Experiment Procedure

For patients who have one limb that is more impaired than the other, there is an opportunity to provide haptic feedback to the less affected limb to improve performance in handling objects. For the experiments in this paper, force sensors are attached to the index, middle and ring fingers of a subject’s more impaired hand and vibrotactile stimulators are attached to the back of the fingernail on the corresponding fingers of the less impaired hand.

The task in the experiments is inspired by the everyday task of lifting a glass of water. The proxy for the glass is a hollow plastic parallelepiped, 5.7mm × 5.7mm × 15.5mm, weighing 73 grams. We asked subjects to grasp this object and raise it several centimeters from a table top, hold it for several seconds and then replace it. We informed the subjects that they should try to hold the object steadily and to balance the forces among their fingers to prevent it from tipping. When subjects reported that they felt the forces were balanced, we recorded the forces for 5 seconds and then asked them to replace the object.

Three haptic feedback modes were selected: no haptic feedback (NHF), amplitude based feedback (ABF), and event-cue feedback (ECF). The characteristics of each feedback mode are discussed in the next subsection. A within subject test was conducted. 24 multiple sclerosis patients were recruited as subjects at the Masku Neurological Rehabilitation Center in Finland. Eight of those subjects are males, sixteen are females. The range of ages is from 33 to 64 with a mean of 56.4. The recruited subjects all have reduced sensation in one hand and good sensation in the other hand. They were all able to fully understand the human consent form and able to follow the simple instructions required to complete the sessions.

Since three different feedback modes were provided, there are six possible orderings of the three feedback modes for each subject (NHF-ABF-ECF, NHF-ECF-ABF,etc...). All 24 subjects completed the designed task under each of the three feedback modes with three trials for each mode, for a total of nine trials in one of six possible orderings. For example, a subject might do three trials with ABF, followed by three with NHF and three with ECF. Before the tests, subjects were given time to get familiar with conducting the task under the three different feedback modes. The pretest practice sessions took from 30 to 60 minutes depending on the individual. Also, whenever subjects switched modes, they were given several practice trials with the new mode.