Scand. J . Psychol., 1977,18, 224-230
Visual control of locomotion DAVID N. LEE ROLY LISHMAN
Absrrucr.-How is locomotion controlled?What information is necessary and how is it used? It is first of all argued that the classical twofold division of information into exteroceptive and proprioceptive is inadequate and confusing, that three fundamental types of information need to be distinguished and that the information is used in a continual process of formulating locomotor programs, monitoring their execution and adjusting them. The paper goes on to show (1) how steering could be controlled on the basis of the “locomotor flow line” in the optic array, which specifies the potential future course, whether curved or straight, and (2) how stopping for an obstacle could be controlled simply on the basis of the information in the optic array about the time-to-collision and its rate of change. The problems inherent in pedestrian locomotion of controlling footing and balance are discussed and an investigation of visual locomotor programming in the long jump reported. Moving around the environment is such a natural as well as a necessary ability that we tend to take it for granted. But if we take a close look at someone engaged in locomotor activity, we cannot be but amazed at the complexity and precision of control. In attempting to understand how such control is achieved, three fundamental questions we may ask are: (1) What types of information are required for controlling locomotion? (2) How is the information obtained? (3) How is the information used? This paper attempts to outline an answer to these questions. THE INFORMATION NECESSARY FOR CONTROLLING LOCOMOTION Locomotion entails solving a hierarchy of locomotor problems. Consider, for example, an orienteer. His overall task is to pass by a series of check points in as short a time as possible. The first locomotor problem is planning a route. For this he needs information about the nature of the terrain and the locations of the start and check points, which he obtains from his map, supplemented Scrmd. J . Psvchol. 18
University of Edinburgh, Scotland
perhaps by his knowledge of the terrain. His planned route is, however, only a coarse prescription of what he must do. Planning specific paths through the terrain is the next locomotor problem. Using his map and compass or his knowledge of the terrain, he has to pick out landmarks which lie on his planned route and head for them. However, rarely will it be possible to head directly for a landmark. In general, he will have to visually assess the terrain and plan an eficient path through it which affords sure footing and avoids such obstacles as rocks, dense vegetation, large drops and unleapable sections of streams. In planning both his general route and his specific paths, the orienteer needs information only about the layout of the terrain. However, in actually guiding himself through the terrain he faces further locomotor problems which require different types of information for their solution. During locomotion his body is in dynamic flux. The positions and movements of the body parts relative to each other are changing continuously as is the position, orientation and movement of the body as a whole relative to the environment. Locomotion essentially entails controlling the dynamic flux of the body parts relative to each other so as to control the dynamic flux of the body as a whole relative to the environment. Three types of information can therefore be distinguished which are necessary for controlling locomotion and, indeed, any activity relative to the environment: Exteroceptive information about the layout of the environment and about objects and events in it, which is necessary for planning a general route as well as specific paths through the terrain; Proprioceptive information about the positions and movements of the body parts relative to each other, which is necessary for controlling bodily actions:
Visual control of locomotion Exproprioceptive information about the position, orientation and movement of the body as a whole, or part of the body, relative to the environment, which is necessary for guiding the body through the terrain. Since the term proprioceptive is here used in a sense somewhat different from its conventional meaning, some justification should perhaps be offered. Following Shemngton (1906), the receptor systems of the body have been classified as exteroceptors and proprioceptors, the assumption being that each receptor system subserves a unique function, exteroceptive or proprioceptive. Gibson (1966) has pointed out the fallacy of this classical view; the receptor systems clearly overlap in the functions they perform. The terms exteroceptor and proprioceptor are therefore misleading and would best be dropped from the vocabulary. Gibson distinguished two types of information: exteroceptive, as defined above, and proprioceptive, which he defined as information about one’s actions. These correspond roughly to the two types of stimulation, exafferent and r e a e r e n t , postulated by von Holst (1954). This is too broad a classification, however, for, as has been pointed out above, it is important to distinguish between sensing one’s bodily actions as such and sensing one’s changing relationship to the environment which those actions are intended to control. Gibson (1958) himself pointed out this distinction nearly twenty years ago, giving the examples of a bird flying in a headwind and a fish swimming against a current; forwarddirected actions do not necessarily propel the body forward. Rather than invent two new terms, Lee (19766) suggested that the term proprioceptive should be used in the more restricted sense given above, which is close to its classical meaning, and proposed the new term exproprioceptive.
