Ecological movement principles and how much ...

Ecological movement principles and how much information matters Reinoud J. Bootsma Movement & Perception Laboratory University of the Mediterranean – CNRS, Marseille A basic tenet of the ecological approach to perception and action is that movement cannot be understood without reference to the function it subserves, that is to say without an embedding in the action goals sought after. The pursuit of action, defined as task-specific interaction patterns between actor and environment, necessitates that the current state of affairs can both be assessed (through detection of information) and influenced (through the production of movement). After a brief discussion of the different ways in which these two constituents have been conceptualized within the ecological approach an inventory is compiled of the issues that a full account of action needs to address.

The making of a movement In spite of the fact that the cold war was not yet to be ended soon, the in 1977 emigrated English triple jumper Michael Turvey officially proclaimed the marriage of the fundamental ideas of the Russian physiologist Nicolai Alexandrovitch Bernstein (1896-1966) and the American psychologist James Jerome Gibson (1904-1979). This union has turned out to have been so fertile that it is difficult today to pinpoint the exact dates of birth of the different descendants, many not having become identifiable as such until after quite some years. In the development of his theory of (direct) perception Gibson had been advocating the attribution of a crucial role to movement for several decades and Bernstein had fought many battles for closing the classical stimulus-response arc. It was not, however, until the late 1970s (e.g., Turvey, Shaw, & Mace, 1978) that a so-called ecological approach to perception and action loud-mouthedly announced its differences with respect to the existing approaches in movement science. Reaching an age reminiscent of the terrible twos by the beginning of the 1980s (e.g., Turvey, Shaw, Reed, & Mace, 1981), an elaborate theoretical framework was being hammered home in books, journals and at conferences all over the world (see Meijer, 1988, for more of an historical overview), upsetting many established scholars and exciting a good number of PhD-students at the same time. My tutor John Whiting’s incitation to go and see what this strange noise was all about eventually led me to embrace the meta-theoretical starting points of this approach. Briefly, these can be characterized by the rejection, based on a firm anchoring in evoluPost, A.A., Pijpers, J.R., Bosch, P., & Boschker, M.S.J. (Eds). (1998). Models in Human Movement Science: Proceedings of the Second Symposium of the Institute for Fundamental and Clinical Human Movement Sciences (pp. 51-63). Enschede: PrintPartners Ipskamp.

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tionism (Michaels & Carello, 1981), of two types of dualism , viz. (a) organism-environment dualism, implying that the scientific solution to control problems should not be sought within the brain (cf. Mace, 1978), and (b) perception-movement dualism, implying that the two cannot be understood or studied in isolation (cf. Turvey & Kugler, 1984). The recourse taken by cognitive science to hypothetical intermediate (Van Wieringen, 1988) concepts (implying underlying structures and processes) such as percepts, memories and movement representations is deemed unsatisfactorily because of the “loans on intelligence” (Kugler & Turvey, 1987) they present. While slowly evolving into several allied schools of thought, working (at least initially) on the same problems from different ends, the approach formed a relatively closed front when it came to responding to attacks from what was considered the “Establishment” (see, for example, the discussion in Meijer & Roth, 1988). Although perhaps theoretically top-heavy in the early 1980s, the implication of an increasingly larger number of decidedly experimentally oriented fresh(wo)men has led to the current situation in which the ecological approach to perception and action has become accepted as a (more or less) serious alternative to the more traditional cognitive approaches. Apart from the articles regularly appearing in the most highly respected journals, the most striking example of the maturity of age that has been achieved is the, to say the least, critical paper by two of the prominent members of the approach that was published recently in the journal of its society (Michaels & Beek, 1995). I do not think that this internal criticism can be taken to reflect an adolescent’s uprise against parental authority, and hence may be ignored as representing but a mere ‘phase they are going through’; in fact the majority of proponents of the ecological approach have not gone into the anticipated fit, but on the contrary seem to have been pleasantly surprised to see a critical analysis of the state of their art come to the front. Perhaps notwithstanding the official discourse on the unity of the approach, many of us felt that the ties linking the direct perception, kinetic/thermodynamics, and dynamical systems currents were becoming less well-integrated than once proclaimed and intended. In fact, Beek, Peper, and Stegeman’s (1995) tentative scheme for ordering approaches to movement control and coordination, in which ecological psychology is intentionally separated from coordination dynamics, can be taken to argue this point. Bernstein’s (1967) fundamental insights concerned the functional non-univocality of central commands and the organizational difficulties posed by the tremendous number of (redundant) degrees of freedom in biological systems. The identification of these problems opened the door for a quest for principles, sufficiently powerful to assure at the same time

