Fahle, Manfred
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Item-typ:Veröffentlichung, Neuronale Korrelate der Konturintegration(2006-12-18); ; ; Three studies are presented:-B Mathes, D Trenner, M Fahle (2006). The electrophysiological correlate of contour integration is modulated by task demands, Brain Research, 1114, 98-112It is shown that contour integration elicits a negative shift over posterior recording sites. This effect started about 150 ms after stimulus onset but was delayed for more difficult contour integration. Difficult contour integration may in addition elicit a frontal selection positivity. The result indicate that contours are processed similar to textures and that decreasing saliency of detectable contours is compensated by increasing processing time which possibly results from increasing allocation of visual selective attention.-B Mathes, M Fahle (2007). The electrophysiological correlate of contour integration is similar for colour and luminance mechanisms, Psychophysiology ,in pressTo investigate how contour integration depends on early sensory processing, the electrophysiological correlate of contour integration of elements defined by luminance or isoluminant colour contrasts was measured. Detectable contours elicit a negative shift over posterior recording sites which is similar for luminance and colour contrasts. This indicates a common physiological processing stream for contour integration of red-and-green and black-and-white elements.-B Mathes, M Fahle (2007). Closure facilitates contour integration in smooth and short contours, Vision Research, in pressThe facilitation of contour integration by closure has been questioned. Our results demonstrate that closure improves contour integration, even when shortcomings of earlier studies are controlled for. However, the advantage of closure might be minor.Dissertation266 110 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Detektion und Identifikation von Figur-Grund-Unterschieden: Psychophysik, Elektrophysiologie und Magnetresonanztomographie(2009-05-25); ; ; The dissertation deals with psychophysical and neural properties of figure-ground-segregation in humans i) using two types of visual cues (orientation and spatial frequency) and ii) two types of tasks (detection and identification). The effect of visual cue combination is investigated separately for detection and identification using psychophysical and ERP measurements to evaluate how the combination itself and/or the perceived figure saliency influence the ERP. As a result, it was found in both studies that the neural response at 200 ms reflected saliency-changes rather than physical cue configuration. The comparison of detection and identification was investigated by combining psychophysics and fMRI. Pronounced differences between detection and identification were found i) in the psychophysical thresholds and the reaction times, and ii) in the BOLD responses. In summary, the results indicate at least partly separated cortical mechanisms for object detection and identification.Dissertation283 223 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Specific Aspects in the Adaptation of Human Eye-Hand-Coordination(2011-12-14); ; ; Sensorimotor control, especially eye-hand-coordination, enables us to interact with our environment. In my dissertation I describe and discuss three specific aspects of the adaptation of eye-hand-coordination: 1) the relative weight of visual and proprioceptive adaptation, 2) spatial and proprioceptive generalization of adaptation, and 3) acquisition of alternative mappings with repeated adaptations. The first aspect of sensorimotor adaptation is how an ambiguous mismatch between visual and proprioceptive coordinate systems is resolved. In my experiment I showed that an auditory pacing signal is an important factor facilitating visual adaptation. As a second aspect I examined the generalization of prism adaptation over space. Based on my results I can explain all previous results as an underestimation of change in the guiding system. The third aspect is the acquisition of alternate mappings (dual-adaptation) together with learning to learn. Confirming the involvement of learning to learn, I also found that the efficiency of dual-adaptation only depends on the total number of trials irrespective of their segmentation in blocks of differing lengths.Dissertation349 169 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Über die Abhängigkeit lokaler Konturintegration von Bottom-up-, Top-down- und globalen Gestaltprozessen(2011-07-01); ; ; Shape perception in natural scenes relies on integration processes which construct a representation from local image features. An example is contour integration, grouping nearly collinear aligned edge elements into coherent forms. At the same time, perception is strongly modulated by the global form or context of a shape. A prominent example is the better visibility for cardinal as opposed to oblique shapes. Another factor modulating contour integration is attention, indicating the importance of the confluence of bottom-up (sensory input) and top-down signals (task demands) for this task. In the first study reported in this dissertation we use psychophysical studies and EEG recordings to investigate how contour integration is influenced by global factors in form and context of the contour. In a second study we investigate the impact of differing task demands (attention) on contour integration.Dissertation309 113 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Lernen und Adaptation im visuellen System des Menschen(2008-04-17); ; ; Neural representations are continuously shaped by experience, and processes like learning and adaptation occur within various brain systems. The process of adapting to a prism-induced displacement of the visual world demonstrates the retention of neural plasticity within the adult visual and sensorimotor systems. The present work addressed specific factors of prism adaptation in humans using psychophysical methods. The first study was conducted to examine the completeness of the adaptation process during throwing or pointing movements. In the second study, the task-specificity of prism adaptation under different sensory feedback conditions was tested. A third study assessed spatial generalization of prism adaptation as a function of specific training conditions. The