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Perception Chris Rorden Lecture 8: Vision and perception

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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)

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