Biomedical subjects

Markus Lappe

Publications and source records attributed to Markus Lappe.

14 recordsLinked to original sources

PubMed2007

Travel distance estimation from visual motion by leaky path integration.

Visual motion can be a cue to travel distance when the motion signals are integrated. Distance estimates from visually simulated self-motion are imprecise, however. Previous work in our labs has given conflicting results on the imprecision: experiments by Frenz and Lappe had suggested a general underestimation of travel distance, while results from Redlick, Jenkin and Harris had shown an overestimation of travel distance. Here we describe a collaborative study that resolves the conflict by tracing it to differences in the tasks given to the subjects. With an identical set of subjects and identical visual motion simulation we show that underestimation of travel distance occurs when the task involves a judgment of distance from the starting position, and that overestimation of travel distance occurs when the task requires a judgment of the remaining distance to a particular target position. We present a leaky integrator model that explains both effects with a single mechanism. In this leaky integrator model we introduce the idea that, depending on the task, either the distance from start, or the distance to target is used as a state variable. The state variable is updated during the movement by integration over the space covered by the movement, rather than over time. In this model, travel distance mis-estimation occurs because the integration leaks and because the transformation of visual motion to travel distance involves a gain factor. Mis-estimates in both tasks can be explained with the same leak rate and gain in both conditions. Our results thus suggest that observers do not simply integrate traveled distance and then relate it to the task. Instead, the internally represented variable is either distance from the origin or distance to the goal, whichever is relevant.

Adult↗

PubMed2006

The fate of object features during perisaccadic mislocalization.

Visual objects flashed before a saccade appear compressed toward the saccade target. Simultaneously flashed objects merge perceptually into one. To better understand cortical interactions in perisaccadic processing, we study the perception of features of mislocalized objects. We report four new findings: First, when multiple objects of different colors are compressed onto a single position, their color attributes remain distinguishable. Second, color attributes of objects compressed onto the same position compete for access to visual awareness. Third, objects presaccadically mislocalized onto a static background of identical color and luminance appear visible on top of that background. Object shape can be determined. Fourth, objects flashed during a saccade become invisible when a larger object is present at the mislocalized position. Thus, perisaccadic mislocalization affects the position of objects but retains other object features. Mislocalization must either occur in parallel to color and shape processing or at late stages of the visual pathway.

Adult↗

PubMed2006

Visual perception of biological motion by form: a template-matching analysis.

Biological motion perception is referred to as the ability to recognize a moving human figure from no more than a few moving point lights. Such point-light stimuli contain limited form information about the shape of the body and local image motion signals from the moving points. The contributions of form and motion to the vivid perception of point-light displays are subject to controversy in the discussion. While some studies claim that local motion signals are critical, others emphasize the role of global form cues. Here, we present a template-matching approach to investigate the role of global form analysis. We used a template-matching method that derives biological motion exclusively from form information. The algorithm used static postures monitored from walking humans as stored templates. We compared the simulation results to psychophysical experiments with the commonly used point-light walker and a variant point-light walker with near-absent local motion signals. The common result in all experiments was a high correlation between simulation results and psychophysical data. The results show that the limited form information in point-light stimuli might be sufficient to perceive biological motion. We suggest that it is possible for humans to extract the sparse form information in point-light walkers and to use it to perceive biological motion by integrating dynamic form information over time.

Discrimination, Psychological↗

PubMed2006

A model of biological motion perception from configural form cues.

Biological motion perception is the compelling ability of the visual system to perceive complex human movements effortlessly and within a fraction of a second. Recent neuroimaging and neurophysiological studies have revealed that the visual perception of biological motion activates a widespread network of brain areas. The superior temporal sulcus has a crucial role within this network. The roles of other areas are less clear. We present a computational model based on neurally plausible assumptions to elucidate the contributions of motion and form signals to biological motion perception and the computations in the underlying brain network. The model simulates receptive fields for images of the static human body, as found by neuroimaging studies, and temporally integrates their responses by leaky integrator neurons. The model reveals a high correlation to data obtained by neurophysiological, neuroimaging, and psychophysical studies.

Adult↗

PubMed2006

Mislocalization of perceived saccade target position induced by perisaccadic visual stimulation.

The perceptual localization of objects flashed at the time of a saccade often shows large spatial distortions. These perisaccadic mislocalizations exhibit different spatial patterns depending on the experimental condition. In darkness, when only extraretinal information is available, mislocalization is spatially uniform. In light and when visual references are available, mislocalization is directed toward the saccade target, resembling a compression of visual space. These patterns are derived from measurements of the absolute perceived position of the flashed object in egocentric space. Here, we report that also the perceived location of the saccade target is altered when an object is flashed perisaccadically. The mislocalization of the target depends on the presentation time of the flashed object and is directed toward the position of the flash. The resulting compression of the relative distance between target and flash is similar in darkness and in light and can also be found during fixation. When the localization of the flashed object is described relative to the perceived location of the saccade target, spatial compression becomes similar in many experimental conditions. We therefore suggest that perisaccadic compression relies on an encoding of relative spatial locations of objects rather than on localizations in egocentric space.

