Tuesday 19 July 2011

Lissajous feedback and coordination stability

Understanding the perceptual information you provide people in a task is a critical element of the perception-action analysis. Last time I talked about the new form of coordination feedback I developed to allow us to train coordinated rhythmic movements without perturbing the task dynamic. Prior to this, the most common form of augmented feedback was the Lissajous plot - these are the result of plotting the displacements of two harmonic oscillators against one another, and the unique shape associated with each relative phase can be used as a template on the screen. People can then try to move so as to make a dot trace that shape.

Lissajous plots (have a play with them in this Excel file) are transformed feedback, because they take a coordinated movement and represent it on the screen as the motion of a single dot. This type of feedback has been used extensively to train people to perform novel coordinations, but until recently no-one had thought to investigate the consequences of transforming the information about relative phase. Kovacs, Buchanan and Shea have recently begun doing exactly this, and, in line with the perception-action approach developed by Bingham and pushed at every opportunity by myself, these authors have found that Lissajous plots completely alter the nature of the task, with serious consequences for the studies that rely on it.

Performing 90° with Lissajous feedback
Kovacs et al (2009a) investigated whether people could produce 90° using Lissajous feedback without extensive practice. It typically requires a least a couple of days practice before people can do this; however, Kovacs et al correctly identified that the Lissajous format display really should be easier to use than that, because it consolidates the coordination information into a readily detected signal. They note that most studies pace frequency with an auditory metronome, and also often allow vision of the limbs. The latter had recently been shown to be a problem (Shea et al, 2008).

They compared performance at 90° with Lissajous feedback, with and without a metronome and with no vision of the hands. Without the metronome, the visual feedback display stabilised performance over 10 30s trials (about 5 mins). With the metronome, performance was terrible, both variable and inaccurate. Both groups suffered when the feedback template was removed for two trials at the end, suggesting they both depended on it and hadn't actually learned to produce 90°, just how to track the feedback.

Summary: Lissajous feedback can quickly stabilise performance of 90°, but the commonly used metronome is a major distractor (to add to 'vision of the limbs' from the previous study).

Performing other novel coordinations
Kovacs et al (2009b) then tested whether this effect generalised to other novel coordinations (0°-180° in 30° increments). One group performed using  Lissajous feedback, no metronome and no vision of the arms. The second group performed using a visual metronome, with vision of the arms (cf the 'scanning' sessions developed by Zanone & Kelso). There were three practice blocks from 0° to 180° and back again, followed by the test block in the same format. All data are from the last block, and the feedback template was always on.

The visual metronome group produced the typical result; 0° and 180° were produced most accurately and stably, with 180° less stable than 0°. All other coordinations were performed very inaccurately and with high variability. In contrast, the Lissjous group performed all 7 coordinations with high accuracy, and while the non-0° conditions were more variable than 0°, they were still more stable than under the metronome. With about 20 minutes practice and no distractions, the Lissajous group were able to reliably produce any coordination.

Summary: Lissajous feedback, in the absence of distractors, can quickly allow stable movement at any relative phase with only slight increases in variability.

Amplitude Variations
One of the ways to reduce the stability of coordinated rhythmic movements is to make people move their limbs at different amplitudes. You typically see amplitude assimilation where the two amplitudes converge on some intermediate value and resisting that makes the coordination harder to maintain. These errors often also lead to problems maintaining the target relative phase, and, on top of all of this, there are asymmetries in all these effects due to handedness.

Kovacs & Shea (2010) tested whether Lissajous feedback helps people moving at different amplitudes at either 0°, 90° or 180°. With Lissajous feedback, movement accuracy was good, although variability looked more typical - higher at 90° than 180° which was higher than 0°. However, movements were more accurate and more stable with than without Lissjous feedback. Lissajous feedback also reduced amplitude assimilation, but only when the dominant right hand was performing the smaller amplitude.

Summary: These results are less clear cut, but they do demonstrate that Lissajous feedback does help to stabilise the coordination, and was sometimes able to help stabilise the amplitude control too. This partial effect may have something to do with the fact that Lissajous feedback represents amplitude differences statically, in terms of the radius of the template figure. We think (from an upcoming paper) that amplitude is actually perceived in terms of energy, the radius of the movement on the phase plane. The trouble people had using the Lissajous plot to control amplitude may have something to do with this. This is just something that has come up very recently, so as yet I don't have a more formal story. However, my hunch is that, again, these results support the broader attempts to keep perception firmly in the story. 

Overall summary
In general, Lissajous feedback stabilises all kinds of otherwise difficult movements. It does this by removing the need to visually perceive a coordination; instead, all you need to do is perceive a single dot which represents that coordination. The perceptual task no longer varies in difficulty across relative phase, and the stable perception of the target allows stable behaviour. This is great evidence in favour of the perception-action approach and (with our coordination feedback data) for the role of the perception of coordination information as the primary source of the HKB phenomena.

Kovacs et al are mostly on board with that analysis; they cite our work and talk about it sensibly, which makes a refreshing change. There's one aspect of their analysis I don't think quite works, though. They note that the dynamic pattern approach suggests the bistability of 0° and 180° is the result of attractor dynamics. They then suggest, however, that, on the basis of their results, the bistability actually seems to be a side effect of attentional confounds (metronomes, vision of the limbs). This I don't agree with; I think our coordination displays, which still produce bistability without metronomes or vision of the limbs, confirm that the bistability emerges from the overall perception-action dynamic, of which information is a key part. These attentional issues clearly matter; but I don't think they are the ultimate source of the key structure. Still, some excellent results which I intend to engage with empirically over the next year; I'm confident I'll be able to extend and support these data.

Kovacs, A., Buchanan, J., & Shea, C. (2009a). Bimanual 1:1 with 90° continuous relative phase: difficult or easy! Experimental Brain Research, 193 (1), 129-136 DOI: 10.1007/s00221-008-1676-2

Kovacs, A., Buchanan, J., & Shea, C. (2009b). Using scanning trials to assess intrinsic coordination dynamics Neuroscience Letters, 455 (3), 162-167 DOI: 10.1016/j.neulet.2009.02.046

Kovacs, A., & Shea, C. (2010). Amplitude differences, spatial assimilation, and integrated feedback in bimanual coordination Experimental Brain Research, 202 (2), 519-525 DOI: 10.1007/s00221-009-2154-1

Shea, C.H., Buchanan, J.J., & Kovacs, A.J. (2008). Cooperation between the limbs is better than we thought. Journal of Sport & Exercise Psychology, 30, S128.   

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