Harry S. Orbach

Maturation of the pattern-reversal VEP in human infants: a theoretical framework

Visual evoked potentials to pattern reversal (PR-VEPs) are used to assess the integrity and maturation of the visual pathways in infants and young children. To establish normal ranges and to facilitate interpolation, we consider the maturation rate of PR-VEPs using published normative data. Curves based on the logistic function (a sigmoid model) are introduced and compared with three other models: (1) the power law function; (2) the sum of two decaying exponentials; and (3) a two-stage linear model. Although methods vary somewhat, remarkable consistency among laboratories is found for the maturation of the major positivity (P1) of PR-VEP. The P1 occurs at approximately 260 ms in neonates and is quite variable. It matures rapidly before 12-14 weeks of age and becomes much less variable. The logistic model provides a parsimonious description of P1 maturation with most rapid maturation at around 6 weeks of age for large patterns and around 9 weeks for small patterns. As inter-laboratory agreement is generally good, the normal ranges based on this model could be used in centres, which do not have their own normative databases for infant VEPs.

Computing feature motion without feature detectors: a model for terminator motion without end-stopped cells.

Pointlike object features such as line-endings, have a privileged position in the computation of the veridical direction of object motion. Experiments confirm that the human visual system relies heavily on such features if they are present. It has been proposed that units such as end-stopped cells might be necessary for the computation of feature motion instead of the simple cells used in plaid motion models. Conventional plaid motion models have not been applied to feature motion. We present here a model, based on ordinary simple cells, using two parallel pathways (Fourier and non-Fourier) for the computation of the direction of two dimensional motion. Although similar in structure to popular models of plaid motion, our model includes a novel scheme for contrast normalisation and incorporates spatial pooling at the level of MT cells. The model predictions are consistent with psychophysical results for plaids. Furthermore, it computes directions within 5 degrees of the physical motion of line-endings. It is shown that the non-Fourier signal is necessary for the computation of veridical motion.

Anisotropy in judging the absolute direction of motion.

The angular dependence of precision measurements is well established as the oblique effect in motion perception. Recently, it has been shown that the visual system also exhibits anisotropic behaviour with respect to accuracy of the absolute direction of motion of random dot fields. This study aimed to investigate whether this angular dependent, directional bias is a general phenomenon of motion perception. Our results demonstrate, for single translating tilted lines viewed foveally, an extraordinary illusion with perceptual deviations of up to 35 degrees from veridical. Not only is the magnitude of these deviations substantially larger than that for random dots, but the general pattern of the illusion is also different from that found for dot fields. Significant differences in the bias, as a function of line tilt and line length, suggest that the illusion does not result from fixed inaccuracies of the visual system in the computation of direction of motion. Potential sources for these large biases are motion integration mechanisms. These were also found to be anisotropic. The anisotropic nature and the surprisingly large magnitude of the effect make it a necessary consideration in analyses of motion experiments and in modelling studies.

Effects of global shape on angle discrimination

Previous studies have been inconclusive as to whether angle discrimination performance can be predicted by the sensitivity of orientation discrimination mechanisms or by that of mechanisms specialised for angle coding. However, these studies have assumed that angle discrimination is independent of the shape of the object of which the angle is a part. This assumption was tested by measuring angle discrimination using angles that were parts of different triangular shapes. Angle discrimination thresholds were lowest when angles were presented in isosceles triangles (sides forming the angle were of identical length). Performance was significantly poorer when angles were presented in scalene triangles (sides of different lengths) and as much as three times worse when the sides forming the angle varied randomly in length between presentations. Comparing orientation discrimination for single lines with angle discrimination for different stimulus conditions (isosceles, scalene and random triangles) leads to conflicting conclusions as to the mechanisms underlying angle perception: line orientation sensitivity correctly predicts angle discrimination for random triangles, but underestimates angle acuity for isosceles triangles. The fact that performance in angle discrimination tasks is strongly dependant on the overall stimulus geometry implies that geometric angles are computed by mechanisms that are sensitive to global aspects of the stimulus.

