Lesson 1.14 · Unit 1 · CED topic 1.6 (part 2)
Vision: the eye, color, and the visual brain
Vision is the sense we rely on most, and it shows nearly every principle of sensation in action: a stimulus (light), transduction (the retina), a pathway to the brain, and an interpretation that is partly built by the brain rather than simply recorded. The exam asks about the functions of the eye's parts rather than their anatomy, so concentrate on what each part does for vision and what goes wrong when it fails. This lesson also covers the two theories of color vision and two disorders that result when the brain's visual areas are damaged.
Learning objective.
- 1.6.B. Explain how the structures and functions of the visual sensory system relate to behavior and mental processes.
Essential knowledge.
- 1.6.B.1. The retina is the photosensitive surface at the back of the eye. Cells in the retina capture visual information that is transduced to the brain for processing. Evidence of incomplete images captured by the retina is demonstrated by the presence of the blind spot, where the visual nerve exits the eye. The brain fills in the gaps in the incomplete retinal images to perceive a relatively complete picture of the world. 1.A 2.B 3.B 3.C
- 1.6.B.2. Visual stimuli are focused onto the retina by the lens via a process called accommodation. When this process is altered, nearsightedness or farsightedness can result.
- 1.6.B.3. Cells that lie in the periphery of the eye and detect shapes and movement, but not color, are called rods. These cells are mainly activated in low-light environments. These cells play a role in light and dark adaptation.
- 1.6.B.4. Color vision is explained by both the trichromatic theory and the opponent-process theory.
- 1.6.B.4.i. Photoreceptor cells located in the fovea of the eye that process color and detail are called cones. Researchers have identified blue (detecting short wavelengths), green (detecting medium wavelengths) and red (detecting long wavelengths) cones in the retina.
- 1.6.B.4.ii. Afterimages result when certain ganglion cells in the retina are activated while others are not. The ganglion cells involved in this opponent process are red/green, blue/yellow/and black/white.
- 1.6.B.4.iii. Color vision deficiency involves damage or irregularities to one or more cones or ganglion cells (red/green, blue/yellow). Color vision deficiency includes dichromatism or monochromatism.
- 1.6.B.5. Damage to parts of the brain responsible for vision (mainly the occipital lobes) can result in disorders such as prosopagnosia (face blindness) and blindsight.
Light and the path to the retina
The stimulus for vision is light, a form of electromagnetic energy that travels in waves. The wavelength of the light determines its hue (color: short wavelengths look blue, long wavelengths look red) and the amplitude (height) of the wave determines its brightness. Light enters the eye through the transparent cornea, which begins to bend (focus) the light. It passes through the pupil, the opening whose size is controlled by the colored iris: the pupil gets smaller in bright light and larger in dim light. Behind the pupil is the lens, which changes shape to focus the light on the back of the eye.
The lens focuses by changing its curvature, a process called accommodation: it flattens to focus on distant objects and thickens to focus on near ones. When the focus does not land exactly on the retina, vision is blurred. In nearsightedness (myopia) the image focuses in front of the retina, so nearby objects are clear but distant ones are blurry. In farsightedness (hyperopia) the image focuses behind the retina, so distant objects are clear and close ones are blurry. Glasses, contact lenses, or surgery correct the focus.
The retina
The retina is the light-sensitive surface at the back of the eye. It contains the photoreceptor cells, rods and cones, that transduce light into neural signals. These signals pass through other neurons (bipolar cells, then ganglion cells) whose axons bundle together to form the optic nerve, which carries the information to the brain.
- Rods are located mostly in the periphery of the retina and detect shapes and movement, but not color. They are very sensitive to dim light, so they work mainly in low-light environments, and they play the major role in dark adaptation (the increasing sensitivity as you stay in the dark, the way your eyes adjust after the lights go out) and light adaptation.
- Cones are concentrated in the fovea, the small central region of the retina, and they detect color and fine detail. They work best in bright light. This is why you look straight at something to see its detail.
The place where the optic nerve leaves the eye contains no receptors, so there is a blind spot in each eye. You do not normally notice it because the brain fills in the gap, using information from the surrounding area and from the other eye, to produce a relatively complete picture of the world. This shows that what we see is partly constructed by the brain.
From the eye to the brain
The optic nerves carry information to the thalamus, which relays it to the visual cortex in the occipital lobes at the back of the brain, where it is processed and interpreted. As a result of the crossing of the optic nerves, information from the left half of each eye's visual field is processed by the right hemisphere and vice versa (this is the contralateral organization seen in split-brain studies).
David Hubel and Torsten Wiesel recorded the activity of single neurons in the visual cortex of anesthetized cats while showing them lines and shapes on a screen. They found that individual cells responded best to specific features, such as a line at a particular angle, an edge, or a particular direction of movement, and different cells responded to different features. The researchers won a Nobel Prize in 1981 for the work.
Reading the research: an animal experiment in which the independent variable was the feature shown (angle of the line, for example) and the dependent variable was the neuron's firing rate. It showed that the visual cortex analyzes the scene in pieces that are later combined, which is the basis for the idea of feature detection. Generalizing from cats to humans requires some caution, but human brain imaging has supported the same ideas.
