You Can Learn a Lot Just by Looking at How People Move
No Motion, No Vision!
The ability to perceive motion is one of the most fundamental aspects of human vision. The reason for this is that motion can be generated in many ways.
In most environments, some kind of motion is likely to be present: whether it be produced by a travelling vehicle, the gentle swaying of a leaf, a fly buzzing around one's head, running water, etc.
Even when no object in our visual field is physically moving, if we move the image of the visual scene that is projected upon the retina at the back of the eye undergoes continuous motion related change. If we stand still, retinal image motion is frequently generated by the movement of our head, and/or of our eyes. Even when we do not move, keep our head immobile, and try and hold our eyes as steadily fixed as possible, the retinal image will still undergo some changes due to the presence of a variety of so-called 'miniature eye' movements.
It was long assumed that these minuscule, nearly invisible movements of the eyes were just 'physiological noise,' resulting from the inability of our eye muscles to keep the eyes absolutely stationary. More recently, however, it has become clear that a subset of these tiny movements are, in fact, essential in enabling us to see anything at all. Researchers had static observers wear a device which compensated for these movements, thereby removing all motion from the retinal image. After a short period, the visual scene began to disintegrate and finally faded altogether, to be replaced by an empty, 'misty' field of vision. This proved conclusively that in the absence of movement on the retinal image vision itself fails.
Motion is so fundamental a part of our visual experience, that under certain conditions we tend to perceive it even in its absence. I am referring here to the vast domain of illusions of motion. One of the most important in today's world is 'apparent motion'. The most common version of this illusion is experienced whenever we are watching a movie in a theater or on television. What we are being presented with is a succession of still pictures of a scene with a short blank interval between them, the presentation rate of these pictures being about 24 frames per second. Yet, despite the physical absence of any motion on the screen, we experience a continually changing visual scene within which the motion of objects and people is demonstrably indistinguishable from that occurring in real life.
Our visual system is not only exquisitely attuned to the the detection of motion; it also makes use of motion related information to extract from the visual scene other aspects of the information it contains. For instance, we use motion to tease out an object from its background. Many animals rely on camouflage to make themselves less conspicuous to their predators by making the color and texture of their body surface (and sometimes its shape) blend into the background. Yet an animal that has thus made itself nearly undetectable becomes instantly noticeable as soon as it moves. Along with other visual cues, we use motion related information to assess the distance between the various components of the visual environment, and in order to recover the three-dimensionality of an object (recall that the projection of a solid object onto the retina results in a two-dimensional image).
Experience Biological Motion
Biological motion is one of the more remarkable aspects of our ability to use motion to gain information about an object's other properties and activities. This phenomenon was first investigated by Swedish psychologist Gunnar Joahnsson (1973) by devising an ingenious experimental setup.
Johansson had his associates wear a black jumpsuit, to which were attached a few small lights (called point-lights) placed mostly at the joints: that is, at those locations in the body whence motion originates. When a person thus equipped stood still on a totally darkened theater stage, all the observers could perceive was a quasi-random arrangement of luminous dots, such as the one shown in the figure. However, as soon as he or she started to move, carrying out ordinary activities such as walking, running, dancing, playing tennis, etc., the observers had no difficulty recognizing the tasks that the person was engaged in. The observers were also able to establish, based upon the pattern of moving point-lights, whether the person wearing them was male or female, young or old, happy or sad, healthy or sick. A few point-lights attached to a person's face made it possible to identify a person's facial expression, and whether a person was lifting a heavy or a light object.
The link 'Experience Biological Motion' allows you to experience some of these effects for yourself.
What these experiments proved is that motion related cues enable us to acquire all sorts of information when no other visual cue is present. No less remarkable is the efficiency of this process since very few small point-lights are sufficient to perceive biological motion. This shows that the human brain can identify complex objects and activities by using a very small subset of the information which is available in the ordinary environment.
The research by Johansson and others also established that the single most critical factor which enables us to do the task is the coordinated timing of the moving points.
The perception of biological motion has been associated with a very specific region of the brain, the posterior superior temporal sulcus.
Johansson, G. (1973). Visual perception of biological motion and a model for its analysis. Perception and Psychophysics, 14 (2): 201–211