Biomedical subjects

J A Beintema

Publications and source records attributed to J A Beintema.

9 recordsLinked to original sources

PubMed2006

Perception of biological motion from limited-lifetime stimuli.

The visual perception of human movement from sparse point-light walkers is often believed to rely on local motion analysis. We investigated the role of local motion in the perception of human walking, viewed from the side, in different tasks. The motion signal was manipulated by varying point lifetime. We found the task of coherence discrimination, commonly used in biological motion studies, to be inappropriate for testing the role of motion. A task requiring temporal information showed a strong performance drop when fewer points were used or when the image sequence was sampled and displayed at a reduced frame rate. Irrespective of the frame rate, performance did not vary with point lifetime. We concluded that local motion is not required for the perception of tested biological movements, suggesting that the analysis of biological motion does not benefit from examining local motion. The reliance of perception on the number of displayed points and frames supports the idea that biological motion is perceived from a sequence of spatiotemporally sampled forms.

Child↗

PubMed2002

Perception of biological motion without local image motion.

A vivid perception of the moving form of a human figure can be obtained from a few moving light points on the joints of the body. This is known as biological motion perception. It is commonly believed that the perception of biological motion rests on image motion signals. Curiously, however, some patients with lesions to motion processing areas of the dorsal stream are severely impaired in image motion perception but can easily perceive biological motion. Here we describe a biological motion stimulus based on a limited lifetime technique that tests the perception of a moving human figure in the absence of local image motion. We find that subjects can spontaneously recognize a moving human figure in displays without local image motion. Their performance is very similar to that for classic point-light displays. We also find that tasks involving the discrimination of walking direction or the coherence of a walking figure can be performed in the absence of image motion. Thus, although image motion may generally aid processes such as segmenting figure from background, we propose that it is not the basis for the precept of biological motion. Rather, we suggest biological motion is derived from dynamic form information on body posture evolving over time.

Humans↗

PubMed2001

Heading and path percepts from visual flow and eye pursuit signals.

The percept of self-motion through the environment is supported by visual motion signals and eye movement signals. The interaction between these signals by decoupling of the eye movement and the pattern of retinal motion during brief simulated ego-movement on straight or circular trajectories was studied. A new response method enabled subjects to report perceived destination and perceived curvature of their future path simultaneously. Various combinations of simulated gaze rotation in the retinal flow and eye pursuit were investigated. Simulated gaze rotation ranged from consistent and larger than, to opponent and larger than eye pursuit. It was found that the perceived destination shifts non-linearly with the mismatch between simulated gaze rotation and eye pursuit. The non-linearity is also revealed in the perceived tangent heading direction and perceived path curvature, although to different extent in different subjects. For the same retinal flow, eye pursuit that is consistent with the simulated gaze rotation reduces heading error and the perceived path straightens out. In contrast, perceived path and/or heading do not become more curved or more biased in the direction opposite to pursuit when the eye -in-head rotation is opposite to the simulated gaze rotation. These observations point to modulation of the effect of the extra-retinal pursuit signal by the visual evidence for eye rotation. In a second experiment, one presented to a stationary eye the sum of a component of simulated gaze rotation and radial flow. It was found that the bi-circular flow component, that characterizes the change in pattern of flow directions by the gaze rotation, induces a shift of perceived heading without appreciable perceived path curvature. Conversely, the complementary component of simulated gaze rotation (bi-radial flow) evokes a percept of motion on a curved path with a small tangent heading error. It was suggested that bi-circular and bi-radial flow components contribute primarily to percepts of heading and path curvature, respectively.

Humans↗

PubMed2001

Pursuit affects precision of perceived heading for small viewing apertures.

We investigated the interaction between extra-retinal rotation signals and retinal motion signals in heading perception during pursuit eye movement. For limited viewing aperture, the variability in perceived heading strongly depends on the pattern of motion directions. Heading towards a point outside the aperture generates nearly parallel aperture flow. This results in lower precision of perceived heading than heading that renders the radial pattern of flow visible. We ask if the precision is limited by the pattern of flow visible on the retina or that on the screen. During fixation, the two patterns are identical. They are decoupled during pursuit, since pursuit changes radial flow within the aperture on the screen into nearly parallel flow on the retina, and vice versa. The extra-retinal signal is known to reduce systematic errors in the direction of pursuit, thus compensating for the rotational flow during pursuit. We now ask if the extra-retinal signal also affects the precision of heading percepts. It might if at the spatial integration stage the rotational flow has been subtracted out already. A compensation beyond the integration stage, however, cannot undo the change in retinal motion directions so that an effect of pursuit on precision cannot be avoided. We measured the variable and systematic errors in perceived heading during fixation and pursuit for a frontal plane approach, while varying duration, dot lifetime and aperture size. We found precision is effected by pursuit as much as predicted from the pattern of retinal flow, while compensation is significantly greater than zero. This means that the interaction between the extra-retinal signal and visual motion signals takes place after spatial integration of local motion signals. Furthermore, compensation increased significantly with longer duration (0.5-3.0 s), but not with larger aperture size (10-50 degrees ). A larger aperture size did increase the eccentricity of perceived heading.

Discrimination, Psychological↗

PubMed2000

Perceived heading during simulated torsional eye movements.

