Limits...
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

Experiment setup. (A) shows frontal and side views of the handle used for the study (“a” denotes force/torque sensors). (B) shows the frontal view of the handle with thumb and index fingertip center of pressure of the reference hand located at the same y-coordinates (vertical height relative to the base of the object) on the graspable surfaces of the handle (collinear dy). The red dots denote the CoP of each digit. (C) shows a top view of the experimental setup. The subject is shown contacting the handle with thumb and index fingertip, while the left hand was kept flat on the table. When relaxing in between trials, both hands were kept flat and relaxed. Note that the table top (gray) was opaque and prevented subjects from seeing their forearms and hands but is shown as transparent for graphical purposes only. Forearms and wrists were strapped to the table to prevent movements within and across trials while the handle was anchored to the table.
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Figure 1: Experiment setup. (A) shows frontal and side views of the handle used for the study (“a” denotes force/torque sensors). (B) shows the frontal view of the handle with thumb and index fingertip center of pressure of the reference hand located at the same y-coordinates (vertical height relative to the base of the object) on the graspable surfaces of the handle (collinear dy). The red dots denote the CoP of each digit. (C) shows a top view of the experimental setup. The subject is shown contacting the handle with thumb and index fingertip, while the left hand was kept flat on the table. When relaxing in between trials, both hands were kept flat and relaxed. Note that the table top (gray) was opaque and prevented subjects from seeing their forearms and hands but is shown as transparent for graphical purposes only. Forearms and wrists were strapped to the table to prevent movements within and across trials while the handle was anchored to the table.

Mentions: We used a custom-made grip handle to measure digit forces and CoP of the thumb and index finger pad for both Experiments 1 and 2 (Figure 1A). The sensorized handle has been described in detail elsewhere (Shibata et al., 2013). Briefly, two six-component force/torques sensors were mounted on each side of the handle (ATI Nano-25 SI-125-3, ATI Industrial Automation, Garner, NC; force range: 125, 125, and 500 N for x-, y- and z-axes, respectively; force resolution: 0.06 N; torque range: 3000 N•mm; torque resolution: 0.378 N•mm; Figure 1A). The vertical coordinate (y) of the CoP of each digit on the contact surface (red dots, Figure 1B) was computed from the force-torque sensor output. We performed calibration of each sensor by applying forces (3, 4, 5, and 6 N) perpendicular to the contact surface mounted on the sensor. This calibration revealed that the force and torque output of the two sensors could be used to compute the vertical coordinate of each digit CoP with a maximum error across all measurements and sensors of ± 1.2 mm (maximum average error ± SD: 0.3 ± 0.4 mm). Error in CoP reconstruction was similar between the two sensors and to the errors found when applying smaller normal forces (i.e., 0.6, 1.0, and 1.4 N; Shibata et al., 2013). During the experimental tasks, subjects exerted normal force with a digit within the 0.6–6.0 N range in 98% of all trials. To prevent the digits from slipping when subjects applied tangential forces up to 3.5 N, the contact surfaces of the handles were covered with 100-grit sandpaper (static friction coefficient range: 1.4–1.5).


Digit forces bias sensorimotor transformations underlying control of fingertip position.

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

Experiment setup. (A) shows frontal and side views of the handle used for the study (“a” denotes force/torque sensors). (B) shows the frontal view of the handle with thumb and index fingertip center of pressure of the reference hand located at the same y-coordinates (vertical height relative to the base of the object) on the graspable surfaces of the handle (collinear dy). The red dots denote the CoP of each digit. (C) shows a top view of the experimental setup. The subject is shown contacting the handle with thumb and index fingertip, while the left hand was kept flat on the table. When relaxing in between trials, both hands were kept flat and relaxed. Note that the table top (gray) was opaque and prevented subjects from seeing their forearms and hands but is shown as transparent for graphical purposes only. Forearms and wrists were strapped to the table to prevent movements within and across trials while the handle was anchored to the table.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 1: Experiment setup. (A) shows frontal and side views of the handle used for the study (“a” denotes force/torque sensors). (B) shows the frontal view of the handle with thumb and index fingertip center of pressure of the reference hand located at the same y-coordinates (vertical height relative to the base of the object) on the graspable surfaces of the handle (collinear dy). The red dots denote the CoP of each digit. (C) shows a top view of the experimental setup. The subject is shown contacting the handle with thumb and index fingertip, while the left hand was kept flat on the table. When relaxing in between trials, both hands were kept flat and relaxed. Note that the table top (gray) was opaque and prevented subjects from seeing their forearms and hands but is shown as transparent for graphical purposes only. Forearms and wrists were strapped to the table to prevent movements within and across trials while the handle was anchored to the table.
Mentions: We used a custom-made grip handle to measure digit forces and CoP of the thumb and index finger pad for both Experiments 1 and 2 (Figure 1A). The sensorized handle has been described in detail elsewhere (Shibata et al., 2013). Briefly, two six-component force/torques sensors were mounted on each side of the handle (ATI Nano-25 SI-125-3, ATI Industrial Automation, Garner, NC; force range: 125, 125, and 500 N for x-, y- and z-axes, respectively; force resolution: 0.06 N; torque range: 3000 N•mm; torque resolution: 0.378 N•mm; Figure 1A). The vertical coordinate (y) of the CoP of each digit on the contact surface (red dots, Figure 1B) was computed from the force-torque sensor output. We performed calibration of each sensor by applying forces (3, 4, 5, and 6 N) perpendicular to the contact surface mounted on the sensor. This calibration revealed that the force and torque output of the two sensors could be used to compute the vertical coordinate of each digit CoP with a maximum error across all measurements and sensors of ± 1.2 mm (maximum average error ± SD: 0.3 ± 0.4 mm). Error in CoP reconstruction was similar between the two sensors and to the errors found when applying smaller normal forces (i.e., 0.6, 1.0, and 1.4 N; Shibata et al., 2013). During the experimental tasks, subjects exerted normal force with a digit within the 0.6–6.0 N range in 98% of all trials. To prevent the digits from slipping when subjects applied tangential forces up to 3.5 N, the contact surfaces of the handles were covered with 100-grit sandpaper (static friction coefficient range: 1.4–1.5).

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