| 1. |
- Burstedt, Magnus K, et al.
(författare)
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Coordination of fingertip forces during human manipulation can emerge from independent neural networks controlling each engaged digit.
- 1997
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Ingår i: Experimental Brain Research. - : Springer Science and Business Media LLC. - 0014-4819 .- 1432-1106. ; 117:1, s. 67-79
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the coordination of fingertip forces in subjects who lifted an object (i) using the index finger and thumb of their right hand, (ii) using their left and right index fingers, and (iii) cooperatively with another subject using the right index finger. The forces applied normal and tangential to the two parallel grip surfaces of the test object and the vertical movement of the object were recorded. The friction between the object and the digits was varied independently at each surface between blocks of trials by changing the materials covering the grip surfaces. The object's weight and surface materials were held constant across consecutive trials. The performance was remarkably similar whether the task was shared by two subjects or carried out unimanually or bimanually by a single subject. The local friction was the main factor determining the normal:tangential force ratio employed at each digit-object interface. Irrespective of grasp configuration, the subjects adapted the force ratios to the local frictional conditions such that they maintained adequate safety margins against slips at each of the engaged digits during the various phases of the lifting task. Importantly, the observed force adjustments were not obligatory mechanical consequences of the task. In all three grasp configurations an incidental slip at one of the digits elicited a normal force increase at both engaged digits such that the normal:tangential force ratio was restored at the non-slipping digit and increased at the slipping digit. The initial development of the fingertip forces prior to object lift-off revealed that the subjects employed digit-specific anticipatory mechanisms using weight and frictional experiences in the previous trial. Because grasp stability was accomplished in a similar manner whether the task was carried out by one subject or cooperatively by two subjects, it was concluded that anticipatory adjustments of the fingertip forces can emerge from the action of anatomically independent neural networks controlling each engaged digit. In contrast, important aspects of the temporal coordination of the digits was organized by a "higher level" sensory-based control that influenced both digits. In lifts by single subjects this control was mast probably based on tactile and visual input and on communication between neural control mechanisms associated with each digit. In the two-subject grasp configuration this synchronization information was based on auditory and visual cues.
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| 2. |
- Edin, Benoni B
(författare)
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Finger joint movement sensitivity of non-cutaneous mechanoreceptor afferents in the human radial nerve.
- 1990
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Ingår i: Experimental Brain Research. - 0014-4819 .- 1432-1106. ; 82:2, s. 417-422
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Tidskriftsartikel (refereegranskat)abstract
- The responses of non-cutaneous receptors in the human hand to normal digit movements were studied using single afferent recordings from the radial nerve. Eight joint-related afferents had thresholds of 50 mN or less. All responded to passive flexion movements within the physiological range of joint rotation and showed predominantly static response sensitivity; none increased its discharge during passive extension. However, only two of these eight afferents showed the same response pattern during active movements; three discharged only during the extension phase whereas the other three discharged both during extension and flexion. No high-threshold, joint-related mechanoreceptive afferents were encountered in a population of 148 afferents recorded from the cutaneous portion of the radial nerve indicating a scarcity of such afferents on the dorsal aspect of finger joints. Seven high-threshold, subcutaneous mechanoreceptive units not related to joints had thresholds for indentations of 50 mN or more and lacked responses to finger movements. Low-threshold mechanoreceptive afferents related to joints in the human hand may thus provide kinematic information in the physiological mid-range of both passive and active movements. Joint position cannot, however, be derived unambiguously from their discharge since the receptor responses may be dramatically altered by muscle activity.
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| 3. |
- Säfström, Daniel, et al.
(författare)
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Acquiring and adapting a novel audiomotor map in human grasping.
- 2006
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Ingår i: Experimental Brain Research. - : Springer Science and Business Media LLC. - 0014-4819 .- 1432-1106. ; 173:3, s. 487-497
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Tidskriftsartikel (refereegranskat)abstract
- For sensorimotor transformations to be executed accurately, there must be mechanisms that can both establish and modify mappings between sensory and motor coordinates. Such mechanisms were investigated in normal subjects using a reach-to-grasp task. First, we replaced the normal input of visual information about object size with auditory information, i.e., we attempted to establish an 'audiomotor map'. The size of the object was log linearly related to the frequency of the sound, and we measured the maximum grip aperture (MGA) during the reaching phase to determine if the subjects had learned the relationship. Second, we changed the frequency-object size relationship to study adaptation in the newly acquired map. Our results demonstrate that learning of an audiomotor map consisted of three distinct phases: during the first stage (approximately 10-15 trials) subjects simply used MGAs large enough to grasp any reasonably sized object and there were no overt signs of learning. During the second stage, there was a period of fast learning where the slope of the relationship between MGA and object size became steeper until the third stage where the slope was constant. In contrast, when sensorimotor adaptation was studied in the established audiomotor map, there was rapid learning from the start of a size perturbation. We conclude that different learning strategies are employed when sensorimotor transformations are established compared to when existing transformations are modified.
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| 4. |
- Säfström, Daniel, et al.
(författare)
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Prediction of object contact during grasping.
- 2008
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Ingår i: Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale. - : Springer Science and Business Media LLC. - 1432-1106. ; 190:3, s. 265-77
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Tidskriftsartikel (refereegranskat)abstract
- The maximum grip aperture (MGA) during prehension is linearly related to the size of objects to be grasped and is adapted to the haptically sensed object size when there is a discrepancy between visual and haptic information. We have investigated what information is used to drive this adaptation process and how the onset of fingertip forces on the object is triggered. Subjects performed a reach-to-grasp task, where the object seen and the object grasped physically never were the same. We measured the movements of the index finger and the thumb and the contact forces between each fingertip and the object. The subjects' adaptation of the MGA was unrelated both to different fingertip velocities at the moment of object contact, or the fingertip forces. Instead, the 'timing' of contact between the fingers and the object was most consistently influenced by introducing a size discrepancy. Specifically, if the object was larger than expected, the moment of contact occurred earlier, and if the object was decreased in size, then the contact occurred later. During adaptation, these timing differences were markedly reduced. Also, the motor command for applying forces on the object seemed to be released in anticipation of the predicted moment of contact. We therefore conclude that the CNS dynamically predicts when contact between the fingertips and objects occur and that aperture adaptation is primarily driven by timing prediction errors.
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