16 February 2011

Diving in manipulations: asymmetric bimanual interactions

Back to the blog after a few weeks off ! In a previous post, we talked about the future of manipulations in NUI and the key role of tools. In this post we’re going to see how we interact with our tools and we’ll talk about the key role played by asymmetric bimanual interactions.

What kinds of interactions ?
First, we should note that manipulation of tools by human beings is often realized with hands. It can seem obvious but this isn’t an absolute rule: try to drive a car without using your feet… Anyway, for this post we’ll focus exclusively on manual interactions.

Different kinds of manual interactions
Human beings interact with tools in many different ways but for this post, we’ll retain a simple classification:
  • Interactions realized with one hand (unimanual interactions)like turning a door-handle, …
  • Interactions realized with two hands (bimanual interactions), which can be divided in:
    • Symmetric bimanual interactions like rope skipping, riding a bicycle, driving a car with 2 hands handling the steering-wheel, …
    • Asymmetric bimanual interactions like playing guitar, eating with knife and fork, driving in a nail, writing, …
At first, writing can seem a strange example of bimanual interaction. Don’t we use only one hand to hold and move a pen ? But if you think about it, the importance of the second hand becomes obvious: this hand is required to immobilize the sheet of paper while the other hand writes. But this hand is also useful to move the sheet of paper in order the other hand stays at the same place on the desktop. You’ll see a perfect sample in following video.

Observation of human activity shows that asymmetric bimanual interactions play a key role in our life. Most skilled manual activities involve two hands playing different roles.

Asymmetric bimanual interactions : the kinematic chain model (Y.Guiard)
Asymmetry in bimanual interactions is not a new subject, but for a long time, it has been considered that this lateralization was caused by preference or superiority of one hand. In 1987, Journal of Motor Behavior published an article written by Yves Guiard, researcher at CNRS, called “Asymmetric Division of Labor in Human Skilled Bimanual Action: The Kinematic Chain as a Model”.This article, famous in academic domain, remains quite undiscovered in software industry and this is a pity. Main idea of this article is that, considering asymmetric bimanual interactions, we shouldn’t oppose the 2 hands but we should consider these two hands as two abstract motors cooperating and acting together to reach a goal.

Y.Guiard proposes 3 principles to characterize asymmetric bimanual interactions:

Principle 1: Right-to-Left Spatial Reference in Manual Motion
Let’s  consider that you’re right-handed. According to this principle, motions of right hand find their spatial references in the results of motions of left hand. This is verified when your left hand moves and immobilizes the sheet of paper before your right hand starts to move the pen in order to write. Let’s have a look to the following picture, showing a balinese craftsman carving a traditional mask. First principle is still verified.

The left hand defines a referential for the right hand which activates the tool (left hand positions and immobilizes the mask. Left hand also stabilizes the tool). You’ll notice that legs and feet are also used in this interaction to help the left hand…

Principle 2: Left-Right Contrast in the Spatial-Temporal Scale of Motion
According to this principle, patterns of motions realized by the two hands show a contrast on spatial and temporal axis. On spatial axis, right hand realizes shorter and more accurate movements. For example, this is the case of right hand writing while left hand shifts the sheet of paper. On temporal axis, right hand realizes movements with a greater frequency. For example, this is the case of your right hand realizing many short movements to write, while your left hand moves at a lower frequency (to shift the sheet of paper).

Principle 3: Left-Hand Precedence in Action
According to this principle, left hand’s contribution to current action starts earlier than that of the right hand. For example, this is the case of your left hand which moves and immobilizes the sheet of paper before your right hand starts to write.

A few additional & personal thoughts
Tools, asymmetric bimanual interactions & interaction patterns
Let’s have a look to this specific scenario: manipulation of a tool to transform an object, with asymmetric  bimanual interactions. Even if some others patterns can be found, I’d say the most common pattern is:
  • Left hand defines and stabilizes position and orientation of the object we want to transform. Incidentally, it also helps to stabilize position and orientation of tool. The left hand decreases number of degrees of freedom of  the object and of the tool. According to Y.Guiard’s model, left hand defines a referential for right hand.
  • Right hand defines position and orientation of tool, activates this tool (it transfers energy) and controls amplitude of motion.
This pattern can be divided in two stages :
  • Preparation of object and tool (left hand and right hand can be used)
  • Transformation of object after activation of tool (right hand is used)
Tools and cyclic interactions
In our everyday life, manipulations of tools with asymmetric bimanual interactions are often cyclic: pattern is repeated several times before we reach our goal.

Asymmetric bimanual interactions &  expertise
Observation of people realizing a task with different levels of expertise is interesting when you take into account the framework defined by Y.Guiard. It shows that we can defines different stages of expertise.

Stage 1 : Easing of Principle 2 (Left-Right Contrast in the Spatial-Temporal Scale of Motion)
After some practice, agility of left hand increases a lot (temporal frequency increases, spatial accuracy increases), often more than agility of right hand. For example, let’s think about a guitarist, a beginner one. Left hand defines chords while right hand transfers energy to strings in order to create sounds.

After some practice, agility of the two hands increases, but except for totally arrythmic people, main initial progression will be the increase of left hand agility to “concatenate” chords. Till the right tempo is reached.

Stage 2 : Inversion of hands in principle 1 (Right-to-Left Spatial Reference in Manual Motion)
We all have a preferred hand, which seems more accurate, more skilful. We often use this hand for accurate operations. But, as stated by Y.Guiard, we shouldn’t think this way. Anyway, experience shows that after some practice, we can invert the usual role of our hands. If you watch the following video (from 0’40 to 2’00), you’ll  notice what I’d name (improperly) a “reversed asymmetric bimanual interaction”.

Newton Faulkner - Guitar 'tutorial' session
envoyé par pandaa_one. - Regardez d'autres vidéos de musique. 

The guitarist uses a technique called « hammer-on », which consists to tap the string with the left hand. Transfer of energy is no more realized by right hand, like in classical technique. If you think again to the three principles defined by Y.Guiard, you’ll see that with this technique, it seems the roles of the 2 hands have been inverted.

Stage 3 : Chaining serialized and parallelized interactions
Watch again the previous video (from 2’00 to 3’30). You’ll see a “reversed asymmetric bimanual interaction” and an unimanual interaction realized in parallel…

To conclude
Asymmetric bimanual interactions play a key role in interactions with our environment. Their importance is not enough reflected in existing Human/Computer interfaces. Let’s hope Natural User Interfaces won’t lose the opportunity…

Post scriptum
Y.Guiard is Directeur de Recherche at CNRS TELECOM-ParisTech. His works are a great source of inspiration for everybody interested in human-computer interactions, from study of Fitts’ law applied to multiscales spaces to the definition of new types of interactions for tactile surfaces like the CycloStar approach.

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