Perception of biological motion by form analysis

Detection of other living beings’ movements is a fundamental property of the human visual system. Viewing their movements, categorizing their actions, and interpreting social behaviors like gestures constitutes a framework of our everyday lives. These observed actions are complex and differences among them are rather subtle. However, humans recognize these actions without ma jor efforts and without being aware of the complexity of the observed tasks. In point-light walkers, the visual information about the human body is reduced to only a handful point-lights placed on the ma jor joints of the otherwise invisible body. But even this sparse information does not effectively reduce humans’ abilities to perceive the performed actions. Neurophysiological and neuroimaging studies suggested that the movement of the human body is represented in specific brain areas. Nonetheless, the underlying network is still issue of controversial discussion. To investigate the role of form information, I developed a model and conducted psychophysical experiments using point-light walkers. A widely accepted theory claims that in point-light walkers, form information is decreased to a non-usable minimum and, thus, the perception of biological motion is driven by the analysis of motion signals. In my study, I could show that point-light walker indeed contain useful form information. Moreover, I could show that temporal integration of this information is sufficient to explain results from psychophysical, neurophysiological, and neuroimaging studies. In opposition to the standard models of biological motion perception, I could also show that all results can be explained without the analysis of local motion signals

Action ability modulates time‑to‑collision judgments

Time-to-collision (TTC) underestimation has been interpreted as an adaptive response that allows observers to have more time to engage in a defensive behaviour. This bias seems, therefore, strongly linked to action preparation. There is evidence that the observer’s physical fitness modulates the underestimation effect so that people who need more time to react (i.e. those with less physical fitness) show a stronger underestimation effect. Here we investigated whether this bias is influenced by the momentary action capability of the observers. In the first experiment, participants estimated the time-to-collision of threatening or non-threatening stimuli while being mildly immobilized (with a chin rest) or while standing freely. Having reduced the possibility of movement led participants to show more underestimation of the approaching stimuli. However, this effect was not stronger for threatening relative to non-threatening stimuli. The effect of the action capability found in the first experiment could be interpreted as an expansion of peripersonal space (PPS). In the second experiment, we thus investigated the generality of this effect using an established paradigm to measure the size of peripersonal space. Participants bisected lines from different distances while in the chin rest or standing freely. The results replicated the classic left-to-right gradient in lateral spatial attention with increasing viewing distance, but no effect of immobilization was found. The manipulation of the momentary action capability of the observers influenced the participants’ performance in the TTC task but not in the line bisection task. These results are discussed in relation to the different functions of PPS

Exciting new advances in oral cancer diagnosis: avenues to early detection

The prognosis for patients with oral squamous cell carcinoma remains poor in spite of advances in therapy of many other malignancies. Early diagnosis and treatment remains the key to improved patient survival. Because the scalpel biopsy for diagnosis is invasive and has potential morbidity, it is reserved for evaluating highly suspicious lesions and not for the majority of oral lesions which are clinically not suspicious. Furthermore, scalpel biopsy has significant interobserver and intraobserver variability in the histologic diagnosis of dysplasia. There is an urgent need to devise critical diagnostic tools for early detection of oral dysplasia and malignancy that are practical, noninvasive and can be easily performed in an out-patient set-up. Diagnostic tests for early detection include brush biopsy, toluidine blue staining, autofluorescence, salivary proteomics, DNA analysis, biomarkers and spectroscopy. This state of the art review critically examines these tests and assesses their value in identifying oral squamous cell carcinoma and its precursor lesions

Meat and Nicotinamide:A Causal Role in Human Evolution, History, and Demographics

