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Two neural pathways involved in human vision
Dorsal pathway is thought to be involved in motion
perception, depth perception, and spatial vision.
Ventral pathway is thought to be involved in form
perception, color, and high acuity.
Picture taken from:
http://www.ssc.education.ed.ac.uk
/courses/pictures/vnov07iv0.jpg
The Dorsal Stream
The Ventral Stream
Picture taken from:
http://thebrain.mcgill.ca/flash/a/a_02/a_02_cr/a_02_cr_vis/a_02_cr_vis.ht
ml
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Within the Lateral geniculate nucleus
(LGN)
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Magnocellular cells (M-cells) are
(according to this article) responsible for
the detection of movement, depth, and
high contrast sensitivity.
o M-cells are believed to feed into the
dorsal stream.
• Kaplan and Shapley along with
others argue that there are a
variety of different M-cells
within the LGN, that have a
variety of different functions.
Parvocellular cells (P-cells) are
(according to this article) responsible for
the color, high acuity, and the detection
of slow moving objects.
o P-cells are believed to feed into the
ventral stream.
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http://antranik.org/peripheral-nervoussystem-cranial-nerves/
Picture from: “X
and Y Cells in the
Lateral Geniculate
Nucleus of
Macaque
Monkeys” by E.
Kaplan and R.M.
Shapely
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The aim of the review was to evaluate whether impairments
within the dorsal stream could explain various visual
problems seen within developmental disorders.
o By learning whether a developmental disorder lies within the dorsal
vs. ventral stream, one could hope to understand where a problem
lies within the brain.
• What are potential problems of this theory?
• One problem with this idea is that the pathways are now thought to be
much more complex than they were originally thought to be.
• Another problem: what if the defects are not localized, but a general
problem throughout the brain?
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Developmental Dyslexia refers to a significant impairment in
reading development, without any known impairment in
cognition.
Is dyslexia caused by an impairment in Magnocellular cells?
The author mentions that most studies have concluded that
dyslexic children are less sensitive to motion stimuli.
There seems to be no impairments within the higher-level
ventral stream processing of dyslexic individuals.
All in all, research seems to be in agreement that M-cell
dysfunction may play an important role within dyslexia.
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An Autism Spectrum Disorder is characterized by delayed
language development along with poor social and
communication skills.
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In autism, the perception of facial and body expression is
impaired. Autistic children also do not react as strongly as
normal children in adjusting their positioning when reacting
to a visual stimulus.
This perceptual problem could arise from autistic children
being unable to understand the movement and spatial
locations of objects.
It could also arise from being unable to globalize.
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The author argues that people with autism may be
unimpaired in the lower ventral stream, but have trouble
with integration in high level ventral processing .
o Bertone et al. studied first and second-order translating, radiating
and rotating stimuli. They considered second-order stimuli to be
more “complex” than first order stimuli. They found no significant
differences in direction discrimination with first-order stimuli, but
the autism group needed higher contrast in the second order
stimuli to be able to spot them.
• The second-order stimuli are not necessarily more complex than the
first order stimuli. They are not necessarily processed in places further
up the hierarchy.
Example of
second order
motion:
http://www.y
outube.com/
watch?v=tGJ
YSIBETho
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The author mentions that many studies have reported that
autistic groups do not tend to differ from control groups in
flicker contrast sensitivity. She suggests that the M-cells of
the lower dorsal stream of autistic individuals are intact.
The author refers to a study by Vandenbroucke et al. which
used plaid stimuli to test whether autistic children would
see global motion.
o ex. of plaid motion:
http://www.brl.ntt.co.jp/people/nishida/demo/motionindex.html
o The autistic group did not differ from the control group, which
suggests that global motion is not impaired in these individuals.
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In other studies, it appears that
individuals with autism are very
skilled at being able to detect
embedded figures.
o It is natural for most people to
view images as a whole; it takes
more effort to distinguish
individual elements.
o According to the Weak Coherence
Theory, children with autism have
difficulty summing up individual
pieces of information in order to
create a global concept.
o Bertone et al. has proposed that
autistic people may have difficulty
integrating information from
multiple cortical regions.
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The author concludes their discussion of autism by stating
that there may be a dysfunction in global processing in the
late stages of the dorsal stream and ventral stream, but
more research needs to be done.
o How much do you think attention plays a role within autism?
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Developmental Dyspraxia is a developmental disorder that
often effects fine and gross motor skills. It is characterized
by a lack in coordination, poor balance, and clumsiness.
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Since moving relies heavily upon vision, visual deficits may
be linked to dyspraxia.
The author indicates that the dorsal stream may be
implicated within dyspraxia.
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O’Brien et al. found that children with dyspraxia had an impaired ability
to detect coherent line-segment structure, but their global motion
processing abilities seemed unhindered.
o Global Dot Motion (GDM) task
o Their data suggested that children with dyspraxia may have a deficit in global
processing in the ventral pathway.
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On the other hand, Sigmundsson et al. tested a classroom of children
on a motor movement test. They compared the extreme 25% lowest
scorers to their peers, and found that they had difficulty with GDM and
coherent organization of static line segments.
o Difficulties in the detection of both global visual motion and the coherent
organization of static line segments
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Evidence supporting a problem within the dorsal pathway is conflicting
and inconclusive, according to the author.
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William’s Syndrome is caused by a deletion in chromosome #7.
Symptoms of William’s Syndrome include cardiovascular disease,
developmental delays, and learning disabilities.
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Do visuo-spatial impairments, seen within William’s Syndrome
arise from dysfunctions within the dorsal stream?
Atkinson et al. found in their study of children with William’s
Syndrome that some of the children had trouble with the GDM,
while another group of children had trouble with both the GDM
task as well as the line segment coherence task.
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o Atkinson et al. also followed up on this study with another one that
included adults with William’s Syndrome. The adults also had higher
thresholds in the GDM and the coherent line segment task.
• The author points out that the coherent line segments could be a V1 task,
whereas the Glass patterns are a V4 task.
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The author concludes that there does seem to be
impairment within the dorsal stream in William’s Syndrome,
but impairments of the disease are not limited to the dorsal
stream.
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Fragile X syndrome (FXS) is a genetic disorder. Some of the
developmental characteristics of the disorder include poor attentional
control, linguistic processing, and visuospatial cognition.
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Kogan et al. studied potential dorsal stream deficits by comparing a
group with FXS individuals to age and (mental age) matched control
groups. They found that the FXS group had reduced sensitivity to global
motion and contrast sensitivity at high flicker frequencies for low spatial
frequency stimuli (M-cells).
o In a follow-up, FXS had higher contrast thresholds for first and second order
dynamic and static stimuli.
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Kogan et al. interpreted their results to show a “clear pervasive
impairment of motion perception in FXS”, yet this result must be taken
with a grain of salt, because less than half of their participants were
able to complete both the first and second order task.
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In conclusion, the authors state there does seem to be a
dorsal stream impairment in developmental dyslexia and
Fragile X Syndrome.
Research is incomplete in the assessment of the two
pathways for William’s Syndrome and Dyspraxia.
In autism, there appears to be a problem with globalization
in the late stages of the dorsal stream, but more research is
needed.
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All of the research evidence presented within this review is
psychophysical. What type of other studies should be done
to support the authors’ claims?
Is motion only associated with the M-cells?
The authors seem to support the idea that globalization
occurs at late stages of the dorsal stream. Can globalization
occur throughout the brain, not just at the end of each
stream?
How segregated are the dorsal and ventral pathways? Can
you really separate them by the tasks the authors mention?
Can one localize a developmental disorder to one region or
pathway within the brain?