Visual representation of malleable and rigid objects that deform as they rotate.

Most studies and theories of object recognition have addressed the perception of rigid objects. Yet, physical objects may also move in a nonrigid manner. A series of priming studies examined the conditions under which observers can recognize novel views of objects moving nonrigidly. Observers were primed with 2 views of a rotating object that were linked by apparent motion or presented statically. The apparent malleability of the rotating prime object varied such that the object appeared to be either malleable or rigid. Novel deformed views of malleable objects were primed when falling within the object's motion path. Priming patterns were significantly more restricted for deformed views of rigid objects. These results suggest that moving malleable objects may be represented as continuous events, whereas rigid objects may not. That is, object representations may be "dynamically remapped" during the analysis of the object's motion.

Surface segmentation cues influence negative priming for novel and familiar shapes.

In a series of experiments, a negative priming paradigm was used to determine how the visual system represents novel shapes under conditions of inattention. Observers in a shape-matching task viewed overlapping shapes with or without surface segmentation cues. Positive priming occurred with opaque and transparent surface-like shapes, whereas negative priming was found with outlined and transparent shapes that lacked surface segmentation cues. This effect generalized to familiar shapes. These results support the importance of segmentation cues in negative priming and suggest that, under otherwise identical conditions, surface segmentation processes can determine whether positive or negative priming occurs in an implicit memory task. Thus, selective attention for overlapping shapes may be best understood in relation to surface segmentation processes.

New aspects of motion perception: selective neural encoding of apparent human movements.

Perception of apparent motion operates somewhat differently for objects and human figures. Depending on the interstimulus interval, the latter d may give rise to either perception of a direct path (i.e. biologically impossible) or indirect path (i.e. biologically possible). Here, PET was used to investigate whether a change in brain activity accompanies this perceptual shift. We found neural encoding of apparent motion to be a function of the intrinsic properties of the stimulus presented (object vs human) as well as the kind of human movement path perceived (biomechanically possible vs impossible). Motor and parietal cortex were only involved for possible motion which suggests that these regions are selectively activated to process actions which conform to the capabilities of the observer.

The visual representation of three-dimensional, rotating objects.

Depth rotations can reveal new object parts and result in poor recognition of "static" objects (Biederman & Gerhardstein, 1993). Recent studies have suggested that multiple object views can be associated through temporal contiguity and similarity (Edelman & Weinshall, 1991; Lawson, Humphreys & Watson, 1994; Wallis, 1996). Motion may also play an important role in object recognition since observers recognize novel views of objects rotating in the picture plane more readily than novel views of statically re-oriented objects (Kourtzi & Shiffrar, 1997). The series of experiments presented here investigated how different views of a depth-rotated object might be linked together even when these views do not share the same parts. The results suggest that depth rotated object views can be linked more readily with motion than with temporal sequence alone to yield priming of novel views of 3D objects that fall in between "known" views. Motion can also enhance path specific view linkage when visible object parts differ across views. Such results suggest that object representations depend on motion processes.

Subconfigurations of the human form in the perception of biological motion displays.

We report four experiments examining processes that contribute to the perception of point-light displays of human locomotion. In three experiments, we employed a simultaneous masking paradigm to examine the visual system's use of configural information in global analyses of biological motion displays. In the fourth experiment, we obtained descriptions of our stimulus displays from naive observers. Performance in both the detection and identification studies suggests that the visual system responded equivalently to figures exhibiting any organization of limbs that is consistent with the human form. Moreover, the subconfigurations best detected were also most likely to be described independently as depicting a human figure. Thus our findings provide evidence that the visual system can exploit characteristic subconfigurations of the human form in the perception of human locomotion.

Dynamic representations of human body movement.

Psychophysical and neurophysiological studies suggest that human body motions can be readily recognized. Human bodies are highly articulated and can move in a nonrigid manner. As a result, we perceive highly dissimilar views of the human form in motion. How does the visual system integrate multiple views of a human body in motion so that we can perceive human movement as a continuous event? The results of a set of priming experiments suggest that motion can readily facilitate the linkage of different views of a moving human. Positive priming was found for novel views of a human body that fell within the path of human movement. However, no priming was observed for novel views outside the path of motion. Furthermore, priming was restricted to those views that satisfied the biomechanical constraints of human movement. These results suggest that visual representation of human movement may be based upon the movement limitations of the human body and may reflect a dynamic interaction of motion and object-recognition processes.

The perception of biological motion across apertures.

To understand the visual analysis of biological motion, subjects viewed dynamic, stick figure renditions of a walker, car, or scissors through apertures. As a result of the aperture problem, the motion of each visible edge was ambiguous. Subjects readily identified the human figure but were unable to identify the car or scissors through invisible apertures. Recognition was orientation specific and robust across a range of stimulus durations, and it benefited from limb orientation cues. The results support the theory that the visual system performs spatially global analyses to interpret biological logical motion displays.

Configural processing in the perception of apparent biological motion.

In classic demonstrations of apparent motion, observers typically report seeing motion along the shortest possible path between 2 sequentially presented objects. However, when realistic photographs of a human body are sequentially presented at slow temporal rates, observers report paths of apparent motion that are consistent with the movement limitations of the human body even when those paths are not the shortest possible. The current set of experiments examined those aspects of the human form that lead to the perception of biomechanically consistent paths of motion. The authors' findings suggest that the perception of apparent biological motion extends to human movements that involve inanimate objects. The authors also report that observers can perceive apparent motion of nonbiological objects in a manner similar to apparent motion of human bodies. However, a global hierarchy of orientation and position cues resembling the human form is required for the perception of these paths.

Increased motion linking across edges with decreased luminance contrast, edge width and duration.

