1. Chapter 3: Introduction to Vision

2. Overview of Questions
How do chemicals in the eye called visual pigments affect our perception?
How does the way neurons are “wired up” affect our perception?
What does it mean to say that perception is indirect?

3. Light is the Stimulus for Vision
Electromagnetic spectrum
Energy is described by wavelength.
Spectrum ranges from short wavelength gamma rays to long wavelength radio waves.
Visible spectrum for humans ranges from 400 to 700 nanometers.
Most perceived light is reflected light

4. Figure 3.1 The electromagnetic spectrum, showing the wide range of energy in the environment and the small range within this spectrum, called visible light, that we can see.

5. Focusing Images on the Retina
The cornea, which is fixed, accounts for about 80% of focusing.
The lens, which adjusts shape for object distance, accounts for the other 20%.
Accommodation results when ciliary muscles are tightened which causes the lens to thicken.
Light rays pass through the lens more sharply and focus near objects on retina.

6. Figure 3.2 An image of the cup is focused on the retina, which lines the back of the eye. The close-up of the retina on the right shows the receptors and other neurons that make up the retina.

7. Figure 3. 3 Focusing of light rays by the eye
Figure 3.3 Focusing of light rays by the eye. (a) Rays of light coming from a small light source that is more further than 20 feet away are approximately parallel. The focus point for parallel light is at A on retina. (b) Moving an object closer to the relaxed eye pushes the focus point back. Here the focus point is at B, but light is stopped by the back of the eye. (c) Accommodation of the eye (indicated by the fatter lens) increases the focusing power of the lens and brings the focus point for a near object back to A on the retina.

8. Focusing Images on Retina - continued
The near point occurs when the lens can no longer adjust for close objects.
Presbyopia - “old eye”
Distance of near point increases
Due to hardening of lens and weakening of ciliary muscles
Corrective lenses are needed for close activities, such as reading

9. Figure 3.4 Vertical lines show how the distance of the near point (green numbers) increases with increasing age. When the near point becomes further than a comfortable reading distance, corrective lenses (reading glasses) become necessary.

10. Focusing Images on Retina - continued
Myopia or nearsightedness - Inability to see distant objects clearly
Image is focused in front of retina
Caused by
Refractive myopia - cornea or lens bends too much light
Axial myopia - eyeball is too long

11. Focusing Images on Retina - continued
Solutions for myopia
Move stimulus closer until light is focused on the retina
Distance when light becomes focused is called the far point.
Corrective lenses can also be used.
LASIK surgery can also be successful.

12. Figure 3. 5 Focusing of light by the myopic (nearsighted) eye
Figure 3.5 Focusing of light by the myopic (nearsighted) eye. (a) Parallel rays from a distant spot of light are brought to a focus in front of the retina, so distant objects appear blurred. (b) As the spot of light is moved closer to the eye, the focus point is pushed back until, at the far point, the rays are focused on the retina, and vision becomes clear. (c) A corrective lens, which bends light so that it enters the eye at the same angle as the light coming from the far point, brings light to a focus on the retina. Angle A is the same in (b) and (c).

13. Focusing Images on Retina - continued
Hyperopia or farsightedness - inability to see nearby objects clearly
Focus point is behind the retina.
Usually caused by an eyeball that is too short
Constant accommodation for nearby objects can lead to eyestrain and headaches.

14. Transduction of Light into Nerve Impulses
Receptors have outer segments, which contain:
Visual pigment molecules, which have two components:
Opsin - a large protein
Retinal - a light sensitive molecule
Visual transduction occurs when the retinal absorbs one photon.
Retinal changes it shape, called isomerization.

15. Figure 3. 6 (a) Rod receptor showing discs in the outer segment
Figure 3.6 (a) Rod receptor showing discs in the outer segment. (b) Close-up of one disc showing one visual pigment molecule in the membrane. (c) Close-up showing how the protein opsin in one visual pigment molecule crosses the disc membrane seven times. The light-sensitive retinal molecule is attached to the opsin at the place indicated.

