Perception

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

Update

1. Error in table from last lecture: Language lateralisation - sodium amytal study.

LH Bilateral RH

Right handers 96% 0% 4%

Left handers 70% 15% 15%

2. Test 1 coming up - details tomorrow

The Visual Perception Hierarchy

1. Initial cortical processing: Primary visual

cortex (V1)

2. Analysis of simple visual features (e.g.

colour, movement, orientation, form):

Extrastriate cortex (V3, V4, V5/MT)

3. Higher perceptual analysis: temporal and

parietal lobes

Two Visual Processing “Streams”

1. The ventral (“what”) stream: recognition of individual objects

2. The dorsal (“where”) stream: spatial processing - Later lectures

Visual perception

• Two “streams” of processing, each hierarchically

organised:

Temporal/parietalTemporal/parietalLobesLobes

RetinaRetina ThalamusThalamus PrimaryPrimaryVisualVisualCortexCortex ExtrastriateExtrastriate

CortexCortex

rods

cones

Magnocellular pathway

Parvocellular pathway

V4

V3V5

Dorsal regions

Ventral regions

• 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)

Two Visual Processing “Streams”

Extrastriate regions in the “What” Extrastriate regions in the “What” stream do simple analysis of stream do simple analysis of colour and formcolour and form

e.g. V4e.g. V4

Lesion involving V4 can cause:Lesion involving V4 can cause:

1. Poor shape discrimination1. Poor shape discrimination

The “What” stream: Extrastriate Cortex

•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".

2. Cortical colour blindness (2. Cortical colour blindness (achromatopsiaachromatopsia))

The “What” stream: Extrastriate Cortex

The “What” stream: Higher analysisA 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 NeuroscienceAdapted from Gazzaniga et al. (2002) Cognitive Neuroscience

The “What” stream”: Object Recognition

Generating a mental representation of an object e.g. How would it look from another angle?

Assigning meaning to the objecte.g. Does it look like anything I know?

What else do know about it?

Can distinguish two processes:Can distinguish two processes:

These processes are dissociableThese processes are dissociable

Generating a Mental Representation

• Involves right parietal lobe

Not contralateral - RH processes both visual fieldsNot contralateral - RH processes both visual fields

Generating a Mental RepresentationTranslating 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""Unusual Views test"

Damage = Apperceptive Agnosia

3.Gollin’s incomplete figures:

1. Unusual Views test

2. Overlapping figures test:

Sufferers can't identify objects in poor Sufferers can't identify objects in poor

conditionsconditions

May fail at:May fail at:

Apperceptive Agnosia

• Tend to make visual errors:

GenerallyGenerally::

Sensitive to visual quality of stimulusSensitive to visual quality of stimulus

Problems when no. of visual features is reducedProblems when no. of visual features is reduced

Apperceptive Agnosia

Bear in mind: Bear in mind:

““Apperceptive agnosia” sometimes used more broadly Apperceptive agnosia” sometimes used more broadly than in this lecturethan in this lecture

e.g. Banich: e.g. Banich: - also includes P’s with extrastriate damage (V4)- also includes P’s with extrastriate damage (V4)

Marr's Theory

Viewer-centred representation

Position of objects, surfaces relative to viewer

Object-centred representation

Position of objects, surfaces relative to each other

Basic 2D image ("Primal sketch")

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:BUT features may also be important:

Optional Resource Hypothesis

A range of cues to the underlying percept

v (axis, features, contours, etc.)

R. Parietal lobe involved only when no. of

cues is reduced

Object Recognition (the “What” stream)

Generating a mental representation of an objecte.g. How would it look from another

angle?

Assigning meaning to the objecte.g. Does it look like anything I know?

What else do know about it?

Can distinguish:Can distinguish:

Assigning meaning to the object• e.g.Does it look like anything I know?• What else do know about it?

Occipito-Temporal border :Occipito-Temporal border :

Both hemispheres seem to be involved, but LH may Both hemispheres seem to be involved, but LH may be most importantbe most important

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.

Associative Agnosia

Failure at semantic categorisation

Can derive percept, but not associate with meaning

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:

Associative Agnosia

Sometimes, have partial info. (e.g. superordinate)

May be OK in other modalities:

e.g. "tell me what you know about tigers"

Copying OK but not drawing-from memory:Copying OK but not drawing-from memory:

Warrington’s model

Face Recognition

• Banich, Ch. 5:

• Faces-humans (pp.

184-8)

• Faces-expertise (pp.

188-191)

Face Recognition

ØInvolves multiple regions:

- general visual processing regions

- PLUS some dedicated areas (ventral RH)

Left HemisphereLeft HemisphereRight HemisphereRight Hemisphere

» good recall poorer recall» strong RH advantage poor lateralisation

•

Face Recognition

e.g. e.g. Inversion effect Inversion effect in normals:in normals:

Face processing has some special propertiesFace processing has some special properties

Face Recognition

Inversion effect is weakened after RH damage

Suggests ventral regions of RH have a role in configuration processing

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 Thirty years ago, Lincoln Holmes was in a car accident. that rendered him completely "face blind". "In those moments when I am suddenly 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 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 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 slides of inanimate objects, he is able to identify them correctly - but finds it completely impossible to recognise a picture of Marilyn Monroe. finds it completely impossible to recognise a picture of Marilyn Monroe. Even when shown a picture of himself, he has to be prompted before 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 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." 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 "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 to that question is 'no' – they don't all look the same, but none of them look like anyone."look like anyone."

Prosopagnosia: Two types

1. Accompanied by more general vis. prob'se.g. Dr. P, The man who

mistook his wife for a hat

2. Selective e.g. Lincoln Holmes

PT no longer recognised the people around him PT no longer recognised the people around him

including his wife. However, as soon as his wife spoke, including his wife. However, as soon as his wife spoke,

he immediately recognised her voice. Indeed, he he immediately recognised her voice. Indeed, he

claimed that on hearing her voice, the visual percept of claimed that on hearing her voice, the visual percept of

her would "fall into place".her would "fall into place".

P's compensate P's compensate

-> use nonfacial info. or other modalities (e.g. voice)-> use nonfacial info. or other modalities (e.g. voice)

Perception vs. Identification

Discriminating unfamiliar faces = more posterior

Associating faces with meaning = more anterior

Are Faces Special?

Faces are important in evolutionary terms

Therefore, humans have developed dedicated regions/processes

Regions involved in face recognition not implicated in other Regions involved in face recognition not implicated in other visual tasksvisual tasks

Propsopagnosics not necc. poor at other discriminations, Propsopagnosics not necc. poor at other discriminations, even when complexity controlled for:even when complexity controlled for:

e.g. cups, cathedrals, similar nonsense drawings, even the e.g. cups, cathedrals, similar nonsense drawings, even the faces of cows!faces of cows!

EvidenceEvidence::

The “Faces as Special” hypothesisThe “Faces as Special” hypothesis::

Are Faces Special?

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

BUT:BUT:Greebles:Greebles:

Are Faces Special?