225
ing relative to the environment, whether actively or passively, it is normally dominated by vision (Lishman & Lee, 1973). Compared with vision, the vestibular system only affords rudimentary exproprioceptive information about the accelerations of the head and its orientation to gravity, while the haptic system of receptors in the joints, muscles and skin affords only local exteroceptive and exproprioceptive information confined to the parts of the environment in physical contact with the body. Proprioceptive information is obtained through the articular system of receptors in the joints and muscles, which is a solely proprioceptive system. The information is, however, also obtainable through vision. Indeed, if one considers tasks involving fine control of the limbs, it is clear that vision affords the more sensitive proprioceptive information. It also appears that the articular proprioceptive system is subject to quite considerable drift, which is normally corrected by vision (Craske, 1%7; Hams, 1%5). Considering the general power of vision vis-a-vis the other perceptual systems, it seems likely that vision nomially functions as an overseer in the control of activity, developing patterns of action and tuning up other perceptual systems and keeping them tuned (Lee, 19766). The power of vision is perhaps nowhere more apparent than in controlling locomotion. It is quite amazing the number of different means of locomotion that a person can master, from normal walking and running to skiing, skating, cycling, driving, flying and so on. They all require different patterns of bodily activity, the proprioceptive aspect varies considerably. What is constant is the controlling visual exteroceptive and exproprioceptive information. LOCOMOTOR PROGRAMS
OBTAINING THE INFORMATION The information is obtainable, to varying degrees, through the different perceptual systems. In man, as in most animals, by far the richest source of the exteroceptive and exproprioceptive information is vision. Even in simply maintaining balance, vision affords the most sensitive exproprioceptive information, making possible very fine control (Lee & Lishman, 1975), and when there is a conflict between visual and non-visual exproprioceptive information a person’s experience of how he is mov15 -77 1948
“Whatever forms of the motor activity of higher organisms we consider, . . . analysis suggests no other guiding constant than the form and sense of the motor problem and the dominance of the required result of its solution, which determine, from step to step, now the fixation and now the reconstruction of the course of the program as well as the realization of the sensory correction.” (Bernstein, 1%7,p. 133.) In this succinct statement, Bernstein lays a foundation for understanding how locomotion is conSuitid. J . P.v?c.hrd. 18
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D. N . Lee and R.Lishman
trolled. Any locomotor act, whether it be short like stepping on a rock or protracted like stopping a vehicle, must be planned and directed by a program of action, a prescription of the course of the activity, for it is the temporal sequence of activity as a whole which achieves the solution to a locomotor problem. This is not to say that a locomotor program, one it has been formulated, is simply run “blindly”, for there will, in general, be unpredictable influences, both from within and without, which will deviate the activity from its intended course. The activity has, therefore, to be monitored in terms of the program and any deviations corrected by adjusting the ongoing prog-am. It is in this continual process of formulating locomotor programs, monitoring their execution and adjusting them that sensory information plays its vital role. The problems encountered during locomotion may be classified under two broad headings: (1) controlling the speed and direction of movement of the body relative to the environment, which is achieved by (2) controlling the external forces acting on the body. The first includes such problems as steering and stopping. The second involves the problem of securing adequate footing and so controlling the dynamic flux of the body parts relative to each other that the reactive forces from the ground can be efficiently utilised. In the case of powered locomotion, the problem is in manipulating the controls of a vehicle appropriately. Let us consider these problems in some detail. CONTROLLING STOPPING Consider someone driving at a steady speed along a straight stretch of road with an obstacle ahead. Assuming that he cannot steer round the obstacle, then clearly, in order to avoid collision, he has to register early enough that he is closing on it. (At night, for example, he might get too close to a stopped or slower moving vehicle before he realises that he is closing on it.) He then has to start braking early enough, and equally if not more importantly, he has to control his deceleration appropriately. The problems are illustrated in Fig. 1. The solid curve corresponds to emergency braking, maintaining the maximum possible deceleration to stop just short of the obstacle. It divides the graph into two zones. If the driver once enters the “crash” zone he will not be able to extricate himself and so will inevitably hit the obstacle. Curve (1) represents a Scund. J . Psychol. 18
Fig. I. The problem of stopping for an obstacle. The solid curve, which corresponds to emergency braking at 0.7 g, divides the graph into a “crash” and a “safe” zone. The driver has to brake both early enough and hard enough to avoid entering the crash zone, from which he cannot escape. Curve (1): a safe driver who starts braking at 0.5 g and later reduces his deceleration to 0.2 g. Curve (3): an unsafe driver who brakes too late at 0.7 g after he has entered the crash zone. Curve (2): another unsafe driver who, though he starts braking at the same time as the safe driver (I), brakes too gently at 0.2 g, enters the crash zone
and then ineffectually applies emergency braking.
safe driver who both starts braking before he enters the crash zone and controls his braking adequately. Curve (3) represents an unsafe driver who starts braking too late, after he has entered the crash zone. Curve (2) represents another unsafe driver who, though he starts braking early enough, does not brake adequately to begin with and so enters the crash zone. Cases (1) and (2) illustrate the importance of the temporal pattern of the deceleration; for a given average deceleration, the higher the deceleration at the beginning the shorter the stopping distance. In general it is wise to brake harder at the beginning. Clearly, in order to avoid collision, the driver has to plan ahead. He has, in some sense, to be able to see his potential course, the stretch of road that he would traverse if the current forces on his vehicle were maintained. In other words, since he himself is controlling the forces by his actions on the vehicle controls, he has to be able to foresee the consequences of his current program of action and if necessary modify it. The driver therefore needs information about his potential future in order to control it. How might he obtain this information? Initially he is travelling
Visual control of locomotion 227
no
backward force
no
that he needs information about all three variables. However, while such information is available in the optic flow field (Lee, 1974), it is not in fact necessary. He could, in principle, control his deceleration simply on the basis of the rate of change of the visual variable which specifies the time-to-collision if his closing velocity were maintained. This is illustrated in Fig. 2. For further details and a discussion of the effects visual limitations would have on a driver's braking performance, the reader is referred to Lee ( 1 9 7 6 ~ ) .
