Influence of stimulus amplitude on unintended visuomotor entrainment

Abstract

Rhythmic limb movements have been shown to spontaneously coordinate with rhythmic environmental stimuli. Previous research has demonstrated how such entrainment depends on the difference between the movement periods of the limb and the stimulus, and on the degree to which the actor visually tracks the stimulus. Here we present an experiment that investigated how stimulus amplitude influences unintended visuomotor entrainment. Participants performed rhythmic forearm movements while visually tracking an oscillating stimulus. The amplitude and period of stimulus motion were manipulated. Larger stimulus amplitudes resulted in stronger entrainment irrespective of how participants visually tracked the movements of the stimulus. Visual tracking, however, did result in increased entrainment for large, but not small, stimulus amplitudes. Collectively, the results indicate that the movement amplitude of environmental stimuli plays a significant role in the emergence of unintended visuomotor entrainment.

Introduction

Rhythmic human movements spontaneously coordinate with those of other individuals or environmental stimuli during visual interaction (e.g., Oullier et al., 2008, Schmidt and O’Brien, 1997, Schmidt et al., 2007, Varlet et al., 2011). Such entrainment can occur in everyday life when an actor is walking, dancing, or just talking with another individual (e.g., Richardson et al., 2005, van Ulzen et al., 2008). Having access to visual information about the movements of an environmental rhythm, however, does not ensure that entrainment will occur (e.g., Shockley, Santana, & Fowler, 2003) and the emergence and stability of spontaneous (or, unintended) coordination can depend on the difference between the natural periods of the movements involved and the degree to which the actor attends to the relevant movement information (e.g., Richardson et al., 2007, Schmidt et al., 2007). The aim of the current study was to investigate whether movement amplitude also influences the occurrence and stability of unintended visuomotor coordination. More specifically, the study examined whether the larger the movement amplitude of a visual stimulus, the greater the occurrence and stability of unintended visuomotor entrainment.

Inspired by the dynamical systems theory, previous research has shown that rhythmic visuomotor coordination between the movements of an individual and an environmental stimulus or rhythm is constrained by the dynamical entrainment processes of coupled oscillators (e.g., Byblow et al., 1995, Richardson et al., 2005, Schmidt et al., 2007, Wimmers et al., 1992). In line with the predictions of the Haken, Kelso, and Bunz (1985) coupled oscillator model that captures the dynamics of bimanual rhythmic coordination (see also, Kelso, 1995) and its derivatives for visual coordination perception (e.g., Bingham, 2004, Bingham et al., 1999, Zaal et al., 2000), visuomotor limb-to-stimulus movements are constrained (without practice) to in-phase and anti-phase patterns of coordination (relative phase values of 0° and 180°, respectively), with in-phase coordination being more stable than anti-phase coordination (e.g., Richardson et al., 2007, Schmidt and O’Brien, 1997, Wimmers et al., 1992). In addition, the stability of in-phase and anti-phase coordination decreases as movement period decreases (i.e., movement becomes faster) and the difference between the natural periods (i.e., detuning) of movement’s involved increases, with anti-phase coordination becoming unstable at fast movement periods and for large magnitudes of detuning (e.g., Richardson et al., 2007, Schmidt et al., 1990; see Schmidt & Richardson (2008) for a review).

Rhythmic visuomotor coordination can occur both intentionally and unintentionally (spontaneously and without awareness). Like bimanual coordination, intended visuomotor coordination is typically absolute, meaning that either in-phase or anti-phase coordination are stably maintained for an extended period of time. In contrast, unintended coordination tends to be relative because of the weak strength of the coupling (e.g., Richardson et al., 2007, Schmidt and O’Brien, 1997, von Holst, 1973) and is characterized by an intermittent attraction toward in-phase and anti-phase patterns of coordination (e.g., Issartel et al., 2007, Schmidt and O’Brien, 1997, Tognoli et al., 2007). Accordingly, unintended visuomotor entrainment is much more affected by differences between the period of the visual stimulus and the preferred period (i.e., comfort mode) of the actor than intended visuomotor coordination, with even small differences in period greatly reducing the chance that entrainment will occur (e.g., Richardson et al., 2007, Schmidt and O’Brien, 1997, Schmidt et al., 2007).

Lopresti-Goodman, Richardson, Silva, and Schmidt (2008) provided a clear demonstration of the significant impact period difference has on the occurrence of unintended visuomotor coordination. In this experiment, participants were instructed to oscillate a wrist pendulum at a self-selected comfort tempo while simultaneously reading letters displayed on an oscillating visual stimulus displayed on a projection screen. The period of the visual stimulus was manipulated as a function of the participant’s preferred movement period as measured from a set of pre-trials. The results demonstrated that the magnitude and stability of entrainment decreased when the stimulus period was faster or slower than the participant’s preferred movement period, with no visuomotor entrainment occurring for stimulus periods that were greater or less than 15% of the participant’s preferred movement period.

