UWORRI fMRI Journal Club

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• UWO/RRI fMRI Journal Club
• Jody Culham
• March 14, 2008

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What is DTI?
Unconstrained diffusion
Constrained diffusion
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Why DTI?

• area connections in normal humans could be
  informative
• help define areas and homologies
• help identify important networks and areal
  interactions
• area connections in neuropsychological patients
  could be informative
• testing hypotheses about residual function
• disconnection syndromes

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Blindsight and Related Phenomena

• Blindsight
• residual visual abilities despite profound visual
  field deficits related to cortical blindness
• may include detection, discrimination, saccades,
  reaching
• many definitions include lack of awareness
• Riddoch Phenomenon (1917)
• spared awareness of moving stimuli despite lack
  of awareness for stationary stimuli

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Blindsight Monkey Helen
Helens visual fields

• bilateral removal of striate cortex

Humphrey, 1974
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Patient D.B.

• early human case of blindsight (Weiskrantz, 1986)

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Patient MC Spared Activation for Motion
May 30, 2007
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Taxonomies of Blindsight

• Weiskrantz (1989)
• Type I blindsight
• residual visual abilities without awareness
• Type II blindsight
• residual visual abilities with awareness
• Danckert Rossetti (2005)
• action blindsight
• ability to saccade or reach in blind field
• attention blindsight
• attentional influences of targets in blind field
  (with or without awareness)
• agnopsia
• ability to guess visual characteristics without
  awareness

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Explanation of Blindsight and Riddoch
Dorsal Stream
PRIMARY VISUAL PATHWAY
Striate Cortex
Retina
LGN
MT
actions
Ventral Stream
Superior Colliculus
Pulvinar
awareness
ALTERNATE VISUAL PATHWAYS
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Hemispherectomies

• removal of one hemisphere of cerebral cortex
• rare treatment for intractable epilepsy
• valuable for interpreting blindsight
• no spared islands of visual cortex
• no LGN pathways

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Spatial Summation
Subjects
blind field
sighted field

• 6 healthy control subjects
• 4 hemispherectomy patients
• 2 with attention blindsight
• spatial summation for black/white targets
• no spatial summation for blue/yellow targets (to
  which the superior colliculus is insensitive)
• 2 without attention blindsight

yields faster reaction times than
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DTI

• 1.5 T Siemens Sonata scanner
• 2.2 mm isotropic voxels
• 60 directions
• b-value of 1000 s/mm2
• FMRIB software (from Oxford)
• probabilistic tract tracing
• allows for an estimation of the most probable
  location of a pathway from a seed point using
  Bayesian techniques
• 5000 samples from each voxel
• threshold results for number of samples that
  arise from seed area
• group analyses
• show voxels in which gt33 of Ss show above
  threshold samples from seed area

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Threshold Doesnt Matter Too Much
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Data Quality Was Validated in Each Subject
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Superior Colliculus Used as Seed
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SC Tracts in Control Subjects
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SC Tracts in IndividualControl Subjects
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Case I (JB) No Attention Blindsight
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Case 2 (FD) No Attention Blindsight
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Case 3 (SE) Attention Blindsight
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Case 4 (DR) Attention Blindsight
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Summary of Results
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Their Conclusions

• patients with attention blindsight show
  connections between intact hemisphere and
  ipsilateral and contralateral SC
• contra connections were stronger than in
  controls
• patients without attention blindsight do not show
  such connections
• conclude that attention blindsight is mediated by
  SC
• caveat DTI can never prove connections are not
  there