ASC disconnectivity theory

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  • Created by: brobs123
  • Created on: 13-01-20 15:38
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  • Disconnectivity Theory of ASC
    • Geschwind & Levitt (2007)
      • ASC = developmental disconnection syndrome
        • Be careful with term because it doesn't mean less connectivity, just different
        • OG theory
          • 3 OG models of dysconnectivity
            • 1) Belmonte et al (2004) ASC as disorder of neural info processing
            • 2) Minshaw et al (1998) ASC as disorder of complex info processing
            • 3)Rubenstein and Merzenich (2003) Imbalance in brain excitation and inhibition
    • Functional connectivity (temporal; EEG)
      • Just et al (2004)
        • Less synchronised activity between frontal and temporal lobes
          • same area used in reading and also found that ASC people less accurate in understanding sentences (although findings weren’t correlated) 
            • Ray et al (2014)
              • other neurodevelopmental disorders (e.g. ADHD/Dyslexia) also show differences in connectivity, what sets them apart from ASC?
                • Reason why dyslexia-adhd-asc all overlap in symptoms?
            • Reason why dyslexia-adhd-asc all overlap in symptoms?
    • Structural connectivity (spatial; fMRI/DTI)
      • Lee et al (2007)
        • low FA seen all over the brain, most commonly reported in association fibres (long-distance cortico-cortical pathways) like superior longitudinal fasciculus (SLF)
          • Noriuchi et al (2010)
            • low FA in SLF associated with social impairments on social responsiveness scale (SRS)
              • SRS doesn't accurately measure genuine social interaction
                • types of people volunteering for studies may have high-functioning ASC as opposed to severity level 3 ASC
      • Walker et al (2012)
        • Low FA in inferior longitudinal fasciculus (connects occipital and temporal lobe), occiptofrontal fasciculus (connects occipital through temporal to frontal lobe) and internal capsule (subcortical pathway that links deep structures like thalamus, putamen to spinal cord and rest of cortex and frontal areas, e.g. frontal motor area)
          • Nair et al (2011)
            • less connections between thalamus and frontal, motor and temporal cortices associated with social and motor impairments as well as general ASC mannerisms – shown in multiple different tests
      • Jou et al (2011)
        • Low FA in unicate fasciculus (connects temporal and frontal lobe; supported by Just et al’s fMRI study), cingulum (around basal ganglia) and corpus collosum
          • Epilepsy is higher in ASC but associated with OC not UC
          • Rudie et al (2012)
            • Low FA in CC associated with social impairments and ASC risk ellele (MET)
              • Jou et al (2011)
                • Found no connection when stats were corrected
    • 3 Cognitive theories of ASC
      • 1. ToM impairments 2. Weak central coherence    3. Executive dysfunction
        • Explaining 1 with UC theory
          • Kana et al (2009)
            • during ‘mental state attribution’ (ToM test; anthropomorphising inanimate shapes), ASC pp’s show less functional connectivity b/w frontal ToM regions and posterior ToM regions
              • BUT only had 12 participants...
        • Explaining 2 with UC theory
          • Damarla et al (2010)
            • found reduced connectivity between frontal and posterior brain regions in ASC, but more activity than controls in visiospatial (posterior visual; short range) areas 
          • Just et al (2012)
            • ASC people excel at activities requiring short range communication but can struggle with complex tasks requiring long range communication
        • Explaining 3 with UC theory
          • Take lang literally
            • Koshino et al (2005)
              • measured EF (exec func) using N-back task. Although they performed similarly, autistic people showed reduced FC between frontal and parietal brain regions
                • Supported by Just et al (2007) who confirmed this
    • Under-connectivity theory of ASC (Just et al - NEW model of dysconnect)
      • Buxhoeveden and Casanova (2002)
        • minicolumns much denser, narrower, excessive in some places and sparser in other, so this local over-connectivity is suggested to upset the balance of excitatory and inhibitory processes in the brain, and to affect long-range communication
      • Rane et al (2015)
        • Reviewed 36 MRI and DTI studies looking at functional connectivity during resting in ASC
          • Found reduced connectivity between resting state regions (DMN) in ASC; these areas also associated with ‘mind-reading’, mentalising and social cognition
          • Also found relationships between lower connectivity (desynchronicity) and ASC symptom severity, or social impairments, language impairments, and restricted behaviours and interests
      • Manella et al (2017)
        • We see reduced connectivity for instance, even when implicitly viewing emotional faces during MEG, autistic people show less synchrony in parts of the social brain and amygdala. 
    • Critical eval of UC
      • Over-connectivity Theory
        • Baron-Cohen (2013)
          • synaesthesia is more common in ASC which is caused by overconnectivity of white matter in sensory pathways (Zamm et al, 2014)
        • Dinstein et al (2011)
          • ASC more interhemispheric communication in the IFG associated with poorer communication skills
        • Monk et al (2009)
          • Greater connection strength between posterior cingulate and right hippocampal gyrus leads to more severe RRBI (restrictive behaviours)
        • Belmonte et al (2004)
          • OC can cause larger noise-signal ratios
            • Mohammad-Rezazadeh et al, 2016
              • Because of this, studies may have innacurate findings
              • results of other studies can’t be generalized considering the vast individuality of ASC symptoms (and the fact you can’t make a level 3 ASC individual sit still in a scanner)
                • Xu et al (2019)
                  • high prevalence of ASC in children (USA), of which 30% weren’t receiving treatment showing a greater need for awareness and education on ASC
        • Power et al (2012)
          • People with ASC move more in scanners which can make brain look UC and appear to have reduced long-range C
            • Di Martino et al (2014)
              • Made ABIDE to compare results and confirmed that ASC shows reduced connectivity in long-range pathways (Picco & Gotts, 2016)
                • Auzias et al (2016)
                  • Still advised caution due to differences between MRI scanners as can distort comparison across data from different places
                  • We need to be very sensitive to age, method, movement, even scanner type when we plan and compare studies.
      • Publication bias
      • Small sample sizes
      • Conti et al (2015)
        • connectivity changes with age (so already hard to generalize) but within first 2 years, ASC children have explosion of hyper-connectivity and brain growth which seems to shift to hypo-connectivity
      • Picci & Gotts (2016)
        • usually studies put wide range of ages into single group rather than looking at trajectories
        • studies supporting long-range underconnectivity have relied on MRI which is less sensitive although MEG has also found patterns of under/over-connectivity
          • No one has yet compared connectivity in the same regions, in the same participants, using different types of neuroimaging!
      • Dajani & Uddin (2016)
        • differences more pronounced in childhood, suggesting that ASC’s develop coping mechanisms OR their brain may reorganise to make up for connectivity issues
    • Conclusion
      • Picci & Gotts (2016)
        • ASC is a mix of both under and over C
          • “targeted under-connectivity among particular cortical regions in combination with subcortico-cortical over-connectivity”
      • Ameis & Catani (2015)
        • ASC accepted as having “altered brain connectivity” and many of the altered connections have been linked to ASC symptoms

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