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Predictive force programming in the grip-lift task in stroke patients: the role of memory links between arbitrary cues and object weight

D Bensmail, AS Sarfeld, F Kemper, H Karbe, G Fink, DA Nowak

Klin Neurophysiol 2009; 40

Objective: It has been shown that healthy subjects are able to rapidly establish an association between an arbitrary sensory cue and a given mass to be lifted. The aim of this study was to test the ability of stroke patients to scale the grip force output in a predictive manner to the mass of an object to be lifted based on learned associations between arbitrary colour cues (ACC) and object masses.

Methods: A clinical evaluation of patients was performed including ARAT, sensibility, aphasia, apraxia and hemineglect tests, MMSE, NIHSS scale. Participants grasped a cylindrical object, mounted on top of a box. The total mass of the configuration was either 0.4 or 0.6kg. The object incorporated a force sensor for grip force registration and sensors for registration of acceleration signals in 3D. In a first experiment, patients performed 10 consecutive lifts of each weight with either hand to assess sensori-motor integration. In a second experiment the ability to use ACC to link them with object masses was tested. In a „no cue“ experiment, a non-informative neutral visual stimulus was presented prior to each lift, thereby not allowing any judgement about which mass to be lifted. In a „cue“ experiment, an ACC provided advance information about the mass patients would have to lift in the subsequent trial. The order of mass changes was pseudo-randomly predetermined and presented in a different order for each subpart of the second experiment. The results were compared with those obtained from healthy subjects.

Results: 24 hemiparetic patients were included in the study with a mean age of 57 years. They had mild motor and sensibility deficit. None had clinical signs of higher cortical functions impairment. Sensori-motor integration was observed in the majority of patients during the first experiment. Patients were, however, unable to establish memory links between ACC and the mass to be lifted, regardless of whether the grip-lift task was performed with the affected or unaffected hand. In contrast, healthy controls were able to scale grip force in a predictive manner based on learned associations between ACC and object mass. Interestingly, we observed a negative correlation between age and the precision of predictive grip force scaling.

Discussion: Stroke patients with mild motor and sensibility deficits are unable to establish a memory link between ACC and the mass of an object to be lifted. The reason for this remains unclear and further studies are needed.