Recession
course
apply forward force
.). Fig. 2. How the visual variable, T , which specifies the time-to-collision if the closing velocity were maintained, could be used by the driver in and its time derivative, i, determining the type of course he is on and hence the action he needs to take: whether to apply a backward force to his vehicle (or reduce the forward force on it) by braking harder or acccelerating less, or to apply a forward force by doing the opposite.
straight at a steady speed; his action program is set to carry him an indefinite distance. Thus, as Gibson (1958) has pointed out, with an open road ahead there will be a radially expanding optic flow field at his eye, the centre of expansion corresponding to the point to which he is heading. When another vehicle lies on his path, its optic image will overlie the centre of expansion. If the vehicle is travelling at the same or a faster speed, its image will be constant or contracting. However, if it is stopped or travelling at a slower speed, its image will be dilating. The change in the optic image of the lead vehicle therefore specifies whether or not the driver is on a collision course with it. Furthermore, the inverse of the rate of dilation of the image specifies the timeto-collision if the current closing velocity were maintained, and this information could be used in judging when to start braking (Lee, 1 9 7 6 ~ ) . Once he has started braking, has set in motion an action program for stopping, he needs information about the adequacy of his deceleration so that if necessary he can adjust his program. Furthermore, as illustrated in Fig. 1 , he must do this early enough, else he will enter the crash zone and not be able to avoid collision. Since the adequacy of his program depends on his closing velocity and deceleration as well as his distance from the obstacle, it might seem
CONTROLLING STEERING Consider a driver approaching a bend along a straight stretch of road. What information does he have, first of all, for maintaining a straight course?
\
/
/
\
0
Fig. 3. The optic flow field, as projected onto a vertical
frontal plane in front of the driver's eye, when driving down a straight stretch of road towards a bend: (a) steering on course; (b) steering straight but off course; ( c ) steering on a curved course. To see how the straight optic flow lines in (a) and (b) are generated, consider the driver to be stationary and the ground moving linearly under him. The points on the ground will be flowing along straight lines parallel to the direction he is moving. The projections of these lines of flow onto the vertical frontal plane in front of the driver's eye give the straight optic flow lines shown. How the curved optic flow lines in (c) are generated is explained in Fig. 4. Scand. J . Psvchol. 18
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D . N . Lee and R . Lishrnan
If he were on course, the optic flow pattern at his eye would be as shown in Fig. 30. Gibson (1958) has suggested that, since the centre from which the flow radiates specifies the point to which he is heading, this could be used in controlling steering. However, this only applies to a straight course; when the driver is on a curved course there is no fixed point to which he is heading. There are, however, two properties of the optic flow field which could be used in steering either a straight course or, as will be shown below, a curved course. The first and more general one, which applies to any terrain, is the optic flow line which passes from view directly beneath the driver. This particular flow line, which we may refer to as the locomotor flow line, specifies his potential path, the path he would follow if the current forces on his vehicle were maintained. The second property is that, when on course, the images of the edges of the road will coincide with optic flow lines. Thus, if he is off course, either steering straight (Fig. 3 b ) or curved (Fig. 3c), the locomotor flow line will not lie down the image of the road, neither will the edges of the road coincide with flow lines. How does the driver negotiate the bend? Steering involves deploying the reactive forces from the ground in such a way that the body is propelled along the intended path. However, as in stopping, the forces that can be deployed are limited. There is, in short, a limit on the speed at which a bend can be taken, which is lower the higher the curvature. Therefore, in general, the driver will have to start negotiating the bend before he reaches it. Based on his assessment of the curvature of the bend and his dynamic relationship to it, he will have to change his locomotor program to one which will reduce his speed to an appropriate level at the bend. While experience probably plays a major part in judging an appropriate speed, the problem of actually reducing his speed is similar to the problem of stopping discussed above and the driver could use similar information. As he is nearing the bend, he has to formulate a locomotor program for adjusting his steering both by the right amount and at the right time. The timing could be based on visual information about time-to-collision, as discussed in the preceding section. If both his steering adjustment and timing were correct, then the optic flow field at his eye would be as shown in Fig. 4a. The locomotor flow line, specifying his potential path, would be down the Scand. J . Psvchol. 18
\
/
\
/
/
\
b
5
Fig. 4. The optic flow field, as projected onto a vertical
frontal plane in front of the driver’s eye, when entering a bend of constant curvature with a sharper bend ahead: ( a ) steering on course; ( b )correct steering angle, but steering adjustment made too late; ( c ) incorrect steering angle. To see how the optic flow field is generated, consider the driver to be stationary and the ground rotating under him about a vertical axis through the centre of curvature of his path (in case ( a ) this corresponds with that of the road). The points on the ground will be flowing along concentric circles. The projections of these circles of flow onto the vertical frontal plane in front of the driver’s eye give the hyperbolic optic flow lines shown.
