In his book, Understanding Balance: The Mechanics of Posture and Locomotion (1995), Roberts concentrates on physics, mechanics, neurophysiology, and the "recognition" of postural states. Yet there is little analysis of what is recognized, or how recognition occurs. There is no explicit discussion of the role of supra-postural goals in shaping postures, and in evaluating their success. Supra-postural goals define "normal" and "abnormal" postures, and so must be taken into account in understanding postural control.
2. Roberts focuses on physics, bio-mechanics, and neurophysiology. While these are undoubtedly important to postural control, they do not exhaust the range of relevant factors. For example, no mention is made of the recent literature on posture and optical flow (e.g., Warren, Kay & Yilmaz, 1996), models of the neural dynamics of posture (e.g., Schoner, 1991), dynamical approaches to the development of posture and locomotion (e.g., Thelen & Smith, 1994), Nashner's influential work with moving platform posturography (e.g., Nashner & McCollum, 1985), recent work on affordances for postural control (e.g., Stoffregen & Riccio, 1988), or non-linear analyses of action (e.g,. Kay, 1988; Riccio & Stoffregen, 1991). No explicit justification is given for the choice of foci, for what is included and what is left out. Inclusion might be motivated on the basis of an explicit, coherent theory, but this is not done. Disturbingly, nowhere is there an explicit statement of what the book is intended to accomplish, beyond a "search for explanatory descriptions" (p. 4). Is the book's purpose to provide a survey of the field? To present a novel theory? To summarize Roberts' own research? The lack of explicitness about both purpose and theory is a nagging problem. I have attempted to derive some of the assumptions that are implicit in the book. This may motivate Roberts to present more explicit statements.
3. In Roberts' analysis "what" takes precedence over "why". This is apparent in his concentration on an attempt to "ascertain just what it is that actually happens" (p. 1). This enterprise provides us with a wealth of information, but there is no discussion of why animals stand up, or of how orientation might be related to the purposes of posture. The only function that receives explicit mention is to "avoid falling over". While this is a real postural goal, it is not the only one that shapes postural control. No explicit consideration is given to the pervasive existence of supra-postural goals (e.g., looking, manual manipulation, etc.). In this, Roberts' treatment resembles much contemporary research (e.g., Dijkstra, Schoner & Gielen, 1994; McCollum & Leen, 1989; Warren, et al., 1996). Silence on the function of postural stability constitutes an implicit theoretical assumption that the goals (intended function) of behavior do not play an important role in the organization or execution of postural control. This assumption, once made explicit, can easily be questioned.
4. Behavioral goals are of great importance for postural control. Stance is not undertaken for its own sake, but in the furtherance of other, supra-postural goals; this is true even in young children (Slobunov & Newell, 1994). Stance is useful to animals only to the extent that upright posture facilitates the accomplishment of other goals (Riccio & Stoffregen, 1988). Roberts acknowledges that animals engage in a variety of postures (e.g., voluntary head movements, running, hopping), but does not say why this is so. Nor does he consider the possibility that supra-postural goals may influence not only the choice of postures but also the way in which posture is controlled (consider variations in posture that result from changes in mood). The disinclination to discuss the function of postural control reflects a general absence in the book of issues relating to evolution, such as the adaptive aspects of behavior. There are only occasional indirect allusions to the evaluation of postural control in terms of its functionality, as in the mention of sensitivity to "normal" and "abnormal" postures (p. 4). Roberts does not define these terms, but it is hard to imagine any definition that does not refer to supra-postural goals. Postures that are "normal" for one behavior would be "abnormal" for another (e.g., contrast ballet with Roberts' example of the initial strides in sprinting). Similarly, Roberts states that "the essence of balancing behavior consists of the appropriate organization of the forces developed between the body and its supports", (p. 4). The use of "appropriate" seems to be an implicit acknowledgment of the centrality of supra-postural goals in the organization of postural control, yet Roberts makes no attempt to define the term. Rather, the issue is shifted to neural mechanisms for the "recognition" of postural states (to be discussed below). This begs the question of what is being recognized.
