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Perception

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:

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Perception

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  1. Perception • Commonalities among Perceptual Systems • Vision • Visual Attention

  2. 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)

  3. 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

  4. Based on sq footage, New Jersey is overrepresented Wyoming is underrepresented in the house in Congress USA USA congressional representation

  5. 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

  6. Explain what is represented by this picture

  7. Cortical Maps: Stimulus properties need to be ‘recreated’ vision: 2D --> 3D audition: 1D --> 2D

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

  9. Commonalities among Perceptual Systems • Sensory receptors do transduction • (ex: mechanical --> electrical) • Sensory receptors elicit graded potentials • response amplitude is equivalent to stimulus intensity

  10. Wave frequency Wave amplitude Purity of the wave Perceptual Dimensions photorreceptors are responsive to a subset of frequencies - different wavelengths induce different color perceptions

  11. 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”

  12. 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

  13. 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.

  14. Lateral visual field Medial Retina

  15. 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)

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

  17. Peak sensitivities of the three cones Tri-chromatic theory • Blue, red, & green “color” receptors • But some colors don’t mix! (-) (+) (-) (+) • Opponent process theory • Red vs. Green; Blue vs. Yellow • - Negative afterimage • - But there is no ‘yellow’ receptor! BLUE YELLOW GREEN RED COLOR VISION

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

  19. 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

  20. Edge detection See example in PsyCog Brightness contrast Fig A.4.1

  21. - - - - - - - - - - - - - - - - - - + + + - - - - - - - - - - - - - - - - - - Hermann Grid Inhibition (-) Excitation (+)

  22. VISUAL PATHWAY

  23. 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 LGN thalamic organization Concentric receptive fields (center surround)

  24. 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 Primary visual cortex (V1)

  25. Neurons respond to specific spatial frequencies ‘Blurry’ As nearsighted

  26. Area MT: motion perception

  27. 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

  28. Dorsal Ventral

  29. Object recognition: Ventral Pathway Lesion of ventral pathway Agnosia fMRI: Object recognition

  30. Cars-Objects Birds-Objects Car Experts Fusiform Gyrus Bird Experts Gauthier et al., 2000 Fusiform Gyrus Neuroimaging of face, bird and car experts Fusiform Gyrus “Face Experts”

  31. Fusiform Gyrus Fusiform Gyrus Children with autism as face “novices” Faces Control Group Autism Group Hypoactivation of fusiform face area Schultz, et al. 2000

  32. synesthesia

  33. Left overs

  34. 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: ??

  35. 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: ??

  36. 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?

  37. 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

  38. 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)

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