Adult↗

PubMed2006

Visual distance estimation in static compared to moving virtual scenes.

Visual motion is used to control direction and speed of self-motion and time-to-contact with an obstacle. In earlier work, we found that human subjects can discriminate between the distances of different visually simulated self-motions in a virtual scene. Distance indication in terms of an exocentric interval adjustment task, however, revealed linear correlation between perceived and indicated distances but with a profound distance underestimation. One possible explanation for this underestimation is the perception of visual space in virtual environments. Humans perceive visual space in natural scenes as curved, and distances are increasingly underestimated with increasing distance from the observer. Such spatial compression may also exist in our virtual environment. We therefore surveyed perceived visual space in a static virtual scene. We asked observers to compare two horizontal depth intervals, similar to experiments performed in natural space. Subjects had to indicate the size of one depth interval relative to a second interval. Our observers perceived visual space in the virtual environment as compressed, similar to the perception found in natural scenes. However, the nonlinear depth function we found can not explain the observed distance underestimation of visual simulated self-motions in the same environment.

Adult↗

PubMed2005

Visual areas involved in the perception of human movement from dynamic form analysis.

The perception of biological motion combines the analysis of form and motion. However, patient observations by Vaina et al. and psychophysical experiments by Beintema and Lappe showed that humans could perceive human movements (a walker) without local image motion information. Here, we examine the specificity of brain regions responsive to a biological motion stimulus without local image motion, using functional magnetic resonance imaging. We used the stimulus from Beintema and Lappe and compared the brain activity with a point-light display that does contain local motion information and was often used in previous studies. Recent imaging studies have identified areas sensitive to biological motion in both the motion-processing and the form-processing pathways of the visual system. We find a similar neuronal network engaged in biological motion perception, but more strongly manifested in form-processing than in motion-processing areas, namely, fusiform-/occipital face area and extrastriate body area.

Adult↗

PubMed2005

Effect of saccadic adaptation on localization of visual targets.

Objects flashed briefly around the time of a saccadic eye movement are grossly mislocalized by human subjects, so they appear to be compressed toward the endpoint of the saccade. In this study, we investigate spatial localization during saccadic adaptation to examine whether the focus of compression tends toward the intended saccadic target or at the endpoint of the actual (adapted) movement. We report two major results. First, that peri-saccadic focus of the compression did not occur at the site of the initial saccadic target, but tended toward the actual landing site of the saccade. Second, and more surprisingly, we observed a large long-term perceptual distortion of space, lasting for hundreds of milliseconds. This distortion did not occur over the whole visual field but was limited to a local region of visual space around the saccade target, suggesting that saccadic adaptation induces a visuo-topic remapping of space. The results imply that the mechanisms controlling saccadic adaptation also affect perception of space and point to a strong perceptual plasticity coordinated with the well-documented plasticity of the motor system.

Adaptation, Physiological↗

PubMed2005

Absolute travel distance from optic flow.

Optic flow fields provide rich information about the observer's self-motion. Besides estimation of the direction of self-motion human observers are also able to discriminate the travel distances of two self-motion simulations. Recent studies have shown that observers estimate the simulated ego velocity of the self-motion simulation and integrate it over time. Thus, observers use a 3-D percept of the ego motion through the environment. In the present work we ask if human observers are able to use this 3-D percept of the motion simulation to build up an internal representation of travel distance and indicate it in a static scene. We visually simulated self-motion in different virtual environments and asked subjects to indicate the perceived distances in terms of static virtual intervals on the ground. The results show that human observers possess a static distance gauge, but that they undershoot the travel distances for short motion simulations. In further experiments we changed the modality of the distance indication but the undershoot in distance estimation remained. This suggests that the undershoot is linked to the perception of the optic flow field.

Cues↗

PubMed2004

Perception of visual space at the time of pro- and anti-saccades.

The localization of peri-saccadically flashed objects shows two types of errors: first, a uniform shift in saccade direction, and second, a compression of visual space around the saccade target. Whereas the uniform shift occurs when the experiment is performed in complete darkness compression occurs when additional visual references are available. Thus peri-saccadic mislocalization contains motor and visual components. To distinguish between both factors we compared peri-saccadic localization errors during pro- and anti-saccades. In the case of anti-saccades, the visual cue that elicits the saccade and the actual eye movement are in opposite directions. We asked whether peri-saccadic compression can be observed with anti-saccades, and if so, whether the compression is directed toward the visual cue or follows the actual eye movement. In blocked trials, subjects performed saccades either toward a visual cue (pro-saccade) or to the mirrored position opposite to a visual cue (anti-saccade). Peri-saccadically, we flashed a thin vertical bar at one of four possible locations. Subjects had to indicate the perceived position of the bar with a mouse pointer about 500 ms after the saccade. Experiments were performed in complete darkness and with visual references. Peri-saccadic mislocalizations occurred during anti-saccades. The mislocalizations were very similar for pro- and anti-saccades in magnitude and direction. For both, pro- and anti-saccades, mislocalizations were directed toward the actual eye movement and not the visual cue.

