Chapter 4: Sensation and Perception

1. Chapter 4: Sensation and Perception

2. Sensation and Perception: The Distinction • Sensation: stimulation of sense organs: for example absorbing energy from light by the eyes. • Perception: selection, organization, and interpretation of sensory input • Psychophysics = the study of how physical stimuli are translated into psychological experience

3. Figure 4.1 The distinction between sensation and perception

4. Psychophysics: Basic Concepts • Sensation begins with a detectable stimulus • Fechner: the concept of the threshold, what is the weakest detectable stimulus • Absolute threshold: absolute threshold is the minimal amount of stimulation that an organism can detect stimulus 50% of the time • Just noticeable difference (JND): smallest difference detectable • Weber’s law: size of JND proportional to size of initial stimulus

5. Figure 4.2 The absolute threshold

6. Psychophysics: Concepts and Issues • Signal-Detection Theory: Signal detection theory holds that the detection of sensory information is influenced by two things…1) noise in the system (irrelevant stimuli in the environment that elicit neural activity), and 2) decision making processes • Subliminal Perception: Many researchers, using very different methods, have demonstrated that perception can occur without awareness • Sensory Adaptation: Decline in sensitivity

7. Figure 4.3 Signal-detection theory

8. Vision: The Stimulus • Light = electromagnetic radiation, that travels as a wave…the wave travels quickly…the speed of light. • Amplitude: perception of brightness • Wavelength: perception of color • Purity: mix of wavelengths • perception of saturation, or richness of colors.

9. Figure 4.5 Light, the physical stimulus for vision

10. The Eye:Converting Light into Neural Impulses • The eye:The eye has two main purposes, providing a “house” for the neural tissue that receives light, the retina, and channeling light toward the retina. • Components: • Cornea: where light enters the eye • Lens: focuses the light rays on the retina • Iris: colored ring of muscle, constricts or dilates via amount of light • Pupil: regulates amount of light

11. Figure 4.7 The human eye

12. The Retina: An Extension of the CNS • Retina: piece of neural tissue that lines the back of the eye, absorbs light, processes images • Optic disk: Axons from the retina to the brain converge at the optic disk, a hole in the retina where the optic nerve leaves the eye optic nerve connection/blind spot • Receptor cells: • Rods: black and white/low light vision • Cones: color and daylight vision • Adaptation: becoming more or less sensitive to light as needed

13. The Retina: An Extension of the CNS • Information processing: • Receptive fields: the collection of rod and cone receptors that funnel signals to a particular visual cell in the retina • Lateral antagonism: or lateral inhibition, occurs when neural activity in a cell opposes activity in surrounding cells.

14. Figure 4.8 Nearsightedness and farsightedness

15. Figure 4.9 The retina

16. Figure 4.10 The process of dark adaptation

17. The Retina and the Brain:Visual Information Processing • Optic Chiasm: the point where the axons cross to different hemispheres • Light  rods and cones  neural signals  bipolar cells  ganglion cells  optic nerve  optic chiasm  opposite half brain • Main pathway: lateral geniculate nucleus (thalamus)  primary visual cortex (occipital lobe) (90% of axons go to LGN) • magnocellular: where (ex brightness) • parvocellular: what (ex color)

18. The Retina and the Brain:Visual Information Processing • Second pathway: superior colliculus thalamus  primary visual cortex • Helps with the perception of motion and coordination of visual input to other senses • These 2 channels use parallel processing • They both extract different info from the same stimulus

19. Figure 4.13 Visual pathways through the brain

20. Figure 4.15 The what and where pathways from the primary visual cortex

21. Hubel and Wiesel:Feature Detectors and the Nobel Prize • Early 1960’s: Hubel and Wiesel • Microelectrode recording of axons in primary visual cortex of animals • Discovered feature detectors: neurons that respond selectively to lines, edges, etc. • Groundbreaking research: Nobel Prize in 1981 • Later research: cells specific to faces in the temporal lobes of monkeys and humans

22. Basics of Color Vision • Wavelength determines color • Longer = red / shorter = violet • Amplitude determines brightness • Purity determines saturation