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the required dimensional reduction (and thus belaborating coordination) and the coupling of organism and environment (thus belaborating control). Whereas the self-organizing principles of non-linear dynamics were rapidly identified as holding the promise of dealing with the first (Kelso, Holt, Kugler, & Turvey, 1980; Kugler, Kelso, & Turvey, 1980), Gibson’s (1979) conception of information was retained by many with respect to the second. I will first briefly address both, before returning to the question of how they may be integrated into a comprehensive whole.

Information for movement According to the Gibsonian analysis, recourse to enrichening internal processes, which form the focus of cognitive perception research, is not necessary for an understanding of perception if one is willing to replace the snapshot conception as the starting point of perception by a transformational basis (Michaels & Carello, 1981). That is to say, by postulating that it are the spatio-temporal changes in the structure of stimulation rather than the static states of stimulation that form the basis of perception, the doctrine of stimulus poverty (Fodor, 1983) can be replaced by a doctrine of informational abundance. Perception of a property of the EnvironmentActor System (EAS) is possible because relevant (and therefore inherently meaningful) properties give rise to distinct patterns of flow in the medium that uniquely correspond to these properties. Detection of the flow pattern by a perceptual system thus allows perception of the corresponding property (Turvey, 1990). The informative flow pattern specifies the relevant EAS-property and is therefore referred to as information about that property. It is important to keep in mind that the specificational step (i.e., from EAS properties to information) does not depend in any way on the characteristics (anatomical, physiological, emotional) of the potential perceiver1; flow patterns should therefore be referred to as being optical, acoustical, mechanical, chemical, rather than as visual, auditory, haptical, olfactorial, etc. (see chapter headings in Bardy, Bootsma, & Guiard, 1995). In the same way, detection of information by a perceptual organ cannot influence

1

Of course, useful descriptions of information take into account the perceptual capabilities of the organism for which the information is supposed to be relevant. Note nevertheless that the ecological conception of information implies that it is a category mistake to refer to optical information as being monocular or binocular: These two modes of vision pertain to the way in which the ambient optic array is sampled by an animal, that is to say to the way in which information is detected, not to the nature of the information available. Although of critical importance for the development of a coherent formalization of information, we will not pursue these issues here, as they are not directly relevant for the present arguments.

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the characteristics of the information itself. This should not be taken to imply that perceptual systems are passive, on the contrary. By producing movement, the actor influences the state of the EAS and thus may actively generate information that can subsequently be used for the guidance of action (see Fig. 1).

Figure 1. Schematic representation of the relations between information, perception, and movement.

On the basis of this type of reasoning, experimental work on movement inspired by direct perception theory has concentrated on establishing the relations existing between information and movement characteristics for different tasks (e.g., Bardy, Warren, & Kay, 1996; Bootsma & Van Wieringen, 1990; Lee, Young, Reddish, Lough, & Clayton, 1983; Michaels & Oudejans, 1992; Savelsbergh, Whiting, & Bootsma, 1991). At best, the purportedly lawful statements that emerge from this type of study describe relations between (kinematic) properties of the flow field and kinematic properties of movement. As argued by Michaels and Beek (1995), among other things, the identification of relevant variables and falsifiability of the assumed causal underpinnings of the relation pose problems. To stay away, for the moment, from the fragile eggs in the ecological basket of the threatened tau species (see Michaels & Beek, 1995, p. 264), the debate on the use of optical acceleration (see Oudejans, 1996, for an overview) in the catching of fly balls may form a nice illustration of these kinds of problems: Following up on the analysis of Chapman (1968), several researchers


