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Simulation of the brain-stem auditory evoked potential
$88 found frequently, ranging from 14.5 to 18.8 msec. The average number of near-field positive peaks preceding the negative 19.8 was three. The clinical value of digital filtering of both near and far-field potentials will be illustrated by several case presentations.
P 3.04 POWER SPECTRAL ANALYSIS OF S H O R T LATENCY S O M A T O S E N S O R Y EVOKED POTENTIALS (SSEPs).
N.F. Hassan, P.J. Maccabee, R.Q. Cracco and J.B. Cracco
expected value m and variance s 2. Considering an ensemble of traces it can be said that the elements of this ensemble are independent and that they obey the same law of probability. The random variable N X ( 1 / N SXi) obeys a Gaussian law with an expected value m i = 1 and variance s 2, according to the central limit theory. The confidence limit can then be estimated for s 2 from the average of the first 32 traces. After a certain number of stimuli it is possible to calculate S / N from the power of the resulting responses to stimuli and knowledge of s 2. If one fixes the value of S / N the optimum number of stimuli for a given intensity of stimuli can be determined. This allows comparison in the same condition of the BAEP and the range of measurements.
(New York, NY, USA) The Fast Fourier Transform (FFT) can be used to analyze evoked potentials. The averaging technique which emphasizes time-locked components provides only limited information regarding the frequency content of the evoked potential. Conversion of evoked potential data from the time to the frequency domain using FFT can result in a comprehensive reflection of the frequencies associated with the waveform in the time domain. In this study the FFT was used to determine the dominant frequency components of median nerve SSEPs. Forty msec epochs of scalp (C 3 or C4) to noncephalic reference recordings obtained from 6 normal subjects were analyzed. In all 6 subjects, 4 dominant frequency components were present. Each of these frequency components was converted to the time domain resulting in a damping sinusoidal wave - unique to each frequency. The addition of these sinusoids resulted in the typical median nerve waveform. This data suggests that median nerve SSEPs are composed of dominant groups of frequencies, each representing a unique damping; sinusoidal wave which may reflect a specific frequency generator.
P 3.06 S I M U L A T I O N OF THE BRAIN-STEM AUDITORY EVOKED POTENTIAL.
E. Ragi (Oxford, UK) A normal brain-stem auditory evoked potential (BAEP) is simulated by adding four sine waves of equal amplitude but of successively doubling frequencies. These sine waves begin simultaneously, less than 0.4 msec after click onset. This suggests that the BAEP originates from a peripheral structure, outside the brain-stem. It also suggests that wave latencies do not indicate nervous conduction time between spatially separate structures.
P 3.07 THE INTER-RATER OBJECTIVITY OF EVOKED POTENTIAL ANALYSIS AND INTERPRETATION SUBJ E C T TO PRIOR INFORMATION.
W. Hacke and K. Willmes (Aachen, West Germany)
P 3.05 O P T I M I Z A T I O N OF T H E RECORDING BRAIN-STEM AUDITORY EVOKED POTENTIALS.
OF
B. Lumeau and G. Rondouin (Montpellier. France) The recording of brain-stem auditory evoked potentials (BAEP) requires the averaging of numerous responses in order to increase the signal/noise ( S / N ) ratio. The number of traces is usually fixed arbitrarily at 2000. We propose a method allowing rapid determination of the S / N ratio and optimization of the number of traces needed to record the BAEP. The potentials recorded are amplified (x 106) by means of a low-noise isolated pre-amplifier (HYPO 2000) with a 100 Hz-10KHz band pass. The average of the acquired signals is made by a Disa 1500 interface and a P D P l l / 2 3 . Each trace has 1024 points and can be considered as a sample of a random variable Xi with an
Evoked potentials (VEP, SEP) and the electric blink reflex help to make the diagnosis of multiple sclerosis (MS) more reliable. In psychology it is well known that the interpretation of test results may be influenced by prior information. We examined whether the interpretation of VEP, SEP and the blink reflex is changed if the examiner knows that a patient is suspected of having MS. We studied the measurement and interpretation of 16 VEP (12 n o r m a l / 4 pathological), 14 SEP (8/6) and 16 blink reflexes (8/8). The printouts from routine examinations were twice evaluated by 8 trained neurophysiologists. In the first session, one half of the patients were labelled "MS?'. In the second session, 8 12 days later, the identical printouts had to be measured and interpreted again, but the raters were not aware of this fact. This time the initially unlabelled half of the potentials were labelled 'MS?'. The results of measurements and interpretations in both sessions were evaluated statistically for inter-rater agreement. While the measurements were highly
P 3.04 POWER SPECTRAL ANALYSIS OF S H O R T LATENCY S O M A T O S E N S O R Y EVOKED POTENTIALS (SSEPs).
