Biomedical subjects

T Kiemel

Publications and source records attributed to T Kiemel.

14 recordsLinked to original sources

PubMed2001

Human multisensory fusion of vision and touch: detecting non-linearity with small changes in the sensory environment.

Previous investigations using relatively large amplitude sensory stimuli or complete removal of sensory input have demonstrated non-linear processing of sensory information for postural control. In the present study, we asked whether a linear range of sensory fusion exists when smaller amplitude stimuli are used. The amplitudes of visual and somatosensory input were simultaneously co-varied within a trial. The postural responses were characterized by analyzing how the Fourier transform of postural sway at the driving frequency varied with sensory movement amplitudes. If the postural control system is linear with constant weighting of sensory inputs, then the pattern of Fourier transforms should be a linear function of movement amplitude. However, in 28 of 58 trials we observed non-linearity in this function. The results clearly show that even at very small amplitudes of sensory change, the nervous system processes multisensory information in a non-linear fashion.

Fourier Analysis↗

PubMed2001

Bending the lamprey spinal cord causes a slowly-decaying increase in the frequency of fictive swimming.

It is well known that rhythmic lateral bending of the isolated lamprey spinal cord/notochord can entrain the central pattern generator (CPG) for locomotion. During entrainment, the CPG's frequency is equal to the bending frequency. We report here that bending can also have a slowly-decaying excitatory effect on the CPG's frequency. Experiments were performed in which the caudal end of a 30-50 segment piece of spinal cord/notochord was repeatedly rhythmically bent for 0.5-12 cycles. A slowly-decaying excitation was said to be present if after the termination of bending the CPG's frequency was significantly greater than baseline and decayed back to baseline with a time constant of one or more cycles. In 14 of 16 animals, a slowly-decaying excitation could be evoked by bending. In five of the nine animals tested, this slowly-decaying excitation could be evoked with bending frequencies both faster and slower than the baseline frequency. Depending on the animal, the slowly-decaying excitation could be elicited by as little as one-half cycle of bending and by bending amplitudes as small as 6-21 degrees peak deflection. We interpret these data as evidence of a second effect of bending distinct from the phase-dependent effect that produces entrainment.

Animals↗

PubMed2001

Impact of movement and movement-related feedback on the lamprey central pattern generator for locomotion.

A semi-reduced, minimally restrained lamprey preparation was used to investigate the impact of movement and movement-related feedback during D-glutamate-induced locomotion. The preparation consisted of the trunk alone with the spinal cord exposed to the bathing solution. Two conditions were compared using electromyography or nerve recording: (i) muscle and spinal cord, (ii) spinal cord alone supported by the notochord. Compared with the isolated spinal cord, movement in the presence of muscle consistently and significantly increased the frequency of the motor output and reduced the phase delay among the segments. In moving preparations, coupling among the segments was reduced by two staggered hemisections to permit the strength and direction of intersegmental coupling to be estimated. The estimates revealed that movement increased the total intersegmental coupling strength and increased the proportion of the coupling that was descending over those of the isolated spinal cord. The effects on the phase and frequency of bursting can be explained in the light of the excitation evoked by bending that we have reported previously. Thus, we demonstrate that movement and movement-related feedback that arise from spinally induced motor patterns can alter the form of the movement and the functional coupling strength among the segments of the lamprey spinal cord.

Animals↗

PubMed2000

Multisensory information for human postural control: integrating touch and vision.

Despite extensive research on the influence of visual, vestibular and somatosensory information on human postural control, it remains unclear how these sensory channels are fused for self-orientation. The focus of the present study was to test whether a linear additive model could account for the fusion of touch and vision for postural control. We simultaneously manipulated visual and somatosensory (touch) stimuli in five conditions of single- and multisensory stimulation. The visual stimulus was a display of random dots projected onto a screen in front of the standing subject. The somatosensory stimulus was a rigid plate which subjects contacted lightly (<1 N of force) with their right index fingertip. In each condition, one sensory stimulus oscillated (dynamic) in the medial-lateral direction while the other stimulus was either dynamic, static or absent. The results qualitatively supported five predictions of the linear additive model in that the patterns of gain and variability across conditions were consistent with model predictions. However, a strict quantitative comparison revealed significant deviations from model predictions, indicating that the sensory fusion process clearly has nonlinear aspects. We suggest that the sensory fusion process behaved in an approximately linear fashion because the experimental paradigm tested postural control very close to the equilibrium point of vertical upright.

