Chapter 6- Vision Flashcards
Statement that whatever excites a particular nerve always sends the same kind of information to the brain
Law of specific nerve energies
The brain somehow interprets the action potential’s from the auditory nerve as sounds, those from the olfactory nerve as odors, and so forth.
For example, if you rub your eyes, you may see spots or flashes of light even in a totally dark room. You applied mechanical pressure, which excited visual receptors in your eyes. Anything that excites those receptors is perceived as light.
You perceive an object when it emits or reflects energy that stimulates receptors that transmit information to your brain. How does your brain make sense of that information?
Your brain encodes the information in a way that doesn’t resemble what you see. Your brain stores a representation of information in terms of altered activity in many neurons. Your brain codes the information in terms of which neurons respond, their amount of response, and the timing of their responses
If someone electrically stimulated the auditory receptors in your ear, what would you perceive?
Because of the law of specific nerve energies, you would perceive it as sound, not as shock. Of course, a strong enough shock would spread far enough to excite pain receptors also
If it were possible to flip your entire brain upside down, without breaking any of the connections to sense organs or muscles, what would happen to your perceptions of what you see, hear, and so forth?
Your perceptions would not change. The way visual or auditory information is coded in the brain does not depend on the physical location within the brain. Seeing something as “on top” or “to the left” depends on which neurons are active but does not depend on the physical location of those neurons
An opening in the centre of the iris where light enters
Pupil
Light enters the eye through the pupil and is focussed by the lens, which is adjustable, and cornea, which is not adjustable, and projected onto the retina.
The rear surface of the eye, which is lined with visual receptors
Retina
Light from the left side of the world strikes the right half of the retina, and vice versa. Light from above strikes the bottom half of the retina, and light from below strikes the top half. The inversion of the image poses no problem for the nervous system.
Type of neuron in the retina that receives input directly from the receptors
Bipolar cells
Type of neuron in the retina that receives input from the bipolar cells
Ganglion cells
Ganglion cell axons that exit through the back of the eye and continue to the brain
Optic nerve
Area at the back of the retina where the optic nerve exits; it is devoid of receptors
Blind spot
Images sometimes disappear because they strike the blind spot, and when the blind spot interrupts a straight line or other regular pattern, your brain fills in the gap
Additional cells besides bipolar cells and ganglion cells called ______ cells get information from bipolar cells and send it to other bipolar, amacrine, and ganglion cells. Various types of these cells refine input to ganglion cells, enabling them to respond specifically to shapes, movements, or other visual features
Amacrine
In every day life, you never notice your blind spot for two reasons:
- Your brain feels in the gap for straight lines or other regular patterns
- Anything in the blind spot of one eye is visible to the other eye
What makes the blind spot of the retina blind?
The blind spot has no receptors because it is occupied by exiting axons and blood vessels
Ganglion cells in the fovea of humans and other primates
Midget ganglion cells
It is called midget ganglion cells because each is small and responds to just a single cone. As a result, each cone in the fovea is connected to the brain with a direct route that registers the exact location of the input. Because the midget ganglion cells provide 70% of the input to the brain, our vision is dominated by what we see in the fovea
A tiny area of the retina specialized for acute, detailed vision
Fovea
Means “pit”
Because blood vessels and ganglion cell axons are almost absent near the fovea, it has nearly unimpended vision. The tight packing of receptors also aids perception of detail
More important for perceiving detail, each receptor in the fovea connects to a single _____ ____, which in turn connects to a single ______ ___, which has an axon to the brain
Bipolar cell; ganglion cell
Describe visual perception for many bird species
Many birds eyes occupy most of the head, compared to only 5% of the head in humans.
Many bird species have two foveas per eye, one pointing ahead and one pointing to the side. The extra foveas enable perception of detail in the periphery.
Hawks and other predatory birds have a greater density of visual receptors on the top half of their retinas, looking down, than on the bottom half, looking up. That arrangement is adaptive because predatory birds spend most of their day soaring high in the air looking down. However, to look up, the bird must turn its head.
