The Archimedes spiral aftereffect: a function of boundary velocity and frequency of stimulation
Larsen, Suzanne Steinbock
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The present stuqy is an investigation of the effects of two parameters of the spiral aftereffect. Clinical psychologists have been interested in aftereffect research because of the possibility of utilizing the phenomenon as a diagnostic tool for brain damage. The parameters which were studied are frequoocy of stmulation (FS) and boundary velocity (BV). Frequency of stimulation is the frequency with which a given retinal element is stimulated by a contour (boundary) of the moving stimulus. The frequency of stimulation for the spiral is defined as the product of number of spiral arms and rotational velocity. In this study, variations in frequenqy of stimulation were effected by varying the number of spiral arms. Boundary velocity is the velocity with which a given contour (boundary) passes across a retinal element. In the spiral, boundary velocity corresponds to the rate of expansion or contraction, and it is proportional to: [(Rotational Velocity) x (Visual Angle Subtended by Spiral)]/(Number of Spiral Turns) Variations in boundary velocity were effected by simultaneously varying number of spiral arms and rotational velocity in such a way that frequency of stimulation remained constant and boundary velocity varied. In this study, a population of responses was sampled from two male subjects, each of Whom viewed five Archimedes spirals rotated at four rotational velocities. The spirals were drawn with one, two, four, eight, and sixteen arms, and were rotated at 40 rpm, 80 rpm, 160 rpn, and (all except the sixteen arm spiral) at 320 rpm. Each spiral-speed combination was repeated three times each day for eight days, resulting in a total of 24 replications for each spiral-speed combination. The dependent variable was the aftereffect, as measured by extent and duration. [TRUNCATED]
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