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234Gestalt Theory, Vol. 25 (2003), No. 4With a congruous palm position, a stability advantage of the symmetrical patternas well as a tendency towards symmetry was expected, as in KELSO’s (1984) results.This was so because here, as in KELSO’s original experiment, symmetrical fingeroscillation goes together with periodic co-activation of homologous muscle groups.The interesting condition concerned the incongruous palm positions, because hereonly the parallel movement mode may involve periodic co-activation of homologousmuscle groups. If there is actually a tendency towards a co-activation of homologousmuscles, then the parallel movement mode should be more stable than the symmetricmode. On the other hand, if there is a tendency towards spatial, and thus perceptual,mirror-symmetry, then the symmetric movement mode should be more stablethan the parallel mode, even though non-homologous muscle groups are periodicallyco-activated.The findings by MECHSNER et al. (2001) were straightforward. With congruousas well as incongruous palm positions, the mirror-symmetrical movement patternwas more stable than the parallel pattern. Independent of palm positions, spontaneoustransitions from an instructed parallel movement into symmetry were observedat increasing oscillation frequencies, but not into the reverse direction. The sameoutcome was revealed if the participants’ view was occluded, thus perception of thehands was restricted to proprioception. MECHSNER et al. concluded that the symmetrytendency is biased towards spatial, perceptual symmetry, without any regardto the involved muscles, or to the corresponding neuronal commands.This is a challenging result. To be fair, it has been known before that processesin the perceptual functional medium can be powerful enough to bring about spontaneousmovement coordination phenomena. For instance, two participants looking ateach other will tend to synchronize swinging limbs as well as swinging pendulums(SCHMIDT et al. 1990,1998; SCHMIDT & O’BRIEN 1997). Even spontaneous transitionsfrom parallel into symmetrical movements occur. Intrapersonal oscillationsof one hand and one foot tend to adopt a parallel oscillation pattern, independent of aprone or supine position of the forearm (BALDISSERA et al. 1982). Manipulation ofhaptic feedback can stabilize, but also reverse the preferred coordination pattern in afinger flexion and extension task (KELSO et al. 2001). Such evidence notwithstandingit has been argued that intrapersonal coupling of homologous limbs is a specialcase (e.g., SWINNEN et al. 1998), as coupling of homologous muscles is possiblehere, and indeed the reason for the symmetry tendency. Until now, there has been noevidence against this common claim.MECHSNER et al.’s (2001) results on finger oscillation demonstrate that there isreason to doubt the role of motoric constraints in bringing about the symmetry tendency.Instead, one may plausibly hypothesize that this tendency is purely perceptualin nature. Confirming results were obtained in the case of a bimanual four-finger tappingmodel (MECHSNER et al. 2001), as well as in a bimanual wrist oscillation model(MECHSNER et al., submitted). It is worth noting that it seems to be no problem toinstantaneously organize the suitable muscular activity that corresponds to the perceptualtendency. The suitable motoric neuronal commands seem to be automaticallyselected and tuned in, always adapted to the perceptual movement goal.
Mechsner, Gestalt Factors in Human Movement Coordination 235One may speculate that not only spontaneous but also voluntary movement patternsare coordinated directly by way of perceptions and perceptual anticipations,whereas corresponding coordinative processes in the motor system are not at allnecessary. In all fairness, in saying so I certainly acknowledge that there are complexpatterns of muscular activity and motoric commands that bring about the movements.However, I assume that such a pattern is not an autonomous entity, which isconceived as such and well organized in and of itself. I rather assume that this patternis organized as a by-product of ongoing processes in a perceptual functional medium.Compare the motor system to a piano, movements to ongoing music, and theplayer to the perceptual-cognitive system. In order to produce music it is possible tomake use of a player piano or electronic equipment that produces rhythms, harmonies,or even produces fully arranged melodies. On the other hand, one may play anormal piano. In this case the complex activity inside the apparatus is not due to thecomplexity of an autonomous internal configuration, but instead is fully dependenton what the player is doing. I am much inclined to hypothesize that the motor systemdoes not work like a player piano, thus its activity is fully dependent on what is goingon in the cognitive and perceptual functional medium.An experiment on bimanual circling by MECHSNER et al. (2001) provides additionalevidence. Participants circled two visible flags by way of two cranks, whichwere hidden under the table (Figure 3a). The left flag circled directly above the leftcrank and hand, i.e., in exact spatio-temporal correspondence to the hand. The rightflag, however, circled in a 4:3 frequency relationship with regard to the right crankand hand, due to a gear system.FIGURE 3: Flag circling apparatus (a) (see text). Instructed flag movement patters. Mirror-symmetry(b). Antiphase (c). Reprinted by permission from Nature 414: 71 copyright 2001 Macmillan PublishersLtd.After twenty minutes of training, primarily with the handling of the right flag, theparticipants were instructed to circle the visible flags inwards either in mirror-symmetryor in antiphase (see Figure 3b,c). They were asked to begin at a slow, comfortablevelocity and then speed up to a velocity they considered fast, but not beyond the
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