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NEUROSCIENCE |
1 Prince of Wales Medical Research Institute and UNSW, Randwick, NSW 2031, Australia
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Abstract |
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(Received 19 December 2006;
accepted after revision 21 March 2007;
first published online 5 April 2007)
Corresponding author S. Gandevia: Prince of Wales Medical Research Institute, Barker Street, Randwick, NSW 2031, Australia. Email: s.gandevia{at}unsw.edu.au
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Introduction |
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The role of cutaneous receptors was initially overlooked, although clues that they may be important existed. Proprioceptive acuity decreased when cutaneous (and joint) feedback was removed by anaesthesia of digital nerves (e.g. Provins, 1958; Gandevia & McCloskey, 1976; Refshauge et al. 2003). Stronger evidence came from recent studies which showed that stretch of the skin over finger joints evoked illusions of movement (Edin & Johansson, 1995; Collins & Prochazka, 1996; Collins et al. 2000, 2005). Furthermore, when muscle vibration and skin stretch were applied together, the illusory movements were augmented (Collins & Prochazka, 1996; Collins et al. 2000, 2005). Thus, just as muscle vibration revealed the proprioceptive role of muscle spindle endings, so skin stretch confirmed a proprioceptive role for cutaneous receptors.
Another role for cutaneous afferents was suggested by the effects of ‘interfering’ stimuli. Contrary to the original view that cutaneous afferents ‘facilitate’ muscle signals by a central action (Gandevia & McCloskey, 1976), movement detection was not impaired after the loss of tonic inputs from skin in adjacent digits (Refshauge et al. 2003). Instead, increasing input from adjacent, but not more remote, digits with innocuous continuous electrical stimulation or repetitive brushing reduced detection of passive movements of the test digit (Refshauge et al. 2003). Thus, while tonic cutaneous inputs from adjacent digits are not required for normal movement detection, sustained enhancement of that input impairs the movement detection in the adjacent digits. This suggests an ‘interference’ or inhibitory interaction between the various proprioceptive inputs (Refshauge et al. 2003). It was reported that signals coding the properties of tactile stimuli can be masked by receptor responses to vibration, and so could lead to sensorimotor alterations (Ribot-Ciscar et al. 1989), although inhibitory interactions could also resemble the impairment by specific interfering stimuli of discrimination of cutaneous vibration in the hand (Ferrington et al. 1977). Cutaneous inputs arising from Pacinian corpuscles elevated detection thresholds and this was consistent with inhibition at synaptic relays along the sensory pathway (Ferrington et al. 1977).
The present study examined whether similar mechanisms were responsible for the proprioceptive impairment caused by interfering cutaneous stimulation. Pacinian corpuscles respond to vibration at high frequencies, 80–450 Hz (Sato, 1961; Talbot et al. 1968; LaMotte & Mountcastle, 1975), whereas rapidly adapting intradermal tactile receptors, thought to be Meissner corpuscles, respond to low frequencies, 10–80 Hz (Talbot et al. 1968). Single Pacinian afferents exert powerful actions on central neurons (McIntyre et al. 1967) and can elicit sensations of tactile vibration (Ochoa & Törebjork, 1983; Vallbo et al. 1984; Macefield et al. 1990). Based on the findings of Ferrington et al. (1977) we hypothesized that proprioceptive performance would decrease with vibrotactile input designed to engage Pacinian, but not Meissner, afferents. The interphalangeal joint of the index finger was tested because previous studies have demonstrated a role for cutaneous feedback in proprioception at this joint (Edin & Johansson, 1995; Refshauge et al. 2003).
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Methods |
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The proprioceptive acuity for detecting the direction of passive movements at the proximal interphalangeal joint of the index finger was measured when a vibrotactile stimulus was delivered at two locations, the tip of the adjacent middle finger or the thenar eminence. A total of 12 healthy subjects participated (5 female, 7 male; age range 22–51 years), with most involved in more than one study. Ten subjects participated in the initial study, and 8 of them then participated in the main studies. Two new subjects participated in these studies, bringing its total number to 10. All experiments conformed to the Declaration of Helsinki, subjects gave written consent to participate and procedures were approved by the ethics committee of the University of New South Wales.
Experimental setup
The right forearm and the hand were supported on a padded splint, with the wrist positioned in neutral supination–pronation and comfortable extension (
15 deg), and the metacarpophalangeal joint of the index finger flexed to 45 deg (Fig. 1A). Flexion and extension movements were imposed about the proximal interphalangeal joint of the right index finger.
