Kurt E. Hecox

Neuropsychological and behavioral status of children with complex partial seizures

Neuropsychological and behavioral status were examined in 57 children aged 7 to 16 years with complex partial seizures (CPS) and compared with 27 sibling control children of the same age. Epilepsy had a significant effect on both cognitive and behavioral adjustment measures. Children with CPS had significant impairment across all seven cognitive domains assessed, reflective of a profile of relatively diffuse and generalized cognitive dysfunction. Age at onset of recurrent seizures was the strongest and most consistent predictor of adequacy of cognitive functioning; earlier age at onset was associated with poorer cognitive status. Children with CPS also had more problems compared with sibling control children on measures of social and school competence and internalizing behavior problems, but not externalizing behaviors. Further, frequency of seizure activity in the past year, rather than age at seizure onset, emerged as the strongest predictor of these behavioral difficulties. These findings are discussed in the context of understanding the impact of CPS on cognition and behavioral adjustment, and identifying the contribution of various aspects of the neurodevelopmental course of CPS to these issues.

Estimation of in vivo human brain-to-skull conductivity ratio from simultaneous extra- and intra-cranial electrical potential recordings.

OBJECTIVE The present study aims to accurately estimate the in vivo brain-to-skull conductivity ratio by means of cortical imaging technique. Simultaneous extra- and intra-cranial potential recordings induced by subdural current stimulation were analyzed to get the estimation. METHODS The effective brain-to-skull conductivity ratio was estimated in vivo for 5 epilepsy patients. The estimation was performed using multi-channel simultaneously recorded scalp and cortical electrical potentials during subdural electrical stimulation. The cortical imaging technique was used to compute the inverse cortical potential distribution from the scalp recorded potentials using a 3-shell head volume conductor model. The brain-to-skull conductivity ratio, which leads to the most consistent cortical potential estimates with respect to the direct intra-cranial measurements, is considered to be the effective brain-to-skull conductivity ratio. RESULTS The present estimation provided consistent results in 5 human subjects studied. The in vivo effective brain-to-skull conductivity ratio ranged from 18 to 34 in the 5 epilepsy patients. CONCLUSIONS The effective brain-to-skull conductivity ratio can be estimated from simultaneous intra- and extra-cranial potential recordings and the averaged value/standard deviation is 25+/-7. SIGNIFICANCE The present results provide important experimental data on the brain-to-skull conductivity ratio, which is of significance for accurate brain source localization using piece-wise homogeneous head models.

Brainstem auditory evoked response in the diagnosis of pediatric neurologic diseases

Brainstem auditory evoked responses (BAERs) were measured in pediatric patients with neurologic diseases. Abnormalities of interwave intervals, amplitude ratios, and response to changing rate of stimulation were found in patients with tumors, myelin disorders, anoxic-ischemic encephalopathy, trauma, and neu-rodegenerative disorders. Reversibility of brainstem abnormalities (excluding multiple sclerosis) and sensitivity to toxic-metabolic disorders are features of the response not previously described in adults with neurologic disorders. The BAER is a promising new tool for the investigation of pediatric neurologic disease.

A spatial feature extraction and regularization model for the head‐related transfer function

A functional representation is proposed for complex valued (amplitude and phase) head-related transfer functions (HRTFs), including both frequency and spatial dependence. The frequency variation is spanned by a set of eigentransfer functions (EFs) that are generated using the Karhunen-Loève expansion. Any HRTF is represented as a weighted combination of the EFs where the weights are functions of the HRTFs spatial location and are termed spatial characteristic functions (SCFs). Samples of the SCFs are obtained by projecting the measured HRTFs onto the EFs. A regularization framework is employed to obtain a functional representation for the SCFs by fitting each set of SCF samples with a two-dimensional spline. Acoustic validation of the models fidelity and predictive capability is provided using 2188 measured HRTFs from a KEMAR manikin and 1816 measured HRTFs from an anesthetized live cat. Errors between measured and modeled HRTFs are generally less than one percent. Larger errors occur in the contralateral regions for KEMAR and lower back regions for the cat as a consequence of the relatively small HRTF amplitudes resulting from head shadowing. Methods for reducing these errors are discussed.

