1. The
Visual System

2. Vision: The Stimulus
Light = electromagnetic radiation; travels as a wave
Amplitude: height of waves; perception of brightness
Wavelength: distance between peaks; perception of color
* The visible spectrum is only a portion of the total range of wavelengths

3. Figure 4.5 Light, the physical stimulus for vision

4. Vision: The Stimulus
*Visual input must be converted into neural impulses that are sent to the brain
Purity: how varied the mix of wavelengths is
Saturation: relative amount of whiteness in a color, or richness of colors

5. The Eye: Converting Light into Neural Impulses
Two purposes of the eyes:
Channeling light to neural tissue that receives it
Housing the tissue

6. The Eye: Converting Light into Neural Impulses
Pathway for Vision:
Cornea: where light enters the eye
Lens: focuses the light rays on the retina

7. The Lens: Converting Light into Neural Impulses
Accommodation: the curvature of the lens adjusts to alter visual focus
Problems with this lead to:
Nearsightedness: close objects seen clearly; distant objects blurry
Farsightedness: distant objects seen clearly; close objects appear blurry

8. The Eye: Converting Light into Neural Impulses
Iris: colored ring of muscle, constricts or dilates via amount of light
Pupil: regulates amount of light coming into the eye

9. The Retina: An Extension of the CNS
Retina: absorbs light, processes images, sends visual info to the brain

10. Optic nerve – Bundle of neurons that carries visual information from the retina to the brain
Optic Disk/Blind Spot – Point where the optic nerve exits the eye and where there are no photoreceptors

12. The retina is composed of specialized photoreceptor cells that convert light energy into nerve energy.
Only about 10% of the light arriving at the cornea
reaches these receptors
Rods: black and white/low light vision; more sensitive in dim light – peripheral vision; greatest density just outside the fovea
Cones: color and daylight vision; do not respond well in dim light

13. **rods outnumber cones by a huge margin
**cones provide better visual acuity (sharpness/precise detail)
Figure 4.9 The retina

14. Retina Review:
Retina – Light-sensitive layer at the back of the eyeball
Photoreceptors – Light-sensitive cells in the retina that convert light energy to neural impulses
Rods – Sensitive to dim light but not colors
Cones – Sensitive to colors but not dim light
Fovea – Area of sharpest vision in the retina; densely packed cones

15. ADAPTATION
Dark Adaptation: eyes become more sensitive to light in low illumination (enter dark theater on bright day)
Complete in 30 minutes; major progress in first 10
minutes
Light Adaptation: eyes become less sensitive to light in high illumination (leaving school to go to your car/bus)

16. Information Processing in the Retina
Light strikes the photoreceptors and triggers neural impulses - - sent to optic nerve
Bipolar Cells: combine info from photoreceptors; send results to ganglion cells
Ganglion Cells: integrate info into single firing rate to optic nerve
Horizontal Cells: connect receptors
Amacrine Cells: connect bipolar to bipolar; ganglion to ganglion

17. Receptive field: area on the retina that, when stimulated, affects the firing of that cell
Come in a variety of shapes & sizes
Lateral antagonism: neural activity in a cell opposes activity in surrounding cells

18. Directions:
Look at this grid – you will see dark spots at the intersections of the white bars, except in the intersection you’re staring at directly.

20. The Retina and the Brain: Visual Information Processing
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)
Magnocellular Channels: where
Parvocellular Channels: what
Second pathway: superior colliculus  thalamus  primary visual cortex

21. Figure 4.13 Visual pathways through the brain

22. Neural Pathways in the Human Visual System

23. Pathways to the Brain

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

25. Hubel and Wiesel: Feature Detectors
Early 1960’s:
Microelectrode recording of axons in primary visual cortex of animals
Discovered feature detectors: neurons that respond selectively to lines, edges, etc.

26. Simple Cells: respond most strongly to bars of light in their “favorite” orientation
Complex Cells: respond most strongly to moving bars of light in their “favorite” orientation
Hypercomplex Cells: respond most strongly to moving bars of light of a particular length or angle
Later research: cells specific to faces in the temporal lobes of monkeys and humans

27. Intensity (amplitude)
Basics of Color Vision
Wavelength
Intensity (amplitude)
Color
Brightness

28. Basics of Color Vision (cont.)
Wavelength determines color
Longer = red / shorter = violet
Amplitude determines brightness
Purity determines saturation

29. Hue: the qualitative experience of color of the light stimulus
Saturation: purity/vividness of color sensations
Brightness: intensity of light
Figure The color solid

30. Color: Psychological sensation derived from the wavelength of visible light – color, itself, is not a property of the external world
Subtractive Color Mixing: remove wavelengths of light leaving less there
Additive Color Mixing: superimposing lights, putting more light in the mixture than exists with one light by itself

31. Theories of Color Vision
Trichromatic Theory - Young and Helmholtz
Receptors for red, green, blue – color mixing
Opponent Process Theory – Hering
All color experiences arise from 3 systems, each of which includes 2 opponent elements
red/green, blue/yellow, black/white
Current perspective: both theories necessary

32. Complementary Colors
Afterimages:
Visual sensations that linger after the stimulus is removed; color of the image is the complement of the color you originally stared at

35. The Queen                                                       

41. Color Blindness the inability to distinguish colors
affects more males than females
most common: inability to distinguish red from green

42. Perceiving Forms, Patterns, and Objects
Reversible figures: drawings that have two interpretations that can shift back and forth

43. Figure 4.31 A famous reversible figure

44. Perceiving Forms, Patterns, and Objects
Perceptual sets
motivational forces can foster perceptual sets
Inattentional blindness - failure to see visible objects because attention is focused elsewhere

45. Perceiving Forms, Patterns, and Objects
Feature detection theory - bottom-up processing
Form perception - top-down processing

46. Figure 4.25 Feature analysis in form perception

47. Perceiving Forms, Patterns, and Objects
Subjective contours – perceive contours 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

48. Figure 4.27 Subjective contours

49. Principles of Perception
Gestalt principles of form perception:
Figure-ground
Proximity
Similarity
Continuity
Closure
Simplicity

51. Principles of Perception
Recent research:
Distal (stimuli outside the body) vs. proximal (stimulus energies impinging on sensory receptors) stimuli
Perceptual hypotheses
Context

52. Depth and Distance Perception
Binocular cues – clues from both eyes together
retinal disparity
convergence
Monocular cues – clues from a single eye
motion parallax
accommodation
pictorial depth cues

53. Stability in the Perceptual World: Perceptual Constancies
Perceptual constancies – stable perceptions amid changing stimuli
Size
Shape
Brightness
Hue
Location in space

54. Optical Illusions: The Power of Misleading Cues
Optical Illusions - discrepancy between visual appearance and physical reality
Cultural differences: perceptual hypotheses at work

55. The Ames Room

56. Muller-Lyer Illusion

57. Ponzo Illusion

58. Poggendorff Illusion

59. Upside-Down T Illusion

60. Zollner Illusion

61. Impossible Figures