The Spatial and Temporal Structure of Reach-to-Grasp Movements

This post is part of my current series where I am developing a research programme to formally connect affordances and motor abundance analyses (see the intro post here). The first thing I need is a task that has a formal affordance analysis; my chosen task is reaching-to-grasp (sometimes called prehension). I'm pivoting to this task from throwing for a few reasons. First, I need a simpler task. The logistics of data collection and analysis for throwing tasks are large and I just don't have the resources right now; I do have the kit for these studies. Second, I want to expand my own skill set to a new domain, and reach-to-grasp is another venerable topic. Finally, it means I get to spend time really grappling with other work by Geoff Bingham, who was my PhD supervisor and who remains the person who's work I admire the most. 

This post is about the basic form of the reach-to-grasp movements studied in the lab, what some of the key measures are, and the fact these are sensitive to task demands (suggesting they are emerging from an active perception-action loop). In the next post I will work through the initial task analysis that leads us to candidate affordance properties. 

Reach-to-Grasp Movements

We have hands with opposable thumbs mounted on extendable arms, so one of the things we do a lot is reach out and grasp objects. There are of course many ways to do this, but the basic form of the movement is captured by the sort of task in the picture. There is an object, with a certain size and location relative to us. We begin with our hand near us, with fingers and thumbs together. This forms a pincer between the fingers and thumb (this is what 'opposable' means). We have to size this gap (called the aperture) to suit the object and then steer that shaped hand to enclose the object. We then need to close the gap to contact the object. We then usually pick it up; a stable lift needs the force vector between the fingers and thumb to be placed across the object, so where we land our fingers matters.You could just open your hands as wide as they'll go and move to enclose the object, but of course we don't work like this. Well-functioning reach-to-grasp movements have a consistent spatial and temporal structure that reflects the online control of the action in the context:

1. The trial begins with the fingers and thumbs together at a start location. 
2. We begin to reach; the hand starts to accelerate and the aperture between thumb and fingers begins to open. Unless there are obstacles, the hand takes a straight line path to the object, with a bell-shaped velocity profile. 
3. At some point, the hand reaches it's maximum speed (MS) and begins to decelerate for the approach to the object.
4. At some point, the hand reaches it's maximum grip aperture (MGA). This is always larger than is required to enclose the object. 
5. As the hand moves to enclose the object, the speed drops to approximately zero and the hand aperture decreases slightly, down from the MGA but still larger than the object; this is the Terminal Grip Aperture (TGA). 
6. Finally, the fingers are placed on the object, which may then be lifted. 

Of course, the details vary from trial to trial. This is due to redundancy, in part, but also, importantly, task details. Like all movements, reach-to-grasp movements are exquisitely scaled to the current demands, which means they are being created online as a function of information about those demands:

1. The movement time, the peak speed, and when that peak speed occurs in the reach all depend on things like object distance.
2. The scaling of the maximum and terminal grip apertures varies with object properties such as size, and also the timing of when these occur is affected by size. 

These movements therefore have a distinct but task sensitive spatiotemporal structure, and we can use variation in that structure as a function of task changes to probe what changes matter and why.