Frank E. Pollick

Perceiving affect from arm movement

We examined the visual perception of affect from point-light displays of arm movements. Two actors were instructed to perform drinking and knocking movements with ten different affects while the three-dimensional positions of their arms were recorded. Point-light animations of these natural movements and phase-scrambled, upside-down versions of the same knocking movements were shown to participants who were asked to categorize the affect of the display. In both cases the resulting confusion matrices were analyzed using multidimensional scaling. For the natural movements the resulting two-dimensional psychological space was similar to a circumplex with the first dimension appearing as activation and the second dimension as pleasantness. For the scrambled displays the first dimension was similar in structure to that obtained for the natural movements but the second dimension was not. With both natural and scrambled movements Dimension 1 of the psychological space was highly correlated to the kinematics of the movement. These results suggest that the corresponding activation of perceived affect is a formless cue that relates directly to the movement kinematics while the pleasantness of the movement appears to be carried in the phase relations between the different limb segments.

Vision in autism spectrum disorders

Autism spectrum disorders (ASDs) are developmental disorders which are thought primarily to affect social functioning. However, there is now a growing body of evidence that unusual sensory processing is at least a concomitant and possibly the cause of many of the behavioural signs and symptoms of ASD. A comprehensive and critical review of the phenomenological, empirical, neuroscientific and theoretical literature pertaining to visual processing in ASD is presented, along with a brief justification of a new theory which may help to explain some of the data, and link it with other current hypotheses about the genetic and neural aetiologies of this enigmatic condition.

Gender recognition from point-light walkers.

Point-light displays of human gait provide information sufficient to recognize the gender of a walker and are taken as evidence of the exquisite tuning of the visual system to biological motion. The authors revisit this topic with the goals of quantifying human efficiency at gender recognition. To achieve this, the authors first derive an ideal observer for gender recognition on the basis of center of moment (J. E. Cutting, D. R. Proffitt, & L. T. Kozlowski, 1978) and, with the use of anthropometric data from various populations, show optimal recognition of approximately 79% correct. Next, they perform a meta-analysis of 21 experiments examining gender recognition, obtaining accuracies of 66% correct for a side view and 71% for other views. Finally, results of the meta-analysis and the ideal observer are combined to obtain estimates of human efficiency at gender recognition of 26% for the side view and 47% for other views.

Exaggerating Temporal Differences Enhances Recognition of Individuals from Point Light Displays

Humans are very good at perceiving each others movements. In this article, we investigate the role of time-based information in the recognition of individuals from point light biological motion sequences. We report an experiment in which we used an exaggeration technique that changes temporal properties while keeping spatial information constant; differences in the durations of motion segments are exaggerated relative to average values. Participants first learned to recognize six individuals on the basis of a simple, unexaggerated arm movement. Subsequently, they recognized positively exaggerated versions of those movements better than the originals. Absolute duration did not appear to be the critical cue. The results show that time-based cues are used for the recognition of movements and that exaggerating temporal differences improves performance. The results suggest that exaggeration may reflect general principles of how diagnostic information is encoded for recognition in different domains.

A motion capture library for the study of identity, gender, and emotion perception from biological motion.

We present the methods that were used in capturing a library of human movements for use in computeranimated displays of human movement. The library is an attempt to systematically tap into and represent the wide range of personal properties, such as identity, gender, and emotion, that are available in a person’s movements. The movements from a total of 30 nonprofessional actors (15 of them female) were captured while they performed walking, knocking, lifting, and throwing actions, as well as their combination in angry, happy, neutral, and sad affective styles. From the raw motion capture data, a library of 4,080 movements was obtained, using techniques based on Character Studio (plug-ins for 3D Studio MAX, AutoDesk, Inc.), MATLAB (The Math Works, Inc.), or a combination of these two. For the knocking, lifting, and throwing actions, 10 repetitions of the simple action unit were obtained for each affect, and for the other actions, two longer movement recordings were obtained for each affect. We discuss the potential use of the library for computational and behavioral analyses of movement variability, of human character animation, and of how gender, emotion, and identity are encoded and decoded from human movement.

Recognising the style of spatially exaggerated tennis serves.

