neglect);
 Cannot turn left or spin counter- clockwise;
 When touched on the affected side may report being touched on the
intact side.
 May both neglect objects to the left of their own midline or see all
the objects, but neglect the left half of each individual object. This
neglect may also occur with slanted, flipped or mirror-image
presentations, after they are mentally adjusted.

4. How has this course allowed me to better analyze the events
and phenomena around me?
I took this course not only because I have general interest in
neurobiology, but also because I had quite a lot of gaps in my knowledge,
which I was able to fill in thanks to the course.
Even though hemineglect may not be an everyday problem, I found
it very interesting and serious. This course helped me to better
understand the role of both brain hemispheres in day-to-day perception
and attention, as well as to better analyze the importance of neurobiology
in various other phenomena around me.
References
1. Husain, M. (2008). Hemineglect. Scholarpedia, 3(2):3681.
2. Kleinman, J.; Newhart, M.; Davis, C.; Heidler-Gary, J.; Gottesman, R;
Hillis, R. (2007). Right hemispatial neglect: Frequency and
characterization following acute left hemisphere stroke. Brain and
Cognition 64 (1): 50–59. The Belknap Press of Harvard University Press.
3. Mason, P. (2014). Understanding the Brain: The Neurobiology of Everyday
Life.
4. Mavlov, L. (2001). Fundamental Neurology. Sofia.
5. Parton, A.; Malhotra, P; Husain, M. (2004). Hemispatial neglect. J Neurol
Neurosurg Psychiatry 75(1): 13–21.
6. Vaishnavi, S. (2001). Binding Personal and Peripersonal Space: Evidence
from Tactile Extinction. Journal of Cognitive Neuroscience 13 (2): 181–
189.
7. Vallar, G. (1998). Spatial hemineglect in humans. Trends in Cognitive
Sciences: 87–95.
Images: Google