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The Two Stream Model . 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.
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The Two Stream Model • 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 Visual Streams 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.html
M Cells vs. P Cells • Within the Lateral geniculate nucleus (LGN) • Magnocellular cells (M-cells) are (according to this article) responsible for the detection of movement, depth, and high contrast sensitivity. • 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. • P-cells are believed to feed into the ventral stream. http://antranik.org/peripheral-nervous-system-cranial-nerves/
Contrast Sensitivity Picture from: “X and Y Cells in the Lateral Geniculate Nucleus of Macaque Monkeys” by E. Kaplan and R.M. Shapely
Review Overview • The aim of the review was to evaluate whether impairments within the dorsal stream could explain various visual problems seen within developmental disorders. • 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?
Dyslexia • 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.
Autism Spectrum Disorders • An Autism Spectrum Disorder is characterized by delayed language development along with poor social and communication skills. • 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.
How Involved is the Ventral Stream? • 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 . • 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.
Bertone’s Stimuli Example of second order motion: http://www.youtube.com/watch?v=tGJYSIBETho
Globalization in Autism… • 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. • ex. of plaid motion: http://www.brl.ntt.co.jp/people/nishida/demo/motionindex.html • The autistic group did not differ from the control group, which suggests that global motion is not impaired in these individuals.
Globalization in Autism… • In other studies, it appears that individuals with autism are very skilled at being able to detect embedded figures. • It is natural for most people to view images as a whole; it takes more effort to distinguish individual elements. • According to the Weak Coherence Theory, children with autism have difficulty summing up individual pieces of information in order to create a global concept. • Bertone et al. has proposed that autistic people may have difficulty integrating information from multiple cortical regions.
Autism Continued… • 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. • How much do you think attention plays a role within autism?
Dyspraxia • 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. • 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.
Dyspraxia Continued… • 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. • Global Dot Motion (GDM) task • Their data suggested that children with dyspraxia may have a deficit in global processing in the ventral pathway. • On the other hand, Sigmundssonet 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. • Difficulties in the detection of both global visual motion and the coherent organization of static line segments • Evidence supporting a problem within the dorsal pathway is conflicting and inconclusive, according to the author.
William’s Syndrome • William’s Syndrome is caused by a deletion in chromosome #7. Symptoms of William’s Syndrome include cardiovascular disease, developmental delays, and learning disabilities. • 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. • 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.
William’s Syndrome in Conclusion… • 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.
Fragile X syndrome (FXS) • 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. • 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). • In a follow-up, FXS had higher contrast thresholds for first and second order dynamic and static stimuli. • Koganet 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.
In Conclusion… • 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.
Things to Question • 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?
