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 âmatching noise problemâ 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 âconstant Q.â Explain what this means.