16. Figure 3. 7 Model of a visual pigment molecule
Figure 3.7 Model of a visual pigment molecule. The horizontal part of the model shows a tiny portion of the huge opsin molecule near where the retinal is attached. The smaller molecule on top of the opsin is the light-sensitive retinal. The model on the left shows the retinal molecule’s shape before it absorbs light. The model on the right shows the retinal molecule’s shape after it absorbs light. This change in shape is one of the steps that results in the generation of an electrical response in the receptor.

17. Psychophysical Experiment by Hecht et al.
Prior physical evidence showed that it takes one photon to isomerize a pigment molecule.
Purpose of experiment - to determine how many pigment molecules need to be isomerized for a person to see
Method of constant stimuli used to determine absolute threshold.
The details of the experiment are in the following figures. They can be used to explain the process in more depth.

18. Experiment by Hecht et al. - continued
Results showed:
a person can see a light if seven rod receptors are activated simultaneously.
a rod receptor can be activated by the isomerization of just one visual pigment molecule.
Good example of using psychophysics to draw conclusions about the physiology of the visual system.

19. Figure 3. 9 The observer in Hecht et al
Figure 3.9 The observer in Hecht et al.’s (1942) experiment could see a spot of light containing 100 photons. Of these, 50 photons reached the retina, and 7 photons were absorbed by visual pigment molecules.

20. Figure 3.10 How Hecht reasoned about what happened at threshold, when observers were able to see a flash of light when 7 photons were absorbed by visual pigment molecules. The 7 photons that were absorbed are shown poised above 500 rod receptors. Hecht reasoned that because there were only 7 photons but 500 receptors, it is likely that each photon entered a separate receptor. Thus, only one visual pigment molecule was isomerized per rod. Because the observer perceived the light, each of 7 rods must have been activated.

21. Physiological Reaction after Isomerization
Current research in physiology and chemistry shows that isomerization triggers an enzyme cascade.
Enzymes facilitate chemical reactions.
A cascade means that a single reaction leads to increasing numbers of chemical reactions.
This is how isomerizing one pigment leads to the activation of a rod receptor.

22. Figure 3.11 This sequence symbolizes the enzyme cascade that occurs when a single visual pigment molecule is activated by absorption of a quantum of light. In the actual sequence of events, each visual pigment molecule activates hundreds more molecules, which, in turn activate about a thousand more molecules. Thus, isomerization of one visual pigment molecule activates about a million other molecules.

23. Retinal Processing - Rods and Cones
Differences between rods and cones
Shape
Rods - large and cylindrical
Cones - small and tapered
Distribution on retina
Fovea consists solely of cones.
Peripheral retina has both rods and cones.
More rods than cones in periphery.

24. Figure 3. 12 The distribution of rods and cones in the retina
Figure 3.12 The distribution of rods and cones in the retina. The eye on the left indicates locations in degrees relative to the fovea. These locations are repeated along the bottom of the chart on the right. The vertical brown bar near 20 degrees indicates the place on the retina where there are no receptors because this is where the ganglion cells leave the eye to form the optic nerve. (Adapted from Lindsay & Norman, 1977.)

25. Retinal Processing - Rods and Cones - continued
Number - about 120 million rods and 5 million cones
Blind spot - place where optic nerve leaves the eye
We don’t see it because:
one eye covers the blind spot of the other.
it is located at edge of the visual field.
the brain “fills in” the spot.

26. Figure There are no receptors at the place where the optic nerve leaves the eye. This enables the receptor’s ganglion cell fibers to flow into the optic nerve. The absence of receptors in this area creates the blind spot.

27. Diseases that Affect the Retina
Macular degeneration
Fovea and small surrounding area are destroyed
Creates a “blind spot” on retina
Most common in older individuals
Retinitis pigmentosa
Genetic disease
Rods are destroyed first
Foveal cones can also be attacked
Severe cases result in complete blindness

28. Measuring Dark Adaptation
Three separate experiments are used.
Method used in all three experiments:
Observer is light adapted
Light is turned off
Once the observer is dark adapted, she adjusts the intensity of a test light until she can just see it.

