(S&P) Ch. 2/3 Study Guide
How would you use the method of limits to measure the spectral sensitivity of the rod system?
By presenting sequences of stimuli, such as a light, in ascending and descending order we can ask the observer to indicate when they see the light in order to determine the threshold. To measure the Rod system of course, this would have to be applied on a subject that is dark adapted. The threshold is determined by locating the average of all the crossover points.
enzyme Cascade
Sequence of reactions triggered by an activated visual pigment molecule that results in transduction
Explain why the "blind spot" exists, and why we are not usually aware of it
-It is the area of the retina where there are no receptors, where the optic nerve leaves the eye. -We aren't aware of the blind spot because it is located off to the side of our visual field, where objects are not is sharp focus. Because of this we don't know exactly where to look for it. --The most important reason that we don't see the blind spot is that some mechanism in the brain "fills in" the place where the image disappears. The brain does not fill in the area served by the blind spot with "nothing"; rather it creates a perception that matches the surrounding pattern
The formation of an image upon the retina depends on what?
-Light must be able to penetrate the eyes -The distribution of light, must be focused -Patterns of light falling on the retina must preserve the spatial structure of the object
Describe how the cornea and lens focus the image on the retina
-The cornea accounts for about 80% of the eye's focusing power, but like the lens in eyeglasses, it is fixed in place so it can't adjust its focus -The lens can change its shape to adjust the eye's focus for objects located at different distances. This change in shape is achieved by the action of ciliary muscles which increase the focusing power of the lens -Accommodation helps prevent blurring by changing the shape of the lens. The increased curvature increases the bending of the light rays passing through the lens so the focus point is pulled back to A to create a sharp image on the retina. This means as you look around at different object your eye is constantly adjusting its focus by accommodating, especially for nearby objects. Ex. When the eye is relaxed in a person with 20/20 vision and they view a small object that is far away. If the object is located more than 20 feet away the light rays that reach the eye are essentially parallel, and the cornea lens combination brings these parallel rays to focus on the retina at point A. But if the object moves closer to the eye, the light rays reflected from this object enter the eye at more of an angle, and this pushes the focus point back so if the back of the eye weren't there, light would be focused at point B (further from retina). Because the light is stopped by the back of the eye before it reaches point B, the image on the retina is out of focus (blurred vision).
Describe the method for measuring dark adaptation, and the overall results
1) Measure the dark adaptation curve. -Method:Have the observer look at a small fixation point while paying attention to the flashing test light that is off to the side. because the observer is looking directly at the fixation point, its image falls on the fovea, so the image of the test light falls on the peripheral retina, which contains both rods and cones. While still in the light, the observer measures his or her threshold for seeing the light by turning a knob that adjusts the intensity of the flashing light until it can just barely be seen. This threshold is converted to sensitivity. because sensitivity = 1/threshold, this means that a high threshold corresponds to a low sensitivity. Sensitivity measured in the light is called light-adapted sensitivity, because it's measured while the eyes are adapted to the light. Because the room (or adapting) lights are on, the intensity of the flashing test light has to be high to be seen. At the beginning of the experiment , then, the threshold is high and the sensitivity is low. Once the light-adapted sensitivity to the flashing test light is determined, the adapting light is extinguished so the observer is in the dark. The observer continues adjusting the intensity of the flashing light so it can just barely be seen, tracking the increase in sensitivity that occurs in the dark. As the observer becomes more sensitive to the light, they must decrease the light's intensity to keep it just barely visible. -Results: Dark adaptation curve shows that as adaptation proceeds, the observer becomes more sensitive to the light. Higher sensitivity is at the bottom of this graph, so movement of the dark adaptation curve downward means that the observer's sensitivity is increasing. The red dark adaptation curve indicates that the observer's sensitivity increases in 2 phases. It increases rapidly for the first 3 or 4 minutes after the light is extinguished and then levels off. At about 7 to 10 minutes, it begins increasing again and continues to do so until observers have been in the dark for about 20 to 30 minutes. The sensitivity at the end of dark adaption (dark adapted sensitivity), is about 100,000 times greater than the light adapted sensitivity measured before dark adaption begins.