Global shape versus local feature: An angle illusion

We have shown previously that the precision of angle judgments depends strongly on the global stimulus configuration: discrimination thresholds for angles that form part of isosceles triangles are up to 3 times lower than for those that form part of scalene triangles [Kennedy, G. J., Orbach, H. S., & Loffler, G. (2006). Effects of global shape on angle discrimination. VisionResearch, 46(8-9), 1530-1539]. Here, we investigated whether or not the perceived size of an angle (accuracy) is also affected by the overall shape of which it forms a part. Observers compared the relative sizes of angles contained in isosceles triangles with those of angles in scalene triangles and points of subjective equality were determined. For a reference angle of 60 degrees , angles embedded in isosceles triangles were judged to be on average 14 degrees larger than angles embedded in scalene triangles. This result is largely independent of the reference angle, triangle orientation and triangle size. Moreover, the effect is present whether or not triangles of different shapes enclose the same area, whether or not the side of the triangle opposite the angle is present and whether the triangle is outlined or defined by dots at its vertexes. In sum, our results provide evidence for a novel illusion where an angle embedded in an isosceles triangle is judged substantially larger than the same angle embedded in a scalene triangle. This finding demonstrates that mechanisms for computing angles are sensitive to the context within which angles are presented.

Set-size effects for sampled shapes: experiments and model

The location of imperfections or heterogeneities in shapes and contours often correlates with points of interest in a visual scene. Investigating the detection of such heterogeneities provides clues as to the mechanisms processing simple shapes and contours. We determined set-size effects (e.g., sensitivity to single target detection as distractor number increases) for sampled contours to investigate how the visual system combines information across space. Stimuli were shapes sampled by oriented Gabor patches: circles and high-amplitude RF4 and RF8 radial frequency patterns with Gabor orientations tangential to the shape. Subjects had to detect a deviation in orientation of one element (“heterogeneity”). Heterogeneity detection sensitivity was measured for a range (7–40) of equally spaced (2.3–0.4°) elements. In a second condition, performance was measured when elements sampled a part of the shapes. We either varied partial contour length for a fixed (7) set-size, co-varying inter-element spacing, or set-size for a fixed spacing (0.7°), co-varying partial contour length. Surprisingly, set-size effects (poorer performance with more elements) are rarely seen. Set-size effects only occur for shapes containing concavities (RF4 and RF8) and when spacing is fixed. When elements are regularly spaced, detection performance improves with set-size for all shapes. When set-size is fixed and spacing varied, performance improves with decreasing spacing. Thus, when an increase in set-size and a decrease in spacing co-occur, the effect of spacing dominates, suggesting that inter-element spacing, not set-size, is the critical parameter for sampled shapes. We propose a model for the processing of simple shapes based on V4 curvature units with late noise, incorporating spacing, average shape curvature, and the number of segments with constant sign of curvature contained in the shape, which accurately accounts for our experimental results, making testable predictions for a variety of simple shapes.

Modeling the integration of motion signals across space.

Experiments by Loffler and Orbach on the integration of motion signals across space [J. Opt. Soc. Am. A 20, 1461 (2003)] revealed that both three-dimensional analysis and object interpretation play a much smaller role than previously assumed. These results motivated the quantitative description of a low-level, bottom-up model presented here. Motion is computed in parallel at different spatial sites, and excitatory interactions operate between sites. The strength of these interactions is determined mainly by distance. Simulations correctly predict behavior for a variety of manipulations on multi-aperture stimuli: aligned and skewed lines, different presentation times, different inter-aperture gaps, and different spatial frequencies. However, strictly distance-dependent mechanisms are too simplistic to account for all experimental data. Mismatches for grossly misoriented lines suggest collinear facilitation as a promising extension. Once incorporated, collinear facilitation not only correctly predicts results for misoriented patterns but also accounts for the lack of motion integration between heterogeneous stimuli such as lines and dots.

