Thesis (Ph.D.)--Boston University / The purpose of the present study was twofold. First, a new technique for assessing the extent of the spiral aftereffect was utilized; and second, one parameter of the spiral, speed of rotation, was systematically examined. The recent literature on the spiral has consisted of establishing the diagnostic ability of the instrument for organic subjects. Examination of the parameters of the instrument itself has been limited by the lack of an adequate technique for assessing the extent of the aftereffect. None of the extant theories were capable of predicting either the direction, of expansion or contraction, or the function of speed on the aftereffect. A post hoc explanation was offered to explain the effect of speed on spiral aftermovement by an extension of the statistical summation theory. The following prediction was based on pilot data with the intent to determine if speed was a stable general function of the after effect: As spiral rotation speed increases from zero to fusion, the aftereffect will first increase and then decrease. [TRUNCATED]

Thesis (Ph.D.)--Boston University / PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / 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] / 2031-01-01

No / After an observer adapts to a moving stimulus, texture within a stationary stimulus is perceived to drift in the opposite direction¿the traditional motion aftereffect (MAE). It has recently been shown that the perceived position of objects can be markedly influenced by motion adaptation [1] and [2]. In the present study, we examine the selectivity of positional shifts resulting from motion adaptation to stimulus attributes such as velocity, relative contrast, and relative spatial frequency. In addition, we ask whether spatial position can be modified in the absence of perceived motion. Results show that when adapting and test stimuli have collinear carrier gratings, the global position of the object shows a substantial shift in the direction of the illusory motion. When the carrier gratings of the adapting and test stimuli are orthogonal (a configuration in which no MAE is experienced), a global positional shift of similar magnitude is found. The illusory positional shift was found to be immune to changes in spatial frequency and to contrast between adapting and test stimuli¿manipulations that dramatically reduce the magnitude of the traditional MAE. The lack of sensitivity for stimulus characteristics other than direction of motion suggests that a specialized population of cortical neurones, which are insensitive to changes in a number of rudimentary visual attributes [3], may modulate positional representation in lower cortical areas.

No / The tilt aftereffect (TAE) has been used previously to probe whether contours defined by different attributes are subserved by the same or by different underlying mechanisms. Here, we compare two types of contours between texture surfaces, one with texture orientation contrast across the edge (orientation contrast contour; OC) and one without, commonly referred to as a subjective contour (SC). Both contour types produced curves of TAE versus adapting angle displaying typical positive and negative peaks at ~15 and 70 deg, respectively. The curves are well fit by difference of Gaussian (DoG) functions, with one Gaussian accounting for the contour adaptation effect and the other accounting for the texture orientation adaptation effect. Adaptation to OC elicited larger TAEs than did adaptation to SC, suggesting that they more effectively activate orientation-selective neurons in V1/V2 during prolonged viewing. Surprisingly, both contour types adapted a luminance contour (LC) as strongly as did an LC itself, suggesting that the second-order orientation cue contained in the texture edge activates the same set of orientation-selective neurons as does an LC. These findings have implications for the mechanisms by which the orientations of texture edges and SCs are encoded

Yes / Individual radial frequency (RF) patterns are generated by modulating a circle's radius as a sinusoidal function of polar angle and have been shown to tap into global shape processing mechanisms. Composite RF patterns can reproduce the complex outlines of natural shapes and examining these stimuli may allow us to interrogate global shape mechanisms that are recruited in biologically relevant tasks. We present evidence for a global shape aftereffect in a composite RF pattern stimulus comprising two RF components. Manipulations of the shape, location, size and spatial frequency of the stimuli revealed that this aftereffect could only be explained by the attenuation of intermediate-level global shape mechanisms. The tuning of the aftereffect to test stimulus size also revealed two mechanisms underlying the aftereffect; one that was tuned to size and one that was invariant. Finally, we show that these shape mechanisms may encode some RF information. However, the RF encoding we found was not capable of explaining the full extent of the aftereffect, indicating that encoding of other shape features such as curvature are also important in global shape processing. / This research was supported by a Biotechnology and Biological Sciences Research Council (BBSRC) grant #BB/L007770/1.

Babyschema är ett fysionomiskt fenomen som innefattar infantila särdrag såsom stor ögon- och huvudform i kombination med liten näsa och haka. Utseendet kan ses hos digitala karaktärer i moderna spel och filmer vilket kan ha en psykologisk effekt hos betraktaren då exponering bevisats influera preferenser. Studien ämnar att utreda uppfattningen av ansikten med babyschema och om visuell exponering kan påverka dessa uppfattningar. Undersökningen utfördes på 70 testdeltagare, där ansikten med varierande grader av babyschema presenterades före och efter exponering innehållande karaktärer med babyschema eller realistiska proportioner. Resultaten indikerar att exponering för babyschema i viss utsträckning resulterar i en positiv förskjutning hos deltagarnas uppfattningar däremot hade exponering ingen positiv inverkan på deltagarnas respons till extrem babyschema. Framtida arbeten skulle kunna undersöka babyschema applicerat på manliga och kvinnliga karaktärer i en spelmiljö. I en förlängning kan studier inom ämnet syfta till att upplysa kreatörer om karaktärsdesign och dess möjliga påverkan på samhället.

Sensory adaptation is an important aspect of perception. A seemingly non-beneficial consequence of adaptation is that it produces perceptual illusions. For instance, following focal adaptation, the perceived separation between stimuli straddling the adapted attribute or region is often exaggerated. This type of illusion, known as perceptual repulsion, is both a consequence of and a clue to the brain’s coding strategies and how they are influenced by recent sensory events. Adaptation-induced perceptual repulsion has been well documented in vision (e.g. the tilt aftereffect) and to a lesser extent in audition, but rarely studied in touch. The present thesis investigated the effects of adaptation on tactile spatial perception using a combination of human psychophysics and computational modeling. In a two-interval forced choice task, participants compared the perceived separation between two point-stimuli applied on the forearms successively. The point of subjective equality was extracted as a measure of perceived two-point distance. We showed that tactile spatial perception is subject to an adaptation-induced repulsion illusion: vibrotactile adaptation focally reduced tactile sensitivity and significantly increased the perceived distance between points straddling the adapted skin site (Chapter 2). This repulsion illusion, however, was not observed when the intervening skin was desensitized with topical anesthesia instead of vibrotactile adaptation, suggesting that peripheral desensitization alone is insufficient to induce the illusion (Chapter 3). With Bayesian perceptual modeling, we showed that the illusion was consistent with the hypothesis that the brain decodes tactile spatial input without awareness of the adaptation state in the nervous system (Chapter 4). Together, the empirical and theoretical work furthers the understanding of dynamic tactile spatial coding as the somatosensory system adapts to the sensory environment. Its main findings are consistent with the adaptation- induced repulsion illusions reported in vision and audition, suggesting that perception in different sensory modalities shares common processing features and computational principles. / Thesis / Doctor of Philosophy (PhD) / Sensory adaptation can shape how we perceive the world. In this thesis, we showed that the perception of space in touch is pliable and subject to the influence of adaptation. Psychophysical testing in human participants showed that vibratory adaptation induced an illusion that expanded the perceived distance between stimuli on the skin. This illusion provides clues into how information about space in touch is normally processed and interpreted by the brain. In addition, we developed a computational model that used a powerful statistical framework – Bayesian inference – to probe touch on a theoretical basis. To the best of our knowledge, the present thesis provides the first combined psychophysical and computational study on the effects of adaptation on tactile spatial perception. Our findings suggest that touch shares some common information processing principles with vision and hearing, and adaptation plays a functionally similar role in mediating this process across the senses.