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have embraced the position that catching fly balls is achieved by zeroing out optical acceleration. Notwithstanding the fact that optical acceleration must approach zero for a caught ball, there is to date a dearth of studies manipulating relevant EAS and/or informational variables to demonstrate that the kinematic movement patterns produced under a variety of such manipulations fit better with use of this particular informational quantity than another. In fact, I suggest that this is so because the way in which the information is used has not been adequately addressed (also see Bootsma, Fayt, Zaal, & Laurent, 1997). Going one step further, by introducing force fields underlying the observed kinematic movement patterns, kinetic theory provides some restrictions that may prove helpful in the search for relevant action parameters. However, once again more often than not only relations are predicted (e.g., Warren, Young & Lee, 1986) and it thus remains unclear how the dynamical unfolding of the action would have to be modeled, a step that in my view cannot be avoided in the light of the need for falsifiability.

Movement dynamics Conceiving of movement as an expression of an underlying dynamical system, defined as a system that evolves over time, holds the promise of being able to provide relatively simple explanations for complicated behavioural phenomena (cf. May, 1976). By allowing for non-linearities, the self-regulating and self-organizing properties of such systems can avoid having to pay the cumulated interests that burden the loans cognitive science has drawn on intelligence. In the domain of movement science, dynamical systems analyses have been particularly successful in addressing the issue of coordination (cf. Haken, Kelso, & Bunz, 1985; Kelso 1995) and Beek et al. (1995) even place the study of coordination dynamics in the middle of their triangular ordering. If one is to take their term ‘coordination’ as the descriptor of how a collective (EAS-related) variable changes as a function of its present state and the state of other relevant (perhaps even perceptual) variables—thus dealing with coordination between effector systems as well as coordination between actor and environment—then such a prominent position does indeed seem to be warranted. While, following Newell (1986), I prefer to refer to the latter as control, this should not be taken to suggest that the tools of (non-linear) dynamics would all of a sudden have become unsuitable when it comes to such issues (see Schöner, 1991, 1994), nor that coordination would not be achieved through informational couplings (see Schmidt, Carello, & Turvey, 1990). I simply want to point out that to date much less empirical work exists dealing with issues of control than with issues of (intra- and

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interlimb) coordination. I contend it is in the integration of ecological psychology’s conception of information into control dynamics that the problems raised by Michaels and Beek (1995) can find their answers.

Information in movement Over the recent years, the most cherished childhood toy of ecological psychology, the optic variable tau, has been subjected to heavy criticism, up to the point that theorizing along these lines would have become merely specious (Wann, 1996)2. It is certainly true that at some point some people may have become so enchanted with this informational quantity that near magical properties were attributed to it. Nevertheless, systematic work on the generalization of the limiting head-on approach case described by Lee (1976) has resulted in adequate formulations for the optical specification of the first-order time to contact ( TC1 in the terminology proposed by Bootsma et al., 1997) for other types of approach (Bootsma, 1988; Bootsma & Oudejans, 1993; Tresilian, 1991). Moreover, the initial, once again too restrictive, conception in which the use of information on TC1 would boil down to initiating a movement at a critical value has been succeeded by several alternative positions (Bootsma et al., 1997; Peper, Bootsma, Mestre, & Bakker, 1994; Schöner, 1994; Tresilian, 1994). For purposes of illustration I briefly present the model proposed by Bootsma et al. (1997) for the information-based regulation of movement in interceptive actions. The modeling attempt was based on the following objectives. First, movement initiation and movement execution should result from the same law of control. That is to say, the principle behind the fact that no movement occurs, that movement commences at some point, and that movement continues or ends should be one and the same. Second, inclusion of variables should be based on clear theoretical and empirical considerations. Third, the model should be falsifiable and therefore explicitly detail how movement unfolds as a function both of the current state as well as the current requirements. Finally, so as to move towards the identification of general operating principles, the model should be able to address several tasks, such as catching (requiring being in the right place at the right time) and hitting (requiring moreover that peak velocity is reached at the time of contact). Lee’s tau, the inverse of the rate of dilation of the optical angle ! subtended by the object at the point of observation, 2

