Sensation and Perception

1. Sensation and Perception

2. Sensation: What is it? • The process by which a stimulus in the environment produces a neural impulse that the brain interprets as a sound, image, odor, etc.

3. Transduction • Transduction – Transformation of one form of energy into another – especially the transformation of stimulus information into nerve impulses • Receptors –Specialized neurons that are activated by stimulation and transduce (convert) it into a nerve impulse

4. Transduction • Sensory pathway – Bundles of neurons that carry information from the sense organs to the brain

5. Sensory Adaptation • Sensory adaptation – Loss of responsiveness in receptor cells after stimulation has remained unchanged for a while

6. Thresholds • Absolute threshold – Amount of stimulation necessary for a stimulus to be detected • Difference threshold – Smallest amount by which a stimulus can be changed and the difference be detected (also called just noticeable difference – JND)

7. Thresholds • Weber’s law – The JND is always large when the stimulus intensity is high, and small when the stimulus intensity is low • E.g., Quarter demonstration

8. The senses all operate in much the same way, but each extracts different information and sends it to its own specialized processing region in the brain How Are the Senses Alike? How Are They Different?

16. Retina – Light-sensitive layer at the back of the eyeball The Anatomy of Visual Sensation Photoreceptors – Light-sensitive cells in the retina that convert light energy to neural impulses Rods – Sensitive to dimlight but not colors Cones – Sensitive tocolors but not dim light Fovea – Area of sharpest vision in the retina

17. The Anatomy of Visual Sensation • Optic nerve – Bundle of neurons that carries visual information from the retina to the brain Blind spot – Point where the optic nerve exits the eye and where there are no photoreceptors

18. Transduction of Light in the Retina

19. The Anatomy of Visual Sensation • Visual cortex –Part of the brain – the occipital cortex – where visual sensations are processed

20. Neural Pathways in the Human Visual System

21. How the Visual System Creates Color Color – Psychological sensation derived from the wavelength of visible light – color, itself, is not a property of the external world

22. How the Visual System Creates Color • Electromagnetic spectrum – Entire range of electromagnetic energy, including radio waves, X-rays, microwaves, and visible light • Visible spectrum – Tiny part of the electromagnetic spectrum to which our eyes are sensitive

23. Two Ways of Sensing Color • Trichromatic theory • Three different types of cones that sense different parts of the visible spectrum (i.e., red, green, & blue) • Explains initial stages of color vision • Opponent Process Theory • From bipolar cells onward, visual system processes color in either-or, complementary fashion (i.e., red vs green or blue vs yellow) • Sensations of one color (e.g., red) inhibits sensation of its complementary color (i.e., green) • Explains negative afterimages & color blindness

24. Afterimages • Afterimages – Sensations that linger after the stimulus is removed • In the following slide, fix your eyes on the dot in the center of the flag

27. Amazing Pinwheel

28. Color Blindness

29. Perception

30. Perception • Same sensory input can give rise to very different perceptions • Perceptual Set • Readiness to perceive stimuli in specific ways • Reversible figures – drawing that is compatible with two different interpretations

31. Reversible Figures

32. Reversible Figure

33. Perception • What is Perception? • Active process in which we organize and interpret sensory information • i.e., How we make sense of (or understand) what we see, hear, feel, taste, and smell

34. Object Perception • Distal Stimulus • Stimuli that lie in the distance (i.e., in the outside world) • Three-dimensional • Proximal Stimulus • Stimulus that impinges directly onto your sensory receptors (i.e., the retina) • Distorted and two-dimensional

35. Distal Stimulus

36. Proximal Stimulus

37. Images Projected on the Retina

38. Object Perception • Feature Analysis • Analyze individual features and put them together to form a whole

39. Start with the elements and progress to the whole Evidence Hubel and Wiesel – cells in the cortex operate as highly specialized feature detectors Bottom-Up Processing

40. Top-Down Processing • Start with the whole and work towards the elements • What we perceive is influenced by what we expect to see • Context and prior experience are important

41. Object Perception • A number of Gestalt psychology principles can help explain how we organize information in order to perceive a coherent whole • Figure/Ground • Proximity • Similarity • Continuity • Common Fate • Closure • Goodness of Form or Pragnanz

42. Figure / Ground

43. Proximity

44. Similarity

45. Continuity

46. Common Fate

47. Closure

48. Goodness of Form or Pragnanz =

49. Images Projected on the Retina

50. Proximal Stimulus