Hunting for meat was a critical step in all animal and human evolution. A key brain-trophic element in meat is vitamin B 3 /nicotinamide. The supply of meat and nicotinamide steadily increased from the Cambrian origin of animal predators ratcheting ever larger brains. This culminated in the 3-million-year evolution of Homo sapiens and our overall demographic success. We view human evolution, recent history, and agricultural and demographic transitions in the light of meat and nicotinamide intake. A biochemical and immunological switch is highlighted that affects fertility in the ‘de novo’ tryptophan-to-kynurenine-nicotinamide ‘immune tolerance’ pathway. Longevity relates to nicotinamide adenine dinucleotide consumer pathways. High meat intake correlates with moderate fertility, high intelligence, good health, and longevity with consequent population stability, whereas low meat/high cereal intake (short of starvation) correlates with high fertility, disease, and population booms and busts. Too high a meat intake and fertility falls below replacement levels. Reducing variances in meat consumption might help stabilise population growth and improve human capital

Visuelle Navigation: Dynamik der Wahrnehmung von Eigenbewegung

Visuelle Navigation ist im täglichen Leben von erheblicher Bedeutung. Ein navigierendes System muss über eine Vielzahl von Möglichkeiten verfügen, dazu gehört das Einschätzen der eigenen Bewegung, das Erkennen von Hindernissen wie auch das Erkennen sich bewegender Objekte, die Bestimmung von Kollisionszeitpunken oder das Auffinden von bestimmten Orten in einer Umgebung. Diese Arbeit hatte die Untersuchung der Dynamik visueller Eigenbewegung in experimenteller wie theoretischer Hinsicht zum Gegenstand. Vordringliches Interesse war dabei auf die Veränderung der Bewegungsdynamik gerichtet und auch darauf, wie das System sich mit der Zeit entwickelt und verändert

Visual perceptual stability and the processing of self-motion information: neurophysiology, psychophysics and neuropsychology

