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Modern progressive lenses are widely used to allow distance and near-view within one lens. Novice users experience motion sickness during adaptation and have problems performing everyday tasks when first using progressive lenses. To evaluate the effects of progressive lens distortion in psychophysical experiments these need to be simulated in real-time. We present a method to generate the distortion pattern of a progressive lens in virtual reality that allows testing perceptual distortions and behavioural consequences of exposure to such distortions. A user study showed that the heading angle on the horizontal axis was significantly underestimated through implemented distortion.

We investigated whether biological motion biases heading estimation from optic flow in a similar manner to nonbiological moving objects. In two experiments, observers judged their heading from displays depicting linear translation over a random-dot ground with normal point light walkers, spatially scrambled point light walkers, or laterally moving objects composed of random dots. In Experiment 1, we found that both types of walkers biased heading estimates similarly to moving objects when they obscured the focus of expansion of the background flow. In Experiment 2, we also found that walkers biased heading estimates when they did not obscure the focus of expansion. These results show that both regular and scrambled biological motion affect heading estimation in a similar manner to simple moving objects, and suggest that biological motion is not preferentially processed for the perception of self-motion.

Saccadic adaptation aims at keeping saccades accurate to enable precise foveation of objects. It has been believed to be a rather low-level adjustment, responding chiefly to direction and magnitude of postsaccadic position error. However, recent studies have shown that image content can modify saccadic adaptation. Adaptation is more complete for saccades toward socially relevant human figures in comparison to noise when time constraints exist. In the present experiment, we show that saccadic adaptation is also susceptible to the novelty of a stimulus. In a scanning adaptation paradigm, 20 subjects participated in two sessions of forward adaptation to one position at which the same human picture was always displayed versus a position at which a new human figure was presented in every trial. Saccadic adaptation was more complete to the novel-target position. This suggests that novelty can increase oculomotor learning and corroborates the claim that saccadic adaptation includes influences that reflect the target's visual properties.

Saccades are fast eye movements that reorient gaze. They can be performed voluntarily—for example, when viewing a scene—but they can also be triggered in reaction to suddenly appearing targets. The generation of these voluntary and reactive saccades have been shown to involve partially different cortical pathways. However, saccades of either type confront the visual system with a major challenge from massive image motion on the retina. Despite the fact that the whole scene is swept across the retina, a saccade usually does not elicit a percept of motion. This saccadic omission has been linked to a transient decrease of visual sensitivity during the eye movement, a phenomenon called saccadic suppression. A passive origin of saccadic suppression based on temporal masking has been proposed as well as an active central process that inhibits visual processing during the saccade. The latter one would need to include an extraretinal signal, which is generated already during saccade preparation. Since saccade generation differs for voluntary and reactive saccades, timing and nature of this extraretinal signal as well as its impact on visual sensitivity might also differ. We measured detection thresholds for luminance stimuli that were flashed during voluntary and reactive saccades and during fixation. Detection thresholds were higher during voluntary than during reactive saccades such that suppression appeared stronger during voluntary saccades. Stronger suppression in voluntary saccades could arise from a stronger extraretinal signal that activates suppression or could indicate that a suppression underlying process itself partially differs between voluntary and reactive saccades.

When we observe a scene, we shift our gaze to different points of interest via saccadic eye movements. Saccades provide high resolution views of objects and are essential for vision. The successful view of an interesting target might constitute a rewarding experience to the oculomotor system. We measured the influence of image content on learning efficiency in saccade control. We compared meaningful pictures to luminance and spatial frequency–matched random noise images in a saccadic adaptation paradigm. In this paradigm a shift of the target during the saccades results in a gradual increase of saccade amplitude. Stimuli were masked at different times after saccade onset. For immediate masking of the stimuli, as well as for their permanent visibility, saccadic adaptation was similar for both types of targets. However, when stimuli were masked 200 ms after saccade onset, adaptation of saccades directed toward the meaningful target stimuli was significantly greater than that of saccades directed toward noise targets. Thus, the percept of a meaningful image at the saccade landing position facilitates learning of the appropriate parameters for saccadic motor control when time constraints exist. We conclude that oculomotor learning, which is traditionally considered a low-level and highly automatized process, is modulated by the visual content of the image.