Adult↗

PubMed1999

Temperature can alter the function outcome of spinal cord regeneration in larval lampreys.

Now that spinal cord regeneration has been demonstrated in mammals [Bregman B. S. et al. (1995) Nature 378, 498-501; Cheng H. et al. (1996) Science 273, 510-513], we must examine the consequences and look for means of avoiding negative outcomes. The larval lamprey, which readily regenerates cut spinal axons, offers a model for this important next step. In the present study, one group of larval lampreys with spinal lesions was kept at room temperature during recovery. Another group was returned to their usual cold room temperature. A majority of animals kept at room temperature recovered full locomotor behavioral function, while a majority of those that recovered at a colder temperature exhibited dysfunctional locomotor behavior. The dysfunction most often consisted of segments rostral and caudal to the lesion site lacking the usual coordination and apparently interfering with each. In both groups, there was a close association between the presence of dysfunction and the quality of the intersegmental coordination as assessed in the isolated spinal cord preparation. These results suggest that a relatively minor difference in conditions under which an animal recovers may drastically alter the likelihood of a favorable functional outcome.

Animals↗

PubMed1998

Estimation of coupling strength in regenerated lamprey spinal cords based on a stochastic phase model.

We present a simple stochastic model of two coupled phase oscillators and a method of fitting the model to experimental spike-train data or to sequences of burst times. We apply the method to data from lesioned isolated lamprey spinal cords. The remaining tracts at the lesion site are either regenerated medial tracts, regenerated lateral tracts, control medial tracts, or control lateral tracts. We show that regenerated tracts on average provide significantly weaker coupling than control tracts. We compare our model-dependent estimate of coupling strength to a measure of coordination based on the size of deflections in the spike-train cross-correlation histogram (CCH). Using simulated data, we show that our estimates are able to detect changes in coupling strength that do not change the size of deflections in the CCH. Our estimates are also more resistant to changes in the level of dynamic noise and to changes in relative oscillator frequency than is the CCH. In simulations with high levels of dynamic noise and in one experimental preparation, we are able detect significant coupling strength although there are no significant deflections in the CCH.

Action Potentials↗

PubMed1996

Interaction between the caudal brainstem and the lamprey central pattern generator for locomotion.

Because of its remarkable simplicity and the robustness of the isolated preparation, the lamprey has been used as a model system to study locomotion and its central pattern generator. The function of the spinal cord is relatively well understood in this context, but the role of the brain or even the caudal brainstem remains less so. We here present a study of the interaction between the caudal brainstem and the spinal pattern generator for locomotion. We show that the interaction is highly complex, with both feedforward input from the brainstem to spinal cord and feedback input from the spinal cord to brainstem playing a significant role in the motor output during locomotion. The brainstem, when diffusely stimulated pharmacologically, can initiate fictive locomotion, or it can disrupt or alter the ongoing D-glutamate initiated motor output. The nature of the disruptions vary greatly, and can induce generalized irregularity, while the alterations can include accelerating or decelerating of the bursting. All behaviors are displayed with spectrograms of the motor nerve discharge. We also show that the unstimulated brainstem can disrupt as well as slow the bursting, but in a complex fashion. Finally, a slow episodic behavior initiated from the caudal brainstem is also described. This can be elicited either by D-glutamate to the brainstem or by ascending activity from the spinal cord pattern generator. Thus, we demonstrate that the interaction between the brainstem and the spinal cord during the production of locomotion is highly complex. The locomotion that is exhibited by the combined brainstem-spinal cord preparation is extremely variable. This is in striking contrast to the variability of the locomotor output pharmacologically induced in the spinal cord alone. The latter preparation exhibits remarkable regularity, or upon occasion, irregularity, but not the routine irregularity or the systemic up and down changes in frequency seen with the brainstem present. However, the pattern of frequency changes induced by the brainstem is not predictable, and remains to be understood.

Animals↗

PubMed1996

Dynamic behavior of a neural network model of locomotor control in the lamprey.

