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The fact that we experience negative afterimages is taken as support for the

A. Notion that the retina contains three types of photopigments

B. Color mixing in the visual system

C. Opponent processing color theory

D. Trichromatic Color theory

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Trinidad Tremblay
Trinidad TremblayLv2
28 Sep 2019
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Related questions

Exercise #5: The Nervous system and Perception

Task #1: Color vision


Introduction

In the human retina, there are two kinds of light-sensitive cells: rods and cones. These cells are located on the outside of the retina, away from the vitreous humor and against the choroid layer, and are covered by a later of bipolar and ganglion cells as well as neurons. Light enters the posterior chamber through the lens and must pass through several cell layers before it is absorbed by the rods and cones.

Cones are involved in color vision at daylight intensities. Generator potentials in a cone excite one neuron, giving high-level resolution vision. Cones will not cause neural firing at low light intensities and are, therefore, nonfunctional at night.

Rods provoke black-and-white vision at low light intensities. A bipolar cell associates with several rods, so that generator potentials sum in their effect. This assures neuronal firing at low light intensities and provides a highly sensitive vision but with less resolution.

Receptor convergence. A ganglion cell, which receives input from a number of rods, spread out over the retina, cannot pinpoint the source of a light stimulus as well as a ganglion cell, which is stimulated by a single photoreceptor. In the case of a ganglion cell receiving the added input of several rods, light hitting any one of the rods can cause a neural spike in the ganglion cell, but the brain has no way to know which rod was excited. It is therefore harder to pinpoint the location of light stimuli on the retina when the stimulus is detected by rods (as compared to cones). The result is that our ability to see detail, or our resolving power, is considerably lower under dim light conditions than under bright light conditions.

Dark adaptation. If you go from a lighted area to a very dark room, you will not be able to see. After about ten minutes, however, you begin to be able to see again. This occurs because the rhodopsin pigment, which gets broken down when it absorbs light, is continually re-synthesized in the rods. When there is very little light, very little rhodopsin gets broken down and the amount of rhodopsin gradually builds up, making the rod much more sensitive to dim light.

When light strikes a receptor cell, it triggers a photo-chemical reaction that leads to neuronal firing. If a rod or cone receives high-intensity light, the visual pigments will be temporarily bleached and will not absorb any more light. Within a matter of a few seconds, enzyme systems in the cells will restore the visual pigment to its light-absorbing form. This can be easily demonstrated. If, after viewing a bright light, the eye is quickly shut or turned to a dark wall, the bright image will persist as a positive afterimage because the high generator potential causes the continued firing of neurons. If the eye is cast on a lighter background after a few seconds, a negative afterimage, or a dark image of the object will appear. This is because the still bleached cells are not receptive to the light coming from the light background.

According to the Young-Helmholtz theory of color vision, there are three types of cone cells, which respond respectively to red, green, and blue light.

All other colors are perceived as the brain interprets impulses coming from a mix of these receptors. If an object of one color is viewed for a long period of time at high light intensity, the cones for that color will become bleached. The afterimage of the object will be seen in the complementary color.

The Exercise 6 PowerPoint will demonstrate afterimages to you. Use the slide show option to project the square of red light on your entire screen in a dark room. After staring intensely at the square for 30 seconds or so, without shifting your gaze, click enter to change the slide to a soft white. Continue staring at the screen as the change is made. What do you see? What was the color of the afterimage?

Repeat this experiment using a blue square. Before doing the experiment, hypothesize what color the afterimage will be according to the Young-Helmholtz theory.

Hypothesis to be tested: (2 pts)

What did you see when the experiment was conducted? (2 pts)

Was you hypotheses accepted or rejected? (1 pt)

Explain the colors seen in these afterimages using the Young-Helmholtz theory. (5 pts)

How is it that a single photopigment can give a different response to different wavelengths, yet an organism with only one receptor type (and one variety of photopigment) does not have true color vision? What is the univariance principle? What do we mean when we say that a particular organism has color vision? How does one establish in a behavioral experiment whether an organism is monochromatic, dichromatic, or trichromatic?

3. Relate the responses of chromatically opponent lateral geniculate neurons (DeValois et al.) to the color naming experiment of Boynton and Gordon. Be certain to indicate how DeValois et al. transformed the color naming data and the LGN responses so that they could be compared. According to DeValois et al., what determines the color name(s) used to describe a monochromatic light? Be certain to indicate the ranges of wavelengths at which color naming can be predicted from the activity of chromatically opponent cells. Also specify the wavelengths at which color naming cannot be predicted from neural activity and describe the inconsistencies.

4. Describe the four types of double opponent chromatic cells that are found in V1. How do they differ from the single-opponent cells that are found in the Lateral Geniculate Nucleus? Describe the responses of cells in V4 to variations in color. Include a description of the data of both Zeki (1973) and Schein and Desimone (1990).

Depth Perception (ESSAY 1 of 2)

1. What is the cue for stereopsis, and how is it generated by points in space at different depths? Be certain to define crossed and uncrossed binocular disparities for objects along the optical axis. Why is it a relative measure of depth? The work of Julesz with random dot stereograms shows that (1) form need not be present in the monocular stimulus for stereopsis to take place and (2) stereopsis is impossible on the basis of point-by-point comparisons. How can both of these statements be true? What is the “matching noise problem” encountered in deriving depth information from random dot stereograms? How do cooperative algorithms potentially solve this problem?

2. We stated that the use of monocular depth cues involved certain heuristic assumptions about the nature of world. What are these heuristics, and how are they used in conjunction with the projectional geometry to derive depth information? Describe each of the static (pictorial) cues for depth. Give three examples of stimuli that could fool the system and explain what heuristic assumptions have been violated.

3. Describe the kinetic cues for depth. Include a description of the projectional geometry when discussing motion parallax. Are they relative or absolute cues for depth?

Auditory Nerve Responses and Masking (ESSAY 1 of 2)

1. Explain how one measures a frequency-tuning curve from an auditory-nerve fiber. What specifically is plotted on the two axes? How does one measure an iso-intensity contour? In what sense do these functions show that a nerve fiber to be frequency selective? Why do they appear to be asymmetric? It is sometimes said that the auditory system is “constant Q.” Explain what this means.

If you were a processing plant manager, and you were noticing a rash of bruises on the carcasses that were causing downgrades, the first thing you would do is:

a. Run to the picking machine and adjust the tension of the support bars that keep the wings from moving

b. Ask the USDA why there are so many bruises

c. Get on the phone to the farm service rep and find out what’s going on at the farm

d. Drop the scalding temp ASAP as this may induce blood under the skin

e. Check the testing chart to be sure no diseases were present

32. What is the general process called when we take a live bird and process for sale as a whole bird in a bag?

a. Further processing

b. Primary killing

c. secondary processing

d. primary processing or slaughter

e. bird baggin’ 101 33.

What is the primary concern you would have when birds are terminated in small operations by cutting off the head first?

a. Off flavors in the meat

b. Meat quality, tenderness reduced

c. Too much blood lost

d. Microbial contamination

e. Loss of ability to remove feathers

38. US consumers tend to prefer yellow color in chicken skin. Why is this becoming less true every day?

a. They fear that the chemical color may cause cancer

b. So much chicken is further processed no one sees the skin anyway

c. The new breeds are not yellow

d. Younger people like the white skin color

e. Some people think the yellow color indicates that the carcass is not fresh

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