Chapter%204:%20Object%20Recognition

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Chapter 4 Object Recognition

• What do various disorders of shape recognition
  tell us about object recognition?
• - Apperceptive visual agnosia (Ch. 2)
• - Associative visual agnosia
• - Perceptual categorization deficit
• What do neuroimaging studies tell us about object
  recognition?
• The computational interpretation What does
  cognitive evidence suggest about constraints on
  the nature of the neural representations
  underlying object recognition?

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Object Recognition

• Dr. Harley has filled you in on the neuroscience
  side of the miracle

Farah (2000) Fig. 4.1
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Visual Agnosias

• Visual Agnosia A blanket term for impaired
  visual object recognition following brain damage
  when elementary visual functions (acuity, visual
  fields) are adequate.
• Apperceptive visual agnosia Inability to group
  the local features into a coherent perceptual
  representation (Ch 2)
• Associative visual agnosia Inability to
  recognize visually presented objects, despite
  having a coherent perceptual representation

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Visual Agnosias Copying and drawing from memory.
Apperceptive agnosia
Associative agnosia
Copying
Copying
Memory
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Associative visual agnosia Criteria

• Patients can form percepts (do perceptual
  grouping) unlike apperceptive agnosia patients.
• They can see an object well enough to describe
  its appearance, to draw it, or to succeed in a
  same/different test of appearances.
• They have difficulty recognizing visually
  presented objects (cant name or sort objects by
  category)
• They can demonstrate knowledge of objects from
  other sensory modalities

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Associative agnosia behavior An impairment of
shape perception

• They can copy complex objects. BUT their copying
  behavior is abnormal HJA took 6 hours to do the
  cathedral.
• They are very sensitive to visual quality of
  stimuli. They recognize real objects better than
  line drawings face recognition of unfamiliar
  people is impaired by changing lighting
  conditions.
• They make visual shape recognition errors they
  might call a baseball bat, a paddle, knife,
  baster, etc.

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Associative Visual Agnosia

• The human analog of the IT-lesioned monkeys.
• They fail to recognize objects because they fail
  to represent shape normally.
• Slow, slavish copying
• Sensitivity to visual quality of the stimuli
• Visual shape errors

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Regions of brain damage associated with different
types of visual agnosia

• Apperceptive Agnosia Diffuse damage to the
  occipital lobe and surrounding areas
• Associative Agnosia Occipitotemporal regions of
  both hemispheres

From Banich (2004)
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Perceptual categorization deficit

• Difficulty recognizing objects viewed from
  unusual perspectives or uneven illumination
  conditions (Warrington Taylor, 1973)
• Initially assumed to reflect an impairment of
  viewpoint-invariant object recognition

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BUT Is Perceptual Categorization Deficit really
about loss of shape representations?

• Not impaired in the real world.
• Not impaired under all viewing conditions per se.
• Mainly impaired when matching an USUAL to an
  UNUSUAL view normal people have similar, less
  serious, type of difficulty.
• Associated with unilateral RH lesions, usually in
  parietal cortex, not inferotemporal cortex

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Perceptual categorization deficit

• Demonstrates value of examining evidence
  carefully and trying to link it with what is
  known to see whether patterns fit or not.
• In this case, the pattern differs greatly from
  other visual agnosias.

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Functional Neuroimaging Studies

• Goal of PET, fMRI studies To localize the
  psychological process(es) of interest
• Research design Measure brain activity in at
  least two conditions a control (baseline)
  condition and an experimental condition.
• Psychological process localized by subtracting
  brain activity in the control (baseline)
  condition from brain activity in the experimental
  condition.

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PET Imaging (Posner Raichle, 1994)
Upper row Control PET scan (resting while
looking at static fixation point) is subtracted
from looking at a flickering checkerboard
stimulus positioned 5.5 from fixation
point. Middle row Subtraction procedure produces
a somewhat different image for each of 5
subjects. Bottom row The 5 images are averaged
to eliminate noise, producing the image at the
bottom.
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Where in the brain does visual recognition occur?

• Visual recognition localized to the posterior
  half of the brain really?
• Whoops!!! Why did these studies fail to localize
  visual recognition?
• Poor methodology!

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Neuroimaging methods The subtraction technique

• Using the subtraction technique effectively
  requires that the baseline and experimental
  conditions differ only in the process of
  interest.
• Requires careful logical analysis of the task to
  determine the best comparison conditions.
• If there are many differences between the
  baseline and experimental conditions, it is
  difficult to interpret the results of the study.

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Neuroimagining Studies of Object Recognition
Comparing baseline and experimental conditions

• Sequence of trial events
• Fixation point
• Stimulus presentation
• Task (two types)
• Passive Viewing Task
• Are stimuli comparable (e.g., size, complexity)
• Mind is not a vacuum
• Active Viewing Task
• What is task?
• What is response?

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Sergent et al (1992a) Active viewing task.

• Baseline condition
• - View fixation point

• Experimental condition
• - View fixation point
• - See line drawing of object
• - Decide whether the object is living or
  non-living
• - Make Yes/No response

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Localizing Visual Recognition

• Is human visual recognition supported by one
  general purpose recognition system or are there
  specialized modules for recognizing objects,
  faces, printed words?
• Good functional neuroimaging studies do exist to
  test these possibilities and will be discussed in
  Chapters 5 6.

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Constraints on the nature of shape
representations in IT Coordinate systems

• Cannot be simple viewer-centered or
  environmentally-centered
• IT cells respond to a given shape over changes
  in the position, size, and picture plane
  orientation of object
• Impairments of IT lesioned monkeys suggest they
  have lost abstract shape representation

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Constraints Coordinate systems cont.

• Two possibilities
• 1. Object-centered coordinate system
• 2. A cluster of multiple viewer-centered object
  representations plus the ability to transform one
  representation to another as necessary (a la
  Multiple Views model of Tarr, 1995).

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Constraints Coordinate systems cont.

• Farah prefers the Multiple Viewer-Centered
  representation option because
• Position, size, and orientation invariance shown
  by IT cells is not perfect consistent with some
  views being better learned than others.
• IT neurons can selectively learn arbitrary
  associations between pairs of stimuli, a
  prerequisite for deriving invariance from
  viewer-centered representations.
• Consequently, because cell activity takes time to
  decay and because seeing different views of an
  object tends to be clustered in time, the
  correlation (association) between several
  different retinotopic views of the same object
  could be learned.

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Constraints Primitives and Organization

• Primitives
• Cannot be contours
• Could be either surface-based (2D) or
  volume-based (e.g., 3D geon-like parts).
• Organization
• Hierarchical? We really dont know much about
  how multipart objects are represented

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Constraints Implementation

• Symbolic Model vs Neural net
• Is a perceptual representation created and then
  compared to a memory representation? (Implies
  that the memory representation can be destroyed,
  yet the perceptual representation remains
  intact). Unlikely, since no such evidence
  exists.
• Does the input get coded and recoded in a
  succession of neural nets as it is processed
  through the visual system? (Implies that
  impairment in test of object memory is associated
  with perceptual impairment). Yes, this seems
  true in associative agnosia.

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Constraints Implementation

• Distributed representation more likely than local
  representation
• Single unit recording
• Graceful degradation following damage to IT