Definitions: Motion, Action, Gesture

I have been discussing definitions of the terms motion/movement, action and gesture several times before on this blog (for example here and here). Here is a summary of my current take on these three concepts:

Motion: displacement of an object in space over time. This object could be a hand, a foot, a mobile phone, a rod, whatever. Motion is an objective entity, and can be recorded with a motion capture system. A motion capture system could be anything from a simple slider (1-dimensional), to a mouse (2-dimensional), to a camera-based tracking system ((3-dimensional)  or an inertial system (6-dimensional: 3D position and 3D orientation). I have previously also discussed the difference between motion and movement. Since motion is a continuous phenomenon, it does not make sense to talk about it in plural form: “motions”. Then it makes more sense to talk about one or more motion sequences, but most probably it makes even more sense to talk about individual actions.

Action: a goal-directed motion (or force) sequence, for example picking up a stone from the ground, playing a piano tone. Actions may have a clear beginning and end, but they may also overlap due to coarticulation, such as when playing a series of tones on the piano. This uncertainty as to how actions should be segmented (or chunked), is what makes them subjective entities. As such, I do not think it is possible to measure an action directly, since there is no objective measure for when an action begins or ends, or how it is organised in relation to other actions. But, based on knowledge about human cognition, it is possible to create systems that can estimate various action features based on measurements of motion.

Gesture: the meaning being expressed through an action or motion. A gesture is not the same as action or motion, although it is related to both of them. As such, a gesture can be seen as a semiotic sign, in which the meaning is conveyed through an action, but it is highly subjective and dependent on the cultural context in which the action is carried out. Also, the same meaning can be conveyed through different types of physical actions. For example, the meaning you convey when you wave “good-bye” to someone may be independent of whether you do it with the left or the right arm, the size of the action, etc.

Unfortunately, with the popularity of motion and gesture studies over the last years, I see that many people use the term gesture more or less synonymously to action or motion. This is particularly the case in the field of “gesture recognition” in various versions of human-computer interaction (HCI).  I think it is unfortunate because we loose the precision with which we can describe the three different phenomena. If we track continuous motion in time and space, it is “motion tracking”. If we aim at recognising certain physical patterns in time and space, I would call it “action recognition” unless we are looking for some meanings attached to the actions. “Gesture recognition” I would only use if we actually recognise the meaning attached to some actions or motion. An example here would be to recognise the emotional quality of the performance  of a violinist. That, however, is something very different than tracking the bowing style.

movement-action
An illustration of my definition of the difference between motion and action

Difference between the terms movement and motion

Terminology is always challenging. I have previously written about definitions of actions and gesture several times (e.g. here,  here, and here) and chapter 2 in the book Musical gestures: sound, movement, and meaning (Routledge, 2010):

 

There are, however, two words/terms that I still find very challenging to define properly and to differentiate: movement and motion. In Norwegian we only have one word (bevegelse) for describing movement/motion, which makes everything much simpler. But when writing in English, which word should be used? and what is the difference?

It only adds to the confusion that Wiktionary defines movement as “physical motion between points in space”. And Wikipedia has a page on motion (in physics), while none of the many movement pages are related to body movement.

During the last years I have asked many native English speakers about the difference between motion and movement, but have not received any good explanations yet. Many of them think they are slightly different, although this is usually based on their feeling rather than on a proper explanation of the difference. Some native speakers think the two words are the same and can be used interchangeably.

I have also asked researchers working on various types of movement-oriented disciplines about their use of the words, and they often tend to stick to one or the other. From these discussions I have come to think that people working in biomechanics and physics prefer motion, while people  working in physiotherapy, dance and music prefer movement. That motion is a more scientific term is is also suggested here. From this we could assume that motion is related to measurable displacement of objects, which the term motion capture attest to, while movement refers to the qualities or meaning of the displacement.

The above assumptions are, however, only my assumptions. So I thought it would be interesting to see if I could get some more empirical data on the topic. So I decided to use the powers of Google to quantify the differences. Here are some figures from google and google scholar:

search term Google Google Scholar
movement 562 000 000 4 120 000
motion 144 000 000 2 210 000
“body movement” 4,830 000 83 000
“body motion” 1 370 000 76 300

So, clearly, movement seems to be used much more frequently than motion in general language, and also in the scientific literature. However, body movement and body motion are used almost the same amount of times in scientific papers.

But what if we search for the use of the two terms in different fields? Then we get these numbers:

search term 1 search term 2 Google Google Scholar search term 2 Google Google Scholar
music +movement 565 000 000 1 960 000 +motion 213 000 000 1 110 000
physics +movement 136 000 000 1 940 000 +motion 64,100 000 1 340 000
mechanics +movement 36 400 000 1 270 000 +motion 46 800 000 1 140 000
biomechanics +movement 6 110 000 163 000 +motion 3 060 000 167 000
physiotherapy +movement 4 580 000 71 200 +motion 2 530 000 38 600
kinesiology +movement 1 690 000 28 900 +motion 1 050 000 20 100

Again, we see that movement is generally used more than motion, even in physics and mechanics. I am quite surprised that music+motion is used so frequently, particularly since movement has a double meaning in music (i.e. parts of a piece).

What to conclude from all of this? I still do not know what the difference between movement and motion is, and the numbers show that movement is used more than motion also in the disciplines that I thought used motion almost exclusively. Still I like the idea that motion is used to describe physical properties, while movement is used to describe the qualities of motion. So I will stick to that for a while myself.

What do you think? Any comments or suggestions are highly welcome!

Sonification of Traveling Landscapes

I just heard a talk called “Real-Time Synaesthetic Sonification of Traveling Landscapes” (PDF) by Tim Pohle and Peter Knees from the Department of Computational Perception (great name!) in Linz. They have made an application creating music from a moving video camera. The implementation is based on grabbing a one pixel wide column from the video, plotting these columns and sonifying the image. Interestingly enough, the images they get out (see below) of this are very close to the motiongrams and videograms I have been working on.

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