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45 deg). The middle phalanx was coupled to a linear servomotor under positional feedback, driven by a variable ramp generator and was attached via a small padded clamp (1.5 cm2 in contact area) over the sides of the digit (Fig. 1A). This was designed to minimize the disturbance of the skin on the dorsal and palmar surfaces of the digit. The clamp was placed 1.5 cm distal to the axis of rotation of the joint (Fig. 1A). The proximal phalanx was stabilized by a similar padded clamp so that movement was confined to the proximal interphalangeal joint (Fig. 1A). A barrier was positioned over the hand so that subjects could not see it or the apparatus. Measures of actual angular displacement and geometric calculation were used to calibrate the equipment prior to data collection. In the main studies, vibratory stimulation with sinusoidal motion of the probe tip was delivered via a feedback-controlled mechanical stimulator (see Fig. 3B; for details see Ferrington & Rowe, 1980; Morley & Rowe, 1990; Coleman et al. 2003). However, in the initial study rectangular inputs were used to generate the vibration of 300 Hz; this produced a complex stimulus containing some components at higher frequencies (see Fig. 3A).
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1 mm. Standard protocol
Flexion and extension movements of 1–3 deg were imposed about the proximal interphalangeal joint of the index finger from the initial mid-position. Each movement was held at its full excursion for 3 s (Fig. 1B). The velocity of the imposed movement was selected as that at which the subject could detect correctly the direction of 60% of the control movements. For each subject this threshold velocity was found through preliminary trials and ranged from 0.3 to 1.0 deg s–1. The velocity of the reset movement back to the starting position was constant (1.25 deg s–1). Subjects received the same instructions before each session. The false positive rate (nomination of the wrong direction) was low (< 10%).
A set of test movements consisted of five extension and five flexion movements in random order. This set was repeated 8 times, four with and four without vibration. Half were completed with vibration of the middle finger and the other half with vibration of the thenar eminence (see below). In both control and vibration runs a standard auditory signal was given during the test movements so that subjects were aware when the movement was occurring (Fig. 1B).
Initial study
The first study used a stimulus of 300 Hz complex vibration (20 µm peak-to-peak amplitude). Subjects (n = 10) were asked to nominate orally the direction of movement (flexion or extension) when they were sure or ‘not sure’ when they were unsure of the direction.
Main studies
A group of 10 subjects participated in these studies. Each subject attended four sessions on separate days. During each session a different vibrotactile stimulus was tested. These comprised sinusoidal vibration at 300 Hz (50 µm peak-to-peak amplitude), 30 Hz (50 µm) and 30 Hz at an amplitude matched in perceived intensity to the 300 Hz stimulus. Unless otherwise indicated, vibration was sinusoidal. First, 300 Hz vibration (50 µm) was used to engage Pacinian corpuscles (PCs) and the 30 Hz vibration of the same amplitude was used to preferentially engage RA receptors (Meissner corpuscles) (Talbot et al. 1968; Jänig, 1971). Since, at this amplitude, 30 Hz vibration was subjectively less intense than the 300 Hz vibration, 30 Hz vibration of equal subjective intensity to the 300 Hz (50 µm) vibration was tested. This intensity was found by giving subjects successive 1 s bursts of 30 Hz and 300 Hz vibration 1 s apart. They indicated which stimulation was of greater intensity. The amplitude of the 30 Hz vibration was incrementally increased until the intensity of the two stimuli was equivalent. The amplitude of 30 Hz vibration that produced an equal subjective intensity was determined separately for the tip of the middle finger (190 ± 35 µm) and the thenar eminence (357 ± 33 µm). In the final study, 300 Hz vibration at two amplitudes (20 and 200 µm) was delivered to the middle finger only. As for the other studies, the same four trial sets were completed twice, but this time once with vibration of 200 µm amplitude and once with vibration of 20 µm amplitude.
In the main studies, subjects nominated the direction of movement using a pad with three buttons labelled ‘flexion’, ‘extension’ and ‘not sure’. They were instructed to signal the direction as soon as they were sure and to press ‘not sure’ if unsure of the direction of movement (Fig. 1B). Subjects were able to nominate the direction of movement during the movement itself or during the 3 s hold period after the movement. However, a response was recorded as ‘not sure’ if given after the index finger began to return to the initial position. Starting from the onset of the imposed movement, the time for the subject to choose a response was recorded as the ‘decision time’.
Spread of vibration
The spread of 300 Hz sinusoidal vibration (50 µm) over the hand was measured in five subjects using an accelerometer. As before, the vibration was delivered to the volar aspect of the middle finger tip at the distal phalanx with an initial skin indentation of 1 mm. The accelerometer was firmly attached at 11 different locations within the hand (Fig. 2A).