While we move through our environment, we constantly have to deal with new sensory input. Especially the visual system has to deal with an ever-changing input signal, since we continuously move our eyes. For example, we change our direction of gaze about three times every second to a new area within our visual field with a fast, ballistic eye movement called a saccade. As a consequence, the entire projection of the surrounding world on our retina moves. Yet, we do not perceive this shift consciously. Instead, we have the impression of a stable world around us, in which objects have a well-defined location. In my thesis I aimed to investigate the underlying neural mechanisms of the visual perceptual stability of our environment. One hypothesis is that there is a coordinate transformation of the retinocentric input signal to a craniocentric (egocentric) and eventually even to a world centered (allocentric) frame of reference. Such a transformation into a craniocentric reference frame requires information about both the location of a stimulus on the retina and the current eye position within the head. The physicist Hermann von Helmholtz was one of the first who suggested that such an eye-position signal is available in the brain as an internal copy of the motor plan, which is sent to the eye muscles. This so-called efference copy allows the brain to classify actions as self-generated and differentiate them from being externally triggered. If we are the creator of an action, we are able to predict its outcome and can take this prediction into consideration for the further processing. For example, if the projection of the environment moves across the retina due to an eye movement, the shift is registered as self-induced and the brain maintains a stable percept of the world. However, if one gently pushes the eye from the side with a finger, we perceive a moving environment. Along the same lines, it is necessary to correctly attribute the movement of the visual field to our own self-motion, e.g. to perform eye movements accounting for the additional influences of our movements. The first study of my thesis shows that the perceived location of a stimulus might indeed be a combination of two independent neuronal signals, i.e. the position of the stimulus on the retina and information about the current eye-position or eye-movement, respectively. In this experiment, the mislocalization of briefly presented stimuli, which is characteristic for each type of eye-movement, leads to a perceptual localization of stimuli within the area of the blind spot on the retina. Yet, this is the region where the optic nerve leaves the eye, meaning that there are no photoreceptors available to convert light into neuronal signals. Physically, subjects should be blind for stimuli presented in this part of the visual field. In fact, a combination of the actual stimulus position with the specific, error-inducing eye-movement information is able to explain the experimentally measured behavior. The second study in my thesis investigates the underlying neural mechanism of the mislocalization of briefly presented stimuli during eye-movements. Many previous studies using animal models (the rhesus monkey) revealed internal representations of eye-position signals in various brain regions and therefore confirmed the hypothesis of an efference copy signal within the brain. Although these eye-position signals basically reflect the actual eye-position with good accuracy, there are also some spatial and temporal inaccuracies. These erroneous representations have been previously suggested as the source of perceptual mislocalization during saccades. The second study of my thesis extends this hypothesis to the mislocalization during smooth pursuit eye-movements. We usually perform such an eye movement when we want to continuously track a moving object with our eyes. I showed that the activity of neurons in the ventral intraparietal area of the rhesus monkey adequately represents the actual eye-position during smooth pursuit. However, there was a constant lead of the internal eye-position signal as compared to the real eye-position in direction of the ongoing eye-movement. In combination with a distortion of the visual map due to an uneven allocation of attention in direction of the future stimulus position, this results in a mislocalization pattern during smooth pursuit, which almost exactly resembles those typically measured in psychophysical experiments. Hence, on the one hand the efference copy of the eye-position signal provides the required signal to perform a coordinate transformation in order to preserve a stable perception of our environment. On the other hand small inaccuracies within this signal seem to cause perceptual errors when the visual system is experimentally pushed to its limits. The efference copy also plays a role in dysfunctions of the brain in neurological or psychiatric diseases. For example, many symptoms of schizophrenia patients could be explained by an impaired efference copy mechanism and a resulting misattribution of agency to self- and externally-produced actions. Following this hypothesis, the typically observed auditory hallucinations in these patients might be the result of an erroneously assigned agency of their own thoughts. To make a detailed analysis of this potentially impaired efference copy mechanism possible, the third study of my thesis investigated eye movements of schizophrenia patients and tried to step outside the limited capabilities of laboratory setups into the real world. This study showed that results of previous laboratory studies only partly resemble those obtained in the real world. For example, schizophrenia patients, when compared to healthy controls, usually show a more inaccurate smooth pursuit eye-movement in the laboratory. Yet, in the real world when they track a stationary object with their eyes while they are moving towards it, there are no differences between patients and healthy controls, although both types of eye-movements are closely related. This might be due to the fact that patients were able to use additional sources of information in the real world, e.g. self-motion information, to compensate for some of their deficits under certain conditions. Similarly, the fourth study of my thesis showed that typical impairments of eye-movements during healthy aging can be equalized by other sources of information available under natural conditions. At the same time, this work underlined the need of eye-movement measurements in the real world as a complement to laboratory studies to accurately describe the visual system, all mechanisms of perception and their interactions under natural circumstances. For example, experiments in the laboratory usually analyze particularly selected eye-movement parameters within a specific range, such as saccades of a certain amplitude. However, this does not reflect everyday life in which parameters like that are typically continuous and not normally distributed. Furthermore, motion-selective areas in the brain might play a much bigger role in natural environments, since we generally move our head and/or ourselves. To correctly analyze the contribution to and influences on eye-movements, one has to perform eye-movement studies under conditions as realistic as possible. The fifth study of my thesis aimed to investigate a possible application of eye-movement studies in the diagnosis of neuronal diseases. We showed that basic eye-movement parameters like saccade peak-velocity can be used to differentiate patients with Parkinson’s disease from patients with an atypical form of Parkinsonism, progressive supranuclear palsy. This differentiation is of particular importance since both diseases share a similar onset but have a considerably different progression and outcome, requiring different types of therapies. An early differential diagnosis, preferably in a subclinical stage, is needed to ensure the optimal treatment of the patients in order to ease the symptoms and eventually even improve the prognosis. The study showed that mobile eye-trackers are particularly well-suited to investigate eye movements in the daily clinical routine, due to their promising results in differential diagnosis and their easy, fast and reliable handling. In conclusion, my thesis underlines the importance of an interaction of all the different neuroscientific methods such as psychophysics, eye-movement measurements in the real world, electrophysiology and the investigation of neuropsychiatric patients to get a complete picture of how the brain works. The results of my thesis contribute to extent the current knowledge about the processing of information and the perception of our environment in the brain, point towards fields of application of eye-movement measurements and can be used as groundwork for future research

