This is a statement
s slightly counter-intuitive
. Indeed it is, I suppose, but the statement is not as strange as it first sounds. There are many separate tasks performed by the visual processing
system, one of which serves to detect movement. But what is this movement information useful for?
We find that the perception of motion contributes to a wide range of perceptual tasks. One of the first of these tasks to be identified was that motion perception helps to segment objects and surfaces. This relies of the Gestalt principle of “common fate”. An object can be defined by the coherent and common motion of its different parts. This effect has been demonstrated numerous times in observations a coherent shape in patterns of flowing dots on a display screen, some with a unified motion to define the perceived shape. So motion perception within this context actually has very little to do with seeing movement par se, rather, from movement information we can identify objects in the world.
Motion perception can also be used to recognise objects, particularly other humans and other creatures. We seem to be very sensitive to the dynamic characteristics of human biological movement. This can be demonstrated by attaching lights to the major joints of a person. When that person walks, or otherwise moves, in a dark environment, it is easy to recognise that it is a person moving even from this rather sparse information. It is even possible to identify the sex of the person. We can see the effects of biological motion in everyday life as we can often recognise people from their gait, particularly if it has a relatively distinctive quality to it. In fact, detecting unusual gait is a useful biological mechanism, since it can be used by predators to pick prey weakened because of a limp or other injury.
But it is not only organisms that can be detected and recognised from motion cues – other objects can be discerned as well. For instance, Johannson conducted a series of important demonstrations of point-light displays consisting of random points of light moving in such a way as to mimic the motion patterns caused by the rotation of three-dimensional objects. In his displays, one gets the vivid impression of seeing a three-dimensional rotating cylinder (for instance), when in fact there are no real three-dimensional cues present – indeed, the display is on a flat two-dimensional screen. These effects, in addition to common fate effects, show that motion sensitivity is being used by some parts of the visual system to detect and recognise objects. Further, Johannson’s experiments demonstrate that motion information can be used to detect three-dimensional structures.
The most significant use of motion perception appears to be in a more global context than object recognition. James Gibson identified the importance of optic flow detection in perceptual tasks. Optic flow fields, which seem to be detected by global motion processing areas in area MST, provide a wealth of information about the environment and does not provide a great deal of useful information regarding the movements of individual objects.
Firstly, information about the organism’s self-motion and how this relates to the visual world can be obtained from optic flow. This is done by analysing the expanding patterns of optic flow, where the expansion emanates from a focal point corresponding to the direction in which the viewer is heading. Although, there is some possible ambiguity here as the same effects could be produced by the surfaces of the world moving relative to a stationary observer. This was strikingly shown by David Lee in his special room where a subject taking a step in one direction would cause the entire room to move twice as far in the opposite direction. Even though the subjects knew they were taking steps in one direction, they got a distinct impression they were in fact walking in the opposite direction. Lee also showed how motion perception can effect our posture. In his famous “swinging room” experiment, subjects were placed inside a room which could be swung gently back and forth. Subjects were found to unconsciously adjust their posture exactly in response to the movements of the room. Children, and even adults forced to stand in an unstable posture, could even be made to fall over. This effect can be explained by the unconscious use of optic flow patterns to maintain balance.
Optic flow can also be a great help in identifying the spatial location, and to a certain extent, the shape, of objects in the visual field. The surfaces of objects at different spatial positions will cause fluctuations in a uniform flow field. So, objects in the foreground will have an optic flow pattern which is distinctive to the flow patterns of the background objects. In addition to spatial location, optic flow patterns can be used to determine the three-dimensional layout of the visual scene. When moving perpendicularly to the direction of viewing, objects in the world move across the retina, those in the foreground moving across the retina more quickly than those in the background. So too when moving parallel to the direction of viewing will background objects move across the retina more slowly than those in the foreground, this time flowing past the observer. The relative speeds of optic flow patterns of objects can be used as a fairly accurate method to attain depth perception. Indeed, this is probably the primary method of depth perception in people without stereoscopic binocular vision. Optic flow produces differential movement gradients when viewing a moving scene, this being the root of the three-dimensional information that can be obtained.
However, is there no detection of movement in the visual system? Not necessarily. We perform tasks all the time where we track the movement of objects with our eyes. This is more of a purely movement detection function to assess the movement of stimuli themselves, rather than the world as a whole, like in optic flow. This appears to be a largely different mechanism to that which is needed to register changes purely in the retinal image caused by the movement of objects. When we move our eyes, we also move the image of the world across the retina, but this is not interpreted as a large-scale movement of the entire visual world due to a compensation mechanism in the brain, which separates movements in the visual field due to eye and body movement, and actual movements of stimuli in the world. We use an “outflow mechanism”, as proposed by Helmholtz in 1866, where neural signals about such movements of the body and eyes feed also into the “eye-head system’s” compensation mechanism. Thus, when we move our eyes so that the moving stimulus is relatively stationary on the retina, we do not see a stationary object, but a moving one.
So, when we track a moving stimulus (such as when monitoring the trajectory of a ball in order to catch it), we are using a purely stimulus movement detection system to do this. However, this might also be achieved with more localised optic flow patterns, but generally the tracking mechanism is used. This eye-head subset of the motion detection system seems to be the part which is concerned with the movement of objects, whereas the more global information obtained from the movements in the retinal image appear to be relatively dissociated from detecting the movement of objects, instead using motion cues to extract other useful information from the visual field.
In conclusion, it is not entirely true to say that the purpose of visual processing is not to see movement. Even though the motion sensitive cells of the visual cortex respond to the movement of stimuli to integrate global movement effects, such as optic flow, there is still a useful mechanism to detect the movement characteristics of an individual moving stimulus. But what is important is how relatively little of the movement information we detect actually serves to detect the movement of objects.