Evaluation of the distortion of EEG signals caused by a hole in the skull mimicking the fontanel in the skull of human neonates
Introduction
Brain functions of human infants, including pre-term and term babies, have been traditionally evaluated with EEG as part of a neurological examination. The evaluation is useful in infants with risk factors such as complications during labor and delivery (Sunshine, 1997). EEG is also useful for staging the development of the nervous system, since differences in the functional organization of certain brain regions are already present in newborns (Eiselt et al., 2001), and for differential diagnosis of seizures from non-seizures in the paroxysmal motor behavior (Tharp, 1997, De Weerd, 1995). However, neonatal EEG monitoring is still in need of development, particularly in relating EEG signals to the underlying brain functions.
Interpretation of the underlying physiology and pathophysiology is complicated by the presence of the fontanels and sutures in the skull. The anterior and posterior fontanels are present at the bregma and the lambda. The skull is not present within the fontanels. The brain below the fontanels is protected by the dura, which is thicker within the opening. The fontanels and sutures are squeezed together during delivery, but become larger during the first months and then eventually close. The anterior fontanel may be large enough to admit an adult's thumb. The posterior fontanel is closed within the first 2 month. The anterior fontanel stays open as long as 16 months or, in abnormal cases, even for several years. The sutures can be quite wide near the fontanels. The mean width of the coronal and lambdoidal sutures at their midpositions is 3–4 mm for infants between 0 and 60 days after birth (Eramie and Ringertz, 1976). The sutures may stay unfused for several years (Hansman, 1966). EEG signals may be profoundly affected by the fontanels and sutures, which are effectively skull defects, since they represent areas of high conductivity relative to the skull. Volume currents might be affected by these openings in the skull. Thus, it is necessary to evaluate the effect of these skull openings on EEG in order to accurately infer physiological functions in human infants.
This report describes distortions of the somatosensory-evoked potentials (SEPs) by a hole in the skull of neonatal pigs. Although the EEG signal distortion by skull defects can be studied using mathematical models of the head as discussed later, we chose to study the problem in an experimental preparation since such study is rare and empirical assessment always provides a solid foundation for future modeling studies. Our results may be useful for interpreting distortions of EEG signals in humans arising from skull defects like the fontanels and sutures in human infants, since the head of the piglet is large and the scalp and skull are comparable to those of human infants in thickness. The neonatal farm swine of 1–3 weeks of age has a skull thickness of 2–3 mm near the vertex, similar to the thickness in human newborns (Hansman, 1966). Unlike human neonates, the piglets do not have a fontanel and the sutures are nearly closed at birth. A square hole was, therefore, created in the skull to mimic a fontanel. The SEP distortion was evaluated as a function of distance of active tissue from the hole and for 3 different conductivities of the hole.
Section snippets
Preparation
The protocol used in this study was approved by the Animal Use Committees of the Albuquerque Veterans Affairs Medical Center and the University of New Mexico School of Medicine. Animals were treated according to the National Institute of Health ‘Guide for the Care and Use of Laboratory Animals, Revised 1996’.
Experiments were carried out on 10 farm swine (Sus scrofa). The weight ranged between 8 and 16 kg and the age between 4 and 6 weeks. Pigs were chosen because of their large gyrencephalic
Experiment 1
The circle in the inset of Fig. 1 shows the estimated source location in the rostrum region (shaded area delineated by the circular coronal sulcus) of the SI cortex activated by the snout stimulation. This area was within the square 12×12 mm hole created in the skull. The SEP measurements were carried out within the dashed square area. The SEP over the hole was larger in the sucrose1–agar condition compared to the air1 condition in which the hole was filled with a non-conducting medium (namely
Discussion
The above results clearly demonstrate that a hole in the skull of the neonatal piglet may produce a strong distortion of EEG signals. The leakage of current through the hole and the surrounding skull was clearly affected by the conductivity of the hole and distance of the active tissue producing the SEP. The distortion was different for the different components of the SEP.