The underlying conceptual error is due to a lack of distinction between the general aspecific question of whether a certain type of information is used and the more relevant specific question of whether information is used in a particular manner. Wann’s (1996) analysis merely shows that movement is not initiated at a constant value of tau, which is exactly what the types of model that we propose predict (Bootsma et al., 1997; Peper, Bootsma, Mestre, & Bakker, 1994; Zaal, Bootsma, & Van Wieringen, in press).


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specifies TC1 along the axis connecting the point of observation and the object. When this axis coincides with the primary movement axis, the type of problem encountered may be considered as being purely temporal (as time and space are connected through relative approach velocity) and ! (" ) might indeed prove to be sufficient. However, the extension to non headon approaches raised the problem of complementing temporal uncertainty with spatial uncertainty. Rather than dealing with each of these separately, a better solution is found in a decidely spatio-temporal approach. On the basis of their work on hand movement in catching, Peper et al. (1994) proposed that movement was guided by information about an EAS property that they termed currently required velocity3. Given that the hand should arrive at the position of the ball when the ball crosses the plane of movement of the hand, at each instance in time the velocity required to achieve this can be obtained from the ratio of the distance4 currently separating hand and ball over the current (first-order) time remaining until the ball will cross the plane of hand movement: X ! Xb X˙h req = h (TC1 )b

(1)

This currently required velocity is evaluated against the existing hand velocity ( X˙h ), so as to determine the changes that are to be made: ˙˙ = !X˙ ˙ X h h req " # Xh

(2)

Movement is initiated when the currently required velocity exceeds a certain threshold and subsequently unfolds as a function of the motion of the ball and hand. Interestingly, adding a noise term, to Equation 2 allows the model to give rise to what we have termed a funnel-like type of control (Bootsma, Houbiers, Whiting, & Van Wieringen, 1991; Bootsma & Peper, 1992) with between-trial variability decreasing as the moment of contact

3

Note that this may in fact be the same in the catching fly balls example: The presence of optical acceleration informs the catcher that the current velocity of locomotion needs to be changed. Zero optical acceleration, on the other hand, specifies that, if current conditions pertain, contact will occur between the actor and the ball when it arrives at eye level. 4

The quantity Xh-Xb denotes the current spatial separation of hand and ball on a com-mon projection axis. Its use in the present formulation should not be taken to imply that Xh and Xb need to be separately assessed (see Peper et al., 1994). Moreover, its optical specification could be related to the magnitude of the angle formed by the direction of movement of the hand and the line connecting the hand and the ball.

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approaches. Variability between tasks and people can be accounted for by virtue of allowable variations in the gain parameters ! and ! . The final point raised by Michaels and Beek (1995), that of learning, can be addressed in the present framework as well, with learning being the establishment and elaboration of the proposed coupling. Modeling attempts as these, although for the moment incomplete because formalization of the informational support is missing, thus allow the problems identified to be dealt with.

Principles for movement Beek et al. (1995) point to the fact that the formulation of the HodgkinHuxley equation in the early 1950s preceded the identification of the structural mechanisms subserving membrane excitation and activation propagation. It is in this light that their comment with respect to the lack of interest on the part of the ecological approach in the neurophysiological underpinnings of the proposed information-movement couplings5 should be qualified. It is, of course, commendable to strive to dissolve the distinction between phenomenological and structural models (e.g., Bullock, this volume). However, to date we have only a very limited understanding of the principles that determine how information and movement are coupled in the pursuit of goal-directed action; Because such principles exist at the behavioral level (where they are selected for their compatibility with the attainment of task goals), it is at this level that adequate models must be initially formulated6. As proved to be the case for the Hodgkin-Huxley model, one would hope that such work may provide a principled basis for addressing the neurophysiological implementation. In my view at least the meta-theoretical basis of the ecological approach provides a sound baseline for such a behavioural modelling enterprise. What then should the research agenda of such an enterprise look like? Bootsma (1994) proposed that at least three steps were implied: (a) formalization of the information that might vehicle the relevant properties of the EAS, (b) demonstration of the sensitivity of observers to these particular informational quantities, and (c) demonstration of the use of the information in the control of movement. However, not only does such a 5

