Jaap A. Beintema

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.

The influence of biological motion perception on structure-from-motion interpretations at different speeds.

Nonrigid point-light representations of biological motion are ideal to test higher level influences on structure-from-motion (SFM) perception. Here, we investigated the influence of biological motion perception on 3D SFM interpretations at different speeds. We presented nonrigid biological motion and rigid structures rotating around the vertical axis. The familiarity of the stimuli was changed by presenting three walker types: normal, inverted, and phase scrambled. Subjects had to discriminate rotation in depth and rigidity. We found that at lower-than-natural gait speeds, subjects perceived nonrigid biological motion to be rotating in depth, especially when the walker type was less familiar. In contrast, the percept of rigidity was correct at all speeds. A second experiment, in which a constant fraction of the gait cycle was presented, confirmed the influence of speed and additionally showed that brief displays of a familiar form at a high speed facilitate biological motion interpretations. The more veridical percept of rotation toward higher speeds fits the idea of biological motion channels tuned to higher-more natural walking-speeds that overrule a default assumption to perceive trajectories in depth. We also speculate that the rotation-in-depth percept at lower speeds points toward the existence of low-speed-tuned object motion channels.

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 observers 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.

Circular receptive field structures for flow analysis and heading detection

Recent years have brought forward different models on how the brain might encode heading from optic flow. Neurons in these models can encode heading for a variety of self-motion conditions, while responding to optic flow stimuli similarly as found in electrophysiological studies. Yet, little attention has been given to the receptive field structure of neurons that integrate local motion signals to analyze the optic flow. Intuitively, radial structures might seem suited for the task of heading detection, since pure observer translation causes flow to emanate from the point of heading (Figure1a). However, rotational flow (Figure 1b) during simultaneous eye rotation can cause the retinal flow to be shifted away from the heading (Figure 1c). Moreover, variation in point distances with respect to the translating eye results in retinal motion differences, called motion parallax, that can cause the flow during eye rotation to deviate even more from being purely radial. This poses the question what would be the optimal receptive field structure to deal with complex flow fields for retrieving heading.

Manipulating saccadic decision-rate distributions in visual search

The Gaussian shape of reciprocal latency distributions typically found in single saccade tasks supports the idea of a race-to-threshold process underlying the decision when to saccade (R. H. Carpenter & M. L. Williams, 1995). However, second and later saccades in a visual search task revealed decision-rate (=reciprocal latency) distributions that were skewed Gamma-like (E. M. Van Loon, I. T. Hooge, & A. V. Van den Berg, 2002). Here we consider a related family of Beta-prime distributions that follows from strong competition with a signal to stop the sequence, and is described by two parameters: a fixate and saccade threshold. In three saccadic search experiments, we tried to manipulate the two thresholds independently, thereby expecting change in shape and mean of the reciprocal latency distribution. Interestingly, rate distributions for later saccades were significantly better fit by Beta-prime than by Gamma functions. Increases in the distributions skew were found with higher display density, but only for second and later saccades. First saccade rate distributions were not altered by the expected target location or by visual information presented prior to the search, but making pre-search saccades did influence both thresholds. The mean rate remained a stereotyped function of ordinal position in the saccade sequence. Our results support strong competition between two decision signals underlying the timing of saccades.