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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

Experimental conditions (Experiment 2). The experimental conditions of Experiment 2 are shown in the same format as those shown in Figure 2 for Experiment 1. The only difference between Experiments 1 and 2 is that for the latter experiment, subjects were required to exert the same thumb and index fingertip normal and tangential forces across reference and test hands (see text for more details).
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Figure 3: Experimental conditions (Experiment 2). The experimental conditions of Experiment 2 are shown in the same format as those shown in Figure 2 for Experiment 1. The only difference between Experiments 1 and 2 is that for the latter experiment, subjects were required to exert the same thumb and index fingertip normal and tangential forces across reference and test hands (see text for more details).

Mentions: Subjects’ thumb and index fingertips of the reference hand were passively moved by an experimenter (“passive dy adjustment” phase, Figure 2A) such that the CoPs of both digits on the graspable surface were at the same vertical height relative to the base of the object. Throughout the manuscript, we will refer to this fingertip position as “collinear” (dy = 0 mm; Figure 1B). During this procedure and while matching dy with the test hand (see below for details), subjects were instructed to extend the middle, ring, and little fingers to prevent them from contacting the handle (Figure 1B). The CoP and forces for each digit and the resultant dy of the reference hand was displayed on a second computer monitor that was not visible to the subject. Once an experimenter visually confirmed compliance of the desired hand posture and dy, a verbal cue was given to generate forces in one of six combinations of direction and magnitude (Figure 2B). Specifically, the reference hand exerted tangential force of thumb and index finger in either the same or opposite directions. When tangential forces were exerted in the same direction, both thumb and index finger exerted the tangential force upward (TUP-IUP) or downward (TDOWN-IDOWN) (“Same”; Figures 2B and 3, left column). When tangential forces were exerted in opposite directions, the thumb and index finger exerted the tangential force either upward and downward (TUP-IDOWN) or downward and upward, respectively (TDOWN-IUP) (“Opposite”; Figures 2B and 3, middle column). The range of the normal and tangential forces exerted by each digit of the reference hand was the same across these four experimental conditions (4–5 N and 2.5–3.5 N, respectively).


Digit forces bias sensorimotor transformations underlying control of fingertip position.

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

Experimental conditions (Experiment 2). The experimental conditions of Experiment 2 are shown in the same format as those shown in Figure 2 for Experiment 1. The only difference between Experiments 1 and 2 is that for the latter experiment, subjects were required to exert the same thumb and index fingertip normal and tangential forces across reference and test hands (see text for more details).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 3: Experimental conditions (Experiment 2). The experimental conditions of Experiment 2 are shown in the same format as those shown in Figure 2 for Experiment 1. The only difference between Experiments 1 and 2 is that for the latter experiment, subjects were required to exert the same thumb and index fingertip normal and tangential forces across reference and test hands (see text for more details).
Mentions: Subjects’ thumb and index fingertips of the reference hand were passively moved by an experimenter (“passive dy adjustment” phase, Figure 2A) such that the CoPs of both digits on the graspable surface were at the same vertical height relative to the base of the object. Throughout the manuscript, we will refer to this fingertip position as “collinear” (dy = 0 mm; Figure 1B). During this procedure and while matching dy with the test hand (see below for details), subjects were instructed to extend the middle, ring, and little fingers to prevent them from contacting the handle (Figure 1B). The CoP and forces for each digit and the resultant dy of the reference hand was displayed on a second computer monitor that was not visible to the subject. Once an experimenter visually confirmed compliance of the desired hand posture and dy, a verbal cue was given to generate forces in one of six combinations of direction and magnitude (Figure 2B). Specifically, the reference hand exerted tangential force of thumb and index finger in either the same or opposite directions. When tangential forces were exerted in the same direction, both thumb and index finger exerted the tangential force upward (TUP-IUP) or downward (TDOWN-IDOWN) (“Same”; Figures 2B and 3, left column). When tangential forces were exerted in opposite directions, the thumb and index finger exerted the tangential force either upward and downward (TUP-IDOWN) or downward and upward, respectively (TDOWN-IUP) (“Opposite”; Figures 2B and 3, middle column). The range of the normal and tangential forces exerted by each digit of the reference hand was the same across these four experimental conditions (4–5 N and 2.5–3.5 N, respectively).

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