Dorsal Stream Vulnerability Flashcards
Outline two streams
The ventral stream is thought to be important for processing global form information which is required for object recognition (Gallant et al, 1993).
The dorsal stream is believed to be important for understanding motion and spatial layout and translating these into the control of action (Mishkin et al, 1983)
Measures of _____ can serve as comparable indicators of ventral and dorsal stream function
Measures of sensitivity to global form and global motion can therefore serve as comparable indicators of ventral and dorsal stream function, respectively.
In ____, Braddick, ____ and _____ suggest the DS is vulnerable during development
How did they evidence this?
Which disorders?
In 2003, Braddick, Atkinson and Wattam-Bell suggested that the dorsal stream (DS) is vulnerable during development.
They evidenced this claim with a body of evidence suggesting that a variety of developmental disorders show anomalies in the detection of global motion
Often, this anomalous motion perception was accompanied with normal performance on tasks thought to tax the ventral stream.
These disorders include developmental dyslexia, autism spectrum disorders, dyspraxia and fragile-x syndrome.
Hansen et al: Dyslexia
For or against DSV?
The majority of studies investigating high-level processing in the DS indicate that children with dyslexia are less sensitive than control to coherent motion stimuli (Hansen et al, 2001).
All studies assessing high-level processing in the VS have found intact abilities when comparing dyslexic PS with controls (Hansen, 2001).
Therefore, in the case of dyslexia, it appears that the visual deficit is restricted to the DS, consistent with DS vulnerability hypothesis.
what is dyslexia?
Individuals with developmental dyslexia have difficulties with accurate and/or fluent word recognition and spelling despite adequate instruction and intelligence and intact sensory abilities
ASD and the VS lower level processing
(Davis et al, 2006) ASD PS showed better performance on contrast sensitivity tests
(de Jonge et al, 2007) equal performance on orientation tasks
Commonly reported motor function deficits in ASD PS are most likely linked to specific impairments in DS perception
Rinehart et al, 2001
ASD performance on coherence thresholds for global structure in ______ ______ tasks
(Spencer et al, 2000)
ASD PS have been found to exhibit comparable performance on coherence thresholds for global structure in line segment tasks when compared to controls
sub-groups of ASD PS impaired at ____ _____ _____ which is consistent with anomalous processing in the VS
Spencer & O’brien, 2006
glass pattern thresholds
ASD and higher-level DS
(Davis et al, 2006)
higher motion coherence thresholds in individuals
with high functioning autism compared to matched control groups on GDM tasks
ASD and lower-level DS
Pellicano et al., 2005
Lower-level dorsal stream functioning: intact flicker contrast sensitivity thresholds in individuals with high functioning autism when compared to age- and non-verbal IQ-matched controls
ASD PS are good at what and impaired at what? Indicative of what?
Impaired global motion thresholds in the presence of intact flicker contrast sensitivity thresholds is indicative of impairment in global processing at the higher levels of the dorsal cortical stream
Two possibilities for ASD discrepancy?
- Assuming DSV hypothesis is correct: shows vulnerability is restricted to high-level of DS
- Assuming this discrepancy highlights failure of DSV hypothesis to account for deficits - This contradiction shows the need to consider alternate theories…
Alternative ASD theory
Evidenced by what?
(Frith, 1989).
In an attempt to account for both the strengths and weaknesses seen in ASDs, Weak Central Coherence theory was proposed:
Under this account, children with ASDs have difficulty combining local information to create a coherent global percept, a consequence of which can be their superior performance on tasks that require attention to details
For example, (Grinter et al, 2009). ASD PS show superior performance in detecting embedded figures and in reproducing block designs compared to controls
overall conclusion for ASD?
The evidence for the DSV is relatively weak in ASD as they show intact low-level functioning in the DS and some sub-groups show impairments in the ventral stream.
However, more work is needed regarding the VS to assess their abilities further.
What is dyspraxia?
Clumsiness, lack of coordination and poor balance (i.e DS) are some of the most noticeable features of developmental dyspraxia (Motohide & Mobs, 1995)
O’Brien () measured thesholds on a ____ task and compared them to thresholds on a ____ _____ ____ task in dyspraxia children
Found what?
Suggests what?
O’Brien et al (2002) measured thresholds on a GDM task and compared them to thresholds on a line-segment contour detection task.