29. Figure 3. 18 Viewing conditions for a dark adaptation experiment
Figure 3.18 Viewing conditions for a dark adaptation experiment. The image of the fixation point falls on the fovea, and the image of the test light falls in the peripheral retina.

30. Measuring Dark Adaptation - continued
Experiment for rods and cones:
Observer looks at fixation point but pays attention to a test light to the side.
Results show a dark adaptation curve:
Sensitivity increases in two stages.
Stage one takes place for three to four minutes.
Then sensitivity levels off for seven to ten minutes - the rod-cone break.
Stage two shows increased sensitivity for another 20 to 30 minutes.

31. Figure 3. 19 Three dark adaptation curves
Figure 3.19 Three dark adaptation curves. The red line is the two-stage dark adaptation curve, with an initial cone branch and a later rod branch. The green line is the cone adaptation curve. The black curve is the rod adaptation curve. Note that the downward movement of these curves represents an increase in sensitivity. The curves actually begin at the points indicating “light-adapted sensitivity,” but there is a slight delay between the time the lights are turned off and when measurement of the curves begins. (Partial data [purple curve] from Ruston, 1961.)

32. Measuring Dark Adaptation - continued
Experiment for cone adaptation
Test light only stimulates cones.
Results show that sensitivity increases for three to four minutes and then levels off.
Experiment for rod adaptation
Must use a rod monochromat
Results show that sensitivity increases for about 25 minutes and then levels off.

33. Visual Pigment Regeneration
Process needed for transduction:
Retinal molecule changes shape
Opsin molecule separates
The retina shows pigment bleaching.
Retinal and opsin must recombine to respond to light.
Cone pigment regenerates in six minutes.
Rod pigment takes over 30 minutes to regenerate.

34. Spectral Sensitivity of Rods and Cones
Sensitivity of rods and cones to different parts of the visual spectrum
Use monochromatic light to determine threshold at different wavelengths
Threshold for light is lowest in the middle of the spectrum
1/threshold = sensitivity, which produces the spectral sensitivity curve

35. Figure 3. 21 (a) The threshold for seeing a light versus wavelength
Figure 3.21 (a) The threshold for seeing a light versus wavelength. (b) Relative sensitivity versus wavelength -- the spectral sensitivity curve. (Adapted from Wald, 1964.)

36. Spectral Sensitivity of Rods and Cones - continued
Rod spectral sensitivity shows:
more sensitive to short-wavelength light.
most sensitivity at 500 nm.
Cone spectral sensitivity shows:
most sensitivity at 560 nm.
Purkinje shift - enhanced sensitivity to short wavelengths during dark adaptation when the shift from cone to rod vision occurs

37. Figure 3.22 Spectral sensitivity curves for rod vision (left) and cone vision (right). The maximum sensitivities of these two curves have been set equal to 1.0. However, the relative sensitivities of the rods and the cones depend on the conditions of adaptation: the cones are more sensitive in the light, and the rods are more sensitive in the dark. The circles plotted on top of the rod curve are the absorption spectrum of the rod visual pigment. (Adapted from Wald, 1964; Wald & Brown, 1958.)

38. Spectral Sensitivity of Rods and Cones - continued
Difference in spectral sensitivity is due to absorption spectra of visual pigments
Rod pigment absorbs best at 500 nm.
Cone pigments absorb best at 419nm, 532nm, and 558nm
Absorption of all cones equals the peak of 560nm in the spectral sensitivity curve

39. Figure 3.24 Absorption spectra of the rod pigment (R), and the short- (S), medium- (M), and long wavelength (L) cone pigments. (Adapted from Dartnall, Bowmaker, & Mollon, 1983.)

40. Convergence in the Retina
Rods and cones send signals vertically through
bipolar cells.
ganglion cells.
ganglion axons.
Signals are sent horizontally
between receptors by horizontal cells.
between bipolar and between ganglion cells by amacrine cells.

