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Digit forces bias sensorimotor transformations underlying control of fingertip position.

Shibata D, Kappers AM, Santello M - Front Hum Neurosci (2014)

Bottom Line: We hypothesized that, when the tangential forces of the digits are produced in opposite directions, matching error (1) would be biased toward the directions of the tangential forces; and (2) would be greater when the remembered relative contact points are matched with negligible digit force production.However, matching error was not dependent on whether the reference and test hand exerted similar or different forces.We propose that the expected sensory consequence of motor commands for tangential forces in opposite directions overrides estimation of fingertip position through haptic sensory feedback.

View Article: PubMed Central - PubMed

Affiliation: Kinesiology Program, School of Nutrition and Health Promotion, Arizona State University Tempe, AZ, USA.

ABSTRACT
Humans are able to modulate digit forces as a function of position despite changes in digit placement that might occur from trial to trial or when changing grip type for object manipulation. Although this phenomenon is likely to rely on sensing the position of the digits relative to each other and the object, the underlying mechanisms remain unclear. To address this question, we asked subjects (n = 30) to match perceived vertical distance between the center of pressure (CoP) of the thumb and index finger pads (dy ) of the right hand ("reference" hand) using the same hand ("test" hand). The digits of reference hand were passively placed collinearly (dy = 0 mm). Subjects were then asked to exert different combinations of normal and tangential digit forces (Fn and Ftan , respectively) using the reference hand and then match the memorized dy using the test hand. The reference hand exerted Ftan of thumb and index finger in either same or opposite direction. We hypothesized that, when the tangential forces of the digits are produced in opposite directions, matching error (1) would be biased toward the directions of the tangential forces; and (2) would be greater when the remembered relative contact points are matched with negligible digit force production. For the test hand, digit forces were either negligible (0.5-1 N, 0 ± 0.25 N; Experiment 1) or the same as those exerted by the reference hand (Experiment 2).Matching error was biased towards the direction of digit tangential forces: thumb CoP was placed higher than the index finger CoP when thumb and index finger Ftan were directed upward and downward, respectively, and vice versa (p < 0.001). However, matching error was not dependent on whether the reference and test hand exerted similar or different forces. We propose that the expected sensory consequence of motor commands for tangential forces in opposite directions overrides estimation of fingertip position through haptic sensory feedback.

No MeSH data available.


Related in: MedlinePlus

Fingertip vertical distance: matching errors. Matching errors were compared across experimental conditions and between experiments. (A) shows average matching error for Experiments 1 and 2 (top and bottom plots, respectively) across matching conditions. The mean matching error in the “Fn only” condition was used as a reference to normalize the matching error in the Same and Opposite conditions (left and middle column, respectively; see text for more details). (B) shows average normalized matching error for the Experiments 1 and 2 (top and bottom plots, respectively) across matching conditions. For all panels, matching and normalized errors were averaged across all subjects (vertical bars denote SE). Asterisks denote significant differences (p < 0.05) from zero, whereas “n.s.” denote non-significant differences.
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Figure 5: Fingertip vertical distance: matching errors. Matching errors were compared across experimental conditions and between experiments. (A) shows average matching error for Experiments 1 and 2 (top and bottom plots, respectively) across matching conditions. The mean matching error in the “Fn only” condition was used as a reference to normalize the matching error in the Same and Opposite conditions (left and middle column, respectively; see text for more details). (B) shows average normalized matching error for the Experiments 1 and 2 (top and bottom plots, respectively) across matching conditions. For all panels, matching and normalized errors were averaged across all subjects (vertical bars denote SE). Asterisks denote significant differences (p < 0.05) from zero, whereas “n.s.” denote non-significant differences.

Mentions: A mixed-design ANOVA on the matching errors in the two control conditions (Figures 2B and 3, right column) revealed no significant difference between matching performance in Experiments 1 and 2 (no main effect of Experiment: F[1,28] = 0.467; p > 0.05) and between the “Fn only” and “No Fn/Ftan” (no main effect of Digit normal force: F[1,28] = 0.004; p > 0.05), and no significant interactions (Digit normal force × Experiment: F[1,28] = 2.516; p > 0.05; Figure 5A). These results indicate that subjects’ ability to reproduce the reference dy with the test hand was not sensitive to whether reference and test hands exerted the same or different digit normal force. As matching error did not differ as a function of digit normal force in either experiment, the mean matching error from the “Fn only” condition was used as a within-subject reference to normalize errors in the other experimental conditions characterized by the same normal force (4–5 N). The normalized matching error was defined as the mean matching error averaged within subjects in the Same and Opposite conditions minus the mean matching error from the “Fn only” condition. This resulted in a “normalized matching error” denoting the effect of tangential force production only on dy matching error.


