Mike G. Harris

Human sensitivity to expanding and rotating motion: effects of complementary masking and directional structure

A comparison of sensitivity to expanding, rotating, translating and random motion suggests the existence of specialised mechanism for the detection of expansion and rotation. Complementary masking shows that the detection of expansion is unaffected by the presence of rotation, and vice versa. These results are interpreted in terms of a Relative Motion System, which combines the outputs of localised motion detectors in a variety of ways, and which functions to analyse complex image motion into simpler, more useful, components.

The Roles of Inducer Size and Distance in the Ebbinghaus Illusion (Titchener Circles)

Although the Ebbinghaus illusion is commonly used as an example of a simple size-contrast effect, previous studies have emphasised its complexity by identifying many factors that potentially influence the magnitude of the illusion. Here, in a series of three experiments, we attempt to simplify this complexity. In each trial, subjects saw a display comprising, on one side, a target stimulus surrounded by inducers and, on the other, an isolated probe stimulus. Their task was to indicate whether the probe appeared larger or smaller than the target. Probe size was adjusted with a one-up, one-down staircase procedure to find the point of subjective equality between probe and target. From these experiments, we argue that the apparent effects of inducer size are often confounded by the relative completeness of the inducing surround and that factors such as the similarity of the inducers and target are secondary. We suggest a simple model that can explain most of the data in terms of just two primary and independent factors: the relative size of the inducers and target, and the distance between the inducers and the target. The balance between these two factors determines whether the size of the target is underestimated or overestimated.

Is Optic Flow Used to Guide Walking While Wearing a Displacing Prism

Rushton et al (1998 Current Biology 8 1191–1194) recently showed that walkers wearing displacing prisms follow curved trajectories determined by the perceived direction of their target. This suggests that optic flow is not important in guidance, since flow cues are unaffected by the prism and should allow a straight, direct trajectory. We replicated Rushton et als result but also tried to rule out an important artifact associated with the prism. Prisms restrict the field of view and, particularly, access to the foreground optic flow that is likely to be important in providing guidance cues. We found that performance did not improve when walkers directed their gaze to include the foreground flow, suggesting that Rushton et als results were not due to this artifact. On the other hand, performance did reliably improve when subjects reduced their viewing height by crawling towards the target. This improvement may be due to coarsening of the visual texture or to increased salience of alignment and motion-parallax cues. Whatever its cause, the improvement demonstrates that guidance is not determined only by perceived target direction and that, under some conditions, flow may be important.

The perception of moving stimuli: A model of spatiotemporal coding in human vision

An empirically plausible extension of the Marr and Ullman (1981) [Proc. R. Soc. Lond. B 211, 151-180 (1981)] model for coding movement direction uses the magnitude of spatial and temporal response derivatives to encode the speed as well as the direction of movement. The performance of the model is illustrated for drifting edges of different spatial scales and the properties of the components which separately compute the two derivatives are respectively compared to individual channels of the conventional pattern and flicker systems.

Tau as a potential control variable for visually guided braking

D. N. Lee (1976) described a braking strategy based on optical expansion in which the driver brakes so that the targets time-to-contact declines around a constant slope in the range -0.5 < or = tau < 0. The present results from a series of braking simulations confirm and extend earlier reports (E. H. Yilmaz & W. H. Warren, 1995) that performance is broadly compatible with the tau hypothesis. However, performance was not enhanced in situations that favored the estimation of tau, and unlike in earlier reports, performance deteriorated in the absence of a ground plane that provided information about speed and target distance. This finding suggests that the tau hypothesis does not provide a complete account of braking control.

Independent Detectors for Expansion and Rotation, and for Orthogonal Components of Deformation

It is well known that optic flow—the smooth transformation of the retinal image experienced by a moving observer—contains valuable information about the three-dimensional layout of the environment. From psychophysical and neurophysiological experiments, specialised mechanisms responsive to components of optic flow (sometimes called complex motion) such as expansion and rotation have been inferred. However, it remains unclear (a) whether the visual system has mechanisms for processing the component of deformation and (b) whether there are multiple mechanisms that function independently from each other. Here, we investigate these issues using random-dot patterns and a forced-choice subthreshold summation technique. In experiment 1, we manipulated the size of a test region that was permitted to contain signal and found substantial spatial summation for signal components of translation, expansion, rotation, and deformation embedded in noise. In experiment 2, little or no summation was found for the superposition of orthogonal pairs of complex motion patterns (eg expansion and rotation), consistent with probability summation between pairs of independent detectors. Our results suggest that optic-flow components are detected by mechanisms that are specialised for particular patterns of complex motion.

