Arbitrary sensory-motor mapping during object manipulation in healthy persons

Background: Grasping and lifting an object is a basic tool in everyday life. In the following experiments we tested the ability of healthy humans to establish an association between arbitrary sensory cues and the weight of an object.

Methods: 10 right-handed healthy subjects accomplished a series of grip-lift tasks of an instrumented object with each hand. In a first experiment, participants lifted two objects of equal visual appearance which unexpectedly and randomly changed their weight. In two subsequent experiments, the change in object weight was indicated by cues, which were presented i) visually or ii) auditorily. The object incorporated a force sensor for grip force registration and linear acceleration sensors for registration of acceleration signals in three dimensions. We analysed the following parameters for every lift: (i) first peak in the rate of grip force increase (first time derivate of grip force); (ii) peak in grip force and iii) first peak in lift force. Peak grip force rate was calculated in addition to peak grip forces, because the former occurs prior to lift-off of an object well before somatosensory feedback about weight becomes available and therefore is considered a sensitive measure of weight prediction.

Results: When no cue about the weight of the object to be lifted was presented, participants programmed grip force according to the most recent lift, regardless of the hand used. In contrast, participants were able to rapidly establish an association between a particular sensory cue with a given weight and scaled grip force precisely to the actual object weight thereafter, regardless of the hand used or the sensory modality of the cue.

Conclusion: We have shown that healthy participants rapidly establish predictive grip force scaling according to the weight of an object to be lifted based on learned associations with arbitrary sensory cues. Most importantly, the use of arbitrary sensory cues is independently of the sensory modality and goes beyond the visual domain. The use of arbitrary sensory cues may be a helpful adjunct to establish predictive force scaling in the pathological state.