Sensation and Perception

1. Sensation and Perception

3. Psychophysics • the study of how physical stimuli are translated into psychological experience • Sensation vs PerceptionOliver Sak’s “Dr. P”

4. Psychophysics • Sensation • the stimulation of sense organs • Perception • the selection, organization and interpretation of sensory input • “Dr. P” • visual sensation was intact • visual perception was severely impaired • “visual agnosia”

5. Basic Concepts of Psychophysics • stimulus • any detectable input from the environment • threshold • dividing point between energy levels that do and do not have a detectable effect • absolute threshold • minimum amount of stimulation that an organism can detect • defined as the level where the stimulus intensity is detected 50% of the time

6. Basic Concepts of Psychophysics • JND (Just Noticeable Difference) • smallest difference in amount of stimulation that a specific sense can detect • Weber’s Law • the size of a JND is in proportion to the size of the initial stimulus • as stimuli increase in magnitude, the JND becomes larger • Fechner’s Law • constant increments in stimulus intensity produce smaller and smaller increases in the perceived magnitude of sensation

7. Subliminal Perception • registration of sensory input without conscious awareness • “Eat popcorn” • rock music “backward masking” • self-help tapes • priming • altering participants’ attitude towards target person depending on type of subliminal photos seen • visual priming/semantic priming

8. Basic Concepts of Psychophysics • Signal Detection Theory • detection of a stimulus involves sensory processes but also emotional and cognitive processes • “noise” from irrelevant stimuli in the environment • Sensory Adaptation • gradual decline in sensitivity to prolonged stimulation • automatic, built-in process designed to keep people tuned in to changes rather than constants in their sensory input • Shouldn’t need constant confirmation that your clothes are still on!

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11. Vision: The Stimulus • Light • electromagnetic waves • amplitude brightness • wavelength colour • purity “richness” or “saturation” • incoming visual input must be converted into neural impulses

12. Nature of the stimulus: -one small band of electromagnetic radiation.

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14. Optic Disk: point where the ganglion cell axons converge and exit the retina. -creates a blind spot in the visual field.

15. Retina -contains: Photoreceptor cells Interneurons. Retinal ganglion cells.

16. } Outer Nuclear Layer 1. Outer Nuclear Layer: -contains the photoreceptor cells.

17. } Inner Nuclear Layer } Outer Nuclear Layer 2. Inner Nuclear Layer: -contains the interneurons: bipolar, amacrine & horizontal cells. -transmit signals from the photoreceptor cells to ganglion cells.

18. } Ganglion Cell Layer } Inner Nuclear Layer } Outer Nuclear Layer 3. Ganglion Cell Layer: -contains ganglion cells. -output cells of the retina. -axons form the optic nerve.

19. Photoreceptor Cells Two types: Rods Cones

20. Rods: -very sensitive to light. -used in dim light. -dense in periphery of retina

21. Fovea: -center of the retina. -densely packed with cones.

22. Cone vs Rod Vision • Perception of an item and the perception of color are separate. • A volunteer please!

23. Trichromatic Theory of Color Vision -three types. -each sensitive to a particular wavelength of light -red, blue, green

24. Trichromatic Theory of Color Vision

25. Yellow on Blue off Red on Green off Green on Red off Blue on Yellow off Opponent Process Theory of Color Vision Ganglion cells (and neurons) depend on receptors that respond best to pairs of colours.

26. Theory of Color Vision • Cones appear to follow trichromatic theory • Lateral Geniculate Nucleus and Visual Cortex appear to follow opponent process theory • so … takes both theories to explain color vision

27. Colour Blindness 2 - 8% of males. .03% of females. due to a loss or mutation of pigment gene. Red/Green is most common.

28. First Stage of Visual Processing

29. Information Processing in the Retina • Info from about 130 million rods and cones converge upon 1 million axons in the optic nerve • bipolar and ganglion cells in the intermediate layers integrate and compress signals from many receptors. • Axons from the ganglion cells form the optic nerve

30. Information Processing in the Cortex • Feature Detectors (Hubel & Wiesel) • neurons respond selectively to very specific features of more complex stimuli • simple cells • respond best to line of correct width, oriented at the correct angle and located in correct position in receptive field • complex cells • respond best to specific width and orientation but any position in receptive field

31. Information Processing in the Cortex • Parallel processing • simultaneously extracting different kinds of info from same input • Parvocellular channel • Magnocellular channel

32. Information Processing in the Cortex • Parallel processing • Parvocellular channel • process details about the objects • “what” channel • handles perception of color, form and texture • __________ lobe • prosopagnosia • Magnocellular channel • processes “where” the ojbects are • perception of motion and depth • __________ lobe

33. Review Dimension Rods Cones

34. CENTRAL VISUAL PATHWAYS Optic nerve: fibres from retina to the optic chiasm. Optic chiasm: optic nerves from both eyes converge and partially cross over. Optic tract: optic fibres beyond the chiasm.

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37. Visual Perception • The same visual input can result in radically different perceptions.

38. 05.26 Perceptual Set How we perceive the stimulus depends on our expectations. W. W. Norton

39. Perceptual Theories • Bottom-up processing • Top-down processing

40. Bottom-up Processing • Feature Analysis • process of detecting specific elements in visual input and assembling them into a more complex form • progression from individual elements to the whole • supported by work of Hubel and Wiesel

41. Top-Down Processing • Gestalt psychology • Step 1: formulate perceptual hypothesis about the nature of the stimulus as a whole • Step 2: select and examine features to check hypothesis • Step 3: recognize stimulus

42. Top-Down Processing • Whole can be greater than the sum of its parts • visual illusions take advantage of this • most common visual illusion is television • see smooth motion but actually objects just moving from slightly different positions in successive frames

43. Gestalt Principles of Top-Down Processing • figure-ground • divide visual displays into figure and ground

45. Gestalt Principles of Top-Down Processing • figure-ground • proximity • things that are near one another seem to belong together • similarity • continuity • tend to follow in whatever direction they’ve been lead

47. Gestalt Principles of Top-Down Processing • figure-ground • proximity • similarity • continuity • simplicity • closure • you may “complete” figures that actually have gaps in them

49. Visual Perception • Need both • top-down • bottom-up

50. Perception of Depth • Monocular depth perception • Binocular depth perception