In the case of a hole filled with air, the volume current from an active tissue below the hole cannot flow through the air
Acknowledgements
This work was supported by the German Academic Exchange Service (DAAD), the European Community (IST 1999-10378), and by the NIH Grant (R01-NS30968). Special thanks go to Yoshio Okada for the great opportunity to perform this scientific work at his laboratory.
References (34)
- et al.
Spline Laplacian estimate of EEG potentials over a realistic magnetic resonance-constructed scalp surface model
Electroenceph Clin Neurophysiol
(1996) - et al.
Magnetic localization of a dipolar current source implanted in a sphere and a human cranium
Electroenceph Clin Neurophysiol
(1986) - et al.
Modeling of post-surgical brain and skull defects in the EEG inverse problem with the boundary element method
Clin Neurophysiol
(2002) - et al.
Functional interaction within the newborn brain investigated by adaptive coherence analysis of EEG
Clin Neurophysiol
(2001) - et al.
High resolution EEG: 124-channel recording, spatial deblurring and MRI integration methods
Electroenceph Clin Neurophysiol
(1994) Multiecho imaging sequence with low refocussing angles
J Magn Reson
(1988)An on-line transformation of EEG scalp potentials into orthogonal source derivations
Electroenceph Clin Neurophysiol
(1975)- et al.
Local estimate of surface Laplacian derivation on a realistically shaped scalp surface and its performance on noisy data
Electroenceph Clin Neurophysiol
(1994) - et al.
Theoretical and experimental study of high resolution EEG based on surface Laplacians and cortical imaging
Electroenceph Clin Neurophysiol
(1994) - et al.
Comparison of MEG and EEG on the basis of somatic evoked responses elicited by stimulation of the snout in the juvenile swine
Clin Neurophysiol
(1999)
Experimental analysis of distortion of MEG signals by the skull
Electroenceph Clin Neurophysiol
Somatosensory evoked potentials and magnetic fields elicited by tactile stimulation of the hand during active and quiet sleep in newborns
Clin Neurophysiol
Conductivities of three-layer live human skull
Brain Topogr
Effects of skull thickness, anisotropy, and inhomogeneity on forward EEG/ERP computations using a spherical three-dimensional resistor mesh model
Hum Brain Mapp
Atlas of EEG in the first months of life
Cited by (54)
Consciousness in the cradle: on the emergence of infant experience
2023, Trends in Cognitive SciencesCortical Source Analysis of Event-Related Potentials: A Developmental Approach
2022, Developmental Cognitive NeuroscienceInfluence of unfused cranial bones on magnetoencephalography signals in human infants
2021, Clinical NeurophysiologyMagnetoencephalography signals are influenced by skull defects
2014, Clinical NeurophysiologyCitation Excerpt :They found no significant difference in MEG signal amplitude or morphology, except for an attenuation of the MEG signal when the skull was removed, which was stronger for deeper sources (25% for a 14 mm-deep source). The limitations of existing experiments are that (1) the skull defect was filled with non-conducting air (except in (Flemming et al., 2005) for EEG); (2) the skull defect was large compared with the sensor planes (skull-on versus skull-off); and (3) that evoked responses were used, which have a high variability with regard to source position, extent, orientation, and amplitude. Therefore, the objective of this study is to experimentally investigate the influence of conducting skull defects on EEG and MEG signals above and around a skull defect, using a well-defined current source under the middle and edge of the defect and next to it, in an in vivo rabbit model.
Neural correlates of music-syntactic processing in two-year old children
2014, Developmental Cognitive NeuroscienceUltra-dense EEG sampling results in two-fold increase of functional brain information
2014, NeuroImageCitation Excerpt :This would produce corresponding changes in EEG at the spatial scales of 0.5–1 cm following the Ramon (2009) study. Analytical modeling of skull inhomogeneity (Nunez, 2006; Ollikainen, 1999; Flemming, 2005) provides another possible explanation. Scalp electric potential above a cortical source is very sensitive to the local skull conductivity.