Finger interaction in a three-dimensional pressing task

Abstract

Accurate control of forces produced by the fingers is essential for performing object manipulation. This study examines the indices of finger interaction when accurate time profiles of force are produced in different directions, while using one of the fingers or all four fingers of the hand. We hypothesized that patterns of unintended force production among shear force components may involve features not observed in the earlier studies of vertical force production. In particular, we expected to see unintended forces generated by non-task fingers not in the direction of the instructed force but in the opposite direction as well as substantial force production in directions orthogonal to the instructed direction. We also tested a hypothesis that multi-finger synergies, quantified using the framework of the uncontrolled manifold hypothesis, will help reduce across-trials variance of both total force magnitude and direction. Young, healthy subjects were required to produce accurate ramps of force in five different directions by pressing on force sensors with the fingers of the right (dominant) hand. The index finger induced the smallest unintended forces in non-task fingers. The little finger showed the smallest unintended forces when it was a non-task finger. Task fingers showed substantial force production in directions orthogonal to the intended force direction. During four-finger tasks, individual force vectors typically pointed off the task direction, with these deviations nearly perfectly matched to produce a resultant force in the task direction. Multi-finger synergy indices reflected strong co-variation in the space of finger modes (commands to fingers) that reduced variability of the total force magnitude and direction across trials. The synergy indices increased in magnitude over the first 30% of the trial time and then stayed at a nearly constant level. The synergy index for stabilization of total force magnitude was higher for shear force components when compared to the downward pressing force component. The results suggest complex interactions between enslaving and synergic force adjustments, possibly reflecting the experience with everyday prehensile tasks. For the first time, the data document multi-finger synergies stabilizing both shear force magnitude and force vector direction. These synergies may play a major role in stabilizing the hand action during object manipulation.

Figure 1

A: The experimental setup. B: Orientation of the 6D sensors. C: A schematic of the feedback screen shown to the subject.

Figure 2

A: Force time series of a typical representative trial by the Index finger in the Down Forward (DF) task. The thin dashed lines indicate the desired force profile. Thick solid lines indicate the force produced by task finger in instructed directions. Thick dashed lines indicate the force produced by the non task (enslaved) fingers along Z axis. F_Z_task: Force template in −Z (down) direction. F_Y_task: Force template in +Y (forward) direction. F_Z _I : Index finger force in −Z direction. F_Y_I: Index finger force in +Y (forward) direction. F_Z_M: Middle finger force in −Z direction. F_Z_R: Ring finger force in −Z direction. B: Force time series of a typical representative trial by the Index finger in the Down task with feedback on the right-left force deviations (D_RL). The thin dashed lines indicate the desired force profile. Thick solid lines indicate the force produced by the subject in the instructed direction and thick dashed line indicates unintended force production (F_UN) along Y axis. F_Z _task: Force template in −Z direction. F_X_task: Force template along X (right-left) axis. F_Z_I : Index finger force in −Z (backward) direction. F_X_I : Index finger force along X (right-left) axis. F_Y_I: Index finger force along Y (forward-backward) axis.

Figure 3

E_j (how a finger enslaves other fingers) values for different conditions. Values have been averaged across all subjects (with standard error bars). D: Down only, D_RL: Down – feedback on the right-left force changes, D_FB: Down – feedback on the forward-backward force changes, DL: Down left, DR: Down right, DF: Down forward, DB: Down backward.

Figure 4

E_i (How a finger is enslaved by other fingers) values for different conditions. Values have been averaged across all subjects (with standard error bars). For abbreviations see Figure 3.

Figure 5

The index of unintended force production F_UN within-a-finger along Y axis for the four fingers, in 3 different pressing tasks involving X axis. Averaged values across subjects with standard error bars are shown. For abbreviations see Figure 3.

Figure 6

The index of unintended force production F_UN within-a-finger along X axis for the four fingers, in 3 different pressing tasks involving Y axis. Averaged values across subjects with standard error bars are shown. For abbreviations see Figure 3.

Figure 7

Time changes of the index of multi-finger synergy (ΔV) stabilizing force magnitude for different task conditions and different force directions. Averages across subjects are shown. For abbreviations see Figure 3.

Figure 8

Time changes of the index of multi-finger synergy (ΔV) stabilizing force direction for different task conditions involving force production in the right-left direction (A) and in the forward-backward direction (B). Averages across subjects are shown. For abbreviations see Figure 3.

Figure 9

Individual finger force vectors for a typical four-finger trial for the Down with feedback on the X-axis (right-left) force task (D_RL) performed by a representative subject. The arrows show the magnitude and the direction of force. The instructed direction corresponds to 90° angle. The insert graph (left, bottom corner) shows standard deviation of force direction computed for the same data set. We used standard deviations rather than circular statistics because of the relatively small variation in the finger force directions. IMRL: total force produced by all four fingers; I, M, R and L: forces produced by the Index, Middle, Ring and Little fingers.

Figure 10