Topic 10: Faces Flashcards
What is Pareidolia?
Where in the brain is involved?
The ability to detect faces and patterns in organisms and nature leads to a phenomenon called Pareidolia.
The L+R fusiform gyrus
New borns have preference for … faces
6 month old can …
9 month old (and adults) can …
New-born babies show preference for (cartoonish) faces
6 month old infants can discriminate both human and monkey faces
9 months old infants (and adults) can discriminate human faces but lose the ability to discriminate monkey faces
Face perception: features vs configuration
Holistic (configural) face processing
Features: Eyes, mouth, nose etc. Each are identifiable parts that vary in subtle ways across individuals
Configuration: the arrangement of face features (spacing, symmetry, position within face outline)
Holistic processing:Involves integrating information from an entire object
There is evidence that faces (but not other objects) are recognized through holistic processing
Do we engage in featural processing or configural processing or both at the same time (holistic processing)?
Are individual features recognised/remembered?
Features are identified regardless of orientation (no inversion effect)
Recognition of face configuration is poor with upside-down faces
Configural processing: Getting a ‘likeness’
Configural processing is crucial for getting a ‘likeness’ in a portrait, meaning it is crucial to person recognition.
Face recognition is difficult when configuration is disrupted.
Experiments use “composite faces” (separates top and bottom)
Recognition is dramatically impaired; arguing for importance of configuration processing.
The details of the features are less important.
‘Likeness’ appears to be largely carried by the configuration
What is the identity after effect?
A 50:50 mixture of two people’s faces looks like both of them or neither of them in equal measure.
We might expect cells sensitive to the two individuals to be equally active.
Prolonged viewing of one of the two individuals results in cells sensitive to that individual adapting – inhibition over time.
Subsequent viewing of the 50:50 mix results in it appearing more like the other individual.
The neural code has been biased in favour of the “less familiar” or previously un-viewed individual.
Evidence for face-identity cells?
Or evidence of adaptation to local features?
What is the emotion after effect?
Adapting to a happy face makes a neutral one look angry
and vice versa.
What is Bruce and Young ‘face model’ A psychological face processing model
Two main stages:
Structural encoding occurs first (deals with viewpoint, lighting and visual analysis of visual object)
Extended processing splits to two separate pathways Expression analysisFace recognition
Coding expression (changeable) and coding identity (fixed) use separate pathways.
Familiar and unfamiliar faces are processed differently.
A model of the distributed human neural system for face perception Haxby, Hoffman, and Gobbini (2000)
Forward-backward interaction.
LTM influences (early) structural encoding stage.
Emotion expression is more tightly linked to person knowledge (identity) and can also influence structural encoding.
ERP’s (Event related potentials) and faces: N170
Put surface electrodes on the scalp; record tiny voltages changes.
Start voltage measurement when a picture onsets.
Do this many times.
Average the signals in a time locked manner.
The resultant waveform is called an ERP.
A strong negative going signal appears about 170ms post-stimulus when faces are presented as stimuli.
This is called the N170 and is an ERP signature for faces.
N170 is more obvious with face pictures than other objects
Assumed to reflect structural encoding because it does not generally vary with
expression
familiarity
viewpoint
But… N170 may be also caused by expertise or simply by highly similar stimuli.
Used different stimuli and recorded ERPs. Real faces and cartoon both yield N170.
Shows that brain processes able to extract “faceness” from abstractions of faces.
fMRI and face processing
Basic experiment
Present a series of images in different categories
Face
Houses
Chairs, etc.
Scrambled non-object images
Subtract activation for ‘scrambled’ from each of the object categories.
Find specific areas of brain that are activated more by one category than other.
Right OFA is critical for part based identification
Is it possible to pinpoint the time this is taking place?
Repetition Suppression Effect (adaptation)
Repetition Suppression (RS): Brain activity reduced for stimulus repetition.
Brain areas that show RS are selectively sensitive to the repeated feature.
Sample study.
Condition 1:
Present different images of the same person making different expressions (ID repetition).
If RS occurs, then the brain area is sensitive to face identity and is not sensitive to facial expression.
Finding: RS is found in FFA and posterior STS.
These areas “see” identity; not expression.
Condition 2:
Present images of different people making the same expression (expression repetition).
If RS occurs, then the area is sensitive to face expression and not sensitive to face identity.
Finding: RS is found in anterior STS. Codes expression, not identity
Conclusion: face identity and face expression are coded by separate networks in the brain.
Prosopagnosia and Capgras delusion
Prosopagnosia results in a failure to overtly recognise people even if covert measures suggest recognition.
Ventral route affected.
May include
FFA.
Capgras delusion or syndrome results in a feeling that people are imposters even though they are overtly recognised. Dorsal route affected. May include STS.
How do we read facial expressions?
Face muscles and eyes play a key role.
Positive expressions:
Involves muscles near eyes & mouth.
A “genuine” smile (b) involves eyes + mouth
a polite smile (a) involves only the mouth (social conversation technique)
Face expression
Face muscles and eyes play a key role.
Negative expressions: eyes, eyebrows, mouth
Different muscles are used for neg versus positive expressions
Reading a face requires attention to eye muscles, mouth position and their effects on nose, cheeks & chin
Eye movements to faces reveal how information is acquired
