Tactile Illusory Movement
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Tactile illusory movement: Effects of spatio-temporal stimulus characteristics on the integrative processing of saltatory and successively activated stimulus patterns Inauguraldissertation zur Erlangung des akademischen Grades eines Doktors der Sozialwissenschaften der Universität Mannheim vorgelegt von Dipl.-Psych. Anja Schwab Fakultät für Sozialwissenschaften der Universität Mannheim Dekan: Prof. Dr. Josef Brüderl Referent: Prof. Dr. Rupert Hölzl Koreferent: Prof. Dr. Roger Cholewiak Tag der Disputation: 18.07.2008 Preface Preface This dissertation emerged at the Spatial Orientation Systems Department at the Navel Aerospace Medical Research Laboratory (NAMRL), located at Navel Air Station in Pensacola, Florida, USA. Here, vibrotactile interfaces like the Tactile Situation Awareness System (TSAS) are developed and evaluated. Based on vibrotactile cues, TSAS provides information about an aircraft’s attitude or threat to the pilot, and thus maintains an operator’s situational awareness. As a member of the German Scientist Exchange Program of the German Armed Forces, I worked as a Research Assistant at NAMRL and was able to collect data for my thesis. Angus H. Rupert, Captain in the US Navy Medical Service Corps, and Head of the Spatial Orientation Systems Department and Roger W. Cholewiak, Ph.D., Director of the Cutaneous Communication Laboratory at Princeton University supervised the experiments at NAMRL and at Princeton. The experiments were supported by ONR Grant No #N0001401WX20204 to NAMRL and the Intergovernmental Personnel Agreement N6645200MD00012 to Princeton University and were approved by the Committee for the Protection of Human subjects. In order to generate percepts of movement, two tactile illusions were examined: Successive activation to produce a phi-like motion and saltation. Tactile movement can be used for example as a directional vector in tactile displays. But the experiments were not only conducted for practical purposes, sensory illusions are of particular interest as they expand our knowledge of human perception in that they help to clarify disparities between physical stimuli and the psychophysiologically measured sensations they evoke. Thus, the tactile illusions used in our experiment may aid in studying the integration of spatio-temporal stimuli into percepts of continuous motion. At the University of Mannheim, the saltatory phenomenon is used to asses dynamic changes in somatosensory maps due to spatio-temporal repetitive stimulation. The results of these studies are discussed in connection with the investigation of chronic pain and other disorders possibly related to deranged “body maps”. Professor Rupert Hölzl, Professor for Clinical Psychology and director of the Otto Selz Institute and the dedicated Laboratory of Clinical Psychophysiology agreed to supervise my dissertation at the University of Mannheim. Part of this thesis has been published: The study to determine vibrotactile thresholds on the torso was published in Perception & Psychophysics 2004, a summary of the results of I Preface Experiment 2 was presented as poster at the Psychonomic Society Meeting, Vancouver, Canada, in 2003. I would like to thank all colleagues at the University of Mannheim and NAMRL, who supported my work: First of all Prof. Rupert Hölzl and Prof. Roger W. Cholewiak, for their patience, their ongoing encouragement, and the technical and scientific steering of my thesis. Also CAPT USN Angus Rupert, who made this research project possible at his department. Kristy Beede, not only assisted me in recruiting and testing subjects, she also became a friend outside the Research Laboratory. I would also like to thank the whole NAMRL staff and CAPT MC USN Anderson, Commanding Officer at NAMRL, who were very supportive and made my stay in Pensacola an interesting and very pleasant one. Sincere thanks are given to my subjects, who participated in my studies. Dr. Annette Stolle, who also studied saltation at the University of Mannheim was a valuable dialog partner, and I would like to thank her for the time she spend with me, discussing the contents of my thesis. Additionally I would like to thank Dr. Dieter Kleinböhl and Dr. Andreas Möltner for their helpful statistical advice and Dipl.