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1 mm. The EMG of the flexor carpi radialis was recorded with surface electrodes (amplified by 10 000; bandpass 16–1000 Hz). This arrangement was designed to maximize the likelihood that a short-latency reflex response was evoked by vibration. Data analysis
Separate two-way repeated measures ANOVAs were performed for the number of correct detections and decision times, assessing the effects of vibration and stimulus location (middle finger/thenar eminence) for both 300 Hz and both 30 Hz stimuli. The effects of vibration on movement detection and decision time were analysed at two frequencies, 30 Hz and 300 Hz, which were matched for subjective intensity. The effect of different vibration amplitudes (20 µm and 200 µm) were also investigated for the 300 Hz vibrotactile stimulus applied to the middle finger. A paired t test was also performed for the number of incorrect responses (not sure/false positive). For all tests, significance was accepted when P < 0.05. The statistical software SPSS V. 14.0 was used.
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Results |
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Spread of vibration
The acceleration induced by 300 Hz vibration (50 µm) decreased proximally along the vibrated middle digit, and was undetectable over the neighbouring index digit, the metacarpophalangeal joint and more proximally at the other sites (Fig. 2B). The frequency of the vibration remained clearly discernable and did not degrade at proximal points along the middle digit (Fig. 2B, inset). The 30 Hz stimulus amplitude showed a similar attenuation.
When 300 Hz vibration (50 µm; 3 pulses) was applied directly over the tendon of the flexor carpi radialis during weak contraction of the wrist flexors, there was no short-latency reflex as detected in unrectified or rectified averages of responses to 500 bursts of stimulation.
Movement detection
Vibration at high frequency (300 Hz). Detection of the direction of passively applied flexion and extension movements at the index finger was poorer when a complex 300 Hz (20 µm) vibration was delivered to the tip of the middle finger or to the thenar eminence (P = 0.001) (Fig. 3A). This impairment in detection of the direction of the applied movements was greater with vibration at the middle finger (24.5 ± 3.6% decrease in detection) compared with the thenar eminence (10.5 ± 5.4% decrease; P = 0.046). In addition, the decrease in detection with middle finger stimulation was a consistent finding, being present in 9 of the 10 subjects.
Because the vibration used in the initial study was not purely sinusoidal and contained some components at frequencies higher than 300 Hz (Fig. 3A, inset), in the main studies we used a special stimulator which produced pure sinusoidal motion of the probe (Fig. 3B, inset; see Methods). This achieves more selective activation of specific classes of cutaneous receptor (e.g. Talbot et al. 1968; Bystrzycka et al. 1977; Ferrington et al. 1977).
When 300 Hz vibration at 50 µm (chosen to activate PC afferents) was applied to the middle finger or the thenar eminence, detection of movements at the index finger was also significantly impaired (P = 0.002; Fig. 3B). The decrease in movement detection was similar when vibration was applied to the middle finger (12.5 ± 2.5%) and thenar eminence (11 ± 4.6%; P = 0.78).
We also determined whether the impairment in movement detection with 300 Hz sinusoidal vibration applied to the nearby skin was greater with higher amplitudes of the stimulus. Vibration of the middle finger (at 300 Hz) at amplitudes of 20 and 200 µm led to considerable impairment in movement detection of the index finger (6.5 ± 5.0 and 20.5 ± 3.0% decrease, respectively), though this impairment was only significant at the 200 µm amplitude (P = 0.002; Fig. 4).
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Vibration at low frequency (30 Hz). In contrast to high-frequency vibration, 30 Hz (50 µm) chosen to activate Meissner afferents had no effect on movement detection at the index finger when delivered to the middle finger or thenar eminence (P = 0.43; Fig. 5A).
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5-fold) so that it was subjectively of equal intensity to the 300 Hz (50 µm) vibration. However, this 30 Hz vibration which was matched in subjective intensity, again had no effect on movement detection when applied either to the middle finger or thenar eminence (P
= 0.64; Fig. 5B). Thus, it is unlikely that the reduced detection during the 300 Hz vibration was due simply to altered attention. Decision times
The time taken to decide the direction of the applied movement (‘decision time’) was also used to assess the difficulty of the proprioceptive task. We reasoned that if vibration to adjacent digits made subjects take longer to nominate with certainty the direction of applied movement, proprioceptive acuity had deteriorated. There were trials in which subjects could detect that movement had occurred, but could not nominate its direction. Depending on the angular velocity at which the subject detected the direction of 60% of trial movements, subjects had a window of 9.7–13 s to decide on the direction of movement from its start (6.7–10 s during the movement, plus 3 s when the joint was maintained at full excursion). The decision times were usually 6–9 s for the detection of the correct direction of movements. The average decision time for detecting movement was 7.2 ± 0.4 s without vibration and this increased to 8.0 ± 0.4 s during 300 Hz vibration (P
= 0.007).
