Biomedical subjects

Harald Frenz

Publications and source records attributed to Harald Frenz.

3 recordsLinked to original sources

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

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↗

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↗