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Attention disorder
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This case consists of the visual system as well as the dorsal and ventral pathways. There is also information about agnosia and visual neglect.
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Task 6:Attention disorder and agnosia
1. How does the perceptual (visual) system in the brain work?
Researchers have identified three functional pathways leaving the posterior parietal region and
traveling to the premotor, prefrontal and medial temporal regions.
i. The parieto-premotor pathway is proposed as the principal “how” pathway
ii. The parieto-prefrontal pathway is proposed to have visuospatial functions, especially
related to visuospatial working memory.
iii. The parieto-medial temporal pathway, which flows directly to the hippocampus and
parahippocampal regions as well as indirectly via the posterior cingulate and
retrosplenial cortex, is proposed to have a role in spatial navigation.
So, the posterior parietal cortex would contribute to the dorsal stream by participating in non-
conscious visuospatial behavior, reaching for and grasping objects.
The goal of all dorsal stream pathways is to guide visuospatial behavior through motor output, so
the parieto-prefrontal and parieto-mediotemporal pathways must eventually influence motor
output, though more indirectly than does the parieto-premotor pathway.
Theory of parietal-lobe function
There are two independent parietal-lobe contributions.
i. The anterior zone processes somatic sensations and perceptions.
ii. The posterior zone specializes primarily in integrating sensory input from the somatic
and visual regions and to a lesser extent from other sensory regions, mostly for
controlling movements, reaching and grasping as well as whole-body movements in
space. It also plays a significant role in mental imagery, especially related to both
object rotation and navigation through space.
Behavioral uses of spatial information
We need spatial information about the location of objects in the world, both to direct actions at
those objects and to assign meaning and significance to them.
Recall the two basic types of form recognition, one for recognizing objects and the other for
guiding movements to objects.
Object recognition
Spatial information needed to determine relations between objects may be very different from the
spatial information needed to guide eye, head or limb movements to objects. In the latter case,
visuomotor control must be viewer-centered; the object’s location and its local orientation and
motion must be determined relative to the viewer.
It appears that the brain operates on a “need-to-know” basis. Having too much information may be
counterproductive for any given system. In contrast with the viewer-centered system, the object-
centered system must be concerned with such properties as the object’s size, shape, color and
relative location so that the objects are recognized when they are encountered in different visual
contexts.
The temporal lobe codes objects’ relational properties. Part of this coding probably occurs in the
polymodal region of the superior temporal sulcus and another part in the hippocampal formation.
,Faces are also considered an object so indeed they also appear in the frontal part of the temporal
cortex. It is part of the ventral stream.
Movement guidance
To accommodate the many differing viewer-centered movements (eyes, head, limbs, body,
separately and in combinations) requires separate control systems.
We have considered many visual areas in the posterior parietal region and multiple projections
from the posterior parietal regions to the frontal lobe motor structures for the eyes and limbs.
Connections to the prefrontal region (area 46) have a role in short-term memory for the location of
events in space.
Studies on the posterior parietal lobes have shown the posterior parietal’s role in visuomotor
guidance. The activity of these neurons depends on the concurrent behavior of an animal with
respect to visual stimulation. Most neurons in the posterior parietal region are active both during
sensory input and during movement.
The responses of posterior parietal neurons have two important characteristics in common:
1. They receive combinations of sensory, motivational and related motor inputs.
2. Their discharge is enhanced when an animal attends to a target or moves towards it.
These neurons are therefore well suited to transforming requisite sensory information into
commands for directing attention and guiding motor output.
Sensorimotor transformation
When we move toward objects, we must integrate movements of various body parts with sensory
feedback of what movements are actually being made (efference copy) and the plans to make the
movements. As we move, the locations of our body parts change, and perceptions of our body
must constantly be updated so that we can make future movements smoothly. These neural
calculations are called sensorimotor transformation. Cells in the posterior parietal cortex produce
both the movement-related and the sensory-related signals to make them.
Andersen et al devised experiments with monkeys. They decoded
from parietal neural activity the animal’s intention to reach to
position a cursor on a screen. Monkeys were first trained to make a
series of reaches to touch different locations on a screen. The
monkeys then were instructed with a briefly flashed cue to plan to
execute a reach to another location but without making a
movement.
Their cellular activity was compared with activity associated with
actual movements to the requested target. If it was the same as in
an actual movement, the monkeys were rewarded with a drop of
juice in the mouth and visual feedback showing the correct
correlation.
A. Monkeys are trained to touch a small central green cue and
to look at a red fixation point. A large green cue flashes,
and the monkeys are rewarded if they reach to the target
after a 1500-millisecond memory period.
, B. Monkeys are rewarded if their brain activity indicates that they are preparing to move to
the correct target location.
Clearly, transforming sensorimotor activity into action using a brain-to-brain interface for real-time
sharing is in its infancy and poised to grow.
Spatial navigation
When we travel, we can take the correct route subconsciously. To do so, we have some type of
cognitive spatial map in our brains as well as a mental list of what we do at each spatial location.
The internal list is referred to as route knowledge.
The route knowledge is unlikely located in a single place in
the brain. Studies suggest that participation of the medial
parietal region (MPR), which includes the parietal region
ventral to the PRR (parietal reach region) as well as the
adjacent posterior cingulate cortex, part of the parieto-
mediotemporal pathway in the dorsal stream.
Neurons in the dorsal visual stream could be expected to
participate in route knowledge, in so far as we must make
specific visually guided movements at specific locations in
our journey. Like the cells in PRR, which control the
planning of limb movements to locations, the cells in MPR
control only body movements to specific locations.
The complexity of spatial information
- The first aspect of our theory of parietal-lobe function considers the uses of spatial
information for recognizing objects and guiding movement.
- The second aspect of spatial representation is complexity.
Some types of viewer-centered representations are complex, such as distinguishing left and right.
Other spatial relations are even move complex, such as visualizing objects and manipulating the
mental images spatially. Patients with posterior parietal lesions are impaired at mental
manipulations.
Other parietal-lobe functions
Three parietal-lobe symptoms do not fit obviously into the simple view of visuomotor control
center: difficulties with arithmetic, aspects of language and movement sequences.
A researcher proposed that mathematics and arithmetic have a quasi-spatial nature analogous to
mentally manipulating concrete shapes but entailing the manipulation of abstract symbols.
From this perspective, parietal-lobe patients experience acalculia (= an inability to perform
mathematical operations because of the task’s spatial nature). Thus, arithmetic operations may
depend on the polysensory tissue at the left temporoparietal junction, a region where the
temporal and parietal lobes meet at the end of the Sylvian fissure.
Language too can be seen as quasi-spatial. Patients may understand individual elements clearly,
but they cannot understand the whole when the syntax becomes important. This ability, too may
depend on the polysensory region at the temporoparietal junction.
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