How we see color
Two theories, which turn out to describe different stages, are needed to explain color vision.
Trichromatic theory (the Young-Helmholtz theory) says that the retina has three kinds of cones, each most sensitive to a different range of wavelengths: blue (short wavelengths), green (medium), and red (long). The brain perceives any color by comparing how strongly each type responds. If all three respond equally strongly, you see white. This theory explains how cones detect color.
Opponent-process theory says that color is also processed in opposing pairs by ganglion cells in the retina: red/green, blue/yellow, and black/white. When one member of a pair is stimulated the other is inhibited, so you cannot see reddish-green or bluish-yellow. This theory explains afterimages: if you stare at a green square and then look at a white surface, you see a red afterimage. Staring at green tires the cells that respond to green, so when you look at white the opposing red response is stronger. The two theories work together: three cone types in the retina, and opponent-process cells that process the signals after them.
Color vision deficiency (color blindness) results from damage or irregularities in one or more types of cones or ganglion cells. Most commonly the red/green or blue/yellow system is affected. A person who is missing one type of cone has dichromatism and can distinguish only two dimensions of color (for example, red/green deficiency, which is more common in males); a person with monochromatism has only one cone type or none and sees no color at all, which is rare.
When the visual brain is damaged
Damage to the parts of the brain responsible for vision, mainly the occipital lobes, can produce disorders even when the eyes work well. Prosopagnosia (face blindness) is the inability to recognize familiar faces, even one's own, although the person can see that it is a face and recognize people by other cues such as voice or clothing. In blindsight, a person with damage to the visual cortex insists they cannot see anything in part of the visual field but can still point to or guess the location of objects there, with accuracy better than chance, because other pathways still process some visual information without conscious awareness.
For each situation, say whether rods or cones are mainly responsible. (a) Reading small print in bright light. (b) Noticing a movement at the edge of your vision at dusk. (c) Telling red from green. (d) Finding your way around a dark bedroom after your eyes have adjusted.
Step 1, (a) fine detail in bright light needs cones, which are concentrated in the fovea.
Step 2, (b) movement in the periphery in dim light is mostly rods.
Step 3, (c) color is detected by cones.
Step 4, (d) dark adaptation is a function of rods.
You stare at a bright yellow flag for 30 seconds, then look at a white wall and see a blue flag. Which theory explains this, and how?
Step 1, the opponent-process theory, because yellow and blue are an opposing pair.
Step 2, staring at yellow fatigues the cells that respond to yellow in the blue/yellow system.
Step 3, when you look at white (which contains all wavelengths), the fatigued yellow response is weak, so the opponent blue response dominates, and you see blue.
Step 4, trichromatic theory explains the first step (the cones that respond to red and green wavelengths together signal yellow) and opponent-process theory explains the afterimage.
A student sees close objects clearly but the whiteboard is blurry. What is the condition, what is happening inside the eye, and what would help?
Step 1, near objects clear and far objects blurry is nearsightedness (myopia).
Step 2, the lens focuses distant images in front of the retina rather than on it.
Step 3, glasses or contact lenses with a lens that shifts the focus back onto the retina would help.
Step 4, this is a problem of focusing (accommodation), not of the retina or brain, so rods and cones are normal.
- Path: cornea, pupil/iris, lens (accommodation), retina (rods, cones), ganglion cells, optic nerve, thalamus, occipital lobe. Nearsighted = near clear; farsighted = far clear.
- Rods: periphery, dim light, shape and movement, dark adaptation, no color. Cones: fovea, color and detail, bright light. The blind spot is where the optic nerve exits; the brain fills it in.
- Trichromatic theory: blue, green, and red cones. Opponent-process theory: red/green, blue/yellow, black/white (explains afterimages). Dichromatism and monochromatism are forms of color vision deficiency.
- Prosopagnosia and blindsight result from damage to the visual brain (occipital lobes).
- What is accommodation, and what happens in nearsightedness?
Show answer
Accommodation is the lens changing shape to focus light on the retina. In nearsightedness the image focuses in front of the retina, so distant objects are blurry.
- Why can't you see color well in dim light?
Show answer
Cones, which detect color, need bright light; in dim light mainly rods are active, and they do not detect color.
- Explain the blind spot and why you don't normally notice it.
Show answer
The optic nerve exits the eye at a point with no receptors, so that spot cannot detect light. The brain fills in the gap using surrounding information and the other eye's input.
- Name the theory that explains each: (a) three types of cones, (b) afterimages.
Show answer
(a) trichromatic theory; (b) opponent-process theory.
- A man cannot recognize his wife's face, but knows her by her voice. Name the disorder and the brain region involved.
Show answer
Prosopagnosia (face blindness), associated with damage to visual areas in the occipital (and temporal) lobes of the brain.
- What did Hubel and Wiesel's cat experiments show about the visual cortex?
Show answer
Individual neurons respond best to specific features such as lines at certain angles, edges, or movement, so the visual system analyzes the scene in parts.