Raumkodierung während glatter Augenfolgebewegungen

Bei der Interaktion mit unserer Umwelt ist die Wahrnehmung von bewegten Objekten mit besonderen Aufgaben verbunden. Da nur in einem kleinen Bereich des visuellen Feldes Objekte mit höchster Auflösung wahrgenommen werden, stellen bewegte visuelle Ziele eine Herausforderung für die visuelle Wahrnehmung dar. Mit sogenannten glatten Augenfolgebewegungen werden bewegte Objekte im Bereich des schärfsten Sehens gehalten. Bei einem stationären Beobachter bleiben während dieser Augenbewegung die Raumkoordinaten der äußeren Welt relativ zum Beobachter konstant, während sie sich relativ zur Blickrichtung ständig verändern. Aus früheren Studien ist bekannt, dass die Ausführung von glatten Augenfolgebewegungen zu systematischen Verschiebungen bei der Lokalisation kurz präsentierter Reize führt. Kurz eingeblendete, visuelle Ziele werden während glatter Augenfolgebewegungen unter Laborbedingungen in Richtung der glatten Augenfolgebewegung verschoben wahrgenommen. Der Ort, relativ zur Blickrichtung an dem ein kurzer visueller Reiz präsentiert wird, hat ebenfalls einen großen Einfluss auf die Lokalisation. Bei Zielen, die in dem Halbfeld des visuellen Feldes in das sich das Auge bewegt präsentiert werden, zeigen Probanden deutlich größere Lokalisationsfehler als bei Zielen in dem anderen Halbfeld. Frühere Studien zeigten auch, dass die Augenbewegung auf die Lokalisation auditorischer Reize einen geringen Effekt hat. In dieser Dissertation habe ich drei Experimente zur Wahrnehmung und Kodierung des Raumes während glatter Augenfolgebewegungen durchgeführt. In der ersten Studie dieser Arbeit untersuchte ich, wie die Lokalisation und Integration von auditorischen und visuellen Reizen während glatter Augenfolgebewegungen erfolgt. Während periodischer glatter Augenfolgebewegungen wurden visuelle und auditorische Reize räumlich kongruent präsentiert und die Lokalisation von menschlichen Probanden untersucht. Dabei wurden sowohl unimodal auditorische oder visuelle Reize lokalisiert, als auch bimodale audiovisuelle Reize. Es zeigte sich dabei, dass die Lokalisation audiovisueller Reize während glatter Augenfolgebewegungen nach einer Maximum-Likelihood-Methode aus den unimodalen Antworten sehr gut vorhergesagt werden kann. Dieses Ergebnis bestätigt zum einen, dass während glatter Augenfolgebewegungen keine supramodale Repräsentation des Raumes existiert, weil die Informationen von unterschiedlichen Modalitäten zu deutlich unterschiedlichen Lokalisationsmustern führen. Die zur Verfügung stehenden Informationen von verschiedenen Modalitäten wurden allerdings nach einem einfachen Maximum-Likelihood-Modell optimal integriert. Physikalische Beschleunigungen werden im Alltag in einer dynamischen Umgebung ständig beobachtet. Es war bekannt, dass das visuelle System für die Diskriminierung unterschiedlicher Beschleunigungen weit weniger sensibel ist als beispielsweise für die Diskriminierung unterschiedlicher Geschwindigkeiten. Es war bislang unklar, ob die Beschleunigung des Auges während glatter Augenfolgebewegungen die Lokalisation beeinflusst. Im zweiten Experiment dieser Dissertation habe ich daher den Einfluss der Beschleunigung des Auges auf die Lokalisation während glatter Augenfolgebewegungen untersucht. Es zeigten sich hier messbare Einflüsse. Die Lokalisationsfehler während positiver Beschleunigungen waren betragsmäßig deutlich geringer als während negativer (abbremsendes Auge) oder nicht beschleunigter glatter Augenfolgebewegungen. Basierend auf physiologischen Daten vorhergehender Studien kann eine mögliche Erklärung für diesen Effekt darin begründet sein, dass positive Beschleunigung neuronal besser kodiert wird. Visuelle Ziele, die sich auf einen Beobachter hinzubewegen, erscheinen dem Betrachter beschleunigt. Sich nähernde