image of the road and the images of the edges of the road would coincide with flow lines, just as when he is on a straight course (Fig. 3a). However, if either his timing were incorrect (Fig. 4b) or his steering adjustment (Fig. 4c), then neither of these optic flow properties would obtain. Thus, in general, whether the road is straight or curved, whenever the driver is steering off course, or is about to go off course because the curvature of the road ahead is changing, the relation between the locomotor flow line and the image of the road affords him information for adjusting his steering. CONTROLLING FOOTING
In addition to the general locomotor problems of steering and stopping, pedestrian locomotion en-
Visual control of locomotion
tails a special problem of its own. Walking, running, leaping, skiing and so on all depend on securing adequate footing and controlling the dynamic flux of the body parts relative to each other so that the reactive forces from the ground can be efficiently utilised. Someone running over rough ground, for example, clearly has to determine where to put his feet, and this may well involve planning several strides ahead. Vision affords the necessary distance information; it is very accurate in the body near-space (Johansson, 1973). The nature of the footing is also important-whether the runner has to step up, down, over to one side, onto the side of a rock, and so on-for he has to strike the ground not only at the right place but also in the right way, otherwise he will not be propelled towards his next footing and might well fall. He has, therefore, to program the dynamic flux of his body parts accurately, probably at least two strides ahead. Precise timing is clearly necessary. A simple everyday example is stepping off the sidewalk. If a person has not noticed the edge and so has adjusted his foot for striking the ground that split second too early, he can get a nasty jolt (see also Melvill Jones & Watt, 1971). Likewise, running over undulating ground is easy enough in the light but in the dark or semi-dark it can be quite jamng. Precise timing of body movements is especially important in high speed activities. The slalom skiier clearly has to time his turns very accurately, and the ski-jumper has to adjust his posture at just the right moment before take-off to avoid unintended aerobatics. Even on level ground where there is no problem in securing adequate footing, precise control is necessary to avoid falling. In walking and running, the centre of gravity of the body has to be shifted towards each foot in turn. Balance will be disturbed if the centre of gravity is shifted too far or too little or if unexpected forces act on the body, as when the foot strikes uneven, loose or compliant ground. Exproprioceptive information about the movement of the body relative to the environment is therefore necessary in order that appropriate adjustments can be made to the ongoing locomotor program. Vision probably affords the most sensitive exproprioceptive information, as it does in static balance control (Lee & Lishman, 1975). Uneven, loose or compliant ground renders the haptic exproprioceptive information through the feet unreliable, since the physical contact reference to the environment is
229
uncertain, which is why a person who has lost his vestibular system can walk over such ground only if he has adequate visual exproprioceptive information. In conclusion, let us consider the problem of leaping. As a person is running towards a stream, for example, he has to adjust his last few strides so that the foot he wishes to jump from strikes the ground as near to the edge of the stream as possible. Frequently this involves a shuffle about five strides away from the edge, as can be observed in athletes running a steeplechase course. A clear example of precisely controlled foot placement is the long jump. A skilled athlete can, after sprinting 40 m. strike the take-off board with a standard error of less than 10 cm-an accuracy better than 0.25 %. How is such accuracy achieved? Since no adjustments to the stride pattern are normally apparent, many coaches and athletes believe that it is all a matter of developing a standard runup. However, a recent study of three female athletes, who, together with their coaches, held such a belief and used no check marks down the track, showed that they were, in fact, visually adjusting their stride patterns (Lee, Lishman & Thomson, 1976). Film analysis showed that their run-ups were nowhere near as standard as they thought. The standard errors of the football positions increased considerably down the track, reaching a peak of 40 cm for one Olympic athlete. However, over the last three strides the standard error decreased dramatically, the lengths of the strides being highly correlated with the distance from the board. It seems clear, therefore, that as the athletes neared the board they visually assessed their distance from it and adjusted their running program accordingly. The work was supported by grant G974/294/C from the Medical Research Council.
REFERENCES Bernstein, N. (1%7). The co-ordination and regulation of movements. Oxford: Pergamon Press. Craske, B. (1%7). Adaptation to prisms: change in internally registered eye-position. Brit. J . Psychol. 58,
329-335. Gibson, J. J. (1958). Visually controlled locomotion and visual orientation in animals. Brit. J. Psychol. 49, 182-1 94. Gibson, J. J. (1%).
The senses considered as perceptual Houghton Mifllin. Hams, C. S. (1%5). Perceptual adaptation to inverted, systems. Boston:
Scand. J . Psychol. 18
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reversed, and displaced vision. Psychol. Rev. 72, 41-44. Holst, E. von (1954). Relations between the central nervous system and the peripheral organs. Brit. J . of Animal Eehaviour, 2 , 89-94. Johansson, G. (1973). Monocular movement parallax and near-space perception. Perception, 2, 135-146. Lee, D. N . (1974). Visual information during locomotion. In R. B. MacLeod and H. L. Pick (Eds.), Perception: Essays in honor ofJames J . Gibson. Ithaca & London: Cornell University Press. Lee, D. N. (1976a). A theory of visual control of braking based on information about time-to-collision. Perception, 5 , 437-459. Lee, D. N. (1976b). The functions of vision. In H. L. Pick & E. Saltzman (Eds.), Modes of perceiving and processing information. Erlbaum Press, in press. Lee, D. N. & Lishman, J. R. (1975). Visual proprioceptive control of stance. Journal of Human Movement Studies, I , 87-95. Lee, D. N., Lishman, J. R. & Thomson, J. A. (1976). Visual guidance in the long jump. In Report of VZZth Annual Coaches’ Convention, Edinburgh. Lishman, J. R. & Lee, D. N. (1973). The autonomy of visual kinaesthesis. Perception, 2, 287-294. Melvill Jones, G. & Watt, D. G. D. (1971). Muscular control of landing from unexpected falls in man. J . Physiol. 219, 729-737. Shemngton, C. S. (1906). The integrative action of the nervous system. Cambridge: University Press. Postal address:
D. N. Lee Department of Psychology University of Edinburgh Edinburgh Scotland
Srond. J . Psychol. 18
Visual control of locomotion DAVID N. LEE ROLY LISHMAN
Absrrucr.-How is locomotion controlled?What information is necessary and how is it used? It is first of all argued that the classical twofold division of information into exteroceptive and proprioceptive is inadequate and confusing, that three fundamental types of information need to be distinguished and that the information is used in a continual process of formulating locomotor programs, monitoring their execution and adjusting them. The paper goes on to show (1) how steering could be controlled on the basis of the “locomotor flow line” in the optic array, which specifies the potential future course, whether curved or straight, and (2) how stopping for an obstacle could be controlled simply on the basis of the information in the optic array about the time-to-collision and its rate of change. The problems inherent in pedestrian locomotion of controlling footing and balance are discussed and an investigation of visual locomotor programming in the long jump reported. Moving around the environment is such a natural as well as a necessary ability that we tend to take it for granted. But if we take a close look at someone engaged in locomotor activity, we cannot be but amazed at the complexity and precision of control. In attempting to understand how such control is achieved, three fundamental questions we may ask are: (1) What types of information are required for controlling locomotion? (2) How is the information obtained? (3) How is the information used? This paper attempts to outline an answer to these questions. THE INFORMATION NECESSARY FOR CONTROLLING LOCOMOTION Locomotion entails solving a hierarchy of locomotor problems. Consider, for example, an orienteer. His overall task is to pass by a series of check points in as short a time as possible. The first locomotor problem is planning a route. For this he needs information about the nature of the terrain and the locations of the start and check points, which he obtains from his map, supplemented Scrmd. J . Psvchol. 18
University of Edinburgh, Scotland
perhaps by his knowledge of the terrain. His planned route is, however, only a coarse prescription of what he must do. Planning specific paths through the terrain is the next locomotor problem. Using his map and compass or his knowledge of the terrain, he has to pick out landmarks which lie on his planned route and head for them. However, rarely will it be possible to head directly for a landmark. In general, he will have to visually assess the terrain and plan an eficient path through it which affords sure footing and avoids such obstacles as rocks, dense vegetation, large drops and unleapable sections of streams. In planning both his general route and his specific paths, the orienteer needs information only about the layout of the terrain. However, in actually guiding himself through the terrain he faces further locomotor problems which require different types of information for their solution. During locomotion his body is in dynamic flux. The positions and movements of the body parts relative to each other are changing continuously as is the position, orientation and movement of the body as a whole relative to the environment. Locomotion essentially entails controlling the dynamic flux of the body parts relative to each other so as to control the dynamic flux of the body as a whole relative to the environment. Three types of information can therefore be distinguished which are necessary for controlling locomotion and, indeed, any activity relative to the environment: Exteroceptive information about the layout of the environment and about objects and events in it, which is necessary for planning a general route as well as specific paths through the terrain; Proprioceptive information about the positions and movements of the body parts relative to each other, which is necessary for controlling bodily actions:
Visual control of locomotion Exproprioceptive information about the position, orientation and movement of the body as a whole, or part of the body, relative to the environment, which is necessary for guiding the body through the terrain. Since the term proprioceptive is here used in a sense somewhat different from its conventional meaning, some justification should perhaps be offered. Following Shemngton (1906), the receptor systems of the body have been classified as exteroceptors and proprioceptors, the assumption being that each receptor system subserves a unique function, exteroceptive or proprioceptive. Gibson (1966) has pointed out the fallacy of this classical view; the receptor systems clearly overlap in the functions they perform. The terms exteroceptor and proprioceptor are therefore misleading and would best be dropped from the vocabulary. Gibson distinguished two types of information: exteroceptive, as defined above, and proprioceptive, which he defined as information about one’s actions. These correspond roughly to the two types of stimulation, exafferent and r e a e r e n t , postulated by von Holst (1954). This is too broad a classification, however, for, as has been pointed out above, it is important to distinguish between sensing one’s bodily actions as such and sensing one’s changing relationship to the environment which those actions are intended to control. Gibson (1958) himself pointed out this distinction nearly twenty years ago, giving the examples of a bird flying in a headwind and a fish swimming against a current; forwarddirected actions do not necessarily propel the body forward. Rather than invent two new terms, Lee (19766) suggested that the term proprioceptive should be used in the more restricted sense given above, which is close to its classical meaning, and proposed the new term exproprioceptive.
