In the target article, Dijkerman & de Haan (D&dH) argue “in normal circumstances, … perceptual and action-based processes must operate in a coordinated fashion” (sect. 7, para. 1). According to their account, healthy participants coordinate their perceptions and actions more than disabled participants do. However, the authors do not explain how perception and action are coordinated. They mostly focus their arguments on how these two processes are separated from each other in the somatosensory modality. We want to focus, instead, on the coupling between perception and action. We argue that to begin any discussion on perception and action, it is necessary to start from how perception and action are linked before knowing how these two systems are subdivided. We attempt to illustrate our argument with two major points.

First, two decades of research have shown that inter-limb relationships and environment-movement relationships can be thoroughly understood if one first considers the so-called macroscopic level of coordination (Kelso Reference Kelso1995; Turvey Reference Turvey, Jirsa and Kelso2004). In multiple experiments (Kelso et al. Reference Kelso, Delcolle, Schöner and Jeannerod1990; Stins & Michaels Reference Stins and Michaels1999; Wimmers et al. Reference Wimmers, Beek and van Wieringen1992), experimenters showed that when one oscillating limb is coordinated with an external stimulus, the coordination is more stable when the phase difference between these two components is in phase (synchronization) rather than in anti-phase (syncopation). For instance, if the two components are in anti-phase, then an abrupt phase transition to in-phase, through a loss of stability, is provoked when the control parameter of frequency (rate) is increased; if the initial phase relation is in-phase, then the pattern of phase difference is maintained over the same range of parameter change. Moreover, at a neurophysiological level, the spontaneous changes from syncopation to synchronization provoked by the variation of the rate of the periodic stimulus is accompanied by a drastic reorganization of the large-scale cortical activity (Fuchs et al. Reference Fuchs, Kelso and Haken1992; 2000; Mayville et al. Reference Mayville, Bressler, Fuchs and Kelso1999). These results are observable because one is looking at the macroscopic level. To do so, one needs a measure that affords a direct quantification of the relation between the components, which corresponds, for instance, to the relative phase between the actor and the environment. If we were to try to study each component separately, we would not have access to the crucial information about the actual stability of the behavior. In the first place, stability indicates the particular relations between movement and environment that can be maintained and, therefore, that are spontaneously selected. Secondly, through stability changes one may address transitions between coordination patterns that occur under variations of environmental or internal parameters (e.g., rate of stimuli, attention load).

Other experiments in interpersonal coordination using a dual EEG set-up bring evidence for neural correlates specific to the actual state of the coordination between the participants, quantified by the variability of the phase difference between participants' movements, not by the variability of the phase for each participant's movements considered in isolation (Tognoli et al. Reference Tognoli, Lagarde, DeGuzman and Kelso2007). In the case of multisensory integration, it was also recently found that this macroscopic level is the relevant level of study to understand the binding of movement and multiple senses. For example, Lagarde and Kelso (Reference Lagarde and Kelso2006) found with a basic paradigm that the particular relationships between action (such as flexion and extension of an oscillating finger) and periodic sensory modalities (such as sound and touch) were determinants of the stability of multisensory integration and its breakdown. Together, this empirical evidence proves that one should not consider lightly the impact that the dynamic link between perception and action has on the adaptive behavior of humans or animals, and the non-trivial qualitative consequences of this link on the functioning of both perception and action.

The second reason that we believe the link between action and perception is a fundamental discussion point, is demonstrated by Pagano and Cabe (Reference Pagano and Cabe2003; see also Pagano et al. Reference Pagano, Fitzpatrick and Turvey1993) in a study where blindfolded participants had to recognize the shape and length of simple rods. Results showed that participants accurately judged the shape and the length of the rod by moving it with one hand. In this case, it is the action that permits the perception recognition process. This illustration shows that not only is perception needed for an action, but also an action is needed for perception. When considering the what (perception for recognition) and how (perception for the guidance of action) of somatosensory processes as proposed by D&dH, it is coordination that prevails: some object's properties are perceived via movement. Following from Gibson's perception view (Gibson Reference Gibson1979), this very relevant example illustrates how movement creates the information required to recognize the object's properties. Going further, Bardy and Laurent (Reference Bardy and Laurent1998) demonstrated that, to control a somersault, gymnasts use peripheral optical flow information. But the optical flow is produced by the somersault itself. This example perfectly illustrates the inextricability of the “what” and “how” and, by extension, perception and action as well. Dissociating the two processes should not come first, and is clearly applying a very restricted understanding of goal-directed behavior's adaptive properties – perception and action are inseparable.

In understanding perception and action, the dynamic relationship between the two components must be considered. From our point of view, it is necessary to start from the coupling among components in order to understand the macroscopic state of the behavior, its stability, and hence its efficiency in a given context. Understanding how perception and action are linked will help us understand how each component works, because coordination is the hallmark of what people do every day in any given movement and is the level at which the stability of behavior ultimately lies.