Toward the periphery of the retina, more and more receptors converge onto bipolar and ganglion cells. As a result, the brain cannot detect the exact location or shape of a peripheral light source. However, the summation enables perception of ______ ______ in the periphery. In short, foveal vision has better ______, or sensitivity to detail, and peripheral vision has better sensitivity to ___ ____
Fainter lights; acuity; dim light
In the periphery, your ability to detect detail is limited by interference from other nearby objects
Type of retinal receptor that detects brightness of light
Rods
Abundant in the periphery of the human retina
Respond to faint light but are not useful in daylight because bright light bleaches them
Type of retinal receptor that contributes to colour perception
Cones
Abundant in and near the fovea
Less active in dim light, more useful in bright light, and essential for colour vision
Chemicals contained in rods and cones that release energy when struck by light
Photopigments
Because of the distribution of rods and cones, you have a good colour vision in the _____ but not in the _____
Fovea; periphery
Although rods out
number cones by about 20 to 1 in the human retina, cones provide about ____% of the brain’s input
90%
You sometimes find that you can see a faint star on a dark night better if you look slightly to the side of the star instead of straight at it. Why?
If you look slightly to the side, the light falls on an area of the retina with more rods and more convergence of input
If you found a species with a high ratio of cones to rods in its retina, what would you predict about its way of life?
We should expect this species to be highly active during the day and seldom active at night
Theory that colour is perceived through the relative rates of response by three kinds of cells, each one maximally sensitive to a different set of wavelengths
Trichromatic theory or young-Helmholtz theory
Short-wavelength, medium-wavelength, and long-wavelength cone types. Each cone responds to a broad range of wavelengths but to some more than others.
According to the trichromatic theory, we discriminate among wave links by the ratio of activity across the three types of cones. For example, light at 550 nm excites the medium-wavelength and long-wavelength receptors about equally and the short-wavelength receptor almost not at all. This ratio of responses among the three cones determines a perception of yellow-green. When all three types of cones are equally active, we see white or gray
Area of the world that an individual can see at any time
Visual field
Result of staring at a coloured object for a prolonged length of time and then looking at a white surface, the image is seen as a negative image, with a replacement of red with green, green with red, yellow and blue with each other, and a black and white with each other
Negative colour after image
Idea that we perceive colour in terms of opposites
Opponent process theory
That is, the brain has a mechanism that perceives colour on a continuum from red to green, and another from yellow to blue, and another from white to black.
Because demonstrations show that you do not always see the correct colour of after image, after images depend on the whole context, not just the light on individual receptors. The cerebral cortex must be responsible, not the bipolar or ganglion cells
The ability to recognize colours despite changes in lighting
Colour constancy
Concept that the cortex compares information from various parts of the retina to determine the brightness and colour for each area
Retinex theory
Proposed by Edwin Land to account for colour and brightness constancy.
Suppose a bipolar cell receives excitatory input from medium-wavelength cones and inhibitory input from all three kinds of cones. When it is highly excited, what colour would one see? When it is inhibited, what colour perception would result?
Excitation of the cell should yield a perception of green under normal circumstances. Inhibition would produce the opposite sensation, red
When a television set is off, it’s screen appears gray. When you watch a program, parts of the screen appear black, even though more light is actually showing on the screen than when the set was off. What accounts for the black perception?
The black experience arises by contrast with the other brighter areas. The contrast occurs by comparison within the cerebral cortex, as in the retinex theory of colour vision
Figure 6.9 shows 500 nm light as blue and 550 nm light as yellow. Why should we nevertheless not call them “blue light” and “yellow light”?
Colour perception depends not just on the wavelength of light from a given spot but also the light from surrounding areas. As in figure 6.14, the context can change the colour perception
Inability to perceive colour differences
Colour vision deficiency
For genetic reasons, some people lack one or two of the types of cones. Some have three kinds of cones, but one kind is abnormal. In red-green colour deficiency, the most common form of colour deficiency, people have trouble distinguishing red from green because their long and medium wavelength cones have the same photopigment instead of different ones.
Most people can use varying amounts of three colours to match any other colour that they see. Who would be an exception to this rule, and how many colours would they need?
Red-green color-deficient people would need only two colors. Women with four kinds of cones might need four