After having used the term perception-action coupling for a number of years, I was recently seduced by Beek and Van Wieringen’s (1994) suggestion to consider perception and movement as the constituent elements of action and to speak, therefore, of perceptionmovement coupling. As will become clear further on (see Figure 2), I believe that the correct term is in fact information-movement coupling. 6

Obviously this position should not degenerate into a lack of interest in ongoing research in other than strictly behavioural areas. Identification of neurophysiological, anatomical, and biomechanical constraints is cleary beneficial to the proper modelling of function.


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scheme suggest that logical progress would proceed from (a) to (c), which need not be true at all, it also fails to provide a framework for the way in which the search for the relevant properties of the EAS, of information, and of movement might proceed. Figure 2(next page) dresses an inventory of the issues that need to be addressed at one point, without specifying the order in which this should be done. While a clockwise progression starting from the top allows a reading along the lines proposed by Bootsma (1994), the networking of the issues to be addressed as depicted in Figure 2 allows for a more coherent and at the same time more encompassing view. Research focussing on each one of the main components identified may serve to provide additional insights and constraints on the others. In the end, a full-fledged account of action necessitates that the relevant properties of the EAS have been identified, that the informational support has been formalized, that the perceptual sensitivity of the observer/actor to this information has been demonstrated (and that the perceptual limitations on its detection have been identified), that the (neuro-skeleto-muscular) executional requireements have been determined and the proper activation dynamics have been identified, and finally that a clear description of how movement is regulated by information has been provided. Note that whereas the perception-actuation couplings (i.e., the way in which detected information is transformed into a behavioural dynamics) are located within the animal and thus speak to neurophysiological issues, the information-movement couplings that they subserve are located in the interaction between the animal and its environment. Importantly, the latter describe the functions of behaviour and capture the behavioral principles that evolution and learning have allowed the animal to discover. Because lawful relations between information and movement may be subserved by different kinds of perception-actuation couplings, the neurophysiological underpinnings are to be considered as constraints on possible information-movement couplings and not as their causal basis.

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Figure 2. Issues for the research agenda of the ecological approach to action. Solid arrows pointing in clockwise directions designate processes involved. SPECIFICATION : Relevant properties of the EAS give rise to flow patterns that are specfic to them, thus constituting information about them. DETECTION: When compatible with perceptual constraints (e.g., observer sensitivity), information can be detected, thus allowing perception of the relevant EAS properties. INFORMATION-MOVEMENT COUPLING: Attaining a task goal requires that the state of the EAS can be modified, which is accomplished by setting up an adequate functional relation between information and movement. PERCEPTIONACTUATION COUPLING: Implementation of the coupling of information detected to a behavioral dynamics ACTIVATION : Movement results from the interplay between internal and external forces. MECHANICS: Movement of the masses involved leads to changes in the EAS. Dotted arrows pointing in counter clockwise direction designate the tools used for studying the processes involved. FLOW PATTERN PHYSICS : The study of ecological optics, acoustics, etc., seeks to identify the flow pattern descriptors that are specific to relevant EAS-properties. PSYCHOPHYSICS: The study of observer sensitivity to flow pattern descriptors. INFORMATIONAL CONTROL: The study of how informational quantities feed into movement. NEUROPHYSIOLOGY : The study of how perceived properties are transformed into muscular activation patterns. ACTIVATION DYNAMICS : The study of the mechanisms underlying the structure of neuro-muscular activation patterns. TASK CONTROL: The study of how pertinent EAS-properties influence movement patterns.


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