Induced motion may account for the illusory transformation of optic flow fields found by Duffy and Wurtz.

Duffy and Wurtz [(1993) Vision Research, 33, 1481-1490] found an illusory shift in the position of the focus of expansion (FOE) of random dot patterns when planar motion was superimposed on expanding radial motion. Subjects indicated that this illusory shift was in the direction of the planar motion. This is the opposite direction to a true shift in the FOE that is perceived when the planar motion is vector summed with the expanding motion. We account for this illusion by suggesting that planar motion induces opposite motion on the expanding dots which after vector summation produces the illusory shift in the FOE. We use a matching technique with a method of adjustment to measure induced motion and perceived FOE in moving random dot patterns and present the results in support of our assertion.

On the Relationship Between Deformation and Perceived Surface Slant

A compelling impression of surface slant is produced by random dot displays depicting deformation and translation alone. A simple model of slant estimation based upon deformation is shown to capture quantitatively both the perceived slant in this situation and the distortion in perceived slant produced when constant deformation is added to random dot displays depicting moving slanted surfaces. The results confirm that deformation provides a simple account of perceived slant.

Speed gradients and the perception of surface slant: Analysis is two-dimensional not one-dimensional

Motion parallax provides cues to the three-dimensional layout of a viewed scene and, in particular, to surface tilt and slant. For example, as a textured surface, inclined around a horizontal axis, translates horizontally relative to an observers view point, then, in the absence of head and eye movements, the observers retinal flow will contain a one-dimensional (1D) vertical speed gradient. The direction of this gradient indicates the direction of surface tilt, and its magnitude and sign can be used in calculating the magnitude and sign of the surface slant. Alternatively, the same retinal flow contains a 1D translating component, plus a two-dimensional (2D) component of rotation (curl), and a 2D component of deformation (def). On this view, the direction of surface tilt is related to the orientation of def and the magnitude and sign of the surface slant is related to the magnitude and sign of def. We used computer generated random dot patterns as stimuli to determine whether the human visual system employs a 1D analysis (i.e. 1D speed gradients) or a 2D analysis (i.e. deformation) of surface slant from motion parallax. Using a matching technique we found compelling impressions of slant when we vector summed a translation field with (i) vertical shear, horizontal shear or deformation (made from vertical and horizontal shear), but not rotation; and (ii) vertical compression, horizontal compression or deformation (made from vertical and horizontal compression), but much less so for expansion. In both cases, the first three conditions contain def, but the fourth does not, and the last three conditions contain 1D speed gradients orthogonal to the perceived axis of inclination, but the first one does not. Therefore, the results from the first and fourth conditions distinguish between the two processing strategies. They support the idea that surface slant is coded by combining both horizontal and vertical speed gradients in a way similar to the 2D differential invariant def and oppose the view that surface slant is encoded by a 1D analysis of motion in a direction orthogonal to the perceived axis of inclination. In a further experiment, we found essentially no effect of reducing the field size from 18 to 9 deg.

Broad direction bandwidths for complex motion mechanisms

Growing evidence from psychophysics and single-unit recordings suggests specialised mechanisms in the primate visual system for the detection of complex motion patterns such as expansion and rotation. Here we used a subthreshold summation technique to determine the direction tuning functions of the detecting mechanisms. We measured thresholds for discriminating noise and signal+noise for pairs of superimposed complex motion patterns (signal A and B) carried by random-dot stimuli in a circular 5 degrees field. For expansion, rotation, deformation and translation we found broad tuning functions approximated by cos(d), where d is the difference in dot directions for signal A and B. These data were well described by models in which either: (a) cardinal mechanisms had direction bandwidths (half-widths) of around 60 degrees; or (b) the number of mechanisms was increased and their half-width was reduced to about 40 degrees. When d=180 degrees we found summation to be greater than probability summation for expansion, rotation and translation, consistent with the idea that mechanisms for these stimuli are constructed from subunits responsive to relative motion. For deformation, however, we found sensitivity declined when d=180 degrees, suggesting antagonistic input from directional subunits in the deformation mechanism. This is a necessary property for a mechanism whose job is to extract the deformation component from the optic flow field.