23. Figure 4.16 The color solid

24. Figure 4.17 Additive versus subtractive color mixing

25. Theories of Color Vision • Trichromatic theory - Young and Helmholtz (mid 1800’s) • This theory holds that the human eye has three types of receptors with differing sensitivities to different light wavelengths…one for red, one for green, and one for blue • Opponent Process theory – Hering (1878) • 3 pairs of antagonistic colors • red/green, blue/yellow, black/white • Current perspective: both theories necessary

26. Figure 4.18 The color circle and complementary colors

27. Perceiving Forms, Patterns, and Objects • Reversible figures: drawing that is compatible with two interpretations that can shift back and forth • Perceptual sets: a readiness to perceive a stimulus in a particular way • Inattentional blindness: failure to see fully visible objects or events in a visual display • Feature detection theory - people detect specific elements in stimuli and build them up into recognizable forms, bottom-up processing

28. Perceiving Forms, Patterns, and Objects • Form perception - involves top-down processing…clearly emphasized by the Gestalt psychologists, who demonstrated that the whole is more than the sum of its parts • Subjective contours: phenomenon whereby contours are perceived where none actually exist • Gestalt psychologists: the whole is more than the sum of its parts • Reversible figures and perceptual sets demonstrate that the same visual stimulus can result in very different perceptions

29. Figure 4.25 The principle of figure and ground

30. Figure 4.28 A famous reversible figure

31. Figure 4.22 Feature analysis in form perception

32. Figure 4.23 Bottom-up versus top-down processing

33. Figure 4.24 Subjective contours

34. Principles of Perception • Gestalt principles of form perception: • figure-ground, proximity, similarity, continuity, closure, and simplicity • The Gestalt emphasis is still felt in the study of perception, as they had useful insights that have stood the test of time, raised important issues

35. Principles of Perception • Recent research: • Distal (stimuli outside the body) vs. proximal (stimulus energies impinging on sensory receptors) stimuli • Perceptual hypotheses • Context, your brain makes a decision as to what an object is • Objects that are thought to be “out of place” are more easily remembered

36. Principles of Perception • Phi Phenomenon • The illusion of movement presented by visual stimuli in rapid succession • Figure and ground: • Figure is the object being looked at • Ground is the background on which it stands • Proximity: we group things that are close together

37. Principles of Perception • Closure: your brain fills in the gaps to complete objects forms • Similarity: we group objects of similar size or shape • Simplicity: we group elements together to form a simple figure • Continuity: we follow objects in the direction we are led

38. Figure 4.26 Gestalt principles of perceptual organization

39. Figure 4.27 Distal and proximal stimuli

40. Figure 4.29 The Necker cube

41. Figure 4.30 Context effects

42. Depth and Distance Perception • Depth perception involves interpretation of visual cues that indicate how near or far away something is • Binocular cues – clues from both eyes together • retinal disparity: objects within 25 feet project images to slightly different locations on the left and right retinas • Convergence: feeling the eyes converge toward each other as they focus on a target.

43. Depth and Distance Perception • Monocular cues – clues from a single eye • motion parallax : having images of objects at different distances moving across the retina at different rates • Accommodation: change in the shape of the lens as the eye focuses • pictorial depth cues: cues about distance that can be given in a flat picture

44. Depth and Distance Perception • Monocular cues (cont.) • pictorial depth cues: cues about distance that can be given in a flat picture • Linear perspective: lines converge in the distance • Texture gradients: small details can not be seen from far away • Interposition: objects that come b/w you and another are closer • Relative size: closer objects are bigger • Height in plane: distant objects are higher in the picture • Light and shadow: gives direction and distance

45. Stability in the Perceptual World:Perceptual Constancies • Perceptual constancies – stable perceptions amid changing stimuli • when a person walks toward you, they get larger perceptually…do you think they are growing? No • Size • Shape • Brightness • Hue • Location in space

46. Optical Illusions:The Power of Misleading Cues • Optical Illusions - discrepancy between visual appearance and physical reality • Famous optical illusions: Muller-Lyer Illusion, Ponzo Illusion, Poggendorf Illusion, Upside-Down T Illusion, Zollner Illusion, the Ames Room, and Impossible Figures • Cultural differences: Perceptual hypotheses at work, Muller-Lyer and Poggendorf demonstrate the importance of perceptual hypotheses.

47. Figure 4.41 The Ames room

48. The Ames Room Mac OS X Windows

49. Figure 4.37 The Muller-Lyer illusion

50. Figure 4.38 Explaining the Muller-Lyer Illusion