Motion trajectories and object properties influence perceived direction of motion

Judging the motion of objects is a fundamental task that the visual system executes in everyday life in order for us to navigate and interact safely with our surroundings. A number of strategies have been suggested to explain how the visual system uses motion information from different points of an object to compute veridical directions of motion. These include combining ambiguous signals from object contours via a vector summation (VS) or intersection of constraints (IOC) calculation, pooling information using a maximum likelihood or tracking object features. We measured the perceived direction of motion for a range of cross-shaped stimuli (composed of two superimposed lines) to test how accurately humans perceive their motion and compared data to predictions from these strategies. Crosses of different shapes (defined by the angle between the component lines) translated along 16 directions of motion with constant speed. The crosses either moved along one of their symmetry axes (balanced conditions with line components equidistant to the direction of motion) or had their symmetry axis tilted relative to the motion (unbalanced conditions) Data show reproducible differences between observers, including occasional bimodal behaviour, and exhibit the following common patterns. There is a general dependence on direction of motion: For all conditions, when motion is along cardinal axes (horizontal and vertical), perception is largely veridical. For non-cardinal directions, biases are typically small (<10 deg) when crosses are balanced but large biases occur (≥30 deg) when crosses are tilted relative to their direction of motion. Factors influencing the pattern of biases are the shape and tilt of the cross as well as the proximity of its direction of motion to cardinal axes. The dependence of the biases on the direction of motion is inconsistent with any isotropic mechanisms including VS, IOC, maximum likelihood or feature tracking. Instead, perception is biased by a number of intrinsic properties of the cross and external references. The strength of these cues depends on the type, with elongation producing the strongest weight, and their proximity to the direction of motion. This suggests that the visual system may rely on a number of static cues to improve the known low precision for non-cardinal directions of motion, a process which can, however, result in large perceptual biases in certain circumstances.

Factors affecting motion integration

The perceived direction of motion of a featureless contour inside a circular aperture is always perpendicular to the contours orientation, regardless of its true motion (the aperture problem). This study investigates the circumstances under which unambiguous feature motion (of line terminators, single dots, or truncations of a D6 pattern) in adjacent apertures can alter the perceived direction of such featureless contours. We find that integration mechanisms responsible for motion capture are fairly robust against misorientations and contrast manipulations of individual components, are sensitive to differences in spatial frequencies, and scale with pattern size. Motion capture is not diminished when a D6 profile is substituted for the square-pulse profile of a line and is independent of the visibility of the apertures, indicating that object interpretations and three-dimensional analyses of a scene are less important than has been postulated previously. These results have strong implications for the neuronal hardware underlying the integration of motion signals across space and provide a framework for global motion models.

Adding Rotation to Translation: Percepts and Illusions

This study investigated how the perception of a translating object is affected by rotation. Observers were asked to judge the motion and trajectory of objects that rotated around their centroid while linearly translating. The expected percept, consistent with the actual dynamics used to generate the movie sequences, is that of a translating and rotating object, akin to a tumbling rugby ball. Observers, however, do not always report this and, under certain circumstances, perceive the object to translate on an illusory curved trajectory, similar to a car driving on a curved road. The prevalence of veridical versus nonveridical percepts depends on a number of factors. First, if the objects orientation remains within a limited range relative to the axis of translation, the illusory, curved percept dominates. If the orientation, at any point of the movie sequence, differs sufficiently from the axis of translation, the percept switches to linear translation with rotation. The angle at which the switch occurs is dependent upon a number of factors that relate to an objects elongation and, with it, the prominence of its orientation. For an ellipse with an aspect ratio of 3, the switch occurs at approximately 45°. Higher aspect ratios increase the range; lower ratios decrease it. This applies similarly to rectangular shapes. A line is more likely to be perceived on a curved trajectory than an elongated rectangle, which, in turn, is more likely seen on a curved path than a square. This is largely independent of rotational and translational speeds. Measuring perceived directions of motion at different instants in time allows the shape of the perceived illusory curved path to be extrapolated. This results in a trajectory that is independent of object size and corresponds closely to the actual object orientation at different points during the movie sequence. The results provide evidence for a perceptual transition from an illusory curved trajectory to a veridical linear trajectory (with rotation) for the same object. Both are consistent with special real-world cases such as objects rotating around a centre outside of the object so that their orientation remains tangent to the trajectory (cheetahs running along a curve, sailboats) or objects tumbling along simple trajectories (a monkey spinning in air, spinning cars on ice). In certain cases, the former is an illusion.