Biological motion is the visual system's ability to perceive object movement by connecting a few small, individual stimuli. Humans use biological motion to identify familiar activities and gestures, which is useful for social interaction. Biological motion processing is a highly routine instance of more general perceptual and attentional processes that construct global form information from constituent parts. Currently, research is being done to uncover the brain structures which allow for biological motion processing.
The phenomenon was first documented by Swedish perceptual psychologist, Gunnar Johansson, in 1973. Early work suggested that the brain may contain mechanisms specialised for the detection of other humans from motion signals, but over the years this claim has been scaled down to the point where some authors now suggest that we have more generalised detectors tuned simply to the characteristic signal generated by the feet of a locomoting animal.
An experiment by Cutting and Kozlowski found that individuals are better at identifying themselves compared to others when looking at gait on a point-light display. In a point-light display, participants are presented with a static or dynamic visual that consists of small, circular light sources placed on the major joints that allow movement for humans. Although these lights are individual, participants are able to perceive motion in the dynamic visuals.  Participants are able to recognize different emotions from the point light-displays. With special attention to body language, one can identify anger, sadness, and happiness. Also, gender is identifiable from these displays.
Humans perception of biological motion matures with age, usually capping at around five years of age. In an experiment by Pavlova et al., 2001, three-year olds, four-year olds, five-year olds, and adults were asked to identify animals (walking human, running dog, flying bird, and walking dog) displayed with moving point-light displays. The adults and five-year olds were able to accurately identify the moving animal, while four-year olds and three-year olds struggled (although 4-year olds were significantly better than 3-year olds). This implies that our perception of motion gets better as we age and reach the cap at five years. While animals tend to recognize their own species over others (for example, cats recognize their own point-light displays over scrambled ones), the three-year olds had the most success identifying a walking dog and least with a walking human. A possible explanation of this might be because of the children's small physical stature and their resulting visual perspectives: dogs are on the same horizontal plane as small kids, while human walkers are a lot taller and harder to perceive. In the next part of the experiment, different participants were asked to identify the same point-light display animals but with static images instead of moving dots. Five-year olds and adults gave results of chance performance, while the younger participants were omitted due to the harder nature of the task. Therefore, this experiment suggests that at five-years old, we are adept at identifying animals with point-light displays and that motion is critical to how we perceive it.
Humans use similar cognitive functions in identifying real verbs and replicable motions. An experiment by Christel Bidet-Ildei and Lucette Toussaint gave participants a lexical and an action decision task to measure how long it took them to identify whether the words were real or the action doable. Participants took much longer to identify pseudo words and actions. The correlation between verbs and actions was found to be rather strong (r= 0.56), while the correlation between nouns and actions was much lower (r= 0.31). This suggests that humans use similar cognitive functions to identify motion, whether it is presented through written language or point-light displays. The author suggests that these findings are in favor of the embodied theory, which states that the processing of actions and words use similar motor functions.
Recent research has begun to focus on the brain structures that are necessary for this processing. Use of Transcranial Magnetic Stimulation gave evidence to suggest that biological motion processing is a dual process that looks at both form and motion, located outside of the MT+/V5 area. Further evidence from another study show that the Default Mode Network is essential in distinguishing between biological and non-biological motion. Such experiments indicate that biological motion perception is a process that pulls on several different brain systems even outside of the usual visual processing structures.
Other research looks into the differences between global and local processing of biological motion. One study investigated the contradiction by replacing the local dots of point-light displays with human images or stick figures; the results showed that the brain allocates fewer resources to the global form processing when the local elements are complex, indicating that the brain uses a similar form-based mechanism for the recognition of both global and local stimuli during processing. The results also show that processing local images is an automatic process that interferes with the subsequent processing of the global form of the stimulus.
Perception of biological motion depends both on the motions of individual dots and the configuration/orientation of the body as a whole, as well as interactions between these local and global cues. Similar to the Thatcher Effect in face perception, inversion of individual points is easy to detect when the entire figure is presented normally, but difficult to detect when the entire display is presented upside-down. However, recent electrophysiological work suggest that the configuration/orientation of the figure might be more important than the figure's motion, at least for early levels of processing.
The superior temporal sulcus is known to be activated for biological motion perception. Also, premotor cortex is important, which indicates that the mirror neuron system is recruited for "filling in" the dots.
In a large study with stroke patients, regions that emerged to be statistically associated with deficient biological motion perception included the superior temporal lobe sulcus and premotor cortex. The cerebellum also is important.
A recent study on a patient with developmental agnosia found intact biological motion, but deficient perception of non-biological form from motion.
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