Children with dyspraxia were impaired in the ability to detect coherent line-segment structure but global motion processing ability was unaffected compared to an age and verbal mental-age matched control group
Suggests an anomaly in the ventral visual stream!
Sigmundsson et al () applied the same measures as were used by Hansen et al () with children that scored highly on ______
Found what?
Therefore?
Sigmundsson et al (2003) applied the same GDM and coherent line segment measures as were used by Hansen et al (2001)
Investigated children with the 25% most extreme scores on the Movement ABC test attending a regular classroom i.e. not dyspraxic children
They reported that developmental clumsiness was associated with difficulties in the detection of both global motion and the coherent organisation of static lines
Therefore, DS deficits arising in these cases are accompanied by VS deficits too
overall conclusion for dyspraxia?
No evidence for vulnerability exclusive to the DS!
What is fragile-x syndrome?
Associated with weaknesses in attentional control (Munir et al, 2000) , linguistic processing (Belser et al, 2001) and visuospatial cognition (Cornish et al, 1998)
Kogan et al () assessed FSX PS on a variety of visual tasks
Found what in terms of DS?
2004
GDM and line segment stimuli
FX PS had reduced sensitivity to global motion
Also showed reduced M cell contrast sensitivity
No differences in P cell contrast sensitivity
FSX group had elevated thresholds on both first and second order motion stimuli for direction discrimination
CONSISTENT with DSV hypothesis
What did Kogan et al () find in terms of the VS and FX?
What does this suggest?
form processing deficit for only second-order stimuli
Suggest that there may be a “generalised cortical dysfunction in integrative mechanisms of early visual input regardless of its source”
Summary of deficits?
It is only Dyslexia which shows deficits confined to the dorsal stream.
However, the fact that all other conditions show contradictory deficits suggests that the the dorsal stream is no more vulnerable than the ventral stream.
methodological critique
The DS vulnerability hypothesis was based mostly on the psychophysical studies that measure coherence thresholds for global motion as an index of dorsal stream functioning…
However, there are multiple stages within both of these pathways and it is unlikely that a single task could capture processing at every level of either stream
What do Braddick et al (2003) suggest in terms of the time course of DS and VS development?
present a body of evidence suggesting that functioning within the ventral visual stream matures earlier than dorsal stream functioning
They propose that the later development of the DS provides a greater opportunity for anomalous development to impair functioning within this pathway
Thus, when a disorder is present, the DS may be more susceptible to impairment
They suggest that this vulnerability is not specific to one particular condition
Evidence against DS developing later?
There is evidence to suggest that for global processing tasks the distinction between DS and VS may be more equivocal, with some studies reporting adult-like performance at similar ages for global dot motion tasks (Gunn et al, 2002) and glass pattern tasks (Lewis et al, 2004)
(Parish et al, 2005) reports that sensitivity to global form develops later than sensitivity to coherent motion
There is evidence from visually deprived populations suggest that the dorsal stream is, on the one hand, highly plastic in early life and on the other hand, very resistant to subsequent experience once it is set
Dormal ()
What evidence?
2012
Fine et al (2003) - Patient MM
Lost vision at 3.5y/o, restored at 46 in one eye.
Results
Normal perception of colour and motion
fMRI: normal cortical responses to visual-motion stimuli
Modest deficit in simple form
Severe deficits in complex form, object and face processing
fMRI: lack of category-selective responses for faces and objects in ventral visual cortex
Ostrovsky and colleagues (2009)
Reported the cases of three supposed congenital blind individuals whose vision was partially restored after years of blindness.
Following cataract removal, these individuals presented marked difficulties in form and depth perception when looking at static images.
However, the introduction of motion cues immediately improved these perception abilities.
Based on sight-recovery patients…
it seems dorsal based abilities e.g. motion are spared and can be recovered, even following blindness.
- Suggests that such abilities don’t require prolonged visual experience in order to crystallise, unlike more ventral-related visual functions e.g. face perception
- Suggests the dorsal stream is actually less vulnerable (at least in cases of visual deprivation) than ventral functions
Conclusion
Evidence from developmental disorders does not show consistent dorsal stream deficits
- Although Dyslexia shows rather consistent dorsal stream deficits, this does not prove that the dorsal stream is vulnerable across all conditions (as suggested by Braddick, 2003)
Evidence from sight-recovery patients in fact show the opposite pattern - one of VS vunerability
Therefore, evidence for DSV hypothesis is weak as shit
