The results in the preceding section indicate that either proportional, amplitude-based feedback (ABF) or eventcue feedback (ECF) in response to excessive variations in forces, can help MS patients to better regulate their grasping forces in handling objects. On average, subjects’ performance in balancing the forces among the index, middle and ring fingers was improved by more than 70% using haptic feedback.

Normalizing each subject’s results obtained with haptic feedback by their no-feedback performance reduces the subject to subject variability sufficiently to reveal further significant trends. In particular, we found that subjects with mild impairment performed best with event-cue feedback while subjects with severe impairment performed better with proportional feedback. This finding also generally matches the preferences voiced by the subjects. A couple of reasons may explain this effect. First, the proportional feedback is always on and can become annoying when it is rarely needed by patients with mild impairment. In contrast, the event-cue feedback is only triggered when a subject is in danger of dropping or bobbling the object, perhaps due to fatigue or a momentary distraction. However, for subjects with severe impairment, the event-cue feedback is triggered frequently and can become distracting. These patients have relatively little sense of how much force their fingers are providing, and proportional haptic feedback provides a straightforward and continuous measure of their activity.

We observed also that the the event-cue feedback produced more failures than the proportional feedback (but fewer failures than no feedback). This result may be a reflection of the delay involved in detecting a dangerous situation (unbalanced forces), alerting the user and allowing the user to respond in time to prevent dropping the object. Providing an earlier warning of impending failures may reduce failure rate with ECF.