Download Motion perception in psychology and more Lecture notes Psychology in PDF only on Docsity!
Level 1
Motion Perception, Psychology of
George W Mather, University of Sussex, Brighton, UK
Motion perception is important for figure±ground segregation, three-dimensional vision, and visual guidance of action. Specialized brain cells detect image motion. Adaptation in these cells leads to illusory motion, such as the motion after-effect.
INTRODUCTION
0581.001 An essential attribute that distinguishes all animals from plants is their capacity for voluntary move- ment. Animals move to find mates, shelter, and food, and to avoid being eaten. But the ability to move brings with it the need to sense movement, whether to navigate through the world, or to detect the movement of other mobile animals such as approaching predators. For sighted animals, this means sensing movement in the visual image that is projected into the eye. The image is formed on a sheet of light-sensitive cells that line the inside of the eye ± the retina. Specialized neural processes are required to detect the presence of movement in the retinal image.
USES OF MOTION INFORMATION
0581.002 Surfaces, shapes, and objects in the scene under viewcreate spatial patterns of light and dark in the retinal image. The image is very rarely still, as in a photograph. Instead, it is in a state of continu- ous change, due to the movement of objects in the scene (e.g. an approaching predator) or to shifts in the position of the observer's eyes, head, or body (e.g. while running away from the predator). Per- ception of movement in the image is crucial, be- cause it can be used in a number of ways.
Figure±Ground Segregation
0581.003 Shapes and objects that are invisible while static (e.g. camouflaged animals) are revealed as soon as they move relative to the background. Many animals have evolved special ways of moving, in
an attempt to defeat figure±ground segregation. For example, prey animals such as lizards and rodents move in short, rapid bursts in between periods of complete stillness, in order to minimize the chances of detection by predators. Predators such as cats tend to move slowly and smoothly to avoid being seen by their prey.
Extraction of Three-dimensional
Structure
When any solid object moves, the images of its 0581. various parts that are cast on the retina move rela- tive to each other. Relative motion of this kind can be used to extract the three-dimensional structure of the object. For example, in a sideways view of a rotating globe, surface markings near the equator move across the field of viewmore rapidly than markings near the poles. In addition, markings near the equator followa linear path, whereas those near the poles followelliptical paths. This highly structured variation in speed and direction is sufficient for the perception of the shape's three- dimensional structure.
Visual Guidance of Action
As the observer moves about the world, image 0581. detail flows' across the image on their retinas, to create a characteristic motion pattern known asoptic flow'. A great deal of information can be extracted from optic flow, including the speed and direction of self-motion. For example, as you run through a wood, looking ahead, image details arising from rocks on the ground, and from trees ahead, appear near the centre of your vision and then flowthrough your field of viewas they disap- pear behind you, creating an expanding flowfield. This pattern of motion flowallows you to navigate a path through the wood without colliding with
CONTENTS
Introduction Uses of motion information Motion cues
The motion after-effect Apparent motion Induced motion
trees or misplacing your feet. The powerful move- ment effects experienced in `Imax' movie theatres are due to optic flow.
MOTION CUES
0581.006 The movement of objects under view, or of the observer through the world, cause the spatial pat- tern of light and dark in the image to fluctuate over time. For example, if you are looking across a rela- tively dark room, and a friend wearing light clothing moves across your line of sight, then the amount of light falling in the small region of the image at the centre of your vision will suddenly increase just as the friend intersects your line of sight, and then decrease again once they have passed through. If the room is empty, and you switch on a light, then again the amount of light falling on the image at the centre of your vision will increase. Howcan the brain distinguish between changes in image intensity due to movement and changes due to other causes, such as changing illu- mination? In order to solve this problem, the brain must combine information from several places in the image, rather than gathering information from just one place at a time. A change in illumin- ation causes a change in intensity everywhere in the image simultaneously, whereas movement causes changes in only a very small part of the image at a time, as Figure 1 demonstrates. 0581.007 The left-hand and middle panels of Figure 1 show two views of a scene containing a light human figure moving across a dark background. It is diffi- cult to tell what movement has occurred between the two views by inspecting them individually. The right-hand panel in Figure 1 shows the changes in light intensity that took place between the two views. Bright areas correspond to places where intensity increased over time from the first view to the second view, and dark areas correspond to places where intensity decreased from the first view to the second view. Grey areas were un- changed between the two views. Notice that the `difference image' on the right effectively isolates the parts of the scene that contained movement. Stationary features disappear. This would allow the observer to detect the presence of movement, perhaps for figure±ground segregation. 0581.008 Is it possible to infer the direction in which the figure was moving? Some parts of the scene in- creased in intensity over time (light in the right- hand panel), and other parts of the scene decreased in intensity over time (dark in the right-hand panel). Increases in intensity occurred where a
bright edge belonging to the figure moved right- ward into a region of the image that was previously dark (e.g. the shin of the leading leg). Decreases occurred where the edge of the figure moved out of a region of the image, returning that region to darkness (e.g. the calf of the leading leg). The brain can therefore infer the direction of a shape's move- ment by finding its edges, and then detecting whether the intensity of the image increases or decreases over time in the region of these edges. Since the 1960s it has been discovered that the brain possesses specialized `motion-detecting' neurons that respond specifically to movement. Each neuron responds only to movement in a specific direction over a small part of the image. Groups of these first-stage neurons are connected to second-stage neurons that integrate information over relatively large areas of the scene, in order to signal the movement of whole shapes and objects.