Observer translation through the environment can be accompanied by rotation of the eye about any axis. For rotation about the vertical axis (horizontal rotation) during translation in the horizontal plane, it is known that the absence of depth in the scene and an extra retinal signal leads to a systematic error in the observer's perceived direction of heading. This heading error is related in magnitude and direction to the shift of the centre of retinal flow (CF) that occurs because of the rotation. Rotation about any axis that deviates from the heading direction results in a CF shift. So far, however, the effect of rotation about the line of sight (torsion) on perceived heading has not been investigated. We simulated observer translation towards a wall or cloud, while simultaneously simulating eye rotation about the vertical axis, the torsional axis or combinations thereof. We find only small systematic effects of torsion on the set of 2D perceived headings, regardless of the simulated horizontal rotation. In proportion to the CF shift, the systematic errors are significantly smaller for pure torsion than for pure horizontal rotation. In contrast to errors caused by horizontal rotation, the torsional errors are hardly reduced by addition of depth to the scene. We suggest the difference in behaviour reflects the difference in symmetry of the field of view relative to the axis of rotation: the higher symmetry in the case of torsion may allow for a more accurate estimation of the rotational flow. Moreover, we report a new phenomenon. Simulated horizontal rotation during simulated wall approach increases the heading-dependency of errors, causing a larger compression of perceived heading in the horizontal direction than in the vertical direction.

Eye Movements↗

PubMed2000

The mechanism of interaction between visual flow and eye velocity signals for heading perception.

A translating eye receives a radial pattern of motion that is centered on the direction of heading. If the eye is rotating and translating, visual and extraretinal signals help to cancel the rotation and to perceive heading correctly. This involves (1) an interaction between visual and eye movement signals and (2) a motion template stage that analyzes the pattern of visual motion. Early interaction leads to motion templates that integrate head-centered motion signals in the visual field. Integration of retinal motion signals leads to late interaction. Here, we show that retinal flow limits precision of heading. This result argues against an early, vector subtraction type of interaction, but is consistent with a late, gain field type of interaction with eye velocity signals and neurophysiological findings in area MST of the monkey.

Eye Movements↗

PubMed1999

Visual search of heading direction.

When we move along we frequently look around. How quickly and accurately can we gaze in the direction of heading? We studied the temporal aspects of heading perception in expanding and contracting patterns simulating self-motion. Center of flow (CF) eccentricity was 15 degrees. Subjects had to indicate the CF by making a saccade to it. A temporal constraint on the response time was introduced, because stimuli were presented briefly (1 s). On average, subjects needed two saccades to find the CF. Initial saccades covered about 50-60% of the distance between the fixation point and the CF. Subjects underestimated the eccentricity of the CF. The systematic radial error ranged from -2.4 degrees to -4.9 degrees. The systematic tangential error was small (about 0.5 degree). The variable radial error ranged from 2.7 degrees to 4.6 degrees. We found a relation between saccade onset time and saccade endpoint error. Saccade endpoint error decreased with increasing saccade onset time, suggesting that saccades were often fired before the heading processing had been completed. From the saccade onset times, saccade endpoint errors and an estimate for the saccadic dead time (interval prior to the saccade during which modification is impossible 70 ms), we estimated the heading processing time (HPT 0.43 s). In three out of four subjects, HPT was longer for trials simulating backward movement than for trials simulating forward movement. For each saccade we determined whether it reduced the distance error. The second saccade reduced the error more effectively per time unit than the initial saccade. On the basis of this finding, we suggest that visual processing that occurs during the saccadic dead time of the first saccade is used in the preparation of the second saccade.

Adult↗

PubMed1998

Heading detection using motion templates and eye velocity gain fields.

Eye or head rotation would influence perceived heading direction if it were coded by cells tuned only to retinal flow patterns that correspond to linear self-movement. We propose a model for heading detection based on motion templates that are also Gaussian-tuned to the amount of rotational flow. Such retinal flow templates allow explicit use of extra-retinal signals to create templates tuned to head-centric flow as seen by the stationary eye. Our model predicts an intermediate layer of 'eye velocity gain fields' in which 'rate-coded' eye velocity is multiplied with responses of templates sensitive to specific retinal flow patterns. By combination of the activities of one retinal flow template and many units with an eye velocity gain field, a new type of unit appears: its preferred retinal flow changes dynamically in accordance with the eye rotation velocity. This unit's activity becomes thereby approximately invariant to the amount of eye rotation. The units with eye velocity gain fields from the motion-analogue of the units with eye position gain fields found in area 7a, which according to our general approach, are needed to transform position from retino-centric to head-centric coordinates. The rotation-tuned templates can also provide rate-coded visual estimates of eye rotation to allow a pure visual compensation for rotational flow. Our model is consistent with psychophysical data that indicate a role for extra-retinal as well as visual rotation signals in the correct perception of heading.

Algorithms↗

PubMed1997

Motion templates with eye velocity gain fields for transformation of retinal to head centric flow.

Heading perception from the optic flow is more difficult during eye rotations than when the eye is stationary, because the centre of the retinal motion identifies the fixation direction rather than the direction of heading. Eye movement signals helps when motion parallax is absent. This paper distinguishes two different possibilities for interactions between eye movement and visual motion signals to perceive heading with a rotating eye. A pre-motion template transformation changes local retinal velocity into head centric velocity. These velocities then feed head centric motion templates. A post-motion template model combines oculomotor signals with retinal motion templates to arrive at head centric flow templates. The latter scheme involves eye velocity gain fields similar to the eye position gain fields as found in area 7a. We propose that the parietal cortex transforms retinal to head centric direction and retinal to head centric flow on the same principle.

Humans↗