Chapter 2 - Study Questions Flashcards
(16 cards)
- Describe the structure of the eye and how moving an object closer to the eye affects how light reflected from the object is focused on the retina.
The eye contains receptors for vision. Light reflected from objects in the environment enters the eye through the pupil and is focused by the cornea and lens to form sharp images of the objects on the retina, the network of neurons that covers the back of the eye and that contains the receptors for vision. The rods and cones contain light-sensitive chemicals called visual pigments that react to light and trigger electrical signals.
Light reflected from an object into the eye is focused onto the retina by a two-element optical system: the cornea and the lens.
When an object moves closer to the eye, the light rays reflected from the object enter the eye at more of an angle and this pushes the focus point back. This change in the lens’s shape that occurs when the ciliary muscles at the front of the eye tighten and increase the curvature of the lens so that It gets thicker is called accommodation.
- How does the eye adjust the focusing of light by accommodation? Describe the following conditions that can cause problems in focusing presbyopia, myopia, hyperopia. Be sure you understand the difference between the near point and the far point and can describe the various solutions to focusing problems, including corrective lenses and surgery.
The eye adjusts the focusing of the light by changing the lens’s shape, this change occurs when the ciliary muscles at the front of the eye tighten and increase the curvature of the lens so that It gets thicker and this change is called accommodation.
Presbyopia is when the distance of the near point increases as the person gets older. This loss of the ability to accommodate occurs because the lens hardens with age and the ciliary muscles become weaker. These changes make it more difficult for the lens to change its shape for vision at close range.
Myopia, also called nearsightedness, is an inability to see distance objects clearly.
Glasses or contact lenses are the major route to clear vision for people with myopia, but surgical procedures in which lasers are used to change the shape of the cornea can enable people to experience good vision without corrective lenses. Laser-assisted in situ keratomileusis ( LASIK) surgery is a procedure that involves sculpting the cornea with a type of laser called an excimer laser, which does not heat tissue. A small flap, less than the thickness of a human hair, is cut into the surface of the cornea. The flap is folded out of the way, the cornea’s curvature is sculpted by the laser so that it focuses light onto the retina, and the flap is then folded back into place.
Hyperopia, or farsightedness, is when a person can see distant objects clearly but has trouble seeing nearby objects. Because the constant need to accommodate when looking at nearby objects can cause eyestrain and headaches, corrective lenses that bring focus point forward onto the retina are available to eliminate this issue.
- Where on the retina does a researcher need to present a stimulus to test dark adaptation of the cones? How is this related to the distribution of the rods and cones on the retina? How can the adaptation of the rods be measured without any interference from the cones?
A researched need to present a stimulus in the fovea to test dark adaptation of the cones as the fovea is the part of the retina that contains only cones. Because of this fact, the rods wouldn’t be interfering, and the dark adaptation of the cones would be properly measured.
The ration of rods and cones depends on the location in the retina.
The peripheral retina, which includes all of the retina outside of the fovea, contains both rods and cones.
The peripheral retina contains many more rods than cones.
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.
- Describe how rod and cone sensitivity changes starting when the lights are turned off and how this change in sensitivity continues for 20 to 30 minutes in the dark. When do the rods begin adapting? When do the rods become more sensitive than the cones?
As soon as the light is extinguished, the sensitivity of both the cones and the rods begins 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. One way to think about this is that the cones have “center stage” at the beginning of dark adaptation, while the rods are working “behind the scenes”. However, after about 3 to 5 minutes in the dark, the cones have reached their 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 rods’ sensitivity finally catches up to the cones’. The rods then become more sensitive than the cones, and rod adaptation becomes visible. It takes the rods 20 to 30 minutes to reach their maximum sensitivity.
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.
- What happens to visual pigment molecules when they (a) absorb light and (b) regenerate?
Light causes the retinal part of the visual pigment molecule to change its shape.
This change in shape and separation from the opsin causes the molecule to become lighter in color, a process called visual pigment bleaching.
As the light remain on, more and more of the pigment’s retinal Is isomerized and breaks away from the opsin, so the retina’s colour changes.
When the pigments are in their lighter bleached state, they are no longer useful for vision. In order to do their job of changing light energy into electrical energy, the retinal needs to return to its bent shape and become reattached to the opsin. This process of reforming the visual pigment molecule is called visual pigment regeneration.
When you are in the light, some of your visual pigment molecules are isomerizing and bleaching, while at the same time, others are regenerating. This means that in most normal light levels, your eye always contains some bleached visual pigment, and some intact visual pigment. When you turn out the lights, the bleached visual pigments continues to regenerate, but there is no more isomerization, so eventually your retina contains only intact (unbleached) visual pigment molecules.
- What is the connection between visual pigment regeneration and dark adaptation?
This 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.
- What is spectral sensitivity? How is a cone spectral sensitivity curve determined? A rod spectral sensitivity curve?
Spectral sensitivity curves - the relationship between wavelength and sensitivity.
We measure the cone spectral sensitivity curve by having an observer look directly at a test light so that it stimulates only the cones in the fovea.
We measure the rod spectral sensitivity curve by measuring sensitivity after the eye I dark adapted ( do the rods control vision because they are the most sensitive receptors) and presenting test flashes in the peripheral retina, off to the side of the fixation point.
- What is an absorption spectrum? How do rod and cone pigment absorption spectra compare, and what is their relationship to rod and cone spectral sensitivity?
Absorption spectrum is a plot of the amount of light absorbed versus the wavelength of the light.