In Fig. 6, changes in the decision times for correct detections are plotted for trials with and without vibration. Decision times were prolonged during vibration of the middle finger (7.2% increase) and the thenar eminence (10.5% increase) when 300 Hz (50 µm) vibration was applied compared with when the stimuli were absent (P = 0.007; Fig. 6). When 300 Hz vibration at amplitudes of 20 and 200 µm was applied to the middle finger, the decision times again increased (by 4.9 and 10.9%, respectively; P = 0.036).
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When subjects were ‘not sure’ or made false positive judgements, the decision times (9–11 s) were longer on average than those for correct responses (6–9 s). However, for these incorrect detections there were no significant changes in decision times with any intervention.
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Discussion |
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The parameters for the applied vibration were selected to activate preferentially particular classes of cutaneous receptor. This was guided by studies in primates (Sato, 1961; Talbot et al. 1968) and human subjects (Mountcastle et al. 1972; LaMotte & Mountcastle, 1975; Bolanowski et al. 1988) which had shown that PC afferents respond to focal stimuli from 10 to 1000 Hz, but they are most sensitive to frequencies of 250–550 Hz. An amplitude of 50 µm was selected for the main experiments as this amplitude was considered sufficient to activate a population of PCs. Within the optimal frequency range, 10 µm is the lowest vibratory amplitude to engage the most sensitive PC afferents (Talbot et al. 1968). The vibration transmitted through the finger to the proximal phalanx attenuated markedly, and was undetectable over the neighbouring index digit, the metacarpophalangeal joint and more proximally (Fig. 2). There was also no evidence of a change in its dominant frequency. Therefore, it is likely that the 300 Hz vibration activated mainly PCs in the distal and middle phalanges of the vibrated finger. The small amplitudes and high frequency of vibration are unlikely to have produced major activation of other cutaneous receptors.
Correlative neural and psychophysical studies indicate that at vibration frequencies above 20 Hz, the slowly adapting cutaneous afferents are less sensitive than other classes and are unlikely to contribute to vibrotactile sensibility (e.g. Talbot et al. 1968; Bolanowski et al. 1988; Bolanowski et al. 1994). Meissner corpuscles are also unlikely to be entrained by 300 Hz stimulation. They are distinguished from PC afferents by their much smaller receptive fields and by being most sensitive to vibratory stimuli around 40–60 Hz (e.g. Talbot et al. 1968; Jänig, 1971; Ferrington & Rowe, 1980).
It seems quite unlikely that muscle spindle endings account for the vibration-induced interference in proprioceptive capacity as there was also no evidence of a short-latency tendon jerk when the flexor carpi radialis tendon was indented and stimulated directly with bursts of 300 Hz vibration (50 µm). This strongly suggests that muscle spindles endings, which are highly sensitive to vibration (Brown et al. 1967), were not engaged by the stimulation. In addition, there were no signs of a tonic vibration reflex, or any reported illusory movements during the proprioceptive experiment. Also, the amplitude of skin vibration that was used (50 µm) was very much lower than that commonly used to evoke illusions of movement when applied directly over tendons (0.5–2 mm) (e.g. Goodwin et al. 1972; Roll & Vedel, 1982; Collins & Prochazka, 1996).
A 300 Hz vibration with a complex non-sinusoidal waveform was used to establish that the interference phenomenon was present with high-frequency stimulation. Although smaller in peak-to-peak amplitude (20 µm), this non-sinusoidal stimulus was more intense than the sinusoidal 300 Hz vibration, presumably due to the higher frequency components in the stimulus (Fig. 3). It led to greater disturbance in movement detection when applied to the tip of the middle finger compared with stimulation over the thenar eminence. This difference was not obvious for the sinusoidal vibration at 300 Hz. It may reflect greater spread of the highest frequency components along the finger. The mechanical impedance of the fingers is lower than the palm of the hand for high-frequency vibration (Dong et al. 2005), hence the complex stimulus may activate more PCs via the finger tip, leading to a greater disturbance in proprioception.
