Finger Displacement Research Articles

Complex oscillations in a human motor system.

Experiments were performed to investigate the oscillations arising in a human motor system with delayed visual feedback. Eight subjects were instructed to maintain a constant finger position relative to a stationary baseline. The finger displacement was measured using a microdisplacement transducer connected to the index finger, and was displayed on an oscilloscope. Time delays between 40 and 1,500 ms were inserted in the visual feedback loop for 100 s. Results show that, as the time delays increase, irregular rhythms appear with short intermittent periods of regular oscillations. These regular low-frequency oscillations have an amplitude that increases with the time delays and a period that is consistently about 2 to 4 times the time delay. Fast Fourier Transforms (FFT) show a peak between 8 and 12 Hz, corresponding to physiological tremor in half the subjects. No systematic variations in the FFT for the 2 to 15 Hz range were observed as time delay increased. In the 0 to 2 Hz range, the FFT show a consistent increase in power with the time delay. These results indicate that, under the conditions of this experiment, tremor is not affected by time delays in the visuomotor system, and time delays in the visuomotor feedback loop give rise to complex oscillatory behaviors.

Factors affecting the accuracy of position matching at the proximal interphalangeal joint in human subjects.

1. The ability of normal subjects to match finger position at the proximal interphalangeal (PIP) joint was tested under a number of different conditions. One finger (target finger) was passively displaced in a pseudorandom manner whilst the other (matching) finger was actively positioned by the subject. 2. Digital nerve block of the target finger resulted in the matching finger only being displaced through a small range ('range compression') whilst anaesthesia of the matching finger produced the opposite effect ('range expansion'). 3. When the finger is unanaesthetized, the velocity of displacement of the target finger from one position to the next does not affect the accuracy of position matching. However, during digital nerve block of the target finger, matching errors increase with decreasing velocity. 4. The impairment of matching performance during digital nerve block is due specifically to loss of input from receptors in and around the PIP joint, as anaesthesia of the tip of the finger to produce any significant decrement in matching accuracy. 5. The results show that muscle receptors contribute to position sense at this joint but are not the exclusive source of proprioceptive sensations.

Time delays, oscillations, and chaos in physiological control systems

Many physiological systems operate by feedback mechanisms in which there are time delays from the sensing of some disturbance to the mounting of an appropriate physiological response. Theoretical studies show that using either “mixed” feedback or multiple negative feedback loops with different delays, it is possible to generate complex periodic and aperiodic, chaotic rhythms. In an experimental study of finger tracking using visual feedback, a variable time delay can be added to the feedback circuit. As this delay is varied, complex rhythms of finger displacement are observed. We suggest that the complex dynamics observed in diverse physiological systems in normal individuals may be associated with chaotic dynamics arising from the interaction of multiple feedback loops.

Position sense at the proximal interphalangeal joint of the human index finger.

1. The ability of eleven normal subjects to match the position of the proximal interphalangeal joints was tested. The right index finger (target finger) was moved to a given position and the subject was required to match this with the left index finger (matching finger). 2. Digital nerve block of the matching finger resulted in substantial impairment in matching performance in seven out of eight subjects. 3. Anaesthesia of the target finger in two subjects also produced obvious position matching deficit, in a pattern consistent with subjective sensation of target finger position. 4. The effects of digital nerve block were specific, as neither injection of saline into the matching finger, nor anaesthesia of adjacent fingers, impaired performance. 5. In one subject, injection of local anaesthetic into the synovial cavity of the matching finger also resulted in impairment of matching performance, indicating that articular receptors contribute to position sense at this joint. 6. Passive displacement of the matching finger did not produce significantly greater errors than those occurring with active finger positioning either under control conditions or during digital nerve block of the matching finger. 7. Although digital nerve block produced substantial impairment of matching performance, there was not complete loss of proprioceptive sensation, suggesting that muscle receptors also contribute. This was confirmed in experiments where isotonic loading of the matching finger while it was anaesthetized resulted in significant alteration in matching performance compared to anaesthesia alone. 8. It is concluded that no one source of afferent input can be excluded from contributing to proprioception at the index finger. It is likely that under different operating conditions several afferent sources are required to provide optimal proprioceptive resolution.

