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Summary Neuropsychology
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Literature Topic 1: Motor and sensory systemLearning goals week 1
TOPIC 1.1 – Brain Anatomy
Chapter 14: The Parietal Lobes
14.1 Parietal lobe anatomy
Subdivisions of the parietal cortex
- Parietal region lies between the frontal and occipital lobes, at the back roof of the
skull
- Principal regions: postcentral gyrus, superior parietal lobule, parietal operculum,
supramarginal gyrus and angular gyrus
- Parietal lobe can be divided into two functional zones:
1. Anterior zone – somatosensory cortex
2. Posterior zone – posterior parietal cortex
- In the human brain, the PG area and the STS (third visual pathway) are increased in
size and because this area is anatomically assymetrical, this leads to unique visual
symptoms after right parietal lesions
- Saccade – series of involuntary, abrupt and rapid small movements or jerks made by
both eyes simultaneously in changing the point of fixation (LIP area is involved)
Connections of the parietal cortex
Anterior parietal cortex makes rather straightforward connections: projections from primary
somatosensory cortex to secondary somatosensory area PE (tactile recognition) and to
motor areas
Connections in monkey brain are summarized, but I don’t think these are important (?)
Anatomy of the dorsal stream
- Dorsal stream stretches from the occipital cortex to the posterior parietal regions.
- It was conceived of as a ‘where pathway’ but now it is seen as a ‘how pathway’ and
its goal is to guide visuospatial behaviour through motor output
This hypothesis of the ‘how pathway’ led to a whole new framework (Kravitz)
- Kravitz: 3 functional pathways leaving the posterior parietal region:
1. Parieto-premotor pathway – ‘how’ pathway
2. Parieto-prefrontal pathway – visuospatial functions (working memory)
3. Parieto-medial temporal pathway – spatial navigation
Posterior parietal pathway contributes to the dorsal stream by participating in
nonconscious visuospatial behaviour (reaching and grasping objects)
, - Apart from these 3 emphasized pathways, others may be found
14.2 A theory of parietal-lobe function
- We can identify two independent parietal-lobe contributions:
1. Anterior zone – processes somatic sensations and perceptions
2. Posterior zone – specializes primarily in integrating sensory input from the
somatic and visual regions, mostly for controlling movements (specific body parts
but also whole-body movements)
Posterior parietal cortex also plays a significant role in mental imagery (object
rotation and navigation through space)
- Even during simple tasks, your brain faces several complex tasks: directing your eyes
towards the right places, coordinating your movements, attending to certain objects
while ignoring others and making movements in the right order
In order to manage these tasks, we need an internal representation of our
surroundings, that is common to all of our senses: the parietal lobe plays a central
role in creating this map
- This map consists of a series of neural representations of space that vary in two
ways:
1. Different representations serve different behavioral needs
2. Spatial representations vary, from simple to abstract ones
Behavioural uses of spatial information
We need spatial information about the location of objects in the world to 1) direct actions at
those objects and 2) to assign meaning and significance to them. Spatial information is then
just another property of visual information.
There are two basic types of form recognition: one for recognizing objects and one for
guiding movements to objects.
Object recognition
- In order to guide eye, head or limb movements to objects, visuomotor control must
be viewer-centered: the objects location and its local orientation must be
determined relative to the viewer
- Computations about orientation must take place everytime we wish to undertake an
acton, since we are constantly moving
- Details of an object’s characteristics are irrelevant to visuomotor guidance of viewer-
centered movements too much information can be counterproductive for a
system (‘need-to-know’ basis)
- For the object-centered system however, properties of an object are important (e.g.
in order to know where the red cup is relative to the green cup, one must identify all
cups)
- Temporal lobe codes object’s relational properties
Movement guidance
- We require seperate control systems in order to accomodate the many different
viewer-centered movements (e.g. head, limbs, etc.)
- The posterior parietal has a role in visuomotor guidance: most neurons in this region
are active during sensory input and during movement
, - Neurons are sensitive to the features that determine for example the hand’s posture
- Response of posterior parietal neurons have two important characteristics in
common:
1. They receive combinations of sensory, motivational and related motor outputs
2. Their discharge is enhanced when an animal attends to a target or moves towards
it
Posterior parietal lesions impair movement guidance
Sensorimotor transformation
- Sensorimotor transformation – neural calculations that
integrate movements of various body parts with sensory
feedback of what movements are actually being made and the
plans to make the movements (updating of the perceptions of
our body)
Cells in posterior parietal cortex produce movement-related
and sensory-related signals to make these calculations
- Movement planning: when someone is prepating for a
movement, the PRR is active which codes the goal of the
movement rather than the movement itself
This type of study is foundational to developing
neuroprosthetic devices that enable paralyzed people and amputees to use mental
activity to move prosthetics and feel what they touch
Spatial navigation
- Route knowledge – internal list of what we do at each spatial location (cognitive
spatial map)
This is likely located in various places in the brain, including the parietal region
- Neurons in the dorsal visual stream could als be expected to participate in route
knowledge
- The cells in MPR control only body movements to specific locations, whereas cells in
the the PRR control the planning of limb movements to locations
The complexity of spatial information
- Two aspects of the theory of parietal-lobe function:
1. Considering the use of spatial information for recognizing objects and guiding
movement
2. Complexity: other types of viewer-centered representations (apart from limb-or
eye movements) are complex
- The ability to manipulate objects mentally is likely an extension of the ability to
manipulate objects with the hands 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 a
visuomotor control center:
1. Difficulties with arithmetic
2. Aspects of language
, 3. Movement sequences
- Acalculia – the inability to perform mathematical operations because of the task’s
spatial nature
This could explain why people with lesions in the parietal lobe experience
arithmetic difficulties
Arithmetic operations may depend on the polysensory tissue at the left
temporoparietal junction (where the temporal and parietal lobe meet)
- Language has many demands similar to those of arithmetic: some words have the
same letters but a different spatial organization that give them different meanings
Language can thus be seen as quasi-spatial
- People with parietal-lobe injuries have difficulty copying movement sequence : the
deficit in organizing individual behavioral elements can be seen in movement as well
as in language and arithmetic
Chapter 13: The Occipital Lobes
13.1 Occipital Lobe Anatomy
- The occipital lobes lie beneath the occipital bone at the back
of the skull, where they form the posterior pole of the
cerebral hemispheres
- There are no clear landmarks that seperate the occipital
cortex from the temporal/parietal cortex
Connections of the visual cortex
- Nowadays, the hierarchical view that was held in the late 1960s is considered too
simple and has been replaced by the notion of a distributed hierarchical process with
multiple parallel and interconnecting pathways at each level
- A few principles can be extracted:
1. V1 (striate cortex) is the first processing level, receiving the largest input from the
thalamus and projecting it to all other occipital
regions
2. V2 is the second processing level, which also projects
to all other occipital regions
3. After V2, three distinct parallel pathways emerge for
further processing en route to:
Parietal cortex
Multimodal superior temporal sulcus (STS)
Inferior temporal cortex
- Next, two pathways emerge:
Dorsal stream (parietal pathway) – visual guidance of movements
Ventral stream (STS and inferior temporal cortex pathway) – object
perception and perceiving certain types of movements
13.2 A theory of occipital-lobe function
- Areas V1 and V2 are functionally heterogeneous: both segregate processing for color,
form, and motion.
This heterogeneity contrasts with the functions of the areas that follow in the
hierarchy
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