When visual stimuli are presented at the onset of a saccadic eye movement they are seen compressed onto the target location of the saccade. This peri-saccadic compression is believed to result from internal feedback pathways between oculomotor and visual areas of the brain. This feedback enhances vision around the saccade target at the expense of localization ability in other regions of the visual field. Although saccades can be targeted at only one object at a time, often multiple potential targets are available in a visual scene, and the oculomotor system has to choose which target to look at. If two targets are available, preparatory activity builds-up at both target locations in oculomotor maps. Here we show that, in this situation, two foci of compression develop, independent of which of the two targets is eventually chosen for the saccade. Our results suggest that theories that use oculomotor feedback as efference copy signals for upcoming eye movements should take the possibility into account that multiple feedback signals from potential targets may occur in parallel before the execution of a saccade.

Recent models of biological motion processing focus on the articulational aspect of human walking investigated by point-light figures walking in place. However, in real human walking, the change in the position of the limbs relative to each other (referred to as articulation) results in a change of body location in space over time (referred to as translation). In order to examine the role of this translational component on the perception of biological motion we designed three psychophysical experiments of facing (leftward/rightward) and articulation discrimination (forward/backward and leftward/rightward) of a point-light walker viewed from the side, varying translation direction (relative to articulation direction), the amount of local image motion, and trial duration. In a further set of a forward/backward and a leftward/rightward articulation task, we additionally tested the influence of translational speed, including catch trials without articulation. We found a perceptual bias in translation direction in all three discrimination tasks. In the case of facing discrimination the bias was limited to short stimulus presentation. Our results suggest an interaction of articulation analysis with the processing of translational motion leading to best articulation discrimination when translational direction and speed match articulation. Moreover, we conclude that the global motion of the center-of-mass of the dot pattern is more relevant to processing of translation than the local motion of the dots. Our findings highlight that translation is a relevant cue that should be integrated in models of human motion detection.

The visual recognition of action is one of the socially most important and computationally demanding capacities of the human visual system. It combines visual shape recognition with complex non-rigid motion perception. Action presented as a point-light animation is a striking visual experience for anyone who sees it for the first time. Information about the shape and posture of the human body is sparse in point-light animations, but it is essential for action recognition. In the posturo-temporal filter model of biological motion perception posture information is picked up by visual neurons tuned to the form of the human body before body motion is calculated. We tested whether point-light stimuli are processed through posture recognition of the human body form by using a typical feature of form recognition, namely size invariance. We constructed a point-light stimulus that can only be perceived through a size-invariant mechanism. This stimulus changes rapidly in size from one image to the next. It thus disrupts continuity of early visuo-spatial properties but maintains continuity of the body posture representation. Despite this massive manipulation at the visuo-spatial level, size-changing point-light figures are spontaneously recognized by naive observers, and support discrimination of human body motion.

Die Doktorarbeit “Oculomotor signals in visual perception” untersucht den Zusammenhang zwischen visueller Raumwahrnehmung und motorischer Koordination des Blickverhaltens. Mittels psychophysischer Experimente wurde die Hypothese, dass die Metrik der visuellen Raumwahrnehmung auf den motorischen Koordinaten der Augenbewegungen beruht, getestet. In den Experimenten wurde das Blickverhalten von Versuchspersonen gemessen, waehrend sie visuelle Stimuli verfolgten, die auf einem Computer-Bildschirm angezeigt wurden. Es wurde dann beobachtet inwieweit die Veraenderung motorischer Parameter der Augenbewegungen die Einschaetzung der Position visueller Objekte beeinflusst. Es zeigte sich, dass diese Veraenderung zu einer gleichartigen Verschiebung in der Lokalisation visueller Objekte im Raum fuehrte. Diese Ergebnisse belegen die Funktion motorischer Koordinaten fuer die visuelle Raumwahrnehmung.

In der vorliegenden Arbeit wurden die Einflüsse von Form- und Bewegunsinformation auf die Wahrnehmung von biologischer Bewegung untersucht. Dafür wurden unterschiedliche Stimuli verwendet, durch die es möglich war Form- und Bewegungsinformation möglichst getrennt zu untersuchen. Die erste Experimentreihe beschäftigt sich mit dem Einfluss von Bewegungsinformation bei klassischen und Sequential-Position Stimuli in unterschiedlichen Ansichten und Projektionen. Im nächsten Kapitel wurde Handlungserkennung bei Sequential-Position Stimuli mit klassischen Stimuli verglichen. In einer weiteren Experimentreihe wurde eine Counter-Change Technik verwendet um lokale Bewegungsinformation im Stimulus zu vermeiden. Die Ergebnisse der durchgeführten Experimente zeigten keinen Einfluss von lokaler Bewegungsinformation und deuten vielmehr auf eine zeitliche Analyse von Forminformation als Grundlage für die Wahrnehmung von biologischer Bewegung hin.