What are the major cell classes in the retina? Begin with photoreceptors, trace the neural connection to the optic nerve
1). Photoreceptors: rods and cones -Contain outer segments then inner segments then cell bodies 2) Horizontal Cells: Neuron that transmits signals laterally across the retina 3) Bipolar Cells: Neuron that receives inputs from visual receptors and sends signals to the retinal ganglion cells 4) Amacrine Cells: Neuron that transmits signals laterally in the retina 5) Ganglion Cells: Neuron that receives input from bipolar and amacrine cells -Axons of these cells are the nerve fibers that travel out of the eye in the optic nerve
Describe the difference in spectral sensitivity curves between rods and cones
Cone and rod spectral sensitivity curves show that rods are more sensitive to short wavelength light than are cones, with the rods being most sensitive to light at 500 nm and the cones being most sensitive to light at 560 nm. This difference in the sensitivity of cones and rods to different wavelengths means that as vision shifts from the cones to the rods during dark adaptation, we become relatively more sensitive to short wavelength light, that is light nearer the blue and green end of the spectrum.
Describe the structure of the human eye. As you mention major structures (ex. lens) you should also tell me the primary role of each.
Light reflected from objects in the environment enters the eye through the PUPIL an is focused by the CORNEA and LENS to form sharp images of the objects on the RETINA. -These visual receptors in the retina, the RODS and CONES, contain light sensitive chemicals called VISUAL PIGMENTS that react to light and trigger electrical signals. Signals from the receptors flow through the network of neurons that make up the retina and emerge from the back of the eye in the OPTIC NERVE. The cornea and lens at the front of the eye and the receptors and neurons in the retina lining the back of the eye shape what we see by creating 2 transformation: -Transformation from light reflected from an object into an image of the object -Transformation from the image of the object into electrical signals1
Name 2 visual conditions that differentially affect the rods and the cones
Macular Degeneration -Destroys the cone rich fovea and a small area that surrounds it, this creates a blind region in central vision, so when a person looks directly at something they lose sight of it. Retinitis Pigmentosa -Degeneration of the retina passed from one generation to the next (although not always affecting everyone in the family). Condition first attacks the peripheral rod receptors and results in poor vision in the peripheral visual field. Eventually, in severe cases, foveal cone receptors are also attacked, resulting in complete blindness
Describe the method for isolating the rod and cone components of the dark adaptation curve, and the results for each
Measuring dark adaptation in cones -We have to ensure that the image of the test light falls only on the cones. We have the observer look directly at the test light so its image will fall on the all-cone fovea, and by making the test light small enough so that its entire image falls within the fovea. Results: The curve matches the initial phase of the original dark adaptation curve but does not include the second phase. This is because the second part of the curve is due to rods Measuring dark adaptation in rods -Because the cones are more sensitive to light at the beginning of dark adaptation, they control our vision during the early stages of adaptation, so we can't see what the rods are doing. In order to reveal how the sensitivity of the rods is changing at the very beginning of dark adaptation, we need to measure dark adaptation in a person who has no cones. The sensitivity we determine indicates that the rods are much less sensitive than the cone light-adapted sensitivity we measured in our original experiment. We can also see that once dark adaptation begins, the rods increase their sensitivity, and reach their final dark adapted level in about 25 minutes. The end of this rod adaptation measured in the monochromat matches the second part of the 2 stage dark adaptation curve. Overall -As soon as the light is extinguished, the sensitivity of both the cones and the rods begin increasing. However, because the cones are much more sensitive than the rods at the beginning of dark adaptation, we see with our cones right after the lights are turned out. So cones have center stage at the beginning of dark adaptation while the rods are working behind the scenes. However after 3 to 5 minutes in the dark, the cones have reached the maximum sensitivity, as indicated by the leveling off of the dark adaptation curve. Meanwhile the rods are still adapting behind the scenes, and by about 7 minutes in the dark, the rod's sensitivity finally catches up to the cones. The rods then become more sensitive than the cones, and rod adaptation, indicated by the second branch of the dark adaptation curve becomes visible the place where the rods begin to determine the dark adaptation curve is called rod-cone break. Visual pigment regeneration occurs much more rapidly in the cones than in the rods and this is why it takes 20-30 minutes for the rods to reach maximum sensitivity compared to the 3-4 minutes it takes cones.
