Chapter 2: Light and Vision-Study Questions

Question 1

Describe the two ways used to conceptualize light

Answer 1

One way is to think of it as a wave that travels through a medium. Another is to think of it as a stream of photons, tiny particles, each consisting of one quantum of energy.

Question 2

Describe the difference between light that is reflected and light that is transmitted.

Answer 2

Reflected light occurs when a ray of light strikes a light-colored surface and then bounces back towards its point of origin. Transmitted light occurs when light is neither reflected nor absorbed by a surface. An example is a transparent window; light passes through the surface and is transmitted to the other side.

Question 3

What is the purpose of the cornea?

Answer 3

The cornea is a transparent surface on the exterior of the eye. It protects the eye from the outside world. Being transparent, it allows light to be transmitted through it and into the eye.

Question 4

What is the purpose of the retina?

Answer 4

The retina is a light-sensitive membrane in the back of the eye that contains rods and cones, which receive an image from the lens and send it to the brain through the optic nerve.

Question 5

How does the process of accommodation take place in the eye?

Answer 5

Accommodation takes place in the lens of the eye. The lens changes its refractive power by changing its shape. This causes the eye to be able to focus on a given object, whether it is near or far.

Question 6

What is astigmatism and how can it be fixed?

Answer 6

Astigmatism is a visual defect caused by the unequal curving of one or more of the refractive surfaces of the eye, usually the cornea. It can be fixed by wearing lenses that have two focal points (that provide different amounts of focusing power in the horizontal and vertical planes).

Question 7

Why are photoreceptors important in the process of seeing?

Answer 7

Photoreceptors are the cells that make up the backmost layer of the retina. They are sensitive to light, and as soon as they sense it, they can cause neurons in the intermediate layers to fire action potentials. Photoreceptors are important in the process of seeing because they transduce the physical energy of light into neural energy that our brains can analyze.

Question 8

What are rods and cones?

Answer 8

Rods and cones are photoreceptors present in the retina. Rods are specialized for night vision, while cones are specialized for daylight vision, fine visual acuity, and color.

Question 9

Explain what happens in the process of hyperpolarization.

Answer 9

Hyperpolarization is an increase in membrane potential in which the inner membrane surface becomes more negative than the outer membrane surface. This process is one in a sequence of events that occur once light is sensed by the photoreceptors.

Question 10

Why can’t rods signal differences in color?

Answer 10

Rods cannot signal differences in color because they only have one type of photopigment. Cones, on the other hand, have three types of photopigments, which help them differentiate between colors.

Question 11

Describe age-related macular degeneration (AMD) and how it affects one’s vision.

Answer 11

AMD is a disease that affects the macula, gradually destroying high resolution central vision. This degeneration in vision makes it difficult to read, drive, or recognize faces.

Question 12

What is the difference between wet and dry age-related macular degeneration (AMD)?

Answer 12

In wet AMD, abnormal blood vessels behind the retina start to grow under the macula and leak blood and fluid. The blood and fluid raise the macula and cause a rapid loss of central vision. In dry AMD (the more common form), cones in the macula degenerate slowly over time. Once dry AMD becomes too advanced, vision loss becomes permanent, however there are treatments to prevent the advancement of dry AMD.

Question 13

What is the role of horizontal cells?

Answer 13

Horizontal cells are specialized retinal cells that contact both photoreceptors and bipolar cells. They produce lateral inhibition, which allows the signals that reach retinal ganglion cells to be based on differences in activations between nearby photoreceptors rather than absolute levels of activation.

Question 14

What is visual acuity?

Answer 14

Visual acuity is a measure of the finest detail that one can resolve.

Question 15

What is the difference between an “ON” bipolar cell and an “OFF” bipolar cell?

Answer 15

An “ON” bipolar cell is a cone bipolar cell that depolarizes in response to an increase in light intensity. An “OFF” bipolar cell is a cone bipolar cell that depolarizes in response to a decrease in light intensity. These two cells have opposite reactions to light.

Question 16

What is a receptive field?

Answer 16

A receptive field is the region on the retina in which stimuli will activate a neuron. Receptive fields vary in size, shape, and complexity.

Question 17

Why is the center–surround organization of retinal ganglion cells so important?

Answer 17

The center–surround organization of retinal ganglion cells is important because it allows for sensitivity to contrast rather than absolute illumination levels. Ganglion cells are most sensitive to differences in the intensity of light in the center and in the surround, and they are relatively unaffected by the average intensity of light. This is useful because the average intensity of light falling on the retina will be quite variable, depending on whether the observer is indoors, outdoors, etc., but contrasts of light are relatively constant.

Question 18

What is a filter and how is it important in vision?

Answer 18

A filter is an acoustic, electrical, electronic, biological, or optical device, instrument, or computer program that allows the passage of some frequencies or digital elements and blocks others. Filters are important in vision because they allow the transformation of raw images into representations in the brain. Filters highlight certain important visual information while eliminating other unimportant information. The center–surround receptive fields of retinal ganglion cells are filters.

Question 19

What are some consequences of the differing sizes of M ganglion cell and P ganglion cell receptive fields?

Answer 19

P ganglion cells have smaller receptive fields than M ganglion cells at all eccentricities. This allows the M ganglion cells to respond to a larger portion of the visual field. In addition, they are much more sensitive to visual stimuli under low lighting conditions than P ganglion cells. P ganglion cells, on the other hand, provide finer resolution (greater acuity) than M ganglion cells, as long as there is enough light for them to operate.

Question 20

Explain how the pupil adapts to dark and light conditions.

Answer 20

The pupil has the ability to dilate and constrict, depending on amount of light. For example, under well-lit conditions, the pupil tends to constrict to let less light into the eye. Under dark conditions, the pupil dilates to allow more light into the eye.

Question 21

Explain why it is that we are generally not bothered by variations in overall light levels.

Answer 21

We are generally not bothered by variations in overall light levels because we have several mechanisms for regulating how much light enters the eye. One mechanism is the pupil size. Another is the regeneration rates of pigments in our photoreceptors. Yet another is the rod/cone dichotomy—cones operate at moderate and high light levels while rods take over for low light levels. Finally, the neural circuitry of the retina itself helps stabilize external light variations by emphasizing contrasts in luminance rather than absolute light levels.

Question 22

Describe the family of diseases known as retinitis pigmentosa.

Answer 22

Retinitis pigmentosa is a family of hereditary diseases that involves the progressive death of photoreceptors and degeneration of the pigment epithelium. The rods are usually affected before the cones. Therefore, people suffering from this disease first notice vision problems in their periphery and under low light conditions (where rods play the dominant role in collecting light).

Question 23

Describe what happens to the visual field of a patient with retinitis pigmentosa.

Answer 23

Patients suffering from retinitis pigmentosa typically exhibit an overall shrinkage of their visual fields, as well as “ring scotomas,” which are bands of blindness between the relatively normal central fields and the periphery.