225
ing relative to the environment, whether actively or passively, it is normally dominated by vision (Lishman & Lee, 1973). Compared with vision, the vestibular system only affords rudimentary exproprioceptive information about the accelerations of the head and its orientation to gravity, while the haptic system of receptors in the joints, muscles and skin affords only local exteroceptive and exproprioceptive information confined to the parts of the environment in physical contact with the body. Proprioceptive information is obtained through the articular system of receptors in the joints and muscles, which is a solely proprioceptive system. The information is, however, also obtainable through vision. Indeed, if one considers tasks involving fine control of the limbs, it is clear that vision affords the more sensitive proprioceptive information. It also appears that the articular proprioceptive system is subject to quite considerable drift, which is normally corrected by vision (Craske, 1%7; Hams, 1%5). Considering the general power of vision vis-a-vis the other perceptual systems, it seems likely that vision nomially functions as an overseer in the control of activity, developing patterns of action and tuning up other perceptual systems and keeping them tuned (Lee, 19766). The power of vision is perhaps nowhere more apparent than in controlling locomotion. It is quite amazing the number of different means of locomotion that a person can master, from normal walking and running to skiing, skating, cycling, driving, flying and so on. They all require different patterns of bodily activity, the proprioceptive aspect varies considerably. What is constant is the controlling visual exteroceptive and exproprioceptive information. LOCOMOTOR PROGRAMS
OBTAINING THE INFORMATION The information is obtainable, to varying degrees, through the different perceptual systems. In man, as in most animals, by far the richest source of the exteroceptive and exproprioceptive information is vision. Even in simply maintaining balance, vision affords the most sensitive exproprioceptive information, making possible very fine control (Lee & Lishman, 1975), and when there is a conflict between visual and non-visual exproprioceptive information a person’s experience of how he is mov15 -77 1948
“Whatever forms of the motor activity of higher organisms we consider, . . . analysis suggests no other guiding constant than the form and sense of the motor problem and the dominance of the required result of its solution, which determine, from step to step, now the fixation and now the reconstruction of the course of the program as well as the realization of the sensory correction.” (Bernstein, 1%7,p. 133.) In this succinct statement, Bernstein lays a foundation for understanding how locomotion is conSuitid. J . P.v?c.hrd. 18
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D. N . Lee and R.Lishman
trolled. Any locomotor act, whether it be short like stepping on a rock or protracted like stopping a vehicle, must be planned and directed by a program of action, a prescription of the course of the activity, for it is the temporal sequence of activity as a whole which achieves the solution to a locomotor problem. This is not to say that a locomotor program, one it has been formulated, is simply run “blindly”, for there will, in general, be unpredictable influences, both from within and without, which will deviate the activity from its intended course. The activity has, therefore, to be monitored in terms of the program and any deviations corrected by adjusting the ongoing prog-am. It is in this continual process of formulating locomotor programs, monitoring their execution and adjusting them that sensory information plays its vital role. The problems encountered during locomotion may be classified under two broad headings: (1) controlling the speed and direction of movement of the body relative to the environment, which is achieved by (2) controlling the external forces acting on the body. The first includes such problems as steering and stopping. The second involves the problem of securing adequate footing and so controlling the dynamic flux of the body parts relative to each other that the reactive forces from the ground can be efficiently utilised. In the case of powered locomotion, the problem is in manipulating the controls of a vehicle appropriately. Let us consider these problems in some detail. CONTROLLING STOPPING Consider someone driving at a steady speed along a straight stretch of road with an obstacle ahead. Assuming that he cannot steer round the obstacle, then clearly, in order to avoid collision, he has to register early enough that he is closing on it. (At night, for example, he might get too close to a stopped or slower moving vehicle before he realises that he is closing on it.) He then has to start braking early enough, and equally if not more importantly, he has to control his deceleration appropriately. The problems are illustrated in Fig. 1. The solid curve corresponds to emergency braking, maintaining the maximum possible deceleration to stop just short of the obstacle. It divides the graph into two zones. If the driver once enters the “crash” zone he will not be able to extricate himself and so will inevitably hit the obstacle. Curve (1) represents a Scund. J . Psychol. 18
Fig. I. The problem of stopping for an obstacle. The solid curve, which corresponds to emergency braking at 0.7 g, divides the graph into a “crash” and a “safe” zone. The driver has to brake both early enough and hard enough to avoid entering the crash zone, from which he cannot escape. Curve (1): a safe driver who starts braking at 0.5 g and later reduces his deceleration to 0.2 g. Curve (3): an unsafe driver who brakes too late at 0.7 g after he has entered the crash zone. Curve (2): another unsafe driver who, though he starts braking at the same time as the safe driver (I), brakes too gently at 0.2 g, enters the crash zone
and then ineffectually applies emergency braking.
safe driver who both starts braking before he enters the crash zone and controls his braking adequately. Curve (3) represents an unsafe driver who starts braking too late, after he has entered the crash zone. Curve (2) represents another unsafe driver who, though he starts braking early enough, does not brake adequately to begin with and so enters the crash zone. Cases (1) and (2) illustrate the importance of the temporal pattern of the deceleration; for a given average deceleration, the higher the deceleration at the beginning the shorter the stopping distance. In general it is wise to brake harder at the beginning. Clearly, in order to avoid collision, the driver has to plan ahead. He has, in some sense, to be able to see his potential course, the stretch of road that he would traverse if the current forces on his vehicle were maintained. In other words, since he himself is controlling the forces by his actions on the vehicle controls, he has to be able to foresee the consequences of his current program of action and if necessary modify it. The driver therefore needs information about his potential future in order to control it. How might he obtain this information? Initially he is travelling
Visual control of locomotion 227
no
backward force
no
that he needs information about all three variables. However, while such information is available in the optic flow field (Lee, 1974), it is not in fact necessary. He could, in principle, control his deceleration simply on the basis of the rate of change of the visual variable which specifies the time-to-collision if his closing velocity were maintained. This is illustrated in Fig. 2. For further details and a discussion of the effects visual limitations would have on a driver's braking performance, the reader is referred to Lee ( 1 9 7 6 ~ ) .