A technique for the construction of exaggerated human movements was developed and its effectiveness tested for the case of categorising tennis serves as flat, slice, or topspin. The technique involves treating movements as points in a high-dimensional space and uses average movements as the basis for constructing exaggerated movements. Exaggerated movements of a particular style are defined as those points in the space of movements which lie on a line originating at the style average and in the direction defined by the difference between the style average and the grand average. In order to visualise the movements, computer animation techniques were employed to transform the three-dimensional coordinates of the movement into the motion of a solid-body figure. These solid-body models were used in perceptual experiments to assess the effectiveness of the exaggeration technique. After an initial training session on the exemplars from the original library, subjects viewed the synthetic tennis-serve motions and in two separate sessions either made three-alternative, categorisation judgments after viewing a single serve or rated dissimilarity after viewing a pair of serves. Results from both accuracy in the categorisation task and structure of a multidimensional scaling solution of the matrix of dissimilarities indicated that, as distance from the grand average increased, the service motion became more distinct and more accurately identified.

Discriminating rigid from nonrigid motion: Minimum points and views

Theoretical investigations of structure from motion have demonstrated that an ideal observer can discriminate rigid from nonrigid motion from two views of as few as four points. We report three experiments that demonstrate similar abilities in human observers: In one experiment, 4 of 6 subjects made this discrimination from two views of four points; the remaining subjects required five points. Accuracy in discriminating rigid from nonrigid motion depended on the amount of nonrigidity (variance ofthe interpoint distances overviews) in the nonrigid structure. The ability to detect a rigid group dropped sharply as noise points (points not part of the rigid group) were added to the display. We conclude that human observers do extremely well in discriminating between nonrigid and fully rigid motion, but that they do quite poorly at segregating points in a display on the basis of rigidity.

When knowing can replace seeing in audiovisual integration of actions

The ability to predict the effects of actions is necessary to behave properly in our physical and social world. Here, we describe how the ability to predict the consequence of complex gestures can change the way we integrate sight and sound when relevant visual information is missing. Six drummers and six novices were asked to judge audiovisual synchrony for drumming point-light displays where the visual information was manipulated to eliminate or include the drumstick-drumhead impact point. In the condition with only the arm information novices were unable to detect asynchrony whereas drummers were able to. Additionally, in the conditions that included the impact point drummers perceived the best alignment when the sight preceded the sound, while in the arm only condition they perceived the best alignment when the sound occurred together with or preceded the sight, as it would be expected if they were predicting the beat occurrence. Taken together these findings suggest that humans can acquire, through practice, internal models of action which can be used to replace missing information when integrating multisensory signals from the environment.

Recognising facial expression from spatially and temporally modified movements.

We examined how the recognition of facial emotion was influenced by manipulation of both spatial and temporal properties of 3-D point-light displays of facial motion. We started with the measurement of 3-D position of multiple locations on the face during posed expressions of anger, happiness, sadness, and surprise, and then manipulated the spatial and temporal properties of the measurements to obtain new versions of the movements. In two experiments, we examined recognition of these original and modified facial expressions: in experiment 1, we manipulated the spatial properties of the facial movement, and in experiment 2 we manipulated the temporal properties. The results of experiment 1 showed that exaggeration of facial expressions relative to a fixed neutral expression resulted in enhanced ratings of the intensity of that emotion. The results of experiment 2 showed that changing the duration of an expression had a small effect on ratings of emotional intensity, with a trend for expressions with shorter durations to have lower ratings of intensity. The results are discussed within the context of theories of encoding as related to caricature and emotion.

Constant affine velocity predicts the 1/3 power law of planar motion perception and generation.

Numerous studies have shown that the power of 1/3 is important in relating Euclidean velocity to radius of curvature (R) in the generation and perception of planar movement. Although the relation between velocity and curvature is clear and very intuitive, no valid explanation for the specific 1/3 value has yet been found. We show that if instead of computing the Euclidean velocity we compute the affine one, a velocity which is invariant to affine transformations, then we obtain that the unique function of R which will give (constant) affine invariant velocity is precisely R1/3. This means that the 1/3 power law, experimentally found in the studies of hand-drawing and planar motion perception, implies motion at constant affine velocity. Since drawing/perceiving at constant affine velocity implies that curves of equal affine length will be drawn in equal time, we performed an experiment to further support this result. Results showed agreement between the 1/3 power law and drawing at constant affine velocity. Possible reasons for the appearance of affine transformations in the generation and perception of planar movement are discussed.