Perception

1. Perception Ref: Banich, Ch. 6 pp. 185-198

2. Update • Error in table from last lecture: Language lateralisation - sodium amytal study. Test 1 coming up - details tomorrow

3. The Visual Perception Hierarchy • Initial cortical processing: Primary visual cortex (V1) • Analysis of simple visual features (e.g. colour, movement, orientation, form): Extrastriate cortex (V3, V4, V5/MT) • Higher perceptual analysis: temporal and parietal lobes

4. Two Visual Processing “Streams” • The ventral (“what”) stream: recognition of individual objects • The dorsal (“where”) stream: spatial processing - Later lectures

5. Visual perception • Two “streams” of processing, each hierarchically organised: V3 V5 Dorsal regions rods Magnocellular pathway cones Parvocellular pathway V4 Ventral regions Thalamus Retina Primary Visual Cortex Extrastriate Cortex Temporal/parietal Lobes

6. Two Visual Processing “Streams” • Division in seen clearly in Extrastriate cortex (= regions surrounding PVC) • “What” stream (receive input from Parvocellular pathway): • V4: colour and form • “Where” stream (input from Magnocellular pathway): • V3: orientation • V5 (or MT): movement • Still contralaterally organised (deficits usually only show after bilateral damage)

7. The “What” stream: Extrastriate Cortex Extrastriate regions in the “What” stream do simple analysis of colour and form e.g. V4 Lesion involving V4 can cause: 1. Poor shape discrimination

8. The “What” stream: Extrastriate Cortex 2. Cortical colour blindness (achromatopsia) • A patient with achromatopsia was described as follows: "Everything looked black or grey. He had difficulty distinguishing British postage stamps of different value which look alike, but are of different colours. He was a keen gardener, but found that he pruned live rather than dead vines. He had difficulty distinguishing certain foods on his plate where colour was a distinguishing mark“ • Other comments by achromatopsic patients: "The world seems drained of colour", "Everything is like a black and white movie".

9. The “What” stream: Higher analysis A few years following his massive stroke, GS had recovered most of his cognitive functions. Nevertheless, he still had severe problems recognising common objects. For example, when shown a candle , he said it was a “long object”. Upon touching it, he suggested it might be a crayon. But after smelling it, he finally came up with the right answer. GS’s could clearly “see”: he could perform many simple visual tasks like judging which of two lines was longer, and could describe the colour and general shape of stimuli. He was also able to name and explain the functions of various objects. Photographs were most difficult for GS. When shown a picture of a combination lock, he was dumbfounded. When prompted, he ventured that it was a telephone. He had perceived the numeric markings around the clock’s circumference and this was enough to make him believe that it was a a telephone. Adapted from Gazzaniga et al. (2002) Cognitive Neuroscience

10. The “What” stream”: Object Recognition Can distinguish two processes: • Generating a mental representation of an object e.g. How would it look from another angle? • Assigning meaning to the object e.g. Does it look like anything I know? What else do know about it? These processes are dissociable

11. Generating a Mental Representation • Involves right parietal lobe Not contralateral - RH processes both visual fields

12. Generating a Mental Representation • Translating flat shapes and contours into a complete, 3D mental image of an object • Achieving “Object constancy” or constructing a “Stable percept” • Example: we instantly recognise these as the same object: "Unusual Views test"

13. Damage = Apperceptive Agnosia Sufferers can't identify objects in poor conditions May fail at: • Gollin’s incomplete figures: Unusual Views test Overlapping figures test:

14. Apperceptive Agnosia • Tend to make visual errors: • Generally: • Sensitive to visual quality of stimulus • Problems when no. of visual features is reduced

15. Apperceptive Agnosia Bear in mind: “Apperceptive agnosia” sometimes used more broadly than in this lecture e.g. Banich: - also includes P’s with extrastriate damage (V4)

16. Marr's Theory

17. Marr’s Theory (cont.) • Apperceptive agnosia: can't construct Object-centered representation? • Support: P’s may be poorer when major axis of symmetry is foreshortened: BUT features may also be important:

18. Optional Resource Hypothesis • A range of cues to the underlying percept • (axis, features, contours, etc.) • R. Parietal lobe involved only when no. of cues is reduced

19. Object Recognition (the “What” stream) Can distinguish: • Generating a mental representation of an object e.g. How would it look from another angle? • Assigning meaning to the object e.g. Does it look like anything I know? What else do know about it?