Recession
course
apply forward force
.). Fig. 2. How the visual variable, T , which specifies the time-to-collision if the closing velocity were maintained, could be used by the driver in and its time derivative, i, determining the type of course he is on and hence the action he needs to take: whether to apply a backward force to his vehicle (or reduce the forward force on it) by braking harder or acccelerating less, or to apply a forward force by doing the opposite.
straight at a steady speed; his action program is set to carry him an indefinite distance. Thus, as Gibson (1958) has pointed out, with an open road ahead there will be a radially expanding optic flow field at his eye, the centre of expansion corresponding to the point to which he is heading. When another vehicle lies on his path, its optic image will overlie the centre of expansion. If the vehicle is travelling at the same or a faster speed, its image will be constant or contracting. However, if it is stopped or travelling at a slower speed, its image will be dilating. The change in the optic image of the lead vehicle therefore specifies whether or not the driver is on a collision course with it. Furthermore, the inverse of the rate of dilation of the image specifies the timeto-collision if the current closing velocity were maintained, and this information could be used in judging when to start braking (Lee, 1 9 7 6 ~ ) . Once he has started braking, has set in motion an action program for stopping, he needs information about the adequacy of his deceleration so that if necessary he can adjust his program. Furthermore, as illustrated in Fig. 1 , he must do this early enough, else he will enter the crash zone and not be able to avoid collision. Since the adequacy of his program depends on his closing velocity and deceleration as well as his distance from the obstacle, it might seem
CONTROLLING STEERING Consider a driver approaching a bend along a straight stretch of road. What information does he have, first of all, for maintaining a straight course?
\
/
/
\
0
Fig. 3. The optic flow field, as projected onto a vertical
frontal plane in front of the driver's eye, when driving down a straight stretch of road towards a bend: (a) steering on course; (b) steering straight but off course; ( c ) steering on a curved course. To see how the straight optic flow lines in (a) and (b) are generated, consider the driver to be stationary and the ground moving linearly under him. The points on the ground will be flowing along straight lines parallel to the direction he is moving. The projections of these lines of flow onto the vertical frontal plane in front of the driver's eye give the straight optic flow lines shown. How the curved optic flow lines in (c) are generated is explained in Fig. 4. Scand. J . Psvchol. 18
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D . N . Lee and R . Lishrnan
If he were on course, the optic flow pattern at his eye would be as shown in Fig. 30. Gibson (1958) has suggested that, since the centre from which the flow radiates specifies the point to which he is heading, this could be used in controlling steering. However, this only applies to a straight course; when the driver is on a curved course there is no fixed point to which he is heading. There are, however, two properties of the optic flow field which could be used in steering either a straight course or, as will be shown below, a curved course. The first and more general one, which applies to any terrain, is the optic flow line which passes from view directly beneath the driver. This particular flow line, which we may refer to as the locomotor flow line, specifies his potential path, the path he would follow if the current forces on his vehicle were maintained. The second property is that, when on course, the images of the edges of the road will coincide with optic flow lines. Thus, if he is off course, either steering straight (Fig. 3 b ) or curved (Fig. 3c), the locomotor flow line will not lie down the image of the road, neither will the edges of the road coincide with flow lines. How does the driver negotiate the bend? Steering involves deploying the reactive forces from the ground in such a way that the body is propelled along the intended path. However, as in stopping, the forces that can be deployed are limited. There is, in short, a limit on the speed at which a bend can be taken, which is lower the higher the curvature. Therefore, in general, the driver will have to start negotiating the bend before he reaches it. Based on his assessment of the curvature of the bend and his dynamic relationship to it, he will have to change his locomotor program to one which will reduce his speed to an appropriate level at the bend. While experience probably plays a major part in judging an appropriate speed, the problem of actually reducing his speed is similar to the problem of stopping discussed above and the driver could use similar information. As he is nearing the bend, he has to formulate a locomotor program for adjusting his steering both by the right amount and at the right time. The timing could be based on visual information about time-to-collision, as discussed in the preceding section. If both his steering adjustment and timing were correct, then the optic flow field at his eye would be as shown in Fig. 4a. The locomotor flow line, specifying his potential path, would be down the Scand. J . Psvchol. 18
\
/
\
/
/
\
b
5
Fig. 4. The optic flow field, as projected onto a vertical
frontal plane in front of the driver’s eye, when entering a bend of constant curvature with a sharper bend ahead: ( a ) steering on course; ( b )correct steering angle, but steering adjustment made too late; ( c ) incorrect steering angle. To see how the optic flow field is generated, consider the driver to be stationary and the ground rotating under him about a vertical axis through the centre of curvature of his path (in case ( a ) this corresponds with that of the road). The points on the ground will be flowing along concentric circles. The projections of these circles of flow onto the vertical frontal plane in front of the driver’s eye give the hyperbolic optic flow lines shown.
