Object and face perception Flashcards
what do we mean by object perception?
we don’t just see features - we automatically put them together into meaningful parts of the scene
what is feature detection?
- the brain is selective for different features i.e. colour, orientation and spatial frequency
low-level analysis:
encoding features: brightness, colour, contrast, spatial detail, orientation, texture
high-level analysis:
understanding the meaning - (the famous smile - da Vinci)
grouping
the brain has automatic rules for deciding how to organise features
what goes together to form an object?
6 features
- good continuation
- closure
- proximity
- similarity
- common fate
- pragnanz (simplest explanation)
what happens at a luminance boundary?
2 points
- in regions of equal luminance, excitation and inhibition cancel each other
- at the boundary, excitation and inhibition are not balanced and thus increase the relative difference of perceived brightness
contours for visual objects
spatial scales
we extract objects as different spatial scales
Mona-Lisa example:
- some neurons respond at low frequencies - regions
- high spatial frequencies - details
importance of shading:
To recognize an object we have to have an idea of its 3D shape
- shading is important: it adds depth and texture
what does ambigious stimuli show?
ambiguous stimuli show that what we expect has a high influence on what we perceive
why consider face perception separately?
2 points
important part of our communication with each other
- evolutionary context
difficult problem that we solve very well
- extremely variable, yet highly constrained
face detection
- first step
- requires extracting the features that all faces have in common
- this could be achieved by simple “ template matching”
face recognition
3 points
- once a face is detected it can be further analysed to categorise it
- we may ascribe various attributes to it (e.g. emotions)
- we may need to identify who it is
challenges for face recognition
2 points
discrimination
generalisation
- lighting conditions
- varying context
- rigid and non-rigid transformation
what is pareidolia?
tendency to percieve a specific, meaningful image in a random or ambiguous visual pattern
what do we have a tendency for?
seeing faces everywhere e.g. pareidolia to make meaning where there is none
upside down and negative faces
2 points
- all local image information is preserved, including shape, the distance between features etc.
- the key is that inversion has a greater effect for faces than other objects
what did Gautheir et al., 1999 discover?
activation of the middle fusiform face area increases with expertise in recognising novel objects - but still less activation than for faces
do our visual systems process faces in a special way?
hollow potato illusion
the perceptual ‘expectation’ for faces to be convex seems to override our knowledge
- The hollow potato illusion demonstrates that the visual system perceives objects as convex, not just faces. However, the illusion is more pronounced for faces
what is prosopagnosia?
the specific inability to recognise familiar faces
- usually their object recognition is unimpaired
what causes prosopagnosia?
right anterior inferior occipital lesions in the region of the occipital-temporal junction
which cells are selective for faces?
cells in the superior temporal sulcus (STS)
object centred vs viewer centred
how do some neurons respond?
some neurons respond in a view indepedent way
object centred vs viewer centred
what do neurons in STS repond to?
neurons in STS respond to facial motion, posture and eye gaze
