perception
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Perception. Commonalities among Perceptual Systems Vision Visual Attention. 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: - PowerPoint PPT PresentationTRANSCRIPT
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)
Cortical Maps: External dimension---> 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
Based on sq footage, New Jersey is overrepresentedWyoming is underrepresented in the house in Congress
USA congressional representationUSA
Primary Visual cortex representationVisual field
Based on visual field, central vision (fovea, in red) is overrepresented in cerebral cortex
peripheral vision (in blue) is underrepresented
Cortical Maps: • Remapping occurs due to lesion, learning, etc.• Phantom limbs, • Sensory deprivation• Learning
Commonalities among Perceptual Systems• Sensory receptors do transduction
– (ex: mechanical --> electrical)
• Sensory receptors elicit graded potentials – response amplitude is equivalent to stimulus intensity
Perceptual Dimensions
Wave amplitude
Purity of the wave
Wave frequency
photorreceptors are responsive to a subset of frequencies - different wavelengths induce different color perceptions
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,
It’s an eye because it sees you”
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
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 isn’t perfect, there is a blindspot where the axons of ganglion cells leave the eye.
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)
One Cones --> one ganglion cell high acuity (fovea) Many Rods --> ganglion cells. High sensitivity (periphery)
(e.g, night vision)
Tri-chromatic theory – Blue, red, & green “color”
receptors– But some colors don’t mix!
Peak sensitivities of the three cones
Opponent process theory Red vs. Green; Blue vs. Yellow- Negative afterimage- But there is no ‘yellow’ receptor!
COLOR VISION
(-)
BLUE
(-)
GREEN
(+)
RED
(+)
YELLOW
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
Hermann Grid
+- -- -
- -
- -
- -- -
+- -- -
- -
- -
- -- -
Inhibition (-)
Excitation (+)+
- -- -
- -
- -
- -- -
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)
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
Area MT: motion perception
• 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
Neuroimaging of face, bird and car experts
“Face Experts”
FusiformGyrus
FusiformGyrus
CarExperts
BirdExperts
FusiformGyrus
Gauthier et al., 2000
Cars-Objects Birds-Objects
ControlGroup
AutismGroup
Hypoactivation of fusiform face area Schultz, et al. 2000
Faces
FusiformGyrus
FusiformGyrus
Children with autism as face “novices”
The creationist claim: ‘the eye, it is perfection, is proof of God’s existence’
The creationist argument:The 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: ??
The evolution argument:The 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: ??
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. It’s like saying that:
- The nose evolved to support glasses- Horses evolved to support saddles, etc.
• Any answer to this criticism?
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
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)