Perception

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Perception

• Commonalities among Perceptual Systems
• Vision
• Visual Attention

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Commonalities among Perceptual Systems

• Subjective experience depends on the path
  stimulated
• All paths (except olfactory) have a relay station
  at the thalamus
• There are Sensory Maps
• A correspondence between external dimension and
  internal representation (ex location,
  orientation, motion, pitch)

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• Cortical Maps External dimension---gt internal
  representation
• Some areas are overrepresented

Somatosensory representation Motor cortex
Representation
Skin surface
Based on skin surface, the hands lips are
overrepresented in cortex, and the back and
arms are underrepresented in cortex
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Based on sq footage, New Jersey is
overrepresented Wyoming is underrepresented in
the house in Congress
USA
USA congressional representation
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Primary Visual cortex representation
Visual field
Based on visual field, central vision (fovea,
in red) is overrepresented in cerebral cortex
peripheral vision (in blue) is underrepresented
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Explain what is represented by this picture
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Cortical Maps Stimulus properties need to be
recreated vision 2D --gt 3D
audition 1D --gt 2D

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• Cortical Maps
• Remapping occurs due to lesion, learning, etc.
• Phantom limbs,
• Sensory deprivation
• Learning

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Commonalities among Perceptual Systems

• Sensory receptors do transduction
• (ex mechanical --gt electrical)
• Sensory receptors elicit graded potentials
• response amplitude is equivalent to stimulus
  intensity

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Perceptual Dimensions
photorreceptors are responsive to a subset of
frequencies - different wavelengths induce
different color perceptions
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Vision

• El ojo que ves no es
• ojo porque tu lo veas,
• es ojo porque te ve
• Antonio Machado
• The eye you see
• is not an eye due to you seeing it,
• Its an eye because it sees you

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Vision Outline

• Eye
• Color vision
• Receptive Field
• Edge Detection
• Visual Path
• thalamus (LGN)
• primary visual cortex
• Orientation sensitive Spatial frequency
• Ventral Pathways
• Area MT (motion), Object Recognition, Area V4
  (color)
• synesthesia
• Dorsal Pathway
• Spatial Attention
• Hemispatial Neglect

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The eye is a device 'designed' to see

• - transparent medium (cornea, aqueous humor,
  lens, vitreous humor)
• variable-focus lens lens
• diaphragm with variable diameter iris
• - a light-sensitive layer of sensors retina
• - which lies at the focal plane of the lens
• - muscles that move the eyes in conjunction
• edge detectors
• But the eye isnt perfect, there is a blindspot
  where the axons of ganglion cells leave the eye.

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Lateral visual field
Medial Retina

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Retina The sensory receptor structure of the
visual system

• photorreceptors cones (color vision) and rods
  night vision
• fovea center of the retina, high concentration
  of cones
• optic disk (blindspot)
• Direct view of arteries (clinical importance)

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One Cones --gt one ganglion cell high acuity
(fovea) Many Rods --gt ganglion cells. High
sensitivity (periphery) (e.g, night
vision)
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COLOR VISION

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See example in PsyCog beta (cd) Fig A.3.1 Color
Negative Afterimage
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Receptive field

• is that portion of the visual field in which the
  presentation of visual stimuli will produce an
  alteration in the firing rate of a particular
  neuron

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Edge detection
See example in PsyCog Brightness contrast Fig
A.4.1
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Hermann Grid
Inhibition (-)
Excitation ()
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VISUAL PATHWAY
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LGN thalamic organization

• Magnocellular
• M ganglion cells
• large receptive fields
• motion detection
• locating stimulus in space
• dorsal cortical stream
• parietal lobe

• Parvocellular
• P ganglion cells
• small receptive fields
• Object identity
• Color recognition
• ventral cortical stream
• temporal lobe

Concentric receptive fields (center surround)
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Primary visual cortex (V1)

• LGN cells with concentric receptive field provide
  input to simple cells in V1
• V1 cells respond to lines
• of particular orientations
• of particular widths.
• There is a topographic map

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Neurons respond to specific spatial frequencies
Blurry As nearsighted
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Area MT motion perception
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• Take home message
• Different regions (modules) of the visual cortex
  respond to particular features of visual
  information such as
• orientation,
• spatial frequency,
• movement,
• color.
• retinal disparity, and even
• Objects
• faces

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Object recognition Ventral Pathway
Lesion of ventral pathway Agnosia
fMRI Object recognition
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Neuroimaging of face, bird and car experts
Fusiform Gyrus
Face Experts
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Children with autism as face novices
Faces
Control Group
Autism Group
Hypoactivation of fusiform face area
Schultz, et al. 2000
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• synesthesia

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Left overs
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The creationist claim the eye, it is
perfection, is proof of Gods existence

• The creationist argumentThe arrangement of
  structures in the eye suggests that it has been
  designed to fulfill a specific function (that is,
  it cannot have occurred by accident)

The flaws ??
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• The evolution argumentThe arrangement of
  structures in the eye suggests that it has been
  designed to fulfill a specific function (that is,
  it cannot have occurred by accident).
• - However, the design occurs not by divine
  intervention but by a combination of random
  variation and natural selection

The flaws ??
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Possible criticism

• Circularity the structure (eye) is said to have
  evolved to perform a function (vision) that we
  know in advance, the structure performs very
  well. Its like saying that
• The nose evolved to support glasses
• Horses evolved to support saddles, etc.
• Any answer to this criticism?

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Answer to the circularity challenge

• Convergent Evolution species phylogenetically
  distant develop a similar device
• Fewer functions than biological systems
• Functions can be related to the overall function
  of survival and reproduction

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Evolution by Natural Selection

• There are heritable traits
• Random Changes There are random and small
  modifications in the heritable traits. Most of
  them are detrimental, but some few are
  beneficial.
• Selection Pressure There is selection pressure
  so that the (few) random changes in heritable
  traits that favor survival and reproduction are
  retained through generations
• natural selection provides a good account of
  adaptive complexity (any system, composed of many
  interacting parts, where the details of the parts
  and arrangements suggest design to fulfill some
  function. Example the eye)