The rod pigment absorbs best at 500nm, the blue-green area of the spectrum.
There are three absorption spectra for the cones because there are three different cone pigments, each contained in its own receptor. The short-wavelength pigment (S) absorbs light best at about 419nm; the medium-wavelength pigment (M) absorbs light best at about 531nm; and the long-wave-length pigment (L) absorbs light best at about 558 nm.
The absorption of the rod visual pigment closely matches the rod spectral sensitivity curve and the short-medium, and long- wavelength cone pigments that absorb best at 419,531, and 558 nm, respectively, add together to result in a psychophysical spectral sensitivity curve that peaks at 560nm.
- Describe the basic structure of a neuron.
The basic structure of the neuron is: the cell body, which contains mechanisms to keep the cell alive; dendrites, which branch out from the cell body to receive electrical signals from other neurons; and the axon, or nerve fiber, which is filled with fluid that conducts electrical signals
- Describe how to record electrical signals from a neuron.
Electrical signals are recorded from the axons ( or nerve fibers) of neurons using small electrodes to pick up the signals. When the axon, or nerve fiber, is at rest, the difference in potential between the tips of the two electrodes is -70 milivolts. This value, which stays the same as long as there are no signals in the neuron, is called the resting potential.
- What are some of the basic properties of action potentials?
Propagated response – once the response is triggered, it travels all the way down the axon without decreasing in size.
Another property is that the action potential remains the same size no matter how intense the stimulus is.
Changing the stimulus intensity does not affect the size of the action potentials but does affect the rate of firing.
Although increasing the stimulus intensity can increase the rate of firing, there is an upper limit to the number of nerve impulses per second that can be conducted down an axon. This limit occurs because of a property of the axon called the refractory period – the interval between the time one nerve impulse occurs and the next one can be generated in the axon.
Action potentials that occur in the absence of stimuli from the environment are called spontaneous activity. This spontaneous activity establishes a baseline level of firing for the neuron.
- Describe what happens when an action potential travels along an axon. In your description, indicate how the charge inside the fiber changes, and how that Is related to the flow of chemicals across the cell membrane.
When the fiber is at rest, there is no flow of ions, and the record indicates the resting potential. The ion flow will occur when an action potential travels down the fiber. As positively charged sodium (Na+) flows into the axon, the inside of the neuron becomes more positive ( rising phase of the action potential). As positively charged potassium (K+) flows out of the axon, the inside of the axon becomes more negative (falling phase of the action potential). The fiber’s charge returns to the resting level after the flow of Na+ and K+ has moved past the electrode.
- How are electrical signals transmitted from one neuron to another? Be sure you understand the difference between excitatory and inhibitory responses.
The neurotransmitter molecules dlow into the synapse to small areas on the receiving neuron called receptor sites that are sensitive to specific neurotransmitters. These receptor sites exist in a variety of shapes that match the shapes of particular neurotransmitter molecules. When a neurotransmitter makes contact with a receptor site matching its shape, it activates the receptor site and triggers a voltage change in the receiving neuron.
Thus, when an electrical signal reaches the synapse, it triggers a chemical process that causes a new electrical signal in the receiving neuron.
Two types of responses can occur at these receptor sites, excitatory and inhibitory. An excitatory response occurs when the inside of the neuron becomes more positive, a process called depolarization.
An inhibitory response occurs when the inside of the neuron becomes more negative, a process called hyperpolarization.
- What is convergence, and how can the differences in the convergence of rods and cones explain (a) the rods’ greater sensitivity in the dark and (b) the cones’ better detail vision?
Convergence occurs when a number of neurons synapse onto a single neuron.
The signals from the rods converge more than do the signals from the cones.
On the average, about 120 rods send their signals to one ganglion cell, but only about 6 cones send signals to a single ganglion cell.
The difference between rod and cone convergence becomes even greater when we consider the cones in the fovea.
The fovea is the small area that contains only cones. Many of these foveal cones have “private lines” to ganglion cells, so that each ganglion cell receives signals from only one cone, with no convergence. The greater convergence of the rods compared to the cones translates into two differences in perception: (1) the rods result in better sensitivity than the cones and (2) the cones result in better detail vision than the rods.
- What does it mean to say that early events are powerful shapers of perception? Give examples
If problems in the eye’s focusing system deliver degraded images to the retina, no amount of processing by the bran can create sharp perception.
It means that, if there is a problem with a lens, such as the problem with the lens at the Hubble telescope, the image we will perceive will be distorted.
- What is the young infant’s visual acuity, and how does it change over the first year of life? What is the reason for (a) low acuity at birth and (b) the increase in acuity over the first 6 to 9 months?
Visual acuity is poorly developed at birth. Acuity increases rapidly over the first 6 to 9 months. This rapid improvement of acuity is followed by a leveling-off period, and full adult acuity is not reached until sometimes after 1 year of age.
Although the rod-dominated peripheral retina appears adultlike in the newborn, the all-cone fovea contains widely spaced and very poorly developed cone receptors.
This, adults have good acuity because the cones have low convergence compared to the rods and the receptors in the fovea are packed closely together. In contrast, the infant’s poor acuity can be traced to the fact that the infant’s cones are spaced far apart. Another reason for the infant’s poor acuity is that the visual area of the brain is poorly developed at birth, with fewer neurons and synapses than in the adult cortex. The rapid increase in acuity that occurs over the first 6 to 9 months of life can thus be traced to the fact that during that time, more neurons and synapses are being added to the cortex, and the infant’s cones are becoming more densely packed.
