Perception of biological motion from size-invariant body representations

Motion information plays only a secondary role in sex identification of walking persons in frontal view

Observers have a success rate above chance in identifying the sex of walking persons on the basis of movies showing only point lights. It has been claimed that observers rely heavily on motion information for their judgment. Here, we studied, for the frontal plane, the added value of motion information over just form information. In the first experiment, we asked 209 observers to identify the sex of frontal-plane still images of point lights of six male and six female walkers. We used two types of point-light images: (1) cloud-like images, showing just point lights, and (2) skeleton-like images with point lights interconnected. On the basis of cloud-like still images, observers had a mean success rate of 63%; on the basis of skeleton-like still images, they had a higher mean success rate of 70% (p < 0.001). In the second experiment, we asked 273 observers to identify the sex of skeleton-like still images and skeleton-like movies of eight full walking strides. The overall success rate based on movies was 73%. Among the observers first presented with still images, the success rate based on still images was 68%, but for observers first presented with movies the success rate based on still images was 74%, not different from that based on movies (p > 0.05). Our interpretation was that motion information revealed what the point lights represented but had no additional value when this became clear. Hence, we concluded that motion information plays only a secondary role in sex identification of walking persons in the frontal plane.

Extended visuomotor experience with inverted movements can overcome the inversion effect in biological motion perception

Studies have demonstrated that perceiving human and animal movements as point-light displays is effortless. However, simply inverting the display can significantly impair this ability. Compared to non-dancers and typical dancers, vertical dancers have the unique experience of observing and performing movements upside down as being suspended in the air. We studied whether this unique visuomotor experience makes them better at perceiving the inverted movements. We presented ten pairs of dance movements as point-light displays. Each pair included a version performed on the ground whereas the other was in the air. We inverted the display in half of the trials and asked vertical dancers, typical dancers, and non-dancers about whether the display was inverted. We found that only vertical dancers, who have extended visual and motor experience with the configural and dynamic information of the movements, could identify the inversion of movements performed in the air. Neither typical dancers nor non-dancers, who have no motor experience with performing the inverted movements, could detect the inversion. Our findings suggest that motor experience plays a more critical role in enabling the observers to use dynamic information for identifying artificial inversion in biological motion.

Perception and prediction of the putting distance of robot putting movements under different visual/viewing conditions

The purpose of this paper is to examine, whether and under which conditions humans are able to predict the putting distance of a robotic device. Based on the “flash-lag effect” (FLE) it was expected that the prediction errors increase with increasing putting velocity. Furthermore, we hypothesized that the predictions are more accurate and more confident if human observers operate under full vision (F-RCHB) compared to either temporal occlusion (I-RCHB) or spatial occlusion (invisible ball, F-RHC, or club, F-B). In two experiments, 48 video sequences of putt movements performed by a BioRob robot arm were presented to thirty-nine students (age: 24.49±3.20 years). In the experiments, video sequences included six putting distances (1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 m; experiment 1) under full versus incomplete vision (F-RCHB versus I-RCHB) and three putting distances (2. 0, 3.0, and 4.0 m; experiment 2) under the four visual conditions (F-RCHB, I-RCHB, F-RCH, and F-B). After the presentation of each video sequence, the participants estimated the putting distance on a scale from 0 to 6 m and provided their confidence of prediction on a 5-point scale. Both experiments show comparable results for the respective dependent variables (error and confidence measures). The participants consistently overestimated the putting distance under the full vision conditions; however, the experiments did not show a pattern that was consistent with the FLE. Under the temporal occlusion condition, a prediction was not possible; rather a random estimation pattern was found around the centre of the prediction scale (3 m). Spatial occlusion did not affect errors and confidence of prediction. The experiments indicate that temporal constraints seem to be more critical than spatial constraints. The FLE may not apply to distance prediction compared to location estimation.

Action Recognition and Movement Direction Discrimination Tasks Are Associated with Different Adaptation Patterns

The ability to discriminate between different actions is essential for action recognition and social interactions. Surprisingly previous research has often probed action recognition mechanisms with tasks that did not require participants to discriminate between actions, e.g., left-right direction discrimination tasks. It is not known to what degree visual processes in direction discrimination tasks are also involved in the discrimination of actions, e.g., when telling apart a handshake from a high-five. Here, we examined whether action discrimination is influenced by movement direction and whether direction discrimination depends on the type of action. We used an action adaptation paradigm to target action and direction discrimination specific visual processes. In separate conditions participants visually adapted to forward and backward moving handshake and high-five actions. Participants subsequently categorized either the action or the movement direction of an ambiguous action. The results showed that direction discrimination adaptation effects were modulated by the type of action but action discrimination adaptation effects were unaffected by movement direction. These results suggest that action discrimination and direction categorization rely on partly different visual information. We propose that action discrimination tasks should be considered for the exploration of visual action recognition mechanisms.