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VANDERBILT PSY 236 - Bonds

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Bonds1. Cite three practical challenges in forming a clear image on theretina and describe briefly how each is met by the biological structureof the eye. Note that by challenges I do not refer to optical flaws(i.e., myopia, etc).This question addresses OPTICAL factors in image formation, not issuesinvolving retinal or other brain structures.a) Image focus requires strong refraction, implemented by multiplerefractive surfaces and aided by the graded refractive index of the lens.b) Focus must be dynamic, i.e., suitable for objects at varying distances,implemented by variable refractive strength of the lens.c) Imaging quality is optimized for available light by variable aperture ofthe iris. (Note that this does not address light adaptation, not an opticalissue.)2. How does knowledge of an optical linespread permit completecharacterization of the behavior of an optical system?The characterization may be done by using a Modulation Transfer Function(MTF), which describes the response of the system to all spatial frequencies.The linespread is the response to an impulse, which contains all spatialfrequencies at uniform power. The inverse Fourier transform of thelinespread is equal to the MTF.3. The title of the course is The Visual System. What is meant bythat? How is the visual system represented in living organisms?The visual system is embodied in an anatomically-diffuse set of mechanisms inthe brain (central nervous system) that are associated with detection,representation, processing and behavioral response associated with image information.While it may seem to be concentrated in the primary visual pathway, perhaps dozensof other brain structures are involved in this complex process.4. Cells in the retina are represented by two general classes, linearand non-linear. State a functional advantage and disadvantage of eachgeneral class in the context of visual processing.Linear (P, X): Advantage: High acuity, linear image representation, dense populationDisadvantage: Slow conduction, slow temporal reponse, low contrast sensitivityNonlinear (M, Y): Advantage: Rapid conduction, sensitive to high temporal frequency/motion,High contrast sensitivityDisadvantage: Low acuity, nonlinear image representation, sparse populationCasagrande5) Identify the following brain areas based upon their function. Do these areas receive directaxonal input from the retina, Yes or No (circle one).A) The suprachiasmatic nucleus (or hypothalamus) an important visual area for the control ofcircadian rhythms . Does it receive direct input from retina? YesB) The pretectum (or midbrain)is important for the control of pupillary reflexes andaccommodation. Does it receive direct input from retina? YesC) The superior colliculus (or midbrain) is important for the control eye movements andorientation in space. Does it receive direct input from retina? YesD) The lateral geniculate nucleus is an important relay for visual information important forconscious visual perception. Does it receive direct input from retina? YesE) Striate cortex (or V1 or area 17). (They could also answer V2)is an important early visualcortical area that receives its primary input from the thalamus. Does it also receive direct inputfrom retina? No6) Describe the path of light through the eye and retina. Identify in order all structures andretinal layers through which the light will pass before it hits the retinal receptors assuming thelight beam you are describing strikes receptors located peripheral to the fovea. Be specific andprecise in your answer.Cornea –(pupil…not essential to put)- aqueous – lens – vitreous – (blood vessels …not essential)ganglion cell layer –inner plexiform layer - inner nuclear layer –outer plexiform layer – outernuclear layer (technically this would do since this layer is the layer containing cell bodies of thereceptors). Some might have put additionally receptors because they don’t understand that thecell bodies of receptors are what make up the outer nuclear layer. This was considered ok.Lappin (Many of the following answers are more detailed than was required or expected on theesam.) 7. A classic study by Hecht, Shlaer, & Pirenne (1942) used psychophysical methods to reachconclusions about responses of rods to photons. Describe very briefly:A) What did they find? Two basic results were: (1) A single photon is sufficient to produce a visually effectiveexcitation of a rod. (Reliable detections, however, require simultaneous excitations ofseveral rods — roughly 3 – 8.) (2) The trial-to-trial variability in individual observers’behavioral detections was attributable primarily to the random physical processes thatgovern the absorption of photons by photoreceptors. The physiological between thephotoreceptors and the observer’s behavior contributed very little additional variability.B) What is the logic that permits this link from psychophysics to physiology of the retina? Behavioral discrimination (e.g., of differences in the number of photons in the stimulus)implies differences in retinal responses (in responses of photoreceptors and ganglion cellsto variations in the stimulus energy). This follows from the 2nd law of thermodynamics:Information about retinal stimulation can only be lost but not increased by visual processes.8. Stuart Anstis (1974) published a frequently reproduced “chart demonstrating variations inretinal acuity with retinal position.” (Actually, he showed three different charts.)A) What was the structure of these charts illustrating this relationship between acuity andretinal position? The sizes of characters in all three of these charts increased in direct proportion toeccentricity in the visual field. That is, spatial resolution decreases in inverse proportion toretinal eccentricity.B) What is thought to be the anatomical basis for this relationship? Two anatomical properties exhibit approximately the same relationship to retinaleccentricity: (a) diameters of dendritic fields of ganglion cells [true for both M-cells and P-cells, though the proportionality is very different for both], and (b) cortical magnification ofthe visual field in area V1, which is inversely proportional to eccentricity. That is, the ratio oflinear extent of primary visual cortex (V1) relative to angular extent of the visual field, in mmof cortex per degree of visual angle, is inversely proportional to retinal eccentricity. [Thisproportionality


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