Gentle Touch

Source: Pixabay

When you walk along, holding hands with your sweetheart, you don't hold that person's hand with an iron grip. Instead, you maintain gentle contact. The touches you share keep you in contact. Your hand-holding is communicative, not mechanical.

What does this reveal about touch more generally? Think about the way you transfer a pot from one place to another on a stove. You hold the handle and move the pot with one hand and keep your other hand on the lid’s knob. The hand gripping the handle drives the action, doing the heavy lifting. The other hand does something less obvious. It goes along for the ride, but not like a dead weight. It moves with the guidance of gentle touch, engaging in an activity that my colleagues and I called haptic tracking (Rosenbaum, Dawson, & Challis, 2007).

How is haptic tracking controlled? One possibility is that it is based on a central plan. According to this view, the haptically tracking hand follows a central plan as much as the hand carrying the pot.

Another possibility is that haptic tracking is mainly reactive. The hand in contact with the knob might respond to the slipping of the surface of the pot from the fingers. If a slip were felt, a quick move could be made to correct the error. No large-scale planning would be needed. The haptically tracking hand would just respond to local errors, nulling shear forces caused by unwanted slips.

Which of these accounts is correct—the active planning account or the reaction-to-felt-slipping account? Answering this question might have important practical applications.

If haptic tracking were reactive, that outcome could help stroke patients or others with movement difficulties caused by high-level motor-planning deficits. Such patients often lose the ability to plan and generate voluntary movements of one arm. As a result, they often stop using that arm, favoring the “good” arm over the “bad” one. The neglect makes a bad situation worse. The affected arm is used less and less after the brain insult and gets weaker as a result. When the brain finally recovers to the point of being able to regain control of the neglected arm, that arm may be so weak it’s of little use.

It would be helpful if there were a way to get stroke patients to keep using their limbs during recovery. Passively moving the affected limb, either with the help of a physical therapist or a robot, has only limited success, however. Forcing the patient to use the bad limb by tying the good limb (so-called constraint-induced therapy) may be logical but is unpleasant and largely resisted by patients.

Haptic tracking could provide a new way of helping stroke patients regain lost function. If these patients could move their limbs without having to rely on parts of the brain that plan extended movements, the movements could still be made. The muscles wouldn't weaken, so the brain, when it recovers, could command healthy, strong muscles, not weak, atrophied ones.

To find out whether haptic tracking is active or reactive, Amanda Dawson, John Challis, and I studied haptic tracking in young healthy adults performing with two hands at once. We reasoned that if people can move their hands while haptically tracking in ways they could not do otherwise, that outcome would suggest that they were relying on reactive rather than active (planned) control.

We asked Penn State University undergraduates to keep their two hands in touch with two moving objects. Think of rubbing your stomach and patting your head; we used movements of those sorts. We found that if the undergraduates tried to haptically track one object that moved in a square path and another object that moved in a circular path, they could do so with no special training. If the same undergraduates tried to draw the same two shapes, however, they would have been unable to do so; the square would have been round and the circle would have been square, as shown in other studies. But no such interference was seen in our haptic tracking task. More recently, Eefje Roelofsen and her colleagues (2016) obtained the same sort of result for foot movements in a study of healthy young-adult participants at Radboud University in the Netherlands.

Physical therapists already use a technique called active assistive motion to help their clients regain movement capabilities. This technique is a kind of light-touch dance between the therapist and client. But therapists can only dance with their clients for so long. Robots could be used instead, specifically for haptic tracking. The robots wouldn’t move the clients; instead, they would lead the clients, who could maintain contact with the moving robots via gentle touch. Haptic tracking might therefore have important clinical applications.