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Control of forces applied by individual fingers engaged in restraint of an active object.
Umeå University, Faculty of Medicine, Integrative Medical Biology, Physiology.
Umeå University, Faculty of Medicine, Integrative Medical Biology, Physiology.
Umeå University, Faculty of Medicine, Integrative Medical Biology, Physiology.
Umeå University, Faculty of Medicine, Integrative Medical Biology, Physiology.
1997 (English)In: Journal of Neurophysiology, ISSN 0022-3077, E-ISSN 1522-1598, Vol. 78, no 1, 117-128 p.Article in journal (Refereed) Published
Abstract [en]

We investigated the coordination of fingertip forces in subjects who used the tips of two fingers to restrain an instrumented manipulandum with horizontally oriented grip surfaces. The grip surfaces were subjected to tangential pulling forces in the distal direction in relation to the fingers. The subjects used either the right index and middle fingers (unimanual grasp) or both index fingers (bimanual grasp) to restrain the manipulandum. To change the frictional condition at the digit-object interfaces, either both grip surfaces were covered with sandpaper or one was covered with sandpaper and the other with rayon. The forces applied normally and tangentially to the grip surfaces were measured separately at each plate along with the position of the plates. Subjects could have performed the present task successfully with many different force distributions between the digits. However, they partitioned the load in a manner that reflected the frictional condition at the local digit-object interfaces. When both digits contacted sandpaper, they typically partitioned the load symmetrically, but when one digit made contact with rayon and the other with sandpaper, the digit contacting the less slippery material (sandpaper) took up a larger part of the load. The normal forces were also influenced by the frictional condition, but they reflected the average friction at the two contact sites rather than the local friction. That is, when friction was low at one of the digit-object interfaces, only the applied normal forces increased at both digits. Thus sensory information related to the local frictional condition at the respective digit-object interfaces controlled the normal force at both digits. The normal:tangential force ratio at each digit appeared to be a controlled variable. It was adjusted independently at each digit to the minimum ratio required to prevent frictional slippage, keeping an adequate safety margin against slippage. This was accomplished by the scaling of the normal forces to the average friction and by partitioning of the load according to frictional differences between the digit-object interfaces. In conclusion, by adjusting the normal:tangential force ratios to the local frictional condition, subjects avoided excessive normal forces at the individual digit-object interfaces, and by partitioning the load according the frictional difference, subjects avoided high normal forces. Thus the local frictional condition at the separate digit-object interfaces is one factor that can strongly influence the distribution of forces across digits engaged in a manipulative act.

Place, publisher, year, edition, pages
1997. Vol. 78, no 1, 117-128 p.
National Category
Medical and Health Sciences
Identifiers
URN: urn:nbn:se:umu:diva-32715PubMedID: 9242266OAI: oai:DiVA.org:umu-32715DiVA: diva2:305306
Available from: 2010-03-23 Created: 2010-03-23 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Tactile Sensory Control of Dexterous Manipulation in Humans
Open this publication in new window or tab >>Tactile Sensory Control of Dexterous Manipulation in Humans
2003 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

During dexterous manipulation with the fingertips, forces are applied to objects' surfaces. To achieve grasp stability, these forces must be appropriate given the properties of the objects and the skin of the fingertips, and the nature of the task. It

has been demonstrated that tactile sensors in the fingertips provide crucial information about both object properties and mechanical events critical for the control of fingertip forces, while in certain tasks vision may also contribute to predictions of required fingertip actions. This thesis focuses on two specific aspects of the sensory control of manipulation: (i) how individual fingers are controlled for grasp stability when people restrain objects subjected to unpredictable forces tangential to the grasped surfaces, and (ii) how tactile sensors in the fingertips encode direction of fingertip forces and shape of surfaces contacted by the fingertips.

When restraining objects with two fingers, subjects adjust the fingertip forces to the local friction at each digit-object interface for grasp stability. This is accomplished primarily by partitioning the tangential force between the digits in a way that reflects the local friction whereas the normal forces at the involved digits are scaled by the average friction and the total load. The neural control mechanisms in this task rely on tactile information pertaining to both the friction at each digit-object interface and the development of tangential load. Moreover, these mechanisms controlled the force application at individual digits while at the same time integrating sensory inputs from all digits involved in the task.

Microneurographical recordings in awake humans shows that most SA-I, SA-II and FA-I sensors in the distal phalanx are excited when forces similar to those observed during actual manipulation are applied to the fingertip. Moreover, the direction of the fingertip force influences the impulse rates in most afferents and their responses are broadly tuned to a preferred direction. The preferred direction varies among the afferents and, accordingly, ensembles of afferents can encode the direction of fingertip forces. The local curvature of the object in contact with the fingertip also influenced the impulse rates in most afferents, providing a curvature contrast signals within the afferent populations. Marked interactions were observed in the afferents' responses to object curvature and force direction. Similar findings were obtained for the onset latency in individual afferents. Accordingly, for ensembles of afferents, the order by which individual afferents initially discharge to fingertip events effectively represents parameters of fingertip stimulation. This neural code probably represents the fastest possible code for transmission of parameters of fingertip stimuli to the CNS.

Place, publisher, year, edition, pages
Umeå: Integrativ medicinsk biologi, 2003. 52 p.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 822
Keyword
Physiology, cutaneous sensibility, tactile afferents, fingertip force, grasp stability, human hand, manipulation, object shape, precision grip, sensorimotor control, coding, Fysiologi
National Category
Physiology
Research subject
Physiology
Identifiers
urn:nbn:se:umu:diva-23 (URN)91-7305-372-4 (ISBN)
Public defence
2003-01-24, Umeå, 10:00 (English)
Available from: 2003-01-24 Created: 2003-01-24 Last updated: 2010-03-24Bibliographically approved

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