41. Convergence in the Retina - continued
126 million rods and cones converge to 1 million ganglion cells.
Higher convergence of rods than cones
Average of 120 rods to one ganglion cell
Average of six cones to one ganglion cell
Cones in fovea have one to one relation to ganglion cells

42. Convergence and Sensitivity
Rods are more sensitive to light than cones.
Rods take less light to respond
Rods have greater convergence which results in summation of the inputs of many rods into ganglion cells increasing the likelihood of response.
Trade-off is that rods cannot distinguish detail

43. At this intensity, the rod ganglion cell receives ten units of excitation and fires, but each cone ganglion cell receives only two units and therefore does not fire. Thus, the rods’ greater spatial summation enables them to cause ganglion cell firing at lower stimulus intensities than the cones’.
Figure The wiring of the rods (left) and the cones (right). The dot and arrow above each receptor represents a “spot” of light that stimulates the receptor. The numbers represent the number of response units generated by the rods and the cones in response to a spot of intensity of 2.0.

44. Convergence and Detail
All-cone foveal vision results in high visual acuity
One-to-one wiring leads to ability to discriminate details
Trade-off is that cones need more light to respond than rods

45. Figure 3. 28 Neural circuits for the rods (left) and the cones (right)
Figure Neural circuits for the rods (left) and the cones (right). The receptors are being stimulated by two spots of light.

46. Lateral Inhibition of Neurons
Experiments with eye of Limulus
Ommatidia allow recordings from a single receptor.
Light shown into a single receptor leads to rapid firing rate of nerve fiber.
Adding light into neighboring receptors leads to reduced firing rate of initial nerve fiber.

47. Figure 3. 31 A demonstration of lateral inhibition in the Limulus
Figure A demonstration of lateral inhibition in the Limulus. The records on the right show the response recorded by the electrode in the nerve fiber of receptor A: (a) when only receptor A is stimulated; (b) when receptor A and the receptors at B are stimulated together; (c) when A and B are stimulated, with B at an increased intensity. (Adapted from Ratliff, 1965.)

48. Figure 3. 29 Circuit with convergence and inhibition form Figure 2. 16
Figure 3.29 Circuit with convergence and inhibition form Figure Lateral inhibition arrives at neuron B from A and from C.

49. Lateral Inhibition and Lightness Perception
Three lightness perception phenomena explained by lateral inhibition
The Hermann Grid: Seeing spots at an intersection
Mach Bands: Seeing borders more sharply
Simultaneous Contrast: Seeing areas of different brightness due to adjacent areas

50. Here the instructor can start by having the students look at the Hermann grid and discuss what they see. Then continue with the explanation using lateral inhibition.
Figure The Hermann grid Notice the gray “ghost images” at the intersections of the white areas, which decrease or vanish when you look directly at the intersection.

51. Hermann Grid
People see an illusion of gray images in intersections of white areas.
Signals from bipolar cells cause effect
Receptors responding to white corridors send inhibiting signals to receptor at the intersection
The lateral inhibition causes a reduced response which leads to the perception of gray.

52. Figure 3.33 (a) Four squares of the Hermann grid, showing five of the receptors under the pattern. Receptor A is located at the intersection, and B, C, D, and E have a black square on either side. (b) Perspective view of the grid and five receptors, showing how the receptors connect to bipolar cells. Receptor A’s bipolar cell receives lateral inhibition from the bipolar cells associated with receptors B, C, D, and E. (c) The calculation of the final response of receptor A’s bipolar cell starts with A’s initial response (100) and subtracts the inhibition associated with each of the other receptors.

53. Figure 3. 34 (a) Four squares of the Hermann grid, as in the Figure 3
Figure 3.34 (a) Four squares of the Hermann grid, as in the Figure 3.33, but now focusing on receptor D, which is flanked by two black squares. Receptor D is surrounded by receptors A, F, G, and H. Notice that receptors F and H are located under two black squares, so they receive less light than the other receptors. (b) Perspective view showing the inhibition received by the bipolar cells associated with receptor D. Notice that D receives less inhibition than A did in the previous example, because two of the bipolar cells that are sending lateral inhibition (F and H) are associated with receptors that are illuminated more dimly. (c) The calculation of the final response of receptor D indicates that it responds more than A in the previous example.