Digit forces bias sensorimotor transformations underlying control of fingertip position.

Shibata D, Kappers AM, Santello M - Front Hum Neurosci (2014)

Fingertip vertical distance: matching errors. Matching errors were compared across experimental conditions and between experiments. (A) shows average matching error for Experiments 1 and 2 (top and bottom plots, respectively) across matching conditions. The mean matching error in the “Fn only” condition was used as a reference to normalize the matching error in the Same and Opposite conditions (left and middle column, respectively; see text for more details). (B) shows average normalized matching error for the Experiments 1 and 2 (top and bottom plots, respectively) across matching conditions. For all panels, matching and normalized errors were averaged across all subjects (vertical bars denote SE). Asterisks denote significant differences (p < 0.05) from zero, whereas “n.s.” denote non-significant differences.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4120687&req=5

Figure 5: Fingertip vertical distance: matching errors. Matching errors were compared across experimental conditions and between experiments. (A) shows average matching error for Experiments 1 and 2 (top and bottom plots, respectively) across matching conditions. The mean matching error in the “Fn only” condition was used as a reference to normalize the matching error in the Same and Opposite conditions (left and middle column, respectively; see text for more details). (B) shows average normalized matching error for the Experiments 1 and 2 (top and bottom plots, respectively) across matching conditions. For all panels, matching and normalized errors were averaged across all subjects (vertical bars denote SE). Asterisks denote significant differences (p < 0.05) from zero, whereas “n.s.” denote non-significant differences.
Mentions: A mixed-design ANOVA on the matching errors in the two control conditions (Figures 2B and 3, right column) revealed no significant difference between matching performance in Experiments 1 and 2 (no main effect of Experiment: F[1,28] = 0.467; p > 0.05) and between the “Fn only” and “No Fn/Ftan” (no main effect of Digit normal force: F[1,28] = 0.004; p > 0.05), and no significant interactions (Digit normal force × Experiment: F[1,28] = 2.516; p > 0.05; Figure 5A). These results indicate that subjects’ ability to reproduce the reference dy with the test hand was not sensitive to whether reference and test hands exerted the same or different digit normal force. As matching error did not differ as a function of digit normal force in either experiment, the mean matching error from the “Fn only” condition was used as a within-subject reference to normalize errors in the other experimental conditions characterized by the same normal force (4–5 N). The normalized matching error was defined as the mean matching error averaged within subjects in the Same and Opposite conditions minus the mean matching error from the “Fn only” condition. This resulted in a “normalized matching error” denoting the effect of tangential force production only on dy matching error.

Bottom Line: We hypothesized that, when the tangential forces of the digits are produced in opposite directions, matching error (1) would be biased toward the directions of the tangential forces; and (2) would be greater when the remembered relative contact points are matched with negligible digit force production.However, matching error was not dependent on whether the reference and test hand exerted similar or different forces.We propose that the expected sensory consequence of motor commands for tangential forces in opposite directions overrides estimation of fingertip position through haptic sensory feedback.

View Article: PubMed Central - PubMed

Affiliation: Kinesiology Program, School of Nutrition and Health Promotion, Arizona State University Tempe, AZ, USA.

ABSTRACT
Humans are able to modulate digit forces as a function of position despite changes in digit placement that might occur from trial to trial or when changing grip type for object manipulation. Although this phenomenon is likely to rely on sensing the position of the digits relative to each other and the object, the underlying mechanisms remain unclear. To address this question, we asked subjects (n = 30) to match perceived vertical distance between the center of pressure (CoP) of the thumb and index finger pads (dy ) of the right hand ("reference" hand) using the same hand ("test" hand). The digits of reference hand were passively placed collinearly (dy = 0 mm). Subjects were then asked to exert different combinations of normal and tangential digit forces (Fn and Ftan , respectively) using the reference hand and then match the memorized dy using the test hand. The reference hand exerted Ftan of thumb and index finger in either same or opposite direction. We hypothesized that, when the tangential forces of the digits are produced in opposite directions, matching error (1) would be biased toward the directions of the tangential forces; and (2) would be greater when the remembered relative contact points are matched with negligible digit force production. For the test hand, digit forces were either negligible (0.5-1 N, 0 ± 0.25 N; Experiment 1) or the same as those exerted by the reference hand (Experiment 2).Matching error was biased towards the direction of digit tangential forces: thumb CoP was placed higher than the index finger CoP when thumb and index finger Ftan were directed upward and downward, respectively, and vice versa (p < 0.001). However, matching error was not dependent on whether the reference and test hand exerted similar or different forces. We propose that the expected sensory consequence of motor commands for tangential forces in opposite directions overrides estimation of fingertip position through haptic sensory feedback.

No MeSH data available.


Related in: MedlinePlus