image of the road and the images of the edges of the road would coincide with flow lines, just as when he is on a straight course (Fig. 3a). However, if either his timing were incorrect (Fig. 4b) or his steering adjustment (Fig. 4c), then neither of these optic flow properties would obtain. Thus, in general, whether the road is straight or curved, whenever the driver is steering off course, or is about to go off course because the curvature of the road ahead is changing, the relation between the locomotor flow line and the image of the road affords him information for adjusting his steering. CONTROLLING FOOTING
In addition to the general locomotor problems of steering and stopping, pedestrian locomotion en-
Visual control of locomotion
tails a special problem of its own. Walking, running, leaping, skiing and so on all depend on securing adequate footing and controlling the dynamic flux of the body parts relative to each other so that the reactive forces from the ground can be efficiently utilised. Someone running over rough ground, for example, clearly has to determine where to put his feet, and this may well involve planning several strides ahead. Vision affords the necessary distance information; it is very accurate in the body near-space (Johansson, 1973). The nature of the footing is also important-whether the runner has to step up, down, over to one side, onto the side of a rock, and so on-for he has to strike the ground not only at the right place but also in the right way, otherwise he will not be propelled towards his next footing and might well fall. He has, therefore, to program the dynamic flux of his body parts accurately, probably at least two strides ahead. Precise timing is clearly necessary. A simple everyday example is stepping off the sidewalk. If a person has not noticed the edge and so has adjusted his foot for striking the ground that split second too early, he can get a nasty jolt (see also Melvill Jones & Watt, 1971). Likewise, running over undulating ground is easy enough in the light but in the dark or semi-dark it can be quite jamng. Precise timing of body movements is especially important in high speed activities. The slalom skiier clearly has to time his turns very accurately, and the ski-jumper has to adjust his posture at just the right moment before take-off to avoid unintended aerobatics. Even on level ground where there is no problem in securing adequate footing, precise control is necessary to avoid falling. In walking and running, the centre of gravity of the body has to be shifted towards each foot in turn. Balance will be disturbed if the centre of gravity is shifted too far or too little or if unexpected forces act on the body, as when the foot strikes uneven, loose or compliant ground. Exproprioceptive information about the movement of the body relative to the environment is therefore necessary in order that appropriate adjustments can be made to the ongoing locomotor program. Vision probably affords the most sensitive exproprioceptive information, as it does in static balance control (Lee & Lishman, 1975). Uneven, loose or compliant ground renders the haptic exproprioceptive information through the feet unreliable, since the physical contact reference to the environment is
229
uncertain, which is why a person who has lost his vestibular system can walk over such ground only if he has adequate visual exproprioceptive information. In conclusion, let us consider the problem of leaping. As a person is running towards a stream, for example, he has to adjust his last few strides so that the foot he wishes to jump from strikes the ground as near to the edge of the stream as possible. Frequently this involves a shuffle about five strides away from the edge, as can be observed in athletes running a steeplechase course. A clear example of precisely controlled foot placement is the long jump. A skilled athlete can, after sprinting 40 m. strike the take-off board with a standard error of less than 10 cm-an accuracy better than 0.25 %. How is such accuracy achieved? Since no adjustments to the stride pattern are normally apparent, many coaches and athletes believe that it is all a matter of developing a standard runup. However, a recent study of three female athletes, who, together with their coaches, held such a belief and used no check marks down the track, showed that they were, in fact, visually adjusting their stride patterns (Lee, Lishman & Thomson, 1976). Film analysis showed that their run-ups were nowhere near as standard as they thought. The standard errors of the football positions increased considerably down the track, reaching a peak of 40 cm for one Olympic athlete. However, over the last three strides the standard error decreased dramatically, the lengths of the strides being highly correlated with the distance from the board. It seems clear, therefore, that as the athletes neared the board they visually assessed their distance from it and adjusted their running program accordingly. The work was supported by grant G974/294/C from the Medical Research Council.
REFERENCES Bernstein, N. (1%7). The co-ordination and regulation of movements. Oxford: Pergamon Press. Craske, B. (1%7). Adaptation to prisms: change in internally registered eye-position. Brit. J . Psychol. 58,
329-335. Gibson, J. J. (1958). Visually controlled locomotion and visual orientation in animals. Brit. J. Psychol. 49, 182-1 94. Gibson, J. J. (1%).
The senses considered as perceptual Houghton Mifllin. Hams, C. S. (1%5). Perceptual adaptation to inverted, systems. Boston:
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reversed, and displaced vision. Psychol. Rev. 72, 41-44. Holst, E. von (1954). Relations between the central nervous system and the peripheral organs. Brit. J . of Animal Eehaviour, 2 , 89-94. Johansson, G. (1973). Monocular movement parallax and near-space perception. Perception, 2, 135-146. Lee, D. N . (1974). Visual information during locomotion. In R. B. MacLeod and H. L. Pick (Eds.), Perception: Essays in honor ofJames J . Gibson. Ithaca & London: Cornell University Press. Lee, D. N. (1976a). A theory of visual control of braking based on information about time-to-collision. Perception, 5 , 437-459. Lee, D. N. (1976b). The functions of vision. In H. L. Pick & E. Saltzman (Eds.), Modes of perceiving and processing information. Erlbaum Press, in press. Lee, D. N. & Lishman, J. R. (1975). Visual proprioceptive control of stance. Journal of Human Movement Studies, I , 87-95. Lee, D. N., Lishman, J. R. & Thomson, J. A. (1976). Visual guidance in the long jump. In Report of VZZth Annual Coaches’ Convention, Edinburgh. Lishman, J. R. & Lee, D. N. (1973). The autonomy of visual kinaesthesis. Perception, 2, 287-294. Melvill Jones, G. & Watt, D. G. D. (1971). Muscular control of landing from unexpected falls in man. J . Physiol. 219, 729-737. Shemngton, C. S. (1906). The integrative action of the nervous system. Cambridge: University Press. Postal address:
D. N. Lee Department of Psychology University of Edinburgh Edinburgh Scotland
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