1. Experimental studies have shown that a central pattern generator in the spinal cord of the lamprey can produce the basic rhythm for locomotion. This pattern generator interacts with the reticular neurons forming a spinoreticulospinal loop. To better understand and investigate the mechanisms for locomotor pattern generation in the lamprey, we examine the dynamic behavior of a simplified neural network model representing a unit spinal pattern generator (uPG) and its interaction with the reticular system. We use the techniques of bifurcation analysis and specifically examine the effects on the dynamic behavior of the system of 1) changing tonic drives to the different neurons of the uPG; 2) altering inhibitory and excitatory interconnection strengths among the uPG neurons; and 3) feedforward-feedback interactions between the uPG and the reticular neurons. 2. The model analyzed is a qualitative left-right symmetric network based on proposed functional architecture with one class of phasic reticular neurons and three classes of uPG neurons: excitatory (E), lateral (L), and crossed (C) interneurons. In the model each class is represented by one left and one right neuron. Each neuron has basic passive properties akin to biophysical neurons and receives tonic synaptic drive and weighted synaptic input from other connecting neurons. The neuron's output as a function of voltage is given by a nonlinear function with a strict threshold and saturation. 3. With an appropriate set of parameter values, the voltage of each neuron can oscillate periodically with phase relationships among the different neurons that are qualitatively similar to those observed experimentally. The uPG alone can also oscillate, as observed experimentally in isolated lamprey spinal cords. Varying the parameters can, however, profoundly change the state of the system via different kinds of bifurcations. Change in a single parameter can move the system from nonoscillatory to oscillatory states via different kinds of bifurcations. For some parameter values the system can also exhibit multistable behavior (e.g., an oscillatory state and a nonoscillatory state). The analysis also shows us how the amplitudes of the oscillations vary and the periods of limit cycles change as different bifurcation points are approached. 4. Altering tonic drive to just one class of uPG neurons (without altering the interconnections) can change the state of the system by altering the stability of fixed points, converting fixed points to oscillations, single oscillations to two stable oscillations, etc. Two-parameter bifurcation diagrams show the critical regions in which a balance between the tonic drives is necessary to maintain stable oscillations. A minimum tonic drive is necessary to obtain stable oscillatory output. With appropriate changes in the tonic drives to the L and C neurons, stable oscillatory output can be obtained even after eliminating the E neurons. Indeed, the presence of active E neurons in the biological system does not prove they play a functional role in the system, because tonic drive from other sources can substitute for them. On the other hand, very high excitation of any one class of neurons can terminate oscillations. Appropriate balance of tonic drives to different neuron classes can help sustain stable oscillations for larger tonic drives. Published experimental results concerning changes in amplitude and swimming frequency with increased tonic drives are mimicked by the model's responses to increased tonic drive. 5. Interconnectivity among the neurons plays a crucial role. The analysis indicates that the C and L classes of neurons are essential components of the model network. Sufficient inhibition from the L to C neurons as well as mutual inhibition between the left and right halves is necessary to obtain stable oscillatory output. When the E neurons are present in the model network, they must receive appropriate tonic drive and provide appropriate excitation

Animals↗

PubMed1995

Topological and phenomenological classification of bursting oscillations.

We describe a classification scheme for bursting oscillations which encompasses many of those found in the literature on bursting in excitable media. This is an extension of the scheme of Rinzel (in Mathematical Topics in Population Biology, Springer, Berlin, 1987), put in the context of a sequence of horizontal cuts through a two-parameter bifurcation diagram. We use this to describe the phenomenological character of different types of bursting, addressing the issue of how well the bursting can be characterized given the limited amount of information often available in experimental settings.

Animals↗

PubMed1995

Correlational analysis of fictive swimming in the lamprey reveals strong functional intersegmental coupling.