Describe why the near point is used as a measure of presbyopia
Presbyopia is a condition where the distance of the near point increases as a person gets older -10 cm for most 20 year olds, but increases to 14 cm by age 30, 22 cm at 40, and 100 cm at 60 -occurs because the lens hardens with age and the ciliary muscles become weaker. Which makes it difficult for the lens to change shape for vision at close range. -Poses little problem for most people before the age of 45, but at this age the near point begins to increase rapidly. -2 solutions: hold things further away or wear reading glasses
Explain how spectral sensitivity curves explain the Purkinje Shift
Purkinje Shift is the enhanced perception of short wave lengths (blue) during dark adaptation. -We can see this in the rod spectral sensitivity curve by noting how the rod is most sensitive at around 500 nm which would land on the color blue. In the example in the book, if one closes one eye and allows it to dark adapt, when viewing red and blue flowers the blue flower is enhanced and appears brighter in the dark adapted eye
Discuss the differences between the distribution of the rods and cones
Ratio of rods to cones depends on the location in the retina. -Fovea: contains only cones. Very small. Contains about 1 % (50,000) of the 6 million cones -Peripheral Retina: All the retina outside the fovea, contains both rods and cones, but contains more rods than cones because there are about 120 million rods and only 6 million cones in the retina
Explain how visual pigment regeneration is related to dark adaptation.
The increase in visual pigment concentration that occurs as the pigment regenerates in the dark is responsible for the increase in sensitivity we measure during dark adaptation. The relationship between pigment adaptation and sensitivity was demonstrated by William Rushton, who devised a procedure to measure the regeneration of visual pigment in humans by measuring the darkening of the retina that occurs during dark adaptation. -Measurements showed that cone pigment takes 6 minutes to regenerate completely, whereas rod pigment takes more than 30 minutes. When he compared the course of pigment regeneration to the dark adaptation curve, he found that the rate of cone dark adaptation matched the rate of cone pigment regeneration and the rate of rod dark adaptation matched the rate of rod pigment regeneration. -These results demonstrated 2 important connections between perception and physiology (1) Our sensitivity to light depends on the concentration of a chemical (visual pigment) (2) The speed at which our sensitivity increases in the dark depends on a chemical reaction (regeneration of the visual pigment)
Describe the role of visual pigments in transduction
The process of transduction for vision is the transformation of light energy in chemical energy, which occurs in the rods and cones -Visual Pigments are light sensitive chemicals contained in the rods and cones that react to light and trigger electrical signals -Visual pigments have 2 parts: opsin (long protein, 100x longer than retinal.) and retinal (smaller light sensitive component). Retinal is the crucial part because when the retinal and opsin are combined, the resulting molecule absorbs visible light. When a visual pigment molecule absorbs one photon of light, the retinal changes its shape, from being bent, to straight. this change of shape (isomerization), creates a chemical chain reaction that activates thousands of charged molecules to create electrical signals in receptors. The importance of this chain reaction is that it amplifies the effect of isomerization. Isomerizing 1 pigment molecule triggers a chain of chemical reactions that release as many as a million charged molecules, which leads to activation of the receptor. -Visual pigments not only create electrical signals in the receptors, they also shape specific aspects of our perceptions. Ex. properties of the pigments help determine how well we are able to adjust to darkness and how well we are able to see light in different parts of the visible spectrum. -Overall: When a light sensitive visual pigment molecule in a receptor absorbs light, it isomerizes and starts a chain reaction that leads to activation of the receptor. Psychophysical experiments have shown that a rod receptor can be activated by the isomerization of on cone molecule, and activation of 7 rod receptors can result in perception
Discuss how the electromagnetic spectrum is related to vision
Vision is based on visible light, Which is a band of energy within the electromagnetic spectrum. Visible light is the energy within the electromagnetic spectrum that humans can perceive, and it has wavelengths ranging from 400 to 700 nm. The wavelength of visible light is associated with the different colors of the spectrum, short (blue), medium(green), and long (yellow, orange, and red). Although we usually describe light based on its wavelength, we can also describe it as consisting of photons (1 photon being the smallest possible packet of light energy)