Researchers who have investigated unintended visuomotor entrainment have also shown how the strength of the visual coupling is mediated by the degree to which an actor attends to the displacements of rhythmic stimuli. Using a similar method to the Lopresti-Goodman et al. (2008) study just described, Schmidt et al. (2007) demonstrated the role that visual tracking plays in the emergence of visuomotor entrainment. Participants were instructed to read letters that were displayed at random intervals either on an oscillating visual stimulus (i.e., tracking condition) or just above the middle of the trajectory of an oscillating visual stimulus (i.e., non-tracking condition). Results demonstrated that entrainment occurred for both non-tracking and tracking conditions, but that significantly stronger entrainment occurred for the tracking condition compared to the non-tracking condition (see also Richardson et al., 2007). One reason why stronger entrainment occurs when participants visually track the stimuli could be that the movement of the eyes establish an intrapersonal eye-limb coupling that, when added to the informational coupling, operates to strengthen the entrainment (Schmidt et al., 2007). Different studies support the existence of such coupling between eyes and limb movements. Better visual tracking of an oscillating stimulus has been demonstrated when its displacement was manually tracked by participants at the same time (Koken and Erkelens, 1992, Leist et al., 1987). Coupling between limb and eye movements has also been reported in other experimental situations such as pointing and reaching tasks (e.g., Lunenburger et al., 2000, Snyder et al., 2002). Previous research has also showed a significant influence of eye movements on the execution of limb movements similar to the findings of Schmidt et al. (2007) (e.g., Henriques and Crawford, 2002, van Donkelaar, 1997). In addition, when tracking visual stimuli, participants have better access to the endpoints of a movement trajectory that could explain stronger entrainment as well. The endpoints of a stimulus trajectory contain important turn-around point information that strengthens visual coupling by anchoring the movements in time and space (e.g., Bingham, 2004, Hajnal et al., 2009, Roerdink et al., 2008, Roerdink et al., 2005, Wilson et al., 2005).

The above-mentioned research has provided clear evidence that both visual tracking and period difference moderate the strength of unintended visuomotor entrainment. Although the influence of the stimulus amplitude on unintended entrainment has never been investigated, different studies have focused on the role of movement amplitude in bimanual rhythmic motor coordination (e.g., Peper and Beek, 1998a, Peper and Beek, 1999, Post et al., 2000). The results of these previous studies have been somewhat varied, however. Specifically, some investigations have shown an increase of the coupling strength in bimanual coordination for larger amplitudes of movement in line with the Haken–Kelso–Bunz model predictions, whereas other studies have found no influence of movement amplitude (e.g., Peper and Beek, 1998b, Peper et al., 2008, Post et al., 2000). It has also been demonstrated that amplitude asymmetry in bimanual coordination produces an asymmetry between the preferred movement periods of the two hands and destabilizes the coordination (de Poel, Peper, & Beek, 2009). For intended visuomotor coordination, manipulations of movement amplitude have been found to have little to no effect on the stability of coordination (de Rugy et al., 2008, Peper and Beek, 1998a). It is possible, however, that the effects of amplitude may only be apparent for weaker states of coordination, that is, for intermittent unintended coordination (Ridderikhoff, Peper, & Beek, 2005). The possibility that stimulus amplitude may influence the strength of unintended visuomotor entrainment is suggested by research that has investigated the spontaneous entrainment that occurs between the postural movements of a standing participant and the movements of a moving room (e.g., Dijkstra et al., 1994a, Dijkstra et al., 1994b). Although the relation between the visual information and movement performed differed in this postural research, motion of the room with larger amplitudes resulted in stronger entrainment. More generally, previous research that has shown how the effects of visual tracking and period difference on the stability of visual coordination are significantly reduced during intended coordination and only clearly apparent during investigations of unintended coordination (e.g., Richardson et al., 2007, Schmidt et al., 2007).

The current study investigated whether stimulus amplitude influences unintended visuomotor entrainment by using a paradigm similar to those previously employed (e.g., Lopresti-Goodman et al., 2008, Schmidt et al., 2007). Participants were instructed to rhythmically oscillate their forearm while viewing an oscillating visual stimulus of various amplitudes. We expected that larger stimulus amplitudes would increase the occurrence and stability of visuomotor entrainment. Alternatively, it was possible that the strongest entrainment would occur when the stimulus amplitude was equal to the preferred movement amplitude of participants, rather than for the largest stimulus amplitude. In fact, stimulus amplitudes larger or smaller than participants’ preferred amplitude could change their movement amplitude and thus their movement period due to the intrinsic frequency–amplitude relation of human movement (Kay et al., 1987, Rosenbaum et al., 1991). Such a change may decrease entrainment by increasing the stimulus-participant period difference in line with the amplitude asymmetry effect observed in bimanual coordination (de Poel et al., 2009, Lopresti-Goodman et al., 2008).

Moreover, we expected in view of previous research that the influence of stimulus amplitude on unintended visuomotor entrainment would depend on the degree to which participants visually tracked the stimulus. Accordingly, stimulus amplitude was manipulated in both tracking and non-tracking conditions (Schmidt et al., 2007). For the tracking condition, we anticipated an increase of the intrapersonal eye-limb coupling for larger stimulus amplitudes (due to an increase in the movement amplitude of the eyes) and that this would, in turn, strengthen visuomotor entrainment. For the non-tracking condition, in which participants only focused their attention on the middle of the stimulus trajectory, it was possible that higher stimulus amplitudes could have a negative effect on the magnitude of visuomotor entrainment because larger amplitudes would decrease the peripheral availability of endpoint information (e.g., Bingham, 2004, Hajnal et al., 2009, Richardson et al., 2007, Wilson et al., 2005).

Further, it was possible that the stimulus period might also moderate the influence of stimulus amplitude on the strength of unintended visuomotor entrainment. Considering that manipulations of stimulus amplitude could affect both the amplitude and period of participant movements, the unintended entrainment observed could be the result of an interaction between the amplitude and the period of the stimulus. For instance, entrainment observed for stimulus amplitudes greater than the participant’s preferred amplitude could be stronger when the stimulus period is also slower than the participant’s preferred period. Therefore, the stimulus period was also manipulated in the experiment along with stimulus amplitude and visual tracking.