When high-frequency (300 Hz) vibration was used as an interfering stimulus it took subjects longer to detect correctly the direction of movements. Presumably subjects were unsure of the direction of movement and they took longer to make a final decision (by 700 ms). Decision times provided an extra measure for gauging subjective judgements. For example, there was a significant rise in decision time with 30 Hz vibration of matched intensity to 300 Hz vibration (which required the amplitude of vibration to be increased) (Fig. 6), although the overall disturbance was not sufficient to affect the number of correct detections (Fig. 5B). Although PCs are optimally sensitive to cutaneous vibration around 300 Hz, they may be activated at low frequencies, for example at 30 Hz, if high amplitudes are used (Talbot et al. 1968). Thus, the present results do not reveal whether this effect was due to increased numbers of PC afferents activated at the higher amplitude of low-frequency vibration, increased recruitment of other rapidly adapting inputs or whether other critical inputs were activated by the high-amplitude vibration.
The impairment in proprioception was graded with the input. Detection accuracy decreased and detection times increased as the amplitude of 300 Hz sinusoidal vibration increased. One explanation is that when receptor thresholds vary widely, as occurs for PC afferents (Mountcastle et al. 1972; Freeman & Johnson, 1982), an increase in the amplitude of vibration increases the population response as the volume in which the vibratory field exceeds the threshold of the most sensitive afferents increases, and less sensitive afferents are recruited. The discharge rate of recruited afferents also increases (Johnson, 1974).
Our findings extend the findings of Refshauge et al. (2003) who found that increasing cutaneous input with a stroking stimulus to digits adjacent to the test finger impaired movement detection. Additional input from adjacent digits may effectively add ‘noise’ to the neural circuits involved in movement detection (Refshauge et al. 2003). In some circumstances noise can enhance the detection of weak signals in sensory systems, via a mechanism known as stochastic resonance (e.g. Cordo et al. 1996; Collins et al. 2003). For example, the detection of subthreshold tactile stimuli can be enhanced by adding mechanical noise which would activate PCs and other rapidly adapting receptors (Collins et al. 1997; Gravelle et al. 2002). In contrast, adding noise through the activation of PCs led to impaired detection of movements in our studies. This is probably because all the interfering stimuli were suprathreshold, and at these levels the enhancement due to stochastic resonance is abolished (Collins et al. 1997).
Our findings, together with those of Refshauge et al. (2003), make it probable that there are interactions among the various proprioceptive inputs evoked by finger movements. Ferrington et al. (1977) showed that a stimulus of 300 Hz, but not of 30 Hz, at the thenar eminence elevated detection thresholds for detection of vibration of the index finger over a wide range of frequencies (10–450 Hz). These results were interpreted as PC-induced inhibition of input from all classes of vibration-sensitive cutaneous receptors. Our results are consistent with PC involvement not only in cutaneous sensation, but also in movement detection. However, whether PCs inhibit only the cutaneous input to proprioception or also the muscle and joint afferent contribution cannot be deduced from the present study.
The emerging view, supported by a combination of psychophysical and microneurographical data (e.g. Collins et al. 2000, 2005; Edin, 2004), is that cutaneous feedback provides proprioceptive information that is integrated with that from muscle spindles to provide judgement of joint position and movement. Our result implies that there is convergence between skin (PC) and proprioceptive projections along the somatosensory pathway. This could be between skin and muscle afferents or among the cutaneous inputs to proprioception. One possible location where this may occur is the dorsal column nuclei as it contains prominent inputs from vibration-sensitive receptors (e.g. Bystrzycka et al. 1977; Douglas et al. 1978; Ferrington & Rowe, 1982; Connor et al. 1984; see also Hummelsheim et al. 1985). Suppression at the thalamic or cortical level is also possible, although there is no definite data for humans.
In conclusion, sustained enhancement of high-frequency cutaneous input to digits adjacent to the moving digit impaired movement detection. However, this proprioceptive interference was not observed for low-frequency vibration. The impairment in movement detection is graded with the input. Taken together, these findings suggest that stimuli that preferentially activate cutaneous PC afferents may produce significant impairment in proprioceptive acuity.
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References |
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) or to the tip of the middle finger (
) compared with control conditions (C). The inset shows an example of the complex 300 Hz waveform (calibration: 3 ms and 10 µm). B, there was also a significant (*) decrease in the incidence of correct detection of movement direction when 300 Hz sinusoidal (50 µm) vibration was delivered to the thenar eminence (
). The inset shows the vibration waveform at one subject's distal (1), middle (2) and proximal (3) phalanx of the middle finger. The amplitude calibration is a percentage of movement of the vibrator probe tip.