Numerical models and experiments on immiscible displacements in porous media

Immiscible displacements in porous media with both capillary and viscous effects can be characterized by two dimensionless numbers, the capillary number C, which is the ratio of viscous forces to capillary forces, and the ratio M of the two viscosities. For certain values of these numbers, either viscous or capillary forces dominate and displacement takes one of the basic forms: (a) viscous fingering, (b) capillary fingering or (c) stable displacement. We present a study in the simple case of injection of a non-wetting fluid into a two-dimensional porous medium made of interconnected capillaries. The first part of this paper presents the results of network simulators (100 × 100 and 25 × 25 pores) based on the physical rules of the displacement at the pore scale. The second part describes a series of experiments performed in transparent etched networks. Both the computer simulations and the experiments cover a range of several decades in C and M. They clearly show the existence of the three basic domains (capillary fingering, viscous fingering and stable displacement) within which the patterns remain unchanged. The domains of validity of the three different basic mechanisms are mapped onto the plane with axes C and M, and this mapping represents the ‘phase-diagram’ for drainage. In the final section we present three statistical models (percolation, diffusion-limited aggregation (DLA) and anti-DLA) which can be used for describing the three ‘basic’ domains of the phase-diagram.

Absence of long latency reflexes to imposed finger displacements in patients with Huntington's disease

Long latency reflexes in the electromyogram (EMG) of the first dorsal interosseus muscle were elicited by short finger displacements under isometric conditions. In all healthy subjects tested the spinal response was followed by a second involuntary component. Patients with Huntington's disease lacked the late EMG response almost completely, but exhibited a spinal component indistinguishable from that of the control group. A spinal mechanism responsible for this result is unlikely, since double stretches evoked two distinct EMG responses in these patients. Moreover, drastically reduced cortical somatosensory evoked potentials in all patients support the notion that the second EMG response seen in our motor paradigm is of supraspinal origin.

Potentiels évoqués et états de vigilance induits au cours d'épreuves de temps de réaction de choix

Characteristics of Visual Evoked Potentials (N120, P200, P300) were investigated during choice reaction time situations in a group of 10 subjects practising meditation (E.S.) versus a matched control group (C.S.) During a series of visual stimuli occuring at fixed intervals, with 10% random omissions, the subjects were asked : 1) to respond by a finger displacement to each visual stimulus; 2) to hold on the response to the stimulus and to respond to omission. Both tasks were recorded before and after the practice of meditation or rest for the controls. The intergroup comparison showed that the experimental subjects had faster RT's with less mistakes, and N120 and P200 of larger amplitude and shorter latency. These differencies were significant before and after meditation. The transient effects of meditation or rest, were opposite for the two groups : whereas after meditation the RT's became longer with less mistakes, and the amplitude of P300 larger, after rest there was a decrease of the P300 amplitude and no change in the RT's of the controls. These results are interpreted in terms of selective attention capacity and information processing strategies, A.S.C. being used as a model for the study of these processes.

Inhibition of cortical evoked potentials and sensation by self-initiated movement in man.

AN imposed, passive or externally paced displacement (EPD) of the index finger in man evokes brain potentials that differ in delay and waveform for postcentral, precentral and prefrental areas1. Displacement so imposed leads to less accurate subjective estimation of limb position from that obtained by self initiated movement2. This behavioural observation as far as we know has not been neurophysiologically investigated with cortical recordings from the human brain. Thus we compared the cortical potentials following passive or externally paced displacement, to those related with a similar but self paced voluntary displacement (SPD) of the left index finger. Not only is self-initiated movement manipulative, however, it also subserves acquisition of sensory information. To examine this aspect of finger movement in a second series of experiments we studied the evoked responses and the subjective sensation to brief electrical stimuli applied to the left median nerve at the wrist or the index finger during SPD and compared them with similar responses elicited with the subject at rest.

Cortical potentials evoked by finger displacement in man.

THE neuropsychophysiology underlying the sense and change of limb position in animals and man is a centre of enquiry by many disciplines ranging from medicine and physiology to psychology and sociology1. Consistent changes in the electrocorticogram (ECoG) following change of position have, however, not yet been reported in man, although recordings of unit activity during neurosurgical operations have demonstrated neuronal activity in cortex2 and sub-cortical structures3–5. Limited data from scalp recordings after passive movement have also been reported6,7.