Importance of faces:

parathyroidsanchovyAI and Robotics

Nov 17, 2013 (3 years and 8 months ago)

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Importance of faces:


Central role in human interactions


Contribute to speech perception (McGurk effect)


Communicate a wealth of social information:


Age, gender, personal identity (physical structure)


Mood and emotional state (facial expression)


Interest / attentional focus (direction of gaze)


Faces as visual stimuli:


Faces as a category

highly homogenous (similar)


Share basic component parts in a fixed configuration


(2 eyes over a nose over a mouth inside an ellipse)


Individual faces

highly different


Vary in many dimensions, including head shape, individual features,
relative feature placement, color, texture, etc.


Dynamic and changeable due to movable parts that change shape
and relative position


Example: a smile vs. an angry frown


the same face?













A single face can produce radically different images on our retina when it changes
expression and/or orientation (Farah, 2000).

Demands of face recognition:


Generally same as for object recognition


Recognition in context (object in a clutter of other objects in the scene)


Object invariance (across different viewpoints, sizes / distances, &
illuminations; in motion, etc.)


Specificity (matching visual object to specific semantic description)


But
--

faces require higher specificity!

(exemplar vs. category level)


In most situations, we need to recognize a
specific individual face

(e.g.,
as Joe Smith) rather than the general category (“face”)


Find the human face in the display as fast as you can. Ready?



Now find the animal face. Ready?




Pop
-
out effect for faces! (Herschler & Hochstein, 2005)





demonstrates our expertise in face processing

Perceptual expertise:


Humans are experts at face processing (Diamond & Carey, 1986)


Effects of accumulated lifelong experience & daily practice


Face representations always at least partially activated?


Biological predisposition?


Newborn infants will detect and track a human face more readily than
another visual object (Johnson & Morton, 1991)


Prosopagnosia:


Impairment in face recognition (“face blindness”)


Cannot

recognize familiar faces or own face in the mirror


Can

recognize faces as a category vs. other objects


Can

recognize familiar people by voice and other non
-
facial clues


Vision otherwise OK


Due to brain injury (typically to the right temporal lobe)


Socially crippling


What is it like to be face blind?





People who are ‘tone deaf’ are not deaf to tones.
They can hear tones, they just can't tell them apart.
People who are ‘color blind’ can see things that are in
color. They just can't tell colors apart. Similarly,
I can
see faces. I just can't tell them apart
.”


“If you are face blind, in social settings, or even
when watching TV, people will have come and gone long
before you can identify them. So you never do. By the
time
eight seconds

have passed, people in your
presence who don’t know of your face blindness will be
offended at your failure to recognize them. And long
before you even get your eight seconds, you know you will
be criticized for ‘staring’…”


--

Bill Choisser,
Face Blind!


www.choisser.com/faceblind/


The Capgras delusion:


A form of delusional misidentification due to brain injury


Patients claim that their relatives have been replaced by identical
-
looking
impostors, clones, robots, Martians, etc. (Capgras & Reboul
-
Lachaux, 1923)


Recognize relatives visually but have
a deeper, overwhelming sense

that they
are unfamiliar, strange, not who they claim to be, etc.


Carries a serious risk of violence


Loss of appropriate
emotional response

to visual stimuli?



Emotional recognition is faster than perceptual recognition!




Theories of face recognition:


1) Specialized face module


Functionally and anatomically separate


Processes faces
only


2) Faces processed by the general visual system (no
specialized face module)


Specialized face
-
module hypothesis:


Fusiform Face Area (FFA)
?


Right inferior temporal cortex


Along the ventral / occipitotemporal “what” pathway


Case studies of prosopagnosia


damage typically to FFA


Single
-
cell recordings in monkeys:
face cells
? (Baylis et al., 1985)


fMRI studies of humans: FFA selectively activated by faces
(Kanwisher et al., 1997)


But




Evidence that prosopagnosia
not

limited to faces


FFA varies in size and location between individuals


FFA also activated for non
-
face objects (e.g. in dog experts)


Alternative hypothesis:


FFA = Flexible Fusiform Area
? (Tarr & Gauthier, 2000)


FFA as a system specialized for fine discriminations / subordinate
categorization


Processes
all complex homogenous objects
(not just faces)


Activation of FFA increases with expertise