Selektive visuelle Aufmerksamkeit bezeichnet im Allgemeinen die zielgerichtete Steuerung des Informationsflusses. Zahlreiche Studien im Bereich der raum- und merkmalsbasierten Aufmerksamkeit haben gezeigt, dass das visuelle System diese Kontrolle durch aktivitätsmodulierende Mechanismen ausübt. Es wird angenommen, dass diese Mechanismen zu einer verstärkten neuronalen Repräsentation von relevanten Stimuli oder Merkmalen führen, während irrelevante Aspekte unterdrückt werden. Dies bedeutet, dass Aufmerksamkeit lediglich die Stärke der neuronalen Repräsentationen, nicht aber die repräsentierten Inhalte selbst ändert. In dieser Arbeit wird argumentiert, dass Aufmerksamkeit die neuronalen Repräsentationen grundlegend sowohl auf Populationsebene als auch auf der Ebene einzelner Neurone verändern kann. Dies wird anhand offener Aufmerksamkeitsverlagerungen und der Ausrichtung von merkmalsbasierter Aufmerksamkeit gezeigt werden. Selective visual attention is generally conceptualized to control the flow of information with respect to the task at hand. Various studies in the space-based and feature-based domain of attention have demonstrated that the visual system achieves this via gain-control mechanisms. These mechanisms are supposed to result in an enhanced neural representation of relevant stimuli or features while irrelevant ones are suppressed. Thus, attention is suggested to modulate the strength of neural representations without altering their content. In this thesis, however, it will be argued that attention is able to change the very nature of these neural representations both at the level of population responses and of single neurons. This will be demonstrated for overt shifts of space-based attention as well as for the directing of feature-based attention.

Sakkaden sind schnelle, ballistische Augenbewegungen. Trotz großer Unterschiede zwischen prä- und post-sakkadischen retinalen Abbildungen, erscheint die visuelle Wahrnehmung über Sakkaden hinweg als kontinuierlich und stabil. Zur Konstruktion dieser räumlichen und zeitlichen Stabilität sind aktive Kompensationsmechanismen notwendig. Unter Laborbedingungen sind die Grenzen dieser Kompensationsmechanismen aufzeigbar. In zeitlicher Nähe zu Sakkaden lassen sich spezifische, charakteristische Wahrnehmungsfehler nachweisen. In dieser Arbeit wurden psychophysische Experimente mit menschlichen Versuchspersonen zu Teilbereichen der zeitlichen und räumlichen, trans-sakkadischen Stabilität durchgeführt. Untersucht wurden dabei die peri-sakkadische Wahrnehmung von Zeit, die perzeptuellen Konsequenzen sakkadischer Adaptation, die peri-sakkadische Repräsentation von Objekteigenschaften sowie der Einfluss emotional relevanter Sakkadenziele.

Ein Forschungsschwerpunkt in der menschlichen Bewegungsanalyse wurde auf die neuronalen Verarbeitungsprozesse biologischer Bewegung gelegt. In den meisten Studien wurde die Wahrnehmung von Lichtpunkt-Läufern untersucht, bei denen Lichtpunkte an festen oder variablen Körperpositionen gezeigt wurden. In den Experimenten dieser Arbeit habe ich mit Hilfe der fMRT herausgefunden, dass speziell formverarbeitende Gehirnregionen an der Verarbeitung biologischer Bewegung beteiligt sind. Dies widerspricht der allgemein angenommenen Theorie zur Wahrnehmung biologischer Bewegung. Für peripher präsentierte Lichtpunkt-Läufer wurde rechtsseitige pSTG und kontralaterale somatosensorische und fusiforme gyrus Aktivierung beobachtet. Zusammenfassend deuten meine Resultate darauf hin, dass ein Netzwerk aus visuellen und nicht-visuellen Arealen benötigt wird, um biologische Bewegung zu erkennen. Damit ist es z.B. möglich die Absichten einer beobachteten Handlung richtig zu interpretieren.