Download Visual Attention Deficits in Patients with Unilateral Visuo

Visual Attention: Jiayong Liu & Ute Kreitz 1
Visual Attention Deficits in Patients with Unilateral Visuo-spatial Neglect
Visual neglect can be defined as a “disorder of spatial attention that generally
follows posterior parietal damage”, which results in patients ignoring objects and
events in the contralesional visual field (Farah, 1994, Marshall et.al., 2002). This
behavior is commonly found in patients who have suffered a stroke in the right
parietal hemisphere. Patients diagnosed with milder cases of neglect are experiencing
extinction. This disorder entails the neglect of a contralesional stimulus only in the
presence of a competing ipsilateral stimulus. Furthermore, extinction can be more
pronounced when two stimuli that are presented simultaneously have identical
attributes than when they have differing attributes.
According to study reviews by Farah (1994) and Driver & Mattingley (1995),
neglect patients are assumed to be able to tacitly process visual stimuli presented to
the impaired hemifield. Farah (1994) describes a study by Berti & Rizzolatti (1992)
where patients were asked to classify pictures in their ipsilateral and measured the
effects of contralesional stimuli on the speed of processing. Results showed that
participants were able to classify more quickly when the stimuli in both hemifields
were from the same category.
Recently, researchers have reported that neglect may be due to damage in
areas other than the parietal lobe. Marshall et.al (2002) note that based on evidence
from other studies, left neglect can also be found in patients with lesions to the right
temporal and right frontal lobe. However, there is substantial consensus that “severe
visuo-spatial neglect is most typically associated with lesion of the supramarginal
gyrus of the right inferior parietal lobule – Brodmann’s area 39/40”(Marshall et.al.,
2002). Furthermore, Marshall et.al. (2002) report that according to Karnath et.al.
(2001) neglect can be further divided into ‘pure’ and ‘impure’ neglect. In pure
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neglect, patients experience a loss of spatial awareness contralateral to the lesion
without having any visual field deficits. The area responsible for this defect is located
in the superior temporal cortex. On the other hand, patients with impure neglect show
a disorder of “directly coding space for action”(direct quote from Marshall et.al.,
2002).
In our experiment, we will investigate what kind of information is preserved in
patients with left hemineglect and at which level they are still able to process visual
stimuli. We will gather data from a population of patients who have suffered a stroke
or have had a tumor removed from the right parietal lobe. In addition to testing
patients for how much information they still can extract from contralesional stimuli,
we will also test tumor patients for symptoms of hemineglect. We suspect that tumor
patients are tested less often for neglect than stroke patients and expect to find –
depending on the size of the tumor and the extent of the surgical procedure –
symptoms of neglect or at least partial neglect in some tumor patients.
The purpose of this experiment is to give further evidence to claims made by
Farah (1994) and Driver & Mattingly (1992) that neglected stimuli are processed at a
higher level without patients being aware of it.
Method
This experiment consists of two parts: First, we will give a standard battery of
tests to both stroke and tumor patients to test them for symptoms of neglect. We will
use a line bisection task, a star cancellation task and an article-reading task. In the first
task, patients will be presented with a horizontal line and will be asked to mark the
middle of the line with a pen. Measured will be the difference between the actual
middle of the line and the mark the patient made. In the second task, patients will be
given a piece of paper with stars among other symbols and will be asked to cross out
Visual Attention: Jiayong Liu & Ute Kreitz 3
all the stars they can see. Scores will be based on how many stars the patient correctly
crossed out. Finally, in the last task patients will be given a short newspaper article
with three columns and will be asked to read it aloud. This time the performance is
scored based on the number of incomplete words or neglected words. In addition, we
will present tumor patients with a test for extinction. In this case, we will present
patients with a test similar to the one done by Marshall & Halligan (1988). Different
stimuli will be presented to both the left and the right visual field to find out which
one will be perceived more prominently.
After this initial screening test, we will select participants, both stroke and
tumor patients, who show symptoms of hemineglect. They will be given a mood
inventory test to assess their current emotional state. Afterwards, we will perform an
emotional priming task by presenting neutral pictures to the intact visual field and fear
provoking pictures to the impaired hemifield. During the presentation, patients will be
asked to focus on a fixation point and to name the perceived objects. Assuming that
they are able to process the stimuli in the left hemifield on a subconscious level, we
expect the patients to be primed for fear/anxiety after this task. We will measure this
with an implicit test whereby the patient is asked to perform a stem-completion task
and a word generation/association task. We assume patients to associate word stems
and sample words to be associated with a negative meaning due to the priming effect
of the previous test.
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References
Driver, J., Mattingley, J.B. (1995). Selective attention in humans: normality
and pathology. Current Opinion in Neurobiology, 5, 191-197.
Farah, M. (1994). Perception and awareness after brain damage. Current
Opinion in Neurobiology, 4, 252-255.
Marshall, J.C. et.al. (2002). Spatial awareness: A function of the posterior
parietal lobe? Cortex, 38, 253-257.