1. 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’

2. 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
Monocular blindness
Bitemporal hemianopia
Homonymous hemianopia
Upper quadrantanopia
Lower quadrantanopia

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:
Subcortical
Dorsal
Ventral
Different streams do different things…

10. Cortical visual processing
Dorsal system is fast, but color blind.
Helps with motor control (Where/How).
Parietal MT V5
Magno
LGN V1 V2 V3
M-ganglion cells
Parvo
LGN
P-ganglion cells V1 V2 IT
cortex V4
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. Three reasons why people might fail to recognize objects:
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. Intact low-level perception
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. Theoretical explanations:
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. Is face processing special?
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. Assal, Faure & Anderes (1984) report zooagnosic farmer MX
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. MX RB
Accuracy RB MX
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
‘man’
Left hand
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.
Helps with motor control (Where/How).
Parietal MT V5
Magno
LGN V1 V2 V3
M-ganglion cells
Parvo
LGN
P-ganglion cells V1 V2 IT
cortex V4
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 shapes (left), DF was poor at saying if the shapes were same or different, RV was good at this task.
100%
chance 0% DF RV
When asked to grasp an object, DF grasped near the centre (like healthy people), RV was poor at this task. DF RV
Control
25%
Frequency 0%
Distance from centre (mm)