Biomedical subjects

J M Kilner

Publications and source records attributed to J M Kilner.

9 recordsLinked to original sources

PubMed2005

Hemodynamic correlates of EEG: a heuristic.

In this note we describe a heuristic, starting with a dimensional analysis, which relates hemodynamic changes to the spectral profile of ongoing EEG activity. In brief, this analysis suggests that 'activation', as indexed by increases in hemodynamic signals, should be associated with a loss of power in lower EEG frequencies, relative to higher frequencies. The fact that activation is expressed in terms of frequency (i.e., per second) is consistent with a dimensional analysis in the sense that activations reflect the rate of energy dissipation (per second). In this heuristic, activation causes an acceleration of temporal dynamics leading to (i) increased energy dissipation; (ii) decreased effective membrane time constants; (iii) increased effective coupling among neuronal ensembles; and (iv) a shift in the EEG spectral profile to higher frequencies. These predictions are consistent with empirical observations of how changes in the EEG spectrum are expressed hemodynamically. Furthermore, the heuristic provides a simple measure of neuronal activation based on spectral analyses of EEG.

Algorithms↗

PubMed2004

Coupling of oscillatory activity between muscles is strikingly reduced in a deafferented subject compared with normal controls.

Oscillatory activity in the primate motor cortex has been shown to be phase locked to oscillations in contralateral hand and forearm muscle activity in the 15- to 30-Hz frequency range. Recent studies have shown that the degree of coupling between the cortex and the periphery is strongly influenced by the type and degree of movements of the digits. It has also been suggested that changes in corticomuscular and muscle-muscle coherence could be modulated by peripheral sensory inputs. In the current study, we investigated task-dependent changes in the coherent coupling of electromyographic (EMG) activity recorded from different intrinsic (abductor pollicis brevis and first dorsal interosseous) and two extrinsic (flexor digitorum superficialis and extensor digitorum communis) hand muscles during performance of a precision-grip task by normal subjects and by a single subject who has a total loss of touch, vibration, pressure, and kinesthetic sensation below the neck. The task required a hold-move-hold pattern of grip force to be exerted on a compliant object with the dominant right hand. We found significant task-related modulation of 15- to 30-Hz coherence between EMG activity in hand muscles in the control subjects. In contrast, the deafferented subject showed very low levels of significant coherence in the 15- to 30-Hz range and no peak at this frequency in the power spectra of her EMG activity. These results suggest that the presence of sensory afferent signals are necessary for the modulation of 15- to 30-Hz oscillations in the motor system.

Adult↗

PubMed2003

An interference effect of observed biological movement on action.

It has been proposed that actions are intrinsically linked to perception and that imagining, observing, preparing, or in any way representing an action excites the motor programs used to execute that same action. There is neurophysiological evidence that certain brain regions involved in executing actions are activated by the mere observation of action (the so-called "mirror system;" ). However, it is unknown whether this mirror system causes interference between observed and simultaneously executed movements. In this study we test the hypothesis that, because of the overlap between action observation and execution, observed actions should interfere with incongruous executed actions. Subjects made arm movements while observing either a robot or another human making the same or qualitatively different arm movements. Variance in the executed movement was measured as an index of interference to the movement. The results demonstrate that observing another human making incongruent movements has a significant interference effect on executed movements. However, we found no evidence that this interference effect occurred when subjects observed a robotic arm making incongruent movements. These results suggest that the simultaneous activation of the overlapping neural networks that process movement observation and execution infers a measurable cost to motor control.

Adult↗

PubMed2003

Task-dependent modulations of cortical oscillatory activity in human subjects during a bimanual precision grip task.

