Implications of a neural network model of early sensori-motor development for the field of developmental neurology.

This paper reports on a neural network model for early sensori-motor development and on the possible implications of this research for our understanding and, eventually, treatment of motor disorders like cerebral palsy. We recapitulate the results we published in detail in a series of papers [1-4]. The neural circuits in the model self-organize on the basis of rhythmic activity spontaneously generated in the model. This indicates the importance of endogenously generated activity in the developing brain. We also show that afferent feed-back from the mechanical part of the model is easily incorporated in the neural part of the model. In this way the model acquires reflex-related properties which have long been demonstrated in man. In the discussion we relate these experimental findings to the variability concept from developmental neurology and show how variable motor performance is important for motor learning. We also discuss possible implications of our modelling effort for movement disorders, specifically spastic cerebral palsy.

Development in a biologically inspired spinal neural network for movement control.

In two phases, we develop increasingly complex neural network models of spinal circuitry that self-organizes into networks with opponent channels for the control of an antagonistic muscle pair. The self-organization is enabled by a Hebbian learning rule operating during spontaneous activity present in the spinal cord. After the self-organized development, the networks enable independent control of the length and tension of the innervated muscles. This allows higher centers to hold joint angle invariant while varying joint stiffness and vice versa. The first network comprises only spontaneous activity generators, motorneurons, and inhibitory interneurons through which the two channels interact. The inhibitory interneurons enhance reciprocal action, and prevent saturation of the motorneuron pools, which is a necessary condition for independent control. In the second network, the neurons in the motorneuron pools obey the size-principle, which, when added by itself, leads to a loss of the desired invariance property. To restore the desired invariance, the second network further incorporated inhibitory interneurons analogous to Renshaw cells. The results obtained from the two models compare favourably with the FLETE-model for spinal circuitry ([Bullock and Contreras-Vidal, 1993]; [Bullock et al., 1992]; [Bullock and Grossberg, 1991]) which, although successful in explaining several phenomena related to motor control, did not self-organize its connection weights. Finally, we suggest ways in which this research could be applied in technology.

Self-organizing effects of spontaneous neural activity on the development of spinal locomotor circuits in vertebrates.

Presented in this paper is a neural network model that can be used to investigate the possible self-organizing mechanisms occurring during the early ontogeny of spinal neural circuits in the vertebrate motor system. The neural circuit is composed of multiple types of neurons which correspond to motorneurons, Renshaw cells and a hypothetical class of interneurons. While the connectivity of this circuit is genetically predetermined, the efficacies of these connections--the synaptic strengths--evolve in accordance with activity-dependent mechanisms which are initiated by the intrinsic, autonomous activity present in the developing spinal cord. Using Oja's rule, a modified Hebbian learning scheme for adjusting the values of the connections, the network stably self-organizes developing, in the process, reciprocally activated motorneuron pools analogous to those which exist in vivo.

Computer-simulated neural networks: an appropriate model for motor development?

The idea of an artificial neural network is introduced in a historical context, and the essential aspect of it, viz., the modifiable synapse, is compared to the aspect of plasticity in the natural nervous system. Based on such an artificial neural network, a model is presented for the way in which (the path along which) the connectivity in the spinal cord is modified during the period that a newborn 'learns' to control the movement of his forearm. In this way an automatic calibration of the receptors and the antagonists' recruitment of motor units is represented. The learning process is described in non-mathematical terminology. The model is then shown to be able after learning to reach target angles outside the training set of angles, and to be able to relearn when an important receptor has been made inoperative. In this way it is shown that the model is able to generalize, and that it is robust against at least some damage.

Effects of prone and supine position on heart rate, respiratory rate and motor activity in fullterm newborn infants.

Polygraphic recordings were obtained for 24 normal full-term neonates on their 4th or 5th day of life. Thirteen of the infants were first fed, laid supine for 3 hours, fed again around noon and laid prone for 3 more hours. The other 11 were first laid prone and then supine. The awake state was observed more in the supine than the prone position (P less than 0.001). Wakefulness occurred at the expense of state 1 and/or state 2. The heart rate (HR) was higher in babies lying prone. It was also higher in the afternoon than in the morning. No such relationship was found for the respiratory rate (RR). There was higher average EMG activity during state 1 in the prone position. The cross-correlation between HR and EMG activity was positive during state 2, and states 4 and 5, regardless of the position. The cross-correlation between the RR and the EMG was usually negative in state 2, and in states 4 and 5, there being many exceptions. The cross-correlation between breathing and heart-beat was mostly negative during state 2, but it eventually became positive due to periodic breathing, central apnoeas with bradycardia, stretches, and crawling movements.

