C82MHA Statistics by HC120728052330

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       Perception
  • Chris Rorden
  • Lecture 8: Vision and perception
    • Low level visual deficits:
       • Visual field defects
           • Blindsight
       • Achromatopsia :: cortical colorblindness (V4)
       • Akinetopsia :: motion perception (MT/V5)
       • Agnosias :: apperceptive, associative, prosop- (FFA)
           • ‘How’ versus ‘what’




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    Vision

Human vision:
 Lots of real-estate
                                3

     Visual Pathway

 Each eye sees both left
  and right visual field.
 Ipsilateral information
  crosses over at optic
  chiasm.
 Some connections to
  superior colliculi.
   – Reflexive eye movments
 Others go to thalamus
  (lateral geniculate nuclei)
  and then cortex.
                             4

       Visual Defects

 Field defects reveal
  anatomical injury
  A. Monocular blindness
  B.
  C. Bitemporal hemianopia
  D. Homonymous
     hemianopia
  E. Upper quadrantanopia
  F. Lower quadrantanopia
  G. Homonymous
     hemianopia
                       5

    V1

Primary visual
 cortex (V1) lies in
 calcarine fissure.
Complete
 damage leads to
 Homonymous
 hemianopia.
Partial damage
 leads to scotomas
                               6

    V1 – retinotopic mapping

V1 is retinotopic:
 distorted spatial map
 of visual scene
Fovea has massively
 over represented.
                                                    7

     V1 damage and blindsight

 People with damage to V1
  fail to report objects
  presented in their field
  defect.
 However, when forced to
  guess, they can accurately
  point to location of unseen
  visual stimulus!
 Can also accurately report
  direction of motion.        Weiskrantz et al., 1974
                                                             8

     Implications of Blindsight

 V1 is crucial for conscious awareness.
 What explains blindsight? Why do only 20% of
  V1 patients show blindsight?
  – Incomplete damage to V1? Islands of spared tissue
    (Gazzaniga, 1994).
  – Typically seen in people who had injury while young
    – neural plasticity?
  – Small number of indirect connections to later cortical
    visual centers?
  – Visual connections to colliculi?
                                         9

       The visual processing stream

 Three major streams of vision:
  1. Subcortical
  2. Dorsal
  3. Ventral




Different streams do different things…
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          Cortical visual processing

  Dorsal system is fast, but color blind.
                                                     Parietal
  Helps with motor control (Where/How).

                                                MT
                                                V5


                    Magno       V1       V2    V3
M-ganglion cells     LGN

P-ganglion cells     Parvo       V1       V2
                     LGN
                                                V4      IT
                                                      cortex
   Ventral system is slow, but detailed.
   Helps with object identification (What).
                                             11

    Achromatopsia :: V4
Achromatopsia is usually caused by
 bilateral damage to V4 - lingual and
 fusiform gyri (occipitotemporal junction)
 and is characterized by an inability to
 identify or discriminate colour
Still able to perceive form and motion
                                                                                 12

          Akinetopsia (Motion Blindness)
 Zilles reported first case of akinetopsia. Pure cases are rare,
  as requires bilateral injury.
   – Case LM - akinetopsia
        43 yr old. Sinus vein thrombosis
        V5 damaged bilaterally - V1 spared
        Could not see movement of objects but could see still objects. People
         would suddenly appear
        Diagnosed as agoraphobic
        Can see movements/reach for/catch very slow moving objects (< 10°/s)
                                                    13

     V5 timecourse

 Beckers & Zeki (1995) examined brief V5
  disruption using TMS.
 Motion perception disrupted most with V5
  stimulation up to 30ms after visual stimulation
  onset
 V1 stimulation also partially disrupts motion
  perception, but later (60-70ms after VS onset).
 Takes 30-50ms for signals to go from V1 to V5
  – Direct route to V5?
  – Reafference to V1?
  – May explain motion performance in blindsight?
                                                       14

    Agnosias

Three reasons why people might fail to
 recognize objects:
  – Perceptual Deficit: e.g. acuity, field cut, loss
    of color vision
  – Apperceptive agnosia: unable to perceive
    full shape of object despite intact low level
    processing.
  – Associative agnosia: ability to perceive
    shape, but unable to recognize it.
                                15

    Apperceptive agnosia

Intact low-level perception
  – acuity
  – brightness discrimination
  – color vision
Unable to recognize objects
Unable to extract global
 structure.
                          16

    Associative agnosia

Able to see whole
 form of shapes
No problem
 copying figures
However, unable to
 recognize the
 objects
                                                  17

    Associative agnosia

Theoretical explanations:

  – Disconnection between visual representation
    and language?

  – Damage to visual memory representation?

