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Biomedical subjects

A J Maas

Publications and source records attributed to A J Maas.

At least 19 recordsLinked to original sources
PubMed2000

The value of the "worst" computed tomographic scan in clinical studies of moderate and severe head injury. European Brain Injury Consortium.

OBJECTIVE: Computed tomographic (CT) scanning can reveal the pattern and severity of structural brain damage after head injury. With the proliferation of CT scanners in general hospitals, and with improvements in patient transport, the interval from injury to the first CT scan is decreasing. The potential result is an "admission" scan missing an evolving and potentially operable lesion. Furthermore, the literature is confusing regarding the timing and coding of CT findings. We sought to establish the frequency of deterioration in CT appearance from an admission scan to subsequent scans and the prognostic significance of such deterioration. METHODS: In a survey organized by the European Brain Injury Consortium, data on initial severity, management, and subsequent outcome were gathered prospectively for 1005 patients with moderate or severe head injury admitted to one of 67 European neurosurgical units during a 3-month period in 1995. The findings of the initial and the final ("worst") CT scan were classified according to the Traumatic Coma Data Bank system and were related to outcome as assessed using the Glasgow Outcome Scale 6 months after injury. RESULTS: Data on an initial and a final CT scan were available for 897 patients; of these, 724 patients were assessed using the Glasgow Outcome Scale at 6 months. The initial CT findings were classified as a diffuse injury for 53% of the cohort, with 16% of these diffuse injuries demonstrating deterioration on a subsequent scan. In 56 (74%) of 76 deteriorations, the change was from a diffuse injury to a mass lesion. When the initial CT scan demonstrated a diffuse injury without swelling or shift, evolution to a mass lesion was associated with a statistically significant increase in the risk of an unfavorable outcome (62% versus 38%). When the initial scan demonstrated evidence of swelling or shift, there was a nonsignificant trend in the opposite direction, although the numbers were limited. CONCLUSION: When an admission CT scan demonstrates evidence of a diffuse injury, follow-up scans should be performed, because approximately one in six such patients will demonstrate significant CT evolution. In studies comparing series of head-injured patients, correspondence of timing of CT scans is necessary for valid comparison.

Brain Injuries↗
PubMed1996

Compression and its effect on the speech signal.

Compression systems are often used in hearing aids to increase the wearing comfort. A patient has to readjust frequently the gain of a linear hearing aid because of the limited dynamic hearing range and the changing acoustical conditions. A great deal of attention has been given to the static parameters but very little to the dynamic parameters. We present a general method to describe the dynamic behavior of a compression system by comparing modulations at the output with modulations at the input. The use of this method resulted in a single parameter describing the temporal characteristics of a compressor, the cut-off modulation frequency. In this paper its value is compared with known properties of running speech. A limitation of this method is the use of only small modulation depths, and the consequence of this limitation is tested. The use of this method is described for an experimental digital compressor developed by the authors, and the effects of some temporal parameters such as attack and release time are studied. This method shows the rather large effects of some of the parameters on the effectiveness of a compressor on speech. This method is also used to analyze two generally accepted compression systems in hearing aids. The theoretical method is next compared to the effects of compression on the distribution of the amplitude envelope of running speech, and it could be shown that single-channel compression systems do not reduce the distribution width of speech filtered in frequency bands. This finding questions the use of compression systems for fitting the speech banana in the dynamic hearing range of impaired listeners.

Correction of Hearing Impairment↗
PubMed1996

Visual processing during high frequency head oscillation.

There is a discrepancy between the averaged amount of retinal slip measured during head movement and the expected level of visual performance. This study offers an explanation based on the sinusoidal velocity profile of the retinal image motion during head oscillation. Contrast thresholds were measured for stationary and moving 0.5 and 6 cycles per degree (c.deg-1) gratings with both passive oscillation (2-6 Hz) and fixation of the head. Eye and head movements were recorded with a magnetic induction coil technique. When expressed in terms of averaged retinal image motion, threshold contrast for the 0.5 c.deg-1 grating remained suboptimal, whereas the blurring of the 6 c.deg-1 grating appeared to be less during the head movement. The data obtained from head movement versus non-head movement conditions could, however, be matched when the oscillatory character of the retinal image motion was taken in to account. The amplitude of gaze movement was small relative to the size of the individual cycles in the 0.5 c.deg-1 grating. This reduced spatiotemporal interaction which rendered their visibility sub-optimal. Contrast thresholds for the 6 c.deg-1 grating were highly correlated to the duration of the time intervals in which the retinal slip was below the critical value of 2 deg.s-1. Intermittent exposure to quasi-stable retinal images explained the relatively high level of contrast sensitivity for the 6 c.deg-1 grating during head oscillations.

Contrast Sensitivity↗
PubMed1993

Syllabic compression and speech intelligibility in hearing impaired listeners.

