Oculomotor contribution to the change in perceived speed with viewing distance.

An array of moving circular stimuli was used to determine whether perceived speed is affected by the oculomotor responses associated with changes in viewing distance. The perceived speed of stimuli viewed at either 0.33 or 1.33 m was compared to the perceived speed of a similar stimulus viewed at a distance of 5.5 m. In addition, a control condition was run in which changes in perceived speed were compared for monocular viewing of the 0.33 m and 5.5 m stimuli. In the binocular condition, there were statistically significant decreases in perceived speed of about 11% for the 0.33 m viewing distance, and about 6.5% for the 1.33 m viewing distance. There was no significant decrease in perceived speed in the monocular condition. This latter finding, along with the similar appearance of the near and far stimuli in the monocular condition, suggests that ocular vergence (as opposed to accommodation or vergence-accommodation) was the primary determinant of the change in perceived speed with changes in binocular viewing distance. Although the change in perceived speed with fixation distance was relatively small, the data from all observers were in the direction of speed constancy. Thus, to the extent that vergence is a cue to egocentric distance, the present data suggest that egocentric distance is used to scale the perceived speed of targets moving at different distances from the observer.

Active heading control in simulated flight based on vertically extended contours.

In two experiments, we manipulated the properties of 3-D objects and terrain texture in order to investigate their effects on active heading control during simulated flight. Simulated crosswinds were used to introduce a rotational component into the retinal flow field that presumably provided the visual cues used for heading control An active control task was used so that the results could be generalized to real-world applications such as flight simulation. In Experiment 1, we examined the effects of three types of terrain, each of which was presented with and without 3-D objects (trees), and found that the presence of 3-D objects was more important than terrain texture for precise heading control In Experiment 2, we investigated the effects of varying the height and density of 3-D objects and found that increasing 3-D object density improved heading control, but that 3-D object height had only a small effect. On the basis of these results, we conclude that the vertical contours improved active heading control by enhancing the motion parallax information contained in the retinal flow.

Head and eye movements in visual search using night vision goggles.

BACKGROUND: Night-vision goggles (NVGs) provide only a restricted field of view and have other characteristics that may affect the head and eye movements used in visual search. METHODS: We measured head scan patterns, the magnitude and duration of gaze saccades, and fixation duration as subjects searched computer generated imagery either with or without NVGs. Subjects searched for either a large (6 degrees) target on a low-detail background (high conspicuity condition) or a small (2 degrees) target on a high-detail background (low conspicuity condition). RESULTS: All subjects displayed head-scan patterns that were qualitatively similar to those reported in the literature. Although both head-scan speed and amplitude were higher for the NVG condition as compared with the no-NVG condition, the difference was not statistically significant. Head-scan speed did vary significantly with target conspicuity, however. Gaze saccade amplitude varied with target conspicuity but only when NVGs were used. Fixation duration did not vary with either NVG use or target conspicuity. A two-parameter (exponent and scaling parameter) power function was fitted to the amplitude-duration data. The power-function exponents varied from about 0.30 to 0.44, but there was a concommitant variation in the scaling parameter, and the result was no significant difference in the form of the power functions fitted to the data. CONCLUSIONS: NVG use did not significantly affect any of the individual head or eye movement variables involved in searching the computer-generated imagery studied here. However, the decrease in gaze-saccade amplitude with increased target conspicuity when NVGs were used is evidence of the sensitivity of the head and eye movement measurement techniques used here, and suggests that all available measures of response efficiency be considered when evaluating NVGs using complex stimuli.