54. Mach Bands
People see an illusion of enhanced lightness and darkness at borders of light and dark areas.
Actual physical intensities indicate that this is not in the stimulus itself.
Receptors responding to low intensity (dark) area have smallest output.
Receptors responding to high intensity (light) area have largest output.
For this example, the explanation is placed first since it is difficult to get a good set of Mach Bands using a projector. It can be useful to ask students to do this outside of class using one of the methods given in the text, or if the class is small enough, the instructor can make a set of cards with light and dark areas that the students can use during class.

55. This slide provides the students with the stimulus so that they can experience it before it is explained to them on the next slide.
Figure Mach bands at a contour between light and dark. (a) Just to the left of the contour, near B, a faint light band can be perceived, and just to the right at C, a faint dark band can be perceived. (b) The physical intensity distribution of the light, as measured with a light meter. (c) A plot showing the perceptual effect described in (a). The bump in the curve at B indicates the light Mach band, and the dip in the curve at C indicates the dark Mach band. The bumps that represent our perception of the bands are not present in the physical intensity distribution.

56. Mach Bands - continued
All receptors are receiving lateral inhibition from neighbors
In low and high intensity areas amount of inhibition is equal for all receptors
Receptors on the border receive differential inhibition

57. Mach Bands - continued
On the low intensity side, there is additional inhibition resulting in an enhanced dark band.
On the high intensity side, there is less inhibition resulting in an enhanced light band.
The resulting perception gives a boost for detecting contours of objects.

58. Figure Circuit to explain the Mach band effect based on lateral inhibition. The circuit works like the one for the Hermann grid in Figure 3.34, with each bipolar cell sending inhibition to its neighbors. If we know the initial output of each receptor and the amount of lateral inhibition, we can calculate the final output of the receptors. (See text for a description of the calculation.)

59. Figure A plot showing the final receptor output calculated for the circuit of figure The bump at B and the dip at C correspond to the light and dark Mach bands, respectively.

60. This slide provides the students with the stimulus so that they can experience it before it is explained to them on the next slide.
Figure Simultaneous contrast. The two center squares reflect the same amount of light into your eyes but look different because of simultaneous contrast.

61. Simultaneous Contrast
People see an illusion of changed brightness or color due to effect of adjacent area
An area that is of the same physical intensity appears:
lighter when surrounded by a dark area.
darker when surrounded by a light area.

62. Simultaneous Contrast - continued
Receptors stimulated by bright surrounding area send a large amount of inhibition to cells in center.
Resulting perception is of a darker area than when this stimulus is viewed alone.
Receptors stimulated by dark surrounding area send a small amount of inhibition to cells in center.
Resulting perception is of a lighter area than when this stimulus viewed alone.

63. Figure How lateral inhibition has been used to explain the simultaneous contrast effect. The size of the arrows indicate the amount of lateral inhibition. Because the square on the left receives more inhibition, it appears darker.

64. Illusions not Explained by Lateral Inhibition
White’s Illusion
People see light and dark rectangles even though lateral inhibition would result in the opposite effect.

65. Figure 3. 41 White’s illusion
Figure 3.41 White’s illusion. The rectangles at A and B appear different, even though they are printed from the same ink and reflect the same amount of light. (From White, 1981.)

66. Figure 3.42 When you mask off part of the White’s illusion display, as shown here, you can see that rectangles A and B are actually the same. (Try it!)

67. Figure 3.43 The arrows indicate the amount of lateral inhibition received by parts of rectangles A and B. Because the part of rectangle B is surrounded by more white, it receives more lateral inhibition. This would predict that B should appear darker than A (as in the simultaneous contrast display in Figure 3.39), but the opposite happens. This means that lateral inhibition cannot explain our perception of White’s illusion.

68. Explanation of White’s Illusion
Belongingness
An area’s appearance is affected by where we perceive it belongs.
Effect probably occurs in cortex rather than retina.
Exact physiological mechanism is unknown.

69. Perception is Indirect
Stimuli in environment impinge on receptors.
Transduction takes place causing electro-chemical impulse in neurons.
Provides accurate information about world
Feels direct but that is an illusion
This is true for all senses