5. The treatment of physics is detailed and accurate, but incomplete. Missing is any discussion of the dynamics of balance. In stance the human body functions as a set of connected inverted pendula (Stoffregen & Riccio, 1988), yet Roberts provides no discussion of inverted pendulum dynamics. One consequence of inverted pendulum dynamics is the existence of a direction of balance (usually, but not always, corresponding to the direction of unstable equilibrium). The direction of balance is a real, physical property of inverted pendulum systems, which has strong consequences for the muscular operations that are needed to establish and maintain balance (Stoffregen & Riccio, 1988). Roberts alludes to related phenomena (e.g., he understands that in a coordinated turn in flight, the vector sum of gravity and inertial force rotates with the aircraft, p. 290), but does not appear to regard this as being consequential for the control of posture. Recent research (Riccio, Martin & Stoffregen, 1992) has shown that human orientation is strongly influenced by the direction of balance. This has considerable implications for the definition of "up" (Riccio & Stoffregen, 1988), and for Roberts' concept of a "behavioral vertical" (Chapter 6). The direction of balance changes frequently as a consequence of muscular forces, such as those exerted in locomotion. Moreover, the animal's orientation relative to the direction of balance is available in sensory stimulation, such that it could, in principle, be perceived directly (Stoffregen & Riccio, 1988). Thus, alignment relative to the direction of balance might provide a concrete underpinning to the largely undefined behavioral vertical.
6. For Roberts, balancing behavior is thought to "depend on the mysterious process of 'recognition'", (p. 93). Much of the burden of Roberts' conception of postural control appears to rest on this concept (e.g., the definition of different postures or motions as "normal", "abnormal", "appropriate", etc.). In Roberts' view successful balancing will depend absolutely on the ability to distinguish stable from unstable postures, that is, to recognize them. The importance placed on recognition would seem to call for a substantial discussion of the concept. It is disappointing, then, that Roberts confines himself to a discussion of possible neurophysiological structures that may achieve recognition. Roberts appeals to computer simulations of neural networks, but is obliged to acknowledge that the brain may not "perform the necessary signal transformations in the same way as the artificial network does," (p. 318). Thus, the vague conclusion that "it might well prove profitable to think of neural behavior in terms of networks of some kind", (p. 318) appears to have little empirical basis. We are left with little understanding of how recognition might work.
7. A larger problem with the focus on neural modeling is that it fails to come to grips with the necessity for a logical and/or theoretical treatment of the concept of recognition. What is recognized, and how is it defined? What does it mean to recognize a postural state? One conclusion that might be drawn from this is that physics, bio-mechanics, and neurophysiology are insufficient to explain the achievement and maintenance of posture. An alternative to recognition can be found in analyses of self-organization in non-linear systems (e.g., Kugler & Turvey, 1987).
8. The discussion of perceptual issues follows standard inferential models. For example, the section on proprioception discusses only the somatosensory and vestibular systems, despite the acknowledgment (p. 164) of powerful proprioceptive sensitivity in the visual system. The proliferation of sensory reference frames on p. 285 is remarkable. Separate reference frames are proposed for each joint, and "perhaps also for each individual sensory receptor ... governed by its anatomical relations to the rest of the tissue in which it is embedded". The existence of thousands (possibly millions) of anatomically distinct somatosensory receptors would, thus, imply a correspondingly large number of sensory reference frames. Reducing these to a single reference frame for action would be a Herculean task, requiring a large, cumbersome, and unspecified neural machinery. This sounds like the inverse of the degrees of freedom problem that has been such a roadblock for motor program theories of motor control (Turvey, Fitch & Tuller, 1982). An implication of this difficulty is that the appeal to multiple, independent sensory reference frames may not be useful in understanding perception (Stoffregen, 1994).
9. On p. 294 there is a reference to "disturbances in balancing behavior that arise when the labyrinth is exposed to unusual conditions of stimulation". Yet in the preceding discussion of the Coriolis effect and "misleading" signals from the labyrinth there is little reference to disturbances in balance. Instead there is a near-total concentration on subjective reports. On pages 289-292 I counted fourteen references to conscious awareness, and only three relating to the control of action (one of which was about flight control). This reflects a long-standing failure in the motion sickness community to address the behavioral correlates and consequences of nauseogenic situations (Stoffregen & Riccio, 1991).
10. In general, the focus on the material underpinnings of balance suggests a reductionist approach to the study of posture. For centuries, reductionism has been so widespread as to require no explicit justification, but in recent decades there have developed approaches in which reductionism is explicitly rejected. Among these are Bernstein's approach to motor behavior (Bernstein, 1996), and the ecological approach to perception and action (Gibson, 1979/1986; Stoffregen & Riccio, 1988), together with syntheses of these views (e.g., Kugler & Turvey, 1987). The success of such theories indicates that reductionism can no longer be taken for granted. Thus, approaches that retain a reductionist perspective can be challenged to present an explicit motivation for this choice.
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