THE MOTION AFTER-EFFECT
The early Greeks were the first to discover a 0581. striking visual illusion nowknown as the motion after-effect (or MAE). The philosopher Aristotle noticed that if he stood in the middle of a river, and directed his gaze down at the fast-flowing water for a short time, when he shifted his gaze towards the riverbank the stationary scene ap- peared to flowbackwards in the opposite direction to the river. This illusion has been rediscovered a number of times, most famously by Thomas Addams, a Scottish scientist, while touring the Scottish Highlands. He visited the Falls of Foyers on the banks of Loch Ness, and noticed that if he fixed his gaze on the falling waters for a short time, and then looked at the rock face beside the falls, the rocks appeared to move upwards for a short time. For this reason the effect is also known as the waterfall illusion. It is powerful, robust, and easily demonstrated. A convenient way to experience the illusion today is to viewthe title credits of a TV programme or movie. It is important to fix one's gaze steadily at the centre of the screen rather than track the credits as they roll by. After about 30 seconds of adaptation, subsequently viewed scenes should appear to move in the opposite direction to the credits. The MAE is thought to arise from adaptation in 0581. motion-detecting neurons in the brain, of the kind described in the previous section. While viewing an image containing contours moving in a particular direction, cells `tuned' to respond to that direction will initially respond quite strongly. However, after
2 Motion Perception, Psychology of
be perceived when mobile shapes or objects in the image capture and hold the attention of the viewer. As the objects change position in the image, one's focus of attention shifts to keep track of them. This shift in attention itself gives rise to the perception of apparent motion. Advocates of such high-level pro- cesses do not see them as inconsistent with the notion of lower-level neural motion detection, but rather as separate processes that co-exist with low- level detection. It therefore seems likely that the perception of apparent movement is mediated both by low-level processes (motion-detecting neurons) and by high-level processes (inference and attention).
INDUCED MOTION
0581.014 On a cloudy, moonlit night, as relatively large clouds move quickly across the face of the moon, it often seems as if the clouds are stationary but the moon is moving. This illusion is an example of `induced motion': the appearance of motion in a stationary stimulus induced by the physical move- ment of another stimulus. Generally, induced motion is most effective when a large, slowly moving shape surrounds a smaller, stationary shape, such as the moon surrounded by clouds. 0581.015 The simplest demonstration of induced motion consists of a small spot surrounded by a large frame, as shown in Figure 4. If the frame moves slowly sideways while the spot remains stationary (left-hand panel), observers tend to perceive the frame as stationary and the spot as moving (right- hand panel). Induced motion highlights the prob- lem of motion attribution. As mentioned earlier, movement in the visual image is detected by spe- cialized motion-detecting neurons. Image motion can arise from two general sources, either move- ment of objects in the scene under view, or movement of the observer's body. It is obviously crucial to attribute motion to the correct source, but responses in motion-detecting neurons cannot dis- tinguish between them. The brain appears to use several strategies to solve the problem of motion attribution. These strategies are usually sufficient to arrive at the correct interpretation, but in certain situations the interpretation may be erroneous, leading to illusions such as induced motion. One strategy is to make use of nonvisual information in order to determine whether the observer is moving through the scene. Movement of the eye, head, and body can be established using information from the muscles (or from commands to move the muscles), and from the balance (vestibular) sense. When
movement in the image can be accounted for en- tirely by bodily movement, then no motion is per- ceived. For example, eye movements create movement in the image. When the eyes turn to the left, the whole image translates to the right on the retina. This translation excites motion-detecting neurons in the brain, yet we do not perceive the world to move. The image motion that results from eye movement is correctly attributed to the eye movement, so no motion is perceived. A second strategy, or heuristic, used in motion attribution relies on assumptions about the nature of the real world. In general, relatively large shapes and objects in a viewed scene tend to remain fixed in position, while small shapes and objects are likely to move. Large areas in a viewed scene may be filled by, for example, the wall of a building or the side of a hill. These shapes are extremely unlikely to move. Small areas, perhaps representing human or animal figures, or vehicles, are very likely to move. Consequently if there is relative movement between a small object and a large surrounding object, then the brain has a tendency to attribute the motion to the small object.
