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Importance of faces: Central role in human interactions Contribute to speech perception (McGurk effect) Communicate a we

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)

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Importance of faces: Central role in human interactions Contribute to speech perception (McGurk effect) Communicate a we

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  1. 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)

  2. 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?

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

  4. 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”)

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

  6. Now find the animal face. Ready?

  7. Pop-out effect for faces! (Herschler & Hochstein, 2005)  demonstrates our expertise in face processing

  8. 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)

  9. 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

  10. 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/

  11. 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!

  12. 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)

  13. 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)

  14. 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)

  15. 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

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