-Psych. Jörg Trojan for his helpful suggestions, as well as the Laboratory Stuff for their administrative support. Last, but not least, I want to thank my husband, who was always understandingly, encouraged me and provided any practical and emotional support, he could. Augsburg, January 08 Anja Schwab II Abstract - Zusammenfassung Abstract Saltation and apparent movement are related phenomena: Both generate illusory stimuli in- between the actually activated stimulus sites. This dissertation examines spatio-temporal stimulus parameters where both phenomena create equivalent percepts. In these cases it is assumed that cortical representations adapt identically to the properties of both stimulus patterns – saltation and apparent movement. Furthermore it is tested whether repetitive stimulation results in plastic changes of the cortical map and related perceptual changes. Another focus lies on the integration of the stimulus patterns over both body halves. We used varying numbers of vibrotactile point stimuli that were presented on a horizontal array around the trunk. In two pilot studies and three main studies with a total of 139 subjects we reached the following conclusions: In accordance with neural network models stimuli presented close together in time are expected to belong together. Therefore both stimulus patterns generate indistinguishable percepts at short interstimulus intervals (<100 ms); the same applies for short stimulus durations (<50 ms). Increasing the number of tactors – and thus decreasing inter-tactor distance – tends to improve the integration of both stimulus patterns into a percept of continuous movement. Vibration frequency affects the quality of illusory movement as well as the similarity of both patterns. Further studies exploring the spatial resolution of low- and high-frequency vibrotactile stimulus patterns on hairy skin are needed to clearly explain this result. An effect of repetitive stimulation on the comparability of both stimulus patterns could not be found – presumably because plastic changes in the cortical map of body sites like the trunk – which are less innervated and rarely used to explore the environment compared to the fingers – might require more stimulus repetitions. Crossing the body midline impairs comparability of both stimulus patterns, however in the majority of cases saltation and apparent movement generate indiscriminable percepts when presented unilaterally or bilaterally indicating that integration of spatio-temporal stimuli occurred over the body midline – yet interstimulus intervals might differ due to interhemispheric transmission time. The results of this dissertation shed further light on the processing of spatio-temporal stimuli and might also be used in the design of tactile torso displays. III Abstract - Zusammenfassung Zusammenfassung Saltation und takile Scheinbewegung sind verwandte Phänomene, da beide illusorische Reize zwischen physikalischen Reizpunkten generieren. Diese Dissertation untersucht die Ausprägungen der spatio-temporalen Reizmerkmale, bei denen beide Phänomene als identisch wahrgenommen werden. In diesen Fällen wird eine identische Anpassung der zerebralen Repräsentation an die beiden taktilen Reizmuster - Saltation und Scheinbewegung - angenommen. Darüber hinaus wird getestet ob eine wiederholte Reizdarbietung eine Veränderung der kortikalen Karte und damit perzeptiver Korrelate nach sich zieht. Die Integration der Reizmuster über beide Körperhälften ist ein weiterer Fokus dieser Arbeit. Stimuli waren punktuelle Vibrationsreize, die in unterschiedlicher Anzahl auf einer horizontal um den Rumpf verlaufenden Stimulus-Matrix vorgegeben wurden. In zwei Pilot- und drei Hauptstudien mit insgesamt 139 Versuchspersonen wurden folgende Ergebnisse erhoben: Im Einklang mit neuronalen Netzwerk-Modellen werden Stimuli, die in engem zeitlichen Abstand präsentiert werden als zusammenhängend wahrgenommen, beide taktilen Reizmuster generieren daher nicht unterscheidbare Perzepte bei kurzen Interstimulus- Intervallen (<100ms); gleiches gilt für eine kurze Stimulusdauer (<50ms). Eine Erhöhung der Anzahl der Reizgeber – und damit eine Verkürzung der Reizdistanz - verbessert tendenziell ebenfalls die Integration der beiden Reizmuster zu einer glatten Scheinbewegung. Die Vibrationsfrequenz beeinflusst die Qualität der Scheinbewegung und damit auch die Vergleichbarkeit der Reizmuster. Weitere Studien zur räumlichen Auflösung hoch- und niedrigfrequenter taktiler Reizmuster auf haariger Haut sind nötig, um dieses Ergebnis eindeutig erklären zu können. Ein Effekt repetitiver Stimulation auf die Vergleichbarkeit der Reizmuster konnte nicht nachgewiesen werden – vermutlich weil plastische Veränderungen der kortikalen Karte bei Körperregionen wie dem Rumpf – die weniger sensibel innerviert und seltener im täglichen Gebrauch genutzt werden wie z.B. Finger - mehrere Reizwiederholungen erfordern. Das Überqueren der Köpermitte beeinträchtigt zwar die Vergleichbarkeit der Reizmuster, jedoch generieren Saltation und Scheinbewegung sowohl bei unilateraler wie bei bilateraler Darbietung überzufällig oft identische Perzepte, so dass von einer Integration spatio-temporaler Reizmuster auch über die Körpermitte hinweg ausgegangen werden kann – evtl. jedoch bei unterschiedlichen Interstimulus-Intervallen