Adult↗

PubMed2004

Perisaccadic mislocalization orthogonal to saccade direction.

Saccadic eye movements transiently distort perceptual space. Visual objects flashed shortly before or during a saccade are mislocalized along the saccade direction, resembling a compression of space around the saccade target. These mislocalizations reflect transient errors of processes that construct spatial stability across eye movements. They may arise from errors of reference signals associated with saccade direction and amplitude or from visual or visuomotor remapping processes focused on the saccade target's position. The second case would predict apparent position shifts toward the target also in directions orthogonal to the saccade. We report that such orthogonal mislocalization indeed occurs. Surprisingly, however, the orthogonal mislocalization is restricted to only part of the visual field. This part comprises distant positions in saccade direction but does not depend on the target's position. Our findings can be explained by a combination of directional and positional reference signals that varies in time course across the visual field.

Adult↗

PubMed2004

Contrast dependency of sacadic compression and suppression.

In the occurrence of a saccadic eye movement vision becomes suppressed. Supra-threshold visual stimuli that are briefly presented at that time become perceptually compressed towards the saccade target (saccadic compression) and shifted in saccade direction (saccadic shift). We show that the strength of saccadic compression, like the strength of saccadic suppression, varies with stimulus contrast. Low contrast stimuli lead to stronger compression than high contrast stimuli. The similarity of contrast dependence and time course suggests that saccadic compression is related to saccadic suppression. Because the saccadic shift did not depend on contrast we suggest that shift and compression are different effects.

Adult↗

PubMed2003

Discrimination of travel distances from 'situated' optic flow.

Effective navigation requires knowledge of the direction of motion and of the distance traveled. Humans can use visual motion cues from optic flow to estimate direction of self-motion. Can they also estimate travel distance from visual motion?Optic flow is ambiguous with regard to travel distance. But when the depth structure of the environment is known or can be inferred, i.e., when the flow can be calibrated to the environmental situation, distance estimation may become possible. Previous work had shown that humans can discriminate and reproduce travel distances of two visually simulated self-motions under the assumption that the environmental situation and the depth structure of the scene is the same in both motions. Here we ask which visual cues are used for distance estimation when this assumption is fulfilled. Observers discriminated distances of visually simulated self-motions in four different environments with various depth cues. Discrimination was possible in all cases, even when motion parallax was the only depth cue available. In further experiments we ask whether distance estimation is based directly on image velocity or on an estimate of observer velocity derived from image velocity and the structure of the environment. By varying the simulated height above ground, the visibility range, or the simulated gaze angle we modify visual information about the structure of the environment and alter the image velocity distribution in the optic flow. Discrimination ability remained good. We conclude that the judgment of travel distance is based on an estimate of observer speed within the simulated environment.

Adult↗

PubMed1998

A model of the combination of optic flow and extraretinal eye movement signals in primate extrastriate visual cortex. Neural model of self-motion from optic flow and extraretinal cues.

The determination of the direction of heading from optic flow is a complicated task. To solve it the visual system complements the optic flow by non-visual information about the occurrence of eye movements. Psychophysical studies have shown that the need for this combination depends on the structure the visual scene. In a depth-rich visual environment motion parallax can be exploited to differentiate self-translation from eye rotation. In the absence of motion parallax, i.e. in the case of movement towards a frontoparallel plane, extraretinal signals are necessary for correct heading perception ([Warren and Hannon, 1990]). [Lappe and Rauschecker (1993b)] have proposed a model of visual heading detection that reproduces many of the psychophysical findings in the absence of extraretinal input and links them to properties of single neurons in the primate visual cortex. The present work proposes a neural network model that integrates extraretinal signals into this network. The model is compared with psychophysical and neurophysiological data from experiments in human and non-human primates. The combined visual/extraretinal model reproduces human behavior in the case of movement towards a frontoparallel plane. Single model neurons exhibit several similarities to neurons from the medial superior temporal (MST) area of the macaque monkey. Similar to MST cells ([Erickson and Thier, 1991]) they differentiate between self-induced visual motion that results from eye movements in a stationary environment, and real motion in the environment. The model predicts that this differentiation can also be achieved visually, i.e. without extraretinal input. Other simulations followed experiments by [Bradley et al. (1996)], in which flow fields were presented that simulated observer translation towards a frontoparallel plane plus an eye rotation. Similar to MST cells, model neurons shift their preference for the focus of expansion along the direction of the eye movement when extraretinal input is not available. They respond to the retinal location of the focus of expansion which is shifted by the eye movement. In the presence of extraretinal input the preference for the focus of expansion is largely invariant to eye movements and tied to the location of the focus of expansion with regard to the visual scene. The model proposes that extraretinal compensation for eye movements need not be perfect in single neurons to achieve accurate heading detection. It thereby shows that the incomplete compensation found in most MST neurons is sufficient to explain the psychophysical data.

Journal Article↗