Ziele erfordern im Alltag eher eine Reaktion als Ziele, die sich entfernen (Kampf oder Flucht). Ein Beobachter kann von einer besseren Einschätzung des Ortes sich nähernder visueller Objekte also profitieren. Psychophysikalische Studien haben kürzlich gezeigt, dass während glatter Augenfolgebewegung isoluminante, chromatische Reize besser detektiert und diskriminisiert werden können als während Fixation. In der dritten Studie dieser Arbeit wurde die Raum- und Farbkodierung während glatter Augenfolgebewegungen im Areal V4 des Rhesusaffens untersucht. Diese Experimentserie hatte zwei Ziele. Erstens sollte untersucht werden ob ein neuronales Korrelat für den Befund der verbesserten Kodierung chromatischer Reize gefunden werden kann. Die zweite Fragestellung betrifft die Raumkodierung und sollte überprüfen, ob sich die Lage der rezeptiven Felder während glatter Augenfolgebewegungen im Vergleich zur Fixation verschiebt. Dem Affen wurden während glatter Augenfolgebewegungen oder Fixation mit jeder Bildwiederholung an einer zufälligen Position im Raum isoluminante, chromatische Reize gezeigt. Die unterschiedlichen Fragestellungen in diesem Experiment führten zu zwei Ergebnissen. (i) Die neuronalen Antworten auf isoluminante, chromatische Reize waren während glatter Augenfolgebewegungen in der Populationsanalyse im Vergleich zur Fixation tatsächlich deutlich stärker. Diese Daten zeigen erstmals ein neuronales Korrelat für die psychophysikalischen Befunde einer erhöhten Sensitivität für chromatische Reize. Die Lage der rezeptiven Felder verändert sich während glatter Augenfolgebewegungen im Vergleich zur Fixation nicht signifikant im Areal V4. Ein ähnliches Ergebnis wurde bereits in einer früheren Studie im visuellen Areal MT gezeigt. Daraus lässt sich der Schluss ziehen, dass die Fehlwahrnehmungen, die während psychophysikalischen Lokalisationsexperimenten zu beobachten sind, vermutlich nicht auf eine veränderte oder verschobene Lage der rezeptiven Felder in visuellen Arealen zurückzuführen sind. Der Ursprung der Fehlwahrnehmungen bleibt damit weiter unklar. Basierend auf Studien zur Fehlwahrnehmungen während Sakkaden und Raumkodierung relativ zur Aufmerksamkeit, erscheint es zunehmend wahrscheinlicher, dass die Lokalisationsergebnisse während glatter Augenfolgebewegungen auf eine fehlerbehaftete Kodierung der Augenposition und räumlich inhomogene Aufmerksamkeitseffekte zurückgeführt werden können. Zusammenfassend zeigen die Ergebnisse dieser Dissertation, dass viele externe Faktoren, wie die Beschleunigung eines visuellen Ziels oder auditorische Reize die Raumwahrnehmung während glatter Augenfolgebewegungen modulieren können. Zusätzlich können interne Signale, die verantwortlich sind für die Kontrolle und Aufrechterhaltung glatter Augenfolgebewegungen, die Kodierung und Wahrnehmung von Farbe oder kurz präsentierten Objekten verändern

Perceptual stability during saccadic eye movements

Humans and other primates perform multiple fast eye movements per second in order to redirect gaze within the visual field. These so called saccades challenge visual perception: During the movement phases the projection of the outside world sweeps rapidly across the photoreceptors altering the retinal positions of objects that are otherwise stable in the environment. Despite this ever-changing sensory input, the brain creates the percept of a continuous, stable visual world. Currently, it is assumed that this perceptual stability is achieved by the synergistic interplay of multiple mechanisms, for example, a reduction of the sensitivity of the visual system around the time of the eye movement ('saccadic suppression') as well as transient reorganizations in the neuronal representations of space ('remapping'). This thesis comprises six studies on trans-saccadic perceptual stability