Oscillations are a widespread feature of normal brain activity and have been reported at a variety of different frequencies in different neuronal systems. The demonstration that oscillatory activity is present in motor command signals has prompted renewed interest in the possible functions of synchronous oscillatory activity within the primate sensorimotor system. In the current study, we investigated task-dependent modulations in coupling between sensorimotor cortical oscillators during a bimanual precision grip task. The task required a hold-ramp-hold pattern of grip force to be exerted on a compliant object with the dominant right hand, while maintaining a steady grip with the nondominant hand. We found significant task-related modulation of 15- to 30-Hz coherence between magnetoencephalographic (MEG) activity recorded from the left sensorimotor cortex and electromyographic (EMG) activity in hand muscles on the right side. This coherence was maximal during steady hold, but disappeared during the ramp movements. Interestingly coherence between the right sensorimotor MEG and left-hand EMG showed a similar, although less deeply modulated, task-related pattern, even though this hand was maintaining a simple steady grip. No significant ipsilateral MEG-EMG coherence was observed in the 15- to 30-Hz passband for either hand. These results suggest that the cortical oscillators in the two sensorimotor cortices are independent to some degree but that they may share a common mechanism that attenuates the cortical power in both hemispheres in the 15- to 30-Hz range during movements of one hand. The results are consistent with the hypothesis that oscillatory activity in the motor system is important in resetting the descending motor commands needed for changes in motor state, such as those that occur in the transition from movement to steady grip.

Adult↗

PubMed2002

Modulation of synchrony between single motor units during precision grip tasks in humans.

During precision grip, coherence between motor cortex and hand muscle EMG oscillatory activity in the 15-30 Hz range covaries with the compliance of the manipulated object. The current study investigated whether short-term synchrony and coherence between discharges of single motor units (SMUs) in the first dorsal interosseous (1DI) muscle were similarly modulated by object compliance during precision grip. Eight subjects used index finger and thumb to grip two levers that were under robotic control. Guided by visual feedback of the lever force levels, subjects held the levers against a steady force of 1.3 N for 8 s; they then linearly increased the force to 1.6 N over a 2 s period and held for a further 8 s before linearly decreasing the force back to the 1.3 N level over another 2 s period. Subjects performed the task at two different levels of compliance, each with identical grip force levels. Both surface EMG and SMU activity were recorded from the 1DI muscle. Short-term synchrony between the discharges of pairs of SMUs was assessed in the time domain by cross-correlation and in the frequency domain by coherence analysis. Coherence was seen in two frequency ranges: 6-12 Hz and 15-30 Hz. The compliance of the gripped object had a significant effect on both short-term synchronisation and coherence in the 15-30 Hz range between SMUs; both were greater for the more compliant condition. There was no change in the 6-12 Hz coherence.

Adult↗

PubMed2002

A novel algorithm to remove electrical cross-talk between surface EMG recordings and its application to the measurement of short-term synchronisation in humans.

Pairs of discharges of single motor units recorded in the same or different muscles often show synchronisation above chance levels. If large numbers of units are synchronous within and between muscles then the synchrony will be measurable in population recordings such as surface EMG. Measuring synchrony between surface EMG recordings has a number of practical and scientific advantages compared with single motor units recorded from intramuscular electrodes. However, the measurement of such synchrony in the time domain between surface EMGs is complicated because the recordings are contaminated by electrical cross-talk. In this study we recorded surface EMG simultaneously from five hand and forearm muscles during a precision grip task. Using a novel 'blind signal separation' algorithm, we were able to remove electrical cross-talk. The cross-talk-corrected EMGs could then be used to assess task-dependent modulation in both oscillatory (15-30 Hz) and non-oscillatory synchrony (all other frequencies). In agreement with previous studies, the oscillatory component was maximal during steady holding but abolished during movement. By contrast, the non-oscillatory component of the EMG synchrony appeared remarkably constant throughout all phases of the task. We conclude that surface EMG recordings can be of considerable use in the assessment of population synchrony changes, providing that electrical cross-talk between nearby channels is removed using a statistical signal processing technique. Our results show a striking difference in the task-dependent modulation of oscillatory and non-oscillatory synchrony between muscles during a dynamic precision grip task.

Adult↗

PubMed2000

Human cortical muscle coherence is directly related to specific motor parameters.

Cortical oscillations have been the target of many recent investigations, because it has been proposed that they could function to solve the "binding" problem. In the motor cortex, oscillatory activity has been reported at a variety of frequencies between approximately 4 and approximately 60 Hz. Previous research has shown that 15-30 Hz oscillatory activity in the primary motor cortex is coherent or phase locked to activity in contralateral hand and forearm muscles during isometric contractions. However, the function of this oscillatory activity remains unclear. Is it simply an epiphenomenon or is it related to specific motor parameters? In this study, we investigated task-dependent modulation in coherence between motor cortex and hand muscles during precision grip tasks. Twelve right-handed subjects used index finger and thumb to grip two levers that were under robotic control. Each lever was fitted with a sensitive force gauge. Subjects received visual feedback of lever force levels and were instructed to keep them within target boxes throughout each trial. Surface EMGs were recorded from four hand and forearm muscles, and magnetoencephalography (MEG) was recorded using a 306 channel neuromagnetometer. All subjects showed significant levels of coherence (0.086-0.599) between MEG and muscle in the 15-30 Hz range. Coherence was significantly smaller when the task was performed under an isometric condition (levers fixed) compared with a compliant condition in which subjects moved the levers against a spring-like load. Furthermore, there was a positive, significant relationship between the level of coherence and the degree of lever compliance. These results argue in favor of coherence between cortex and muscle being related to specific parameters of hand motor function.