Information statistics as applied to social interaction between mother and infant: synergic covariability vs. partial cross-covariability.

Behaviour can, in general, not be quantified on an equal-interval scale; therefore, the description of sequential aspects of the behaviour of two interacting individuals can not be formulated on the basis of the usual cross-correlation techniques. If the information content of behaviour, however, is defined analogously to the entropy in a message, this allows a quantification of the contribution of either participant to the interaction process, and the identification of a non-additive (synergic) component in addition to the auto- and cross-covariability terms. Such a framework, based on the concept of transinformation or mutual information, was developed and realised in a suite of three FORTRAN programs. These were then applied in a longitudinal study on the development of early mother-infant interaction. The synergic term appears to increase with age in the first five months of life. This application illustrates the usefulness of information statistics for quantifying the ways in which participants contribute to a process of interaction.

EEG coherence functions for normal newborns in relation to their sleep state.

While power spectra provide little information about the properties of the newborn EEG, the coherence functions (especially interhemispheric) have a characteristic shape for each of the 2 sleep states. Coherence functions may therefore be clinically more important than spectra. For this reason, baseline data for normal newborns have been produced as a reference against which coherences from patients can be compared. It is stressed that for a sleeping newborn the behavioural state must be known for the coherence function to be interpretable. Furthermore, since intraindividual variability is large, data from long recordings are required. The results are discussed in the light of the existing literature.

Coherence patterns of the infant sleep EEG in absence of the corpus callosum.

The role of the corpus callosum in the relationship between the EEGs of the two cerebral hemispheres was studied by comparing 3 infants with congenital agenesis of the corpus callosum with a group of neurologically normal infants. Coherence functions were computed for symmetrical pairs of EEG derivations; in the frequency bands below 4 Hz they were in general much lower in the acallosal infants than in the normals.

Electrical activity in the red nuclei of rats and the effects of hemicerebellectomy at young ages.

Extracellular activity in the red nuclei in different behavioural states was investigated in control adult rats and in adults rats who had previously undergone a cerebellar hemispherectomy on their 2nd, 5th, 10th or 20th day. In addition to visual analysis, spectra and coherence functions were computed from the signals during quiescence and movements. Preceding and during movements a particular pattern, characterized by a regular rhythm between 7.5 and 8.0 Hz occurs. Inter-rubral coherences are high at the specific pattern frequency. In rats with a cerebellar hemispherectomy - irrespective of the age at lesioning - power spectra from both red nuclei differ markedly from each other during quiescence. When movements occur, rats operated at the 20th day show a pattern in both red nuclei similar to that found in control rats. However, in rats lesioned at the 5th or 10th day the specific pattern frequency is much lower while in rats operated at the 2nd day no specific patterning of red nuclei activity is evident.

Relative contributions of intracortical and thalamo-cortical processes in the generation of alpha rhythms, revealed by partial coherence analysis.

The thalamo-cortical relationships of alpha rhythms have been analysed in dogs using partial coherence function analysis. The objective was to clarify how far the large intracortical coherence commonly recorded between different cortical sites could depend on a common thalamic site. It was found that the alpha rhythms of the LGN influenced only moderately the coherence between cortical alpha rhythms whereas that of the pulvinar had much more influence, at least in relation to some cortical areas. Significant phase shifts between thalamus and cortex were also measured. The results demonstrate that there are thalamo-cortical and cortico-cortical components which interact in the generation of cortical alpha rhythms.

[Postural scintimyelography of the cervical spine (author's transl)]. / Haltungsszintimyelographie des Zervikalkanals

This study discusses the clinical validity of 101 postural scintimyelograms as a screening method for cervical myelopathy. The RIHSA or Ytterbium isotope-examination are not an extra burden to the patient if the radiopharmacon is introduced into the dural sac following Queckenstedt's test. If there is a normal rise and fall of CSF pressure after bilateral jugular compression and postural scintimyelography is judged normal as well, a cervical subarachnoid obstruction may be excluded. If both examinations yield pathological results an obstructive process in the cervical spinal canal is made certain; and in cases of spondylotic myelopathy a positive contrast myelogrpahy may thus be superfluous. If the results of these two examinations are contradictory then further neuroradiolocal studies are indicated.