Seizure anticipation in pediatric epilepsy: use of kolmogorov entropy

The purpose of this paper is to demonstrate feasibility of using trends in Kolmogorov entropy to anticipate seizures in pediatric patients with intractable epilepsy. Surface and intracranial recordings of preseizure and seizure activity were obtained from five patients and subjected to time series analysis using Kolmogorov entropy. This metric was compared with correlation dimension and power indices, both known to predict seizures in some adult patients. We used alarm levels and introduced regression analysis as a quantitative approach to the analysis of trends. Surrogate time series evaluated data nonlinearity, as a precondition to the use of nonlinear measures. Seizures were anticipated before clinical or electrographic seizure onset for three of the five patients from the intracranial recordings, and in two of five patients from the scalp recordings. Anticipation times varied between 2 and 40 minutes. This is the first report in which simultaneous surface and intracranial recording are used for seizure prediction in children. We conclude that the Kolmogorov entropy and power indices were as effective as the more commonly used correlation dimension in anticipating seizures. Further, regression analysis of the Kolmogorov entropy time series is feasible, making the analysis of data trends more objective.

Prognostic importance of brainstem auditory evoked responses after asphyxia

Brainstem auditory evoked responses were recorded after acute asphyxia in 126 infants, ages birth through 18 months. Of these, 21 had markedly abnormal amplitude ratios, and all infants with abnormal amplitude ratios had severe neurologic handicaps. An abnormal amplitude ratio predicts long-term neurologic sequelae of acute asphyxia in infants. Normal amplitude ratios did not, however, ensure normal neurologic outcome; 10 infants with normal responses were severely handicapped.

Brainstem auditory evoked responses in man. I. Effect of stimulus rise–fall time and duration

Short latency (under 10 msec) evoked responses elicited by bursts of white noise were recorded from the scalp of human subjects. Response alterations produced by changes in the noise‐burst duration (on time), interburst interval (off time), and onset and offset shapes are reported and evaluated. The latency of the most prominent response component, wave V, was markedly delayed with increases in stimulus rise time but was unaffected by changes in fall time. Increase in stimulus duration and therefore loudness resulted in a systematic increse in latency, probably due to response recovery processes, as this effect was eliminated with increases in stimulus off time. The amplitude of wave V was insensitive to changes in signal rise and fall times, while increasing signal on time produced smaller amplitude responses only for sufficiently short off times. It is concluded that wave V of the human auditory brainstem‐evoked response is solely an onset response.Subject Classification :[43]65.59.[43].65.75.

DEVELOPMENTAL DEPENDENCIES OF THE HUMAN BRAINSTEM AUDITORY EVOKED RESPONSE

The technical and methodological challenges to the study of sensory development are multiple. Measurements must often be performed on a small scale, the phenomena under study are in constant evolution, there is increased behavioral variability, and the relative contributions of genetic versus experiential factors must be defined. For these reasons, the introduction of a rapid technique able to circumvent the vagaries of behavioral instability without the sacrifice of quantification is usually enthusiastically received. It is not surprising, therefore, that the introduction of the human brainstem auditory evoked response (BAER). as a quantitative measure of auditory maturation, is rapidly gaining in popularity. This chapter reviews the BAER as applied to the study of auditory development. There are three main divisions in this article. The first is a description of the age dependence of response parameters, the second examines the effect of stimulus variables on response parameters, and the third summarizes selected issues surrounding the application of the brainstem auditory evoked response to the study of human auditory development.

The effect of broadband noise on the human brainstem auditory evoked response. I. Rate and intensity effects

A series of experiments investigated the effects of continuous broadband noise (ipsilateral) on wave V of the click-evoked brainstem auditory evoked response (BAER). In general, a broadband noise masker increases the latency and decreases the amplitude of wave V. Varying both click and noise intensity, it was found that noise levels above about 40 dB SPL increase the latency and decrease the amplitude of wave V, regardless of click intensity. The effects of noise on wave V amplitude appear constant across click intensity, whereas the effects of a constant noise level on wave V latency decrease at higher click intensities. Both masking and adaptation increase wave V latency, but their combined effects are occlusive: rate-induced wave V latency shift decreases in the presence of continuous broadband noise. The clinical and theoretical implications of these findings are discussed.

High-resolution EEG: Cortical potential imaging of interictal spikes

BACKGROUND It is of clinical importance to localize pathologic brain tissue in epilepsy. Noninvasive localization of cortical areas associated with interictal epileptiform spikes may provide important information to facilitate presurgical planning for intractable epilepsy patients. METHODS A cortical potential imaging (CPI) technique was used to deconvolve the smeared scalp potentials into the cortical potentials. A 3-spheres inhomogeneous head model was used to approximately represent the head volume conductor. Five pediatric epilepsy patients were studied. The estimated cortical potential distributions of interictal spikes were compared with the subsequent surgical resections of these same patients. RESULTS The areas of negativity in the reconstructed cortical potentials of interictal spikes in 5 patients were consistent with the areas of surgical resections for these patients. CONCLUSIONS The CPI technique may become a useful alternative for noninvasive mapping of cortical regions displaying epileptiform activity from scalp electroencephalogram recordings.