Adult↗

PubMed1999

Task-dependent modulation of 15-30 Hz coherence between rectified EMGs from human hand and forearm muscles.

1. Recent reports have shown task-related changes in oscillatory activity in the 15-30 Hz range in the sensorimotor cortex of human subjects and monkeys during skilled hand movements. In the monkey these oscillations have been shown to be coherent with oscillatory activity in the electromyographic activity of hand and forearm muscles. 2. In this study we investigated the modulation of oscillations in the electromyogram (EMG) of human volunteers during tasks requiring precision grip of two spring-loaded levers. 3. Two tasks were investigated: in the 'hold' task, subjects were required to maintain a steady grip force (ca 2.1 N or 2.6 N) for 8 s. In the 'ramp' task, there was an initial hold period for 3 s (force ca 2.1 N) followed by a linear increase in grip force over a 2 s period. The task ended with a further steady hold for 3 s at the higher force level (ca 2.6 N). 4. Surface EMGs were recorded from five hand and forearm muscles in 12 subjects. The coherence of oscillatory activity was calculated between each muscle pair. Frequencies between 1 and 100 Hz were analysed. 5. Each subject showed a peak in the coherence spectra in the 15-30 Hz bandwidth during the hold task. This coherence was absent during the initial movement of the levers. During the ramp task the coherence in the 15-30 Hz range was also significantly reduced during the movement phase, and significantly increased during the second hold period, relative to the initial hold. 6. There was coherence between the simultaneously recorded magnetoencephalogram (MEG) and EMG during steady grip in the hold task; this coherence disappeared during the initial lever movement. Using a single equivalent current dipole source model, the coherent cortical activity was localized to the hand region of the contralateral motor cortex. This suggests that the EMG-EMG coherence was, therefore, at least in part, of cortical origin. 7. The results are discussed in terms of a possible role for synchrony in the efficient recruitment of motor units during maintained grip.

Adult↗

PubMed1999

The role of synchrony and oscillations in the motor output.

There is currently much interest in the synchronisation of neural discharge and the potential role it may play in information coding within the nervous system. We describe some recent results from investigations of synchronisation within the motor system. Local field potentials (LFPs) and identified pyramidal tract neurones (PTNs) were recorded from the primary motor cortex of monkeys trained to perform a precision grip task. The LFPs showed bursts of oscillatory activity at 20-30 Hz, which were coherent with the rectified electromyographs (EMG) of contralateral hand and forearm muscles. This oscillatory synchronisation showed a highly specific task dependence, being present only during the part of the task when the animal maintained a steady grip and not during the movement phases before or after it. PTNs were phase-locked to LFP oscillations, implying that at least part of the coherence between cortical activity and EMG was mediated by corticospinal fibres. The phase locking of the PTNs to LFP oscillations produced task-dependent oscillatory synchronisation between PTN pairs, as assessed by the single-unit cross-correlation histogram. Recordings were also made from normal human subjects performing a precision grip similar to that used in the monkey recordings. Pairs of EMGs recorded from intrinsic hand and forearm muscles showed 20-30 Hz coherence, which modulated during task performance, being present only during periods of steady contraction. We suggest that these changes in EMG-EMG synchronisation reflect changing levels of synchronous drive from the corticospinal system. The generation of oscillations in the cortex is discussed in the light of results from a model of local cortical circuits. Other modelling work has shown that synchrony in the corticospinal inputs could act to recruit motoneurones more efficiently, producing more output force from a muscle than asynchronous inputs firing at the same mean rate. A speculative hypothesis is presented on the role of synchronous oscillations in the motor system, which is consistent with experimental observations to date.

Animals↗