results are discussed with respect to the mechanisms the brain uses in order to rapidly adjust sensorimotor representations in response to altered environmental conditions.Dissertation408 278 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Figur-Grund Segmentierung durch zeitliche Hinweise(2003-01-30); ; ; Visual figure-ground segregation can rely on differences in luminance, texture, colour, motion, depth, and also on asynchronous changes in luminance and motion direction. However, since these displays were suspected to contain global motion and contrast artefacts, we created a new stimulus showing constantly 20 by 20 randomly oriented dipoles ("colons") flipping (i.e. rotating instantaneously by 90 deg) with a defined frequency. All dipols flip synchronously within both areas, figure and ground but with a defined delay between them. Human subjects had to localize the targets in a 4-alternative forced-choice task.Given a high luminance contrast between dipols and monitor surround subjects could detect the target up to a threshold frequency of 23 Hz and down to a threshold delay of 14 ms. Replacing the luminance-defined dots by contrast-defined Mexican Hats led to comparable results. Although both tasks are contrast sensitive the segregation can still be detected when dots are isoluminant, whereas dichoptic presentation doesn´t allow for segregation at all.Visual Evoked Potentials and Functional Magnetic Resonance Imaging studies showed VEP components and fMRI locations previously identified with form-from-motion stimuli.In the last study we re-examined form from asynchronous motion-reversals using defined frequencies and delays, and demonstrate that segregation can rely on short intervals (15 ms) of opposing motion directions between figure and ground and on longer (40 ms) intervals of differing contrast cues (i.e. motion energy cues), but not on asynchronous motion reversals per se.In conclusion, we confirm that segregation can rely solely on local changes occuring temporally asynchronously in figure and ground. We propose a segregation mechanism consisting of a set of local (monocular) motion detectors at the front end and a second stage reading and globally comparing their output with a high temporal resolution.Dissertation217 119 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Wahrnehmungskonflikte: Binokulare Rivalität, Stereopsis, Aufmerksamkeit(2011-08-03); ; ; In our everyday live we experience many conflict situations. In this work three different types of conflict are presented: Binocular rivalry, Stereopsis and Attention. Binocular rivalry ensues when both eyes are confronted with two sufficient different pictures. This is resolved by a temporal segregation of both pictures in awareness. The pupil reacts to a change in perceived brightness and does so earlier as an actual behavioral report. The pupil is an indicator for upcoming rivalry shifts. Under normal viewing condition the visual system is provided by many different depth cues, as disparity and perspective. A combination of cues, indicting for the same or different spatial orientation, revealed, that cues are used to estimated depth; without regard to a potential antagonism. The visual system is flooded by information. In order to separate relevant and irrelevant data, attention is employed. Neuronal networks are compared between humans and monkeys performing the same task under fMRI.Dissertation426 181 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Hemodynamic response to oscillatory EEG rhythms in the human visual cortex(2010-07-08); ; ; Understanding the relationship between electrophysiological and hemodynamic signals is of superior importance to draw inferences from modern vascular based imaging techniques back to the underlying neuronal brain activity. Non-invasive studies fusing direct and indirect neuronal based methods such as electroencephalography (EEG) coupled to functional magnetic resonance imaging (fMRI) or near-infrared spectroscopy (NIRS) in human has become a significant approach to elucidate the neurovascular mechanisms and to validate invasive findings in animal. Currently, it is still unclear which electrophysiological components or which combinations of these components have the strongest influence on the hemodynamic signal. Therein, oscillatory brain activity in the alpha- and gamma à ¬range represents fruitful neuronal predictors because of their involvement in a widespread number of cognitive tasks and the assumption of oscillatory activity as a multifunctional coding mechanism in the visual system. The present dissertation aims to shed light on the coupling mechanisms between event-related oscillatory activity and the concomitant hemodynamic response in human. For this purpose, three studies with different experimental designs were performed in order to selectively induce oscillatory activity in the alpha- and gamma-range in the visual cortex and to test the influence of a particular oscillatory band on the hemodynamic response. To adequately address this neurovascular relationship EEG and NIRS techniques were applied simultaneously in all studies. The first study showed that the resonance phenomenon, a local maximum that appears when the stimulation frequency of flicker-light matches the endogenous alpha-frequency, is not accompanied by an increase in vascular parameters. Neither evoked potentials nor ongoing alpha-power or even a combination of both electrophysiological parameters predicted the magnitude of the hemodynamic response. It was therefore suggested that the resonance phenomenon is caused by a low energy demanding phase-resetting mechanism. Furthermore, it could be shown that the resonance boost observed in electrophysiological studies and the 8 Hz peak response observed with hemodynamic based techniques represent independent phenomena. Study II provided evidence for a predictive link between resting state alpha-parameters and evoked signals during flicker-light stimulation. It was shown that resting alpha-frequency is negatively related to the amplitude of evoked potential, to the induced alpha-power and to the magnitude of the hemodynamic response upon stimulation. The results provide further support for the assumption of a functional linkage between evoked potentials and alpha-rhythm (Study I&II). Study III showed a tight coupling between oscillatory activity