20. Assigning meaning to the object • e.g. Does it look like anything I know? • What else do know about it? Occipito-Temporal border : Both hemispheres seem to be involved, but LH may be most important

21. Damage = Associative Agnosia • In 1887, an 80-year-old salesman GL, returned home exhausted after a severe storm had blown him against a wooden fence, knocking his head. After a few days bed rest, GL got up to discover that although he could still see, he no longer recognised the things about him. Instead, he looked around in a perplexed manner, as if everything was unfamiliar. He thought that pictures in his room were boxes and tried to search in them for things he could not find. He mistook his jacket for a pair of trousers, and at mealtimes he could not recognise the pieces of cutlery on the table. Detailed investigations showed that GL still had almost normal visual acuity for his age, and that he could draw quite accurate copies of seen objects that he could not recognise. Neither had he lost his knowledge of objects; he referred to them appropriately in conversation, and he could recognise them immediately if he handled them, or from characteristic sounds. When shown a whistle, for example, he had no idea what it was, yet he recognised it straight away when it was blown. A post-mortem performed on GL's brain revealed a lesion in the left hemisphere located a the junction between the temporal and occipital lobes.

22. Associative Agnosia • Failure at semantic categorisation • Can derive percept, but not associate with meaning

23. Associative Agnosia • Can't name or describe object • OK at purely perceptual tasks (e.g. unusual Views) • Visual quality much less impt. • Errors not visual, but semantic • Failure at Tests of matching by meaning or function:

24. Associative Agnosia • Copying OK but not drawing-from memory: • Sometimes, have partial info. (e.g. superordinate) • May be OK in other modalities: e.g. "tell me what you know about tigers"

25. Warrington’s model

26. Face Recognition • Banich, Ch. 5: • Faces-humans (pp. 184-8) • Faces-expertise (pp. 188-191)

27. Involves multiple regions: - general visual processing regions - PLUS some dedicated areas (ventral RH) Face Recognition Right Hemisphere Left Hemisphere

28. Face Recognition Face processing has some special properties • good recall poorer recall • strong RH advantage poor lateralisation e.g. Inversion effect in normals:

29. Face Recognition • Inversion effect is weakened after RH damage • Suggests ventral regions of RH have a role in configuration processing

30. Prosopagnosia • Prosopagnosia = inability to recognise faces • Can't tell face from others and/or identify person Thirty years ago, Lincoln Holmes was in a car accident. that rendered him completely "face blind". "In those moments when I am suddenly alone, and I don't know where anybody that I am with is, there can be a surge of fear, and it is lonely in that sense" When shown a series of slides of inanimate objects, he is able to identify them correctly - but finds it completely impossible to recognise a picture of Marilyn Monroe. Even when shown a picture of himself, he has to be prompted before he realises he is staring at his own image. "For me it is a face, it is not my face, and there is some sense of incompleteness there. So be it." "When I am asked by people, 'do faces all look the same?', the answer to that question is 'no' – they don't all look the same, but none of them look like anyone."

31. Prosopagnosia: Two types • Accompanied by more general vis. prob's e.g. Dr. P, The man who mistook his wife for a hat • Selective e.g. Lincoln Holmes P's compensate -> use nonfacial info. or other modalities (e.g. voice) PT no longer recognised the people around him including his wife. However, as soon as his wife spoke, he immediately recognised her voice. Indeed, he claimed that on hearing her voice, the visual percept of her would "fall into place".

32. Perception vs. Identification • Discriminating unfamiliar faces = more posterior • Associating faces with meaning = more anterior

33. Are Faces Special? The “Faces as Special” hypothesis: • Faces are important in evolutionary terms • Therefore, humans have developed dedicated regions/processes Evidence: • Regions involved in face recognition not implicated in other visual tasks • Propsopagnosics not necc. poor at other discriminations, even when complexity controlled for: e.g. cups, cathedrals, similar nonsense drawings, even the faces of cows!

34. Are Faces Special? BUT: • Inversion effect for faces not present in young children • Skills may be applied to related domains e.g. Dogs for show judges • Other types of well-learned complex discriminations show similar effects/engage same regions Greebles:

35. Are Faces Special?