Syllabic compression has not been shown unequivocally to improve speech intelligibility in hearing-impaired listeners. This paper attempts to explain the poor results by introducing the concept of minimum overshoots. The concept was tested with a digital signal processor on hearing-impaired subjects. The results show that moderate syllabic compression may raise speech intelligibility, as long as overshoots are minimized and relatively short time constants are used. Frequency equalization also contributes to speech intelligibility.

Acoustic Stimulation↗
PubMed1988

Gain of the adaptation mechanism in the human vestibulo-ocular reflex system.

The response of the vestibulo-ocular reflex to constant angular acceleration was investigated in healthy subjects using various stimulus amplitudes and stimulus durations. For stimulus amplitudes of 1.06 and 2.13 degrees/s2 the peracceleratory response maximum at 43 s was always followed by a decline in response. This decay was less pronounced for the longer stimulus duration suggesting a nonreturn to the zero baseline. From a comparison with the theoretical model it can be deduced that the gain of the adaptation mechanism is less than unity.

Acceleration↗
PubMed1987

Maximal expiratory and inspiratory flow-volume curves in bilateral vocal-cord paralysis. Changes after surgical treatment and comparison with glottic resistance characteristics.

The maximal expiratory flow-volume (MEFV) and maximal inspiratory flow-volume (MIFV) curve present maximal attainable flows, plotted against the displaced volume at the mouth during a forced expiratory manoeuvre from total lung capacity (TLC) and a subsequent forced inspiratory manoeuvre from residual volume (RV), respectively. Depending on the glottic resistance characteristics, the usual flow limitation may be absent during forced expiration, drastically influencing the form of MEFV curves. During forced inspiration however, the flow remains effort-dependent. We tested this hypothesis by comparing the form of MEFV and MIFV curves, and the glottic resistance characteristics, before and after an endolaryngeal superolateralization of a vocal cord, in 12 patients with bilateral vocal-cord paralysis. Peak expiratory and inspiratory flows were estimated with the aid of the measured glottic resistance characteristics on the assumption that the maximal alveolar pressures were normal during the manoeuvres. The estimated values agreed well with measured values. The form of the MEFV and MIFV curves was also found to be closely linked to the glottic resistance characteristics. It is concluded that the MEFV and MIFV curves are sensitive indicators of flow limitation in patients with upper-airway obstructions.

Adult↗
PubMed1985

Comparison of the adaptation time constants of the vestibulo-ocular reflex and of the sensation of rotation during sinusoidal stimulation.

The influence of adaptation on the phase characteristic of both the vestibular-ocular reflex and the sensation of rotation is investigated. The experimental data can be described by a second-order model with an adaptation term. The adaptation time constant Ta is found to be 135 s for the vestibulo-ocular reflex while it has a much lower value, 47 s, for the sensation of rotation. The difference is only found in the low-frequency region.

Adaptation, Physiological↗
PubMed1983

Influence of adaptation upon the impulsive time constant of the human vestibulo-ocular reflex system. I. Theoretical study.

From the impulse response of the human vestibulo-ocular reflex system the value of the long time constant T1 can be calculated in two different ways: (a) by plotting the decay of the response on log-linear graph paper; (b) by plotting the duration of the response against the logarithm of the impulse amplitude for various impulse amplitudes, i.e. the cupulometric method. Using mathematical models, with and without adaptation, we studied the effect of the two methods of calculation on the value of the response T1. The adaptation term was demonstrated to give a distinct reduction in the value of the response T1, compared with that of the cupular T1, especially when the cupulometric method (b) was used. The extent of this reduction was calculated.

Adaptation, Physiological↗
PubMed1983

Influence of adaptation upon the impulsive time constant of the human vestibulo-ocular reflex system. II. Experimental investigation.

The response of the vestibulo-ocular reflex (VOR) system was measured after a start or stop in angular velocity around the vertical axis. All determinations were carried out six times for 5 subjects. In a previous study, the mean value of the response T1 was calculated from the slope of the decay of the slow phase eye velocity (method a) and found to be 13.2 +/- (SD) 1.4 s for CW stimulation, and 12.8 +/- 1.7 s for CCW stimulation. Using the cupulometric method (b) with a fixed threshold for the slow phase eye velocity, these values are 12.1 +/- 2.0 and 11.2 +/- 1.4 s, respectively. The difference between the values calculated by method (a) and those calculated by method (b) is in line with the theoretical predictions as formulated in section I of this paper.

Adaptation, Physiological↗
PubMed1983

Frequency response of the human vestibulo-ocular reflex system at low frequencies: the effect of adaptation. I. Theoretical study.

From the mathematical model of the human vestibulo-ocular reflex (VOR) system, the frequency response has been calculated for harmonic sinusoidal acceleration. The addition of an adaptation term to the second order model, describing the