Accurate interpretations of image require the segmentation of motion signals produced by different objects with the simultaneous integration of motion signals produced by the same object. We investigated a motion integration paradigm in which the direction of an object's motion could only be determined from an integration of motion signals across the disconnected object edges. In a series of experiments we show that observers' ability to determine object motion depends significantly upon stimulus duration, luminance contrast and edge width. These effects suggest that the visual system, after some delay, relies upon relatively thick, luminance defined contour discontinuities to segment moving images.

Disambiguating velocity estimates across image space.

A translating homogeneous edge viewed through an aperture is an ambiguous stimulus, while a translating edge discontinuity is unambiguous. Under what conditions does the visual system use unambiguous velocity estimates to interpret ambiguous velocity estimates? We considered a translating rectangle visible through a set of stationary apertures. One aperture displayed a rectangle edge while the other apertures displayed corners. Observers reported the direction in which the edge appeared to translate. The results suggest that collinearity and terminator proximity determine whether the unambiguous corner velocity was used to interpret the ambiguous edge velocity. These results suggest some of the ways in which the visual system controls the integration of velocity estimates across image space.

Motion integration across differing image features.

To interpret the projected image of a moving object, the visual system must integrate motion signals across different image regions. Traditionally, researchers have examined this process by focusing on the integration of equally ambiguous motion signals. However, when the motions of complex, multi-featured images are measured through spatially limited receptive fields, the resulting motion measurements have varying degrees of ambiguity. In a series of experiments, we examine how human observers interpret images containing motion signals of differing degrees of ambiguity. Subjects judged the perceived coherence of images consisting of an ambiguously translating grating and an unambiguously translating random dot pattern. Perceived coherence of the dotted grating depended upon the degree of concurrence between the velocities of the grating terminators and dots. Depth relationships also played a critical role in the motion integration process. When terminators were suppressed with occlusion cues, coherence increased. When dots and gratings were presented at different depth planes, coherence decreased. We use these results to outline the conditions under which the visual system uses unambiguous motion signals to interpret object motion.

Perceived speed of moving lines depends on orientation, length, speed and luminance.

In this study, the perceived speed of a tilted line translating horizontally (for a duration of 167 msec) is evaluated with respect to a vertical line undergoing the same translation. Perceived speed of the oblique line is shown to be underestimated when compared to the vertical line. This bias increases: (1) when the line is further tilted, (2) with greater line lengths, (3) with lower contrasts, and finally (4) with a speed of 2.1 deg/sec as compared to a higher speed of 4.2 deg/sec. These results may be accounted for by considering that two velocity signals are used by the visual system to estimate the speed of the line: the translation of this line (this signal does not depend on the line's orientation) and the motion component normal to the line (this signal depends on orientation). We suggest that these two signals are encoded by different types of units and that the translation signal is specifically extracted at the line endings. We further suggest that these signals are integrated by a weighted average process according to their perceptual salience. Other interpretations are considered at the light of current models dealing with the two-dimensional integration of different velocity signals.

Different motion sensitive units are involved in recovering the direction of moving lines.

We studied direction discrimination for lines moving obliquely relative to their orientation. Manipulating contrast, length and duration of motion, we found systematic errors in direction discrimination at low contrast, long length and/or short durations. These errors can be accounted for by a competition between ambiguous velocity signals originating from contour motion processing units and signals from line terminator processing units. The dynamic of this competition can be described by a simple model involving two different classes of processing units with different contrast thresholds, different integration time constants and different levels of response saturation.

The influence of terminators on motion integration across space.

Individual motion measurements are inherently ambiguous since the component of motion parallel to a homogeneous translating edge cannot be measured. Numerous models have proposed that the visual system solves this ambiguity through the integration of motion measurements across disparate contours. To examine this proposal, subjects observed a translating diamond through four stationary apertures. Since the diamond's motion could not be determined from any single contour, motion integration across contours was required to determine the diamond's direction of motion. We demonstrate that observers have difficulty accurately integrating motion information across space. Performance improved when the diamond stimulus was presented at 7 degrees eccentricity, through jagged apertures, or at low contrast. Taken together, these results imply that integration across space is more likely when the motion of contour terminators is less salient or reliable.

Percepts of rigid motion within and across apertures.

Humans consistently err in their percepts of rotational motion viewed through an aperture. Such errors provide insight into the constraints observers use to interpret retinal images. In the 1st of 2 experiments, Ss consistently perceived the fixed center of rotation for an unmarked line viewed through an aperture as located on the line, regardless of its actual location. Accuracy greatly improved with visible line endings. This finding was extended to explain why a square appears nonrigid when it rotates behind a partial occluder. This illusion may result from observers misperceiving the center of rotation of the unmarked square sides. In this situation, Ss seemed unable to apply an object rigidity constraint across apertures. These findings support a conceptualization of the visual system in which consistent local information must be clearly present before prior knowledge can be used to interpret retinal stimulation.

Comparison of cube rotations around axes inclined relative to the environment or to the cube.

Observers judged whether 2 successive computer-displayed rotations of a cube were the same or different. With respect to the observers, each rotation was about a vertical axis (Y), a horizontal (line-of-sight) axis (Z), an axis tilted just 10 degrees from vertical or horizontal, or a maximally oblique axis. Independently, with respect to the cube, each rotation was about a symmetry axis through opposite faces (F) or through opposite corners (C), an axis tilted 10 degrees from one of these symmetry axes, or an axis of extreme nonsymmetry. Speed and accuracy of comparison decreased as the axes of the successive rotations departed from the canonical axes of the environment (Z, or especially, Y), or even more sharply, from the symmetry axes of the cube (C, or especially, F). The internalized principles that guide the perceptual representation of rigid motions evidently are ones of kinematic geometry more than of physics.