in the gamma-range and the hemodynamic response during parametrical contrast variation of a visual moving grating. Also, it could be shown that during constant contrasts behavioural performance to the accompanied task was linked to the magnitude of gamma-activity and the hemodynamic response. Here, faster response times were preceded by a phasic enhancement of gamma-band activity. Thus, Study III provides further evidence for the superior role of gamma-oscillations in visual processing and behavioural performance and validates the close coupling between hemodynamic signal and gamma-activity noninvasively in human. In conclusion, by focussing on the same task with complementary methods, the studies of the present dissertation provide a further insight into the relationship between electrophysiological components and hemodynamic signals. Herein, simultaneous assessment of EEG and NIRS techniques provide a powerful tool to study the relationship between direct and indirect neuronal signals in humans.Dissertation248 103 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Neuronale Korrelate zerebral bedingter visueller Wahrnehmungsstörungen(2013-11-29); ; ; Two aspects of visual processing are investigated in this work. First, the impact of unilateral occipito-temporal cortical lesions on object categorization, and second, the influence of top-down manipulation on object categorization in healthy subjects. Major visual object recognition deficits (object agnosia) are rare, and occur mainly after bilateral damage to ventral visual cortices. Most patients suffering from unilateral ventral lesions are clinically non-agnostic. We studied the effect of unilateral occipito-temporal lesions on object categorization and its underlying neural correlates. The findings indicate that unilateral ventral lesions lead to disturbed processing in the lesioned hemisphere, which affects object categorization in both visual hemifields. In healthy object categorization bottom-up and top-down influences interact. We investigated the combination of different top-down influences and their effect on rapid object categorization. The results implicate that different top-down manipulations interact strongly, which should be taken into account when interpreting categorization results.Dissertation321 166 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Zerebral bedingte Sehstörungen(2008-12-05); ; ; The present work concentrates on different aspects of disturbed visual information processing. The patients examined have either a cerebral lesion or a psychiatric disease (schizophrenia). Patients and groups of patients are distinguished based on supposed site of disruption on an early or else a late visual processing stage.The second chapter considers disturbances of early vision. It is shown 1) that a lesion of extrastriate cortex is sufficient to cause a visual field defect and 2) that processing of specified stimulus features within the remaining visual field of the affected quadrant is reduced in a patient with a circumscribed occipital lesion, as revealed by both functional imaging and psychophysics. These results suggest that essential differences exist between species (e.g. human and macaque) already on early visual processing stages.Moreover, in a group study it can be shown that unilateral occipital lesions of both hemispheres always cause a visual exploration deficit. This deficit manifests itself as a reduced processing speed of all patients in an attention demanding visual search task. The results furthermore demonstrate that the performance is worse for the contralesional attentional window, if the size of this window is scaled according to task demands.In the third chapter patients with cerebral lesions causing a late visual processing deficit are described. For the first time a substantial reduction of size constancy is shown in a patient (HE) with a unilateral lesion of right occipito-parietal cortex. HE is not able to adjust the veridical size of two stimuli, if these stimuli are presented at different distances. Especially under monocular viewing conditions, the estimated size of a stimulus is based mainly on retinal image size. This strengthens the hypothesis that the right parietal cortex, which is involved in processing of spatial information, must be involved in the representation of object size, too. Managing precise (grasping) movements requires knowledge of true object size. Impairment of size constancy after occipito-parietal lesion is consistent with the idea that dorsal information processing is responsible for visually guided actions.The second part of the third chapter concentrates on visual exploration performance within a group of patients with right middle cerebral artery stroke. Due to systematic variation of target position in a parallel and in a serial visual search task, the group can be splitted according to systematic schemes of target detection, which in turn can be linked to damage of specific cortical areas. In patients showing an egocentric serial search deficit, the premotor cortex and probably the frontal eye field are damaged. In contrast, the centre of lesions in patients with an allocentric serial search deficit is located in the ventro-medial cortex near the parahippocampal gyrus. These results confirm the theoretical distinction between ventral and dorsal processing, with the first representing detailed object representations within an allocentric reference frame and the latter mediating visual guided actions within an egocentric reference system.Two aspects of the well-known backward masking deficit in schizophrenic patients are investigated in the last chapter. By means of psychophysical measurements it is shown in the first part that the deficit is neither due to prolonged visible persistence nor to a general slowing down of information processing. The difficulty for the patient appears to lie rather in the processing of stimuli with high temporal vicinity when they are additionally presented close together. In the second part it is shown that schizophrenic patients can perform as well as healthy controls in a backward masking task with additionally presented collinear contextual elements, when the stimulus configuration is individually adjusted. This suggests that in schizophrenic patients excitatory but not inhibitory neural connections are modified.Dissertation299 114