  – Slightly impaired perception?
                                   18

    Anatomical considerations

Apperceptive agnosia:
  – right inferior parietal lobe
    (Middle Cerebral Artery)




Associative agnosia:
  – left occipitotemporal
                                            19

    Prosopagnosia

Wigan (1844), Quaglino & Borelli (1867),
 Hughlings Jackson (1872), Charcot &
 Bernard (1883), Wilbrand (1892)
Inability to visually recognize faces
Even a spouse’s face does not seem
 familiar
                                               20

    Prosopagnosia - specificity

Seems specific to faces. Patients can still
 recognize others by:
  – Silhouette
  – Voice
  – Clothing
Note: not like amnesia
                                             21

    Prosopagnosia

Is face processing special?
Or, are faces simply the most difficult
 objects we discriminate?
Most people withprosopagnosia have
 difficulty recognizing differences within
 categories:
  – types of car
  – porcelain fixtures
  – breed of dog
Also, often suffer achromatopsia
                                          22

     Faces are difficult

 Most objects are identified by unique
  components
 However, faces have the same basic
  components:nose, eyes,ears, hair
                                                       23

     Are faces special?

 Farah tried to find objects as difficult as faces:
   – Spectacle frames
   – Undergrads recognized 87% of faces, 67%
     of eyeglass frames (faces easier)
   – LH recognized only 64%
     of faces, and 63% of
     eyeglass frames
                     24

Are faces special?
                     25

Are faces special?
                                    26

     Are faces special?

 Farah examined inversion effect
 Sequential matching task
 Undergrads:
  – Upright: 94% correct
  – Inverted: 82% correct
 Prosopagnosic LH
  – Upright: 58%
  – Inverted: 72%
                                        27

    Double dissociations

Assal, Faure & Anderes (1984) report
 zooagnosic farmer MX
  – Lost ability to recognise cows
  – Still recognises faces
Bruyer et al (1983) report reverse
  – Fails to recognise faces
  – Intact perception of cows
                                                      28

     Double Dissociation

 If faces are simply difficult, we should not find
  patients with spared face recognition who are
  impaired on other tasks.
      Accuracy



                  MX




                                MX
                         RB




                                     RB
                 Faces          Cows
                                                    29

     Prosopagnosia

 Selective to faces in a few patients
 Unable to recognize faces
 Able to discriminate equally difficult objects:
   – cows
   – office furniture
   – spectacle frames
 Why are ‘pure’ prosopagnosics so rare?
   – Lesions tend to be large?
   – Overlap in processing in most patients?
 Functional imaging can resolve this question
                                30

    Anatomical considerations

 Fusiform gyrus.
 Usually bilateral,
  occasionally right
  hemisphere only (Landis
  et al. 1986)
 Near V4 (color vision)
 Functional imaging
  gives convergent
  evidence (Sergent &
  Signoret 1992)
                                       31

      Vision in split brain patients

 Commissurotomy is
  neurosurgical treatment
  for intractable epilepsy
  where the Corpus
  callosum is completely
  divided.
 Allows systematic
  investigation of
  hemispheric
  specialization and
  integration
                                                     32

      Split brain patients

 By using rapid
  (tachistoscopic) stimuli
  we can avoid eye
  movements.
 Using chimeric faces,
  Sperry projected
  different images to each
  hemisphere.
 Most able to return to
  work within 2 years of
  surgery.
 Typically, appear healthy

                              Language   Left hand
                                ‘man’     woman
                                   33

      Split brain patients

 Picture presented in
  RVF (i.e. to LH)
   – Patient could name or
     reach for the object
     correctly with right hand.
 Picture presented in LVF
  (i.e. to RH)
   – Patients could not
     name/describe the object
   – Subjects could reach for
     the correct
   – object with their left hand
 Likewise, unable to find
  a object felt with one
  hand by using the other
  hand.
                                                        34

       Split brain patients
 Left hemisphere clearly specialized in language.
 Right hemisphere appears better at copying designs,
  reading facial expressions, fitting forms in molds
 Similar effects can be seen in healthy people, e.g.
  most think A and C look more similar than A and B
                                                                35

          Cortical visual processing

  Dorsal system is fast, but color blind.
                                                     Parietal
  Helps with motor control (Where/How).

                                                MT
                                                V5


                    Magno       V1       V2    V3
M-ganglion cells     LGN

P-ganglion cells     Parvo       V1       V2
                     LGN
                                                V4      IT
                                                      cortex
   Ventral system is slow, but detailed.
   Helps with object identification (What).
                                                                  36

       Visual Form Agnosia

 DF has ventral damage
  – Profound agnosia :: can not even tell orientation of object
  – Motor control accurate :: motor system functions
    accurately.
       Posting task                     Patient DF   Controls

                           Perceptual
                            matching




                             Posting
                                                                        37
Ventral vs Dorsal damage (Goodale et al. [1994]
Curr Biol. 4:604-610)


           When shown two                                         100%
            shapes (left), DF was
            poor at saying if the                                 chance
            shapes were same or
            different, RV was                                     0%
            good at this task.                      DF RV
When asked to grasp                       Control   DF          RV
 an object, DF               25%

 grasped near the         Frequency

 centre (like healthy
 people), RV was
 poor at this task.          0%
                                      0        15 0     15 0      15     30
                                                 Distance from centre (mm)

								
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