N.F. Hassan, P.J. Maccabee, R.Q. Cracco and J.B. Cracco
expected value m and variance s 2. Considering an ensemble of traces it can be said that the elements of this ensemble are independent and that they obey the same law of probability. The random variable N X ( 1 / N SXi) obeys a Gaussian law with an expected value m i = 1 and variance s 2, according to the central limit theory. The confidence limit can then be estimated for s 2 from the average of the first 32 traces. After a certain number of stimuli it is possible to calculate S / N from the power of the resulting responses to stimuli and knowledge of s 2. If one fixes the value of S / N the optimum number of stimuli for a given intensity of stimuli can be determined. This allows comparison in the same condition of the BAEP and the range of measurements.
(New York, NY, USA) The Fast Fourier Transform (FFT) can be used to analyze evoked potentials. The averaging technique which emphasizes time-locked components provides only limited information regarding the frequency content of the evoked potential. Conversion of evoked potential data from the time to the frequency domain using FFT can result in a comprehensive reflection of the frequencies associated with the waveform in the time domain. In this study the FFT was used to determine the dominant frequency components of median nerve SSEPs. Forty msec epochs of scalp (C 3 or C4) to noncephalic reference recordings obtained from 6 normal subjects were analyzed. In all 6 subjects, 4 dominant frequency components were present. Each of these frequency components was converted to the time domain resulting in a damping sinusoidal wave - unique to each frequency. The addition of these sinusoids resulted in the typical median nerve waveform. This data suggests that median nerve SSEPs are composed of dominant groups of frequencies, each representing a unique damping; sinusoidal wave which may reflect a specific frequency generator.
P 3.06 S I M U L A T I O N OF THE BRAIN-STEM AUDITORY EVOKED POTENTIAL.
E. Ragi (Oxford, UK) A normal brain-stem auditory evoked potential (BAEP) is simulated by adding four sine waves of equal amplitude but of successively doubling frequencies. These sine waves begin simultaneously, less than 0.4 msec after click onset. This suggests that the BAEP originates from a peripheral structure, outside the brain-stem. It also suggests that wave latencies do not indicate nervous conduction time between spatially separate structures.
P 3.07 THE INTER-RATER OBJECTIVITY OF EVOKED POTENTIAL ANALYSIS AND INTERPRETATION SUBJ E C T TO PRIOR INFORMATION.
W. Hacke and K. Willmes (Aachen, West Germany)
P 3.05 O P T I M I Z A T I O N OF T H E RECORDING BRAIN-STEM AUDITORY EVOKED POTENTIALS.
OF
B. Lumeau and G. Rondouin (Montpellier. France) The recording of brain-stem auditory evoked potentials (BAEP) requires the averaging of numerous responses in order to increase the signal/noise ( S / N ) ratio. The number of traces is usually fixed arbitrarily at 2000. We propose a method allowing rapid determination of the S / N ratio and optimization of the number of traces needed to record the BAEP. The potentials recorded are amplified (x 106) by means of a low-noise isolated pre-amplifier (HYPO 2000) with a 100 Hz-10KHz band pass. The average of the acquired signals is made by a Disa 1500 interface and a P D P l l / 2 3 . Each trace has 1024 points and can be considered as a sample of a random variable Xi with an
Evoked potentials (VEP, SEP) and the electric blink reflex help to make the diagnosis of multiple sclerosis (MS) more reliable. In psychology it is well known that the interpretation of test results may be influenced by prior information. We examined whether the interpretation of VEP, SEP and the blink reflex is changed if the examiner knows that a patient is suspected of having MS. We studied the measurement and interpretation of 16 VEP (12 n o r m a l / 4 pathological), 14 SEP (8/6) and 16 blink reflexes (8/8). The printouts from routine examinations were twice evaluated by 8 trained neurophysiologists. In the first session, one half of the patients were labelled "MS?'. In the second session, 8 12 days later, the identical printouts had to be measured and interpreted again, but the raters were not aware of this fact. This time the initially unlabelled half of the potentials were labelled 'MS?'. The results of measurements and interpretations in both sessions were evaluated statistically for inter-rater agreement. While the measurements were highly