1. Cycle-to-cycle fluctuations in cycle periods and intersegmental burst delays of ventral root activity were studied during stable fictive swimming in the adult lamprey spinal cord. High spatial resolution was obtained by recording from 16 ventral roots, 1/2 on each side of the spinal cord. For ipsilateral ventral roots, correlations between cycle periods on the same cycle were high (0.50 +/- 0.16, mean +/- SD), and autocorrelations of intersegmental delays with a shift of one cycle were low (0.03 +/- 0.06). Correlations between cycle periods tended to decrease with intersegmental spacing but remained positive over the range of spacings tested (1-47 segments). 2. Sinusoidal movement imposed on the caudal end of the spinal cord/notochord was used to entrain the fictive swimming rhythm. When the phase between the movement and the rhythm was perturbed, several cycles were required for the phase to return to its preferred value, indicating that in these experiments the effect of the movement was weak. In the absence of external perturbations, autocorrelations of delays between the movement and ventral root bursts with a shift of one cycle were high. 3. Numerical simulations with a simple stochastic phase model of the lamprey central pattern generator (CPG) were conducted. High correlations of periods and low autocorrelations of delays, the pattern observed experimentally, emerged as intersegmental connection strengths were increased. Coupling including both long and short connections produced this pattern of correlations with an average connection strength less than that required by short connections alone. 4. It is concluded that functional intersegmental coupling in the lamprey CPG during stable fictive swimming is strong. Specifically, the low autocorrelations of intersegmental delays indicate that intersegmental coupling is sufficiently strong to ensure that perturbations are almost completely corrected within one cycle.

Animals↗

PubMed1993

Pattern switching in human multilimb coordination dynamics.

A relative phase model of four coupled oscillators is used to interpret experiments on the coordination between rhythmically moving human limbs. The pairwise coupling functions in the model are motivated by experiments on two-limb coordination. Stable patterns of coordination between the limbs are represented by fixed points in relative phase coordinates. Four invariant circles exist in the model, each containing two patterns of coordination seen experimentally. The direction of switches between two four-limb patterns on the same circle can be understood in terms of two-limb coordination. Transitions between patterns in the human four-limb system are theoretically interpreted as bifurcations in a nonlinear dynamical system.

Animals↗

PubMed1992

Modelling of intersegmental coordination in the lamprey central pattern generator for locomotion.

Rhythmic motor activity requires coordination of different muscles or muscle groups so that they are all active with the same cycle duration and appropriate phase relationships. The neural mechanisms for such phase coupling in vertebrate locomotion are not known. Swimming in the lamprey is accomplished by the generation of a travelling wave of body curvature in which the phase coupling between segments is so controlled as to give approximately one full wavelength on the body at any swimming speed. This article reviews work that has combined mathematical analysis, biological experimentation and computer simulation to provide a conceptual framework within which intersegmental coordination can be investigated. Evidence is provided to suggest that in the lamprey, ascending coupling is dominant over descending coupling and controls the intersegmental phase lag during locomotion. The significance of long-range intersegmental coupling is also discussed.

Animals↗

PubMed1990

The development of the lamprey pattern generator for locomotion.

The life cycle of the lamprey includes a larval stage that can last for several years. The motor behavior of the larval lamprey, the ammocoete, has been only minimally studied and little is known of the neural correlates of that behavior. Comparison of known larval behavior to that of adults leaves unclear whether there are large or small changes in the spinal nervous system during transformation. The motor output of isolated larval and transforming spinal cords when stimulated to "swim" with D-glutamate has some differences from that of comparable adult preparations, but shares many important features with adults. Primarily, the fictive swimming is less well regulated and less stable than adults of the same species. We propose that a major difference in the structure and organization of the central pattern generator for locomotion between adults and ammocoetes is a relative lack or immaturity of some cell types that participate in the coordination of the segments and the generation of the rhythm of the periodic bursting.

Animals↗

PubMed1987

A model for the periodic synaptic inhibition of a neuronal oscillator.

We develop a simple, piecewise linear differential equation with discontinuous jumps, which captures the essential characteristics of more complicated equations modelling the dynamics of neuronal oscillators, such as those due to Hodgkin & Huxley (1952), Fitzhugh (1960, 1961), and Nagumo et al. (1962). We investigate the effects of periodically applied stimuli of various durations and compare phase-transition curves or Poincaré maps for our model with numerically computed maps from the 'full' equations. We describe some aspects of the qualitative behaviour and bifurcations of these iterated one-dimensional mappings and attempt to relate them to experimental observations.

Action Potentials↗