Further Reading Mather G, Verstraten F and Anstis S (eds) (1998) The Motion Aftereffect: A Modern Perspective. Cambridge, MA: MIT Press. Palmer SE (1999) Vision Science: Photons to Phenomenology. Cambridge, MA: MIT Press. Smith AT and Snowden RJ (eds) (1994) Visual Detection of Motion. London: Academic Press. Watanabe T (ed.) (1998) High-level Motion Processing. Cambridge, MA: MIT Press.
Glossary Balance sense The sensory system that provides infor- mation about the attitude of the body relative to gravi- tational vertical, and about acceleration of the body through space; its sense organ (vestibular organ) forms part of the inner ear. Motion attribution The perceptual process by which visual movement on the retina is attributed to move- ment of a particular shape or object in the environment, or to movement of the observer through the environ- ment, or some combination of the two. Motion-detecting neuron A neuron in the central ner- vous system (in the cortex in primates) that is special- ized to respond only to visual stimuli that move across the retina in a particular direction and at a particular speed. Optic flow The expandingpattern of light on the retina that results from relative motion between the observer and the environment (e.g. walking, driving).
4 Motion Perception, Psychology of
Perceptual inference An explanation for perception in which our sensory experience is the outcome of a reasoning-like process that operates at an unconscious level. Phi movement An illusion of movement in which obser- vers report the appearance of movement between two
spatial positions, yet cannot perceive an object moving across the gap between the positions. Retina Light-sensitive inner surface of the eye, onto which the visual image is projected; it contains photo- receptors that produce electrical signals when struck by light.
Keywords: (Check) motion detection; adaptation; optic flow; figure±ground segregation; illusion
0581f0010581f001 Figure 1. Cues for motion detection. The left and middle panels show two views of a human figure moving across the field of view, taken at slightly different times. The right panel shows the changes in light intensity that took place between the two views. This difference image' was created simply by subtracting the light intensities in the first image from the intensities in the second image, at each point in the image. Bright areas correspond to places where intensity increased over time from the first view to the second view, and dark areas correspond to places where intensity decreased from the first view to the second view. Grey areas were unchanged between the two views. Thedifference image' effectively selects only the parts of the scene that moved in between the first view and the second view
Motion Perception, Psychology of 5
Real Movement Space
1
4
8
12
Time
16
Apparent Movement Space
0581f0030581f003 Figure 3. Real movement versus apparent movement. Space is plotted horizontally and time is plotted vertically. In real movement (left) the disk changes position smoothly and continuously over time as it moves to the right. In apparent movement (right) the disk changes position just once over a relatively large distance, and is stationary at all other times. Both stimuli lead to the perception of motion
Stimulus Perception
0581f0040581f004 Figure 4. Induced motion. The physical stimulus con- sists of a small stationary spot surrounded by a slowly moving frame (left). Perceptually (right), the frame appears to be stationary while the spot appears to move
Motion Perception, Psychology of 7
ECSaq
Queries for Macmillan, ECS paper no. 581
Title: Motion perception, psychology of
Author: Mather
Figures 1 and 2 captions, cf. text discussions: there is a lot of duplication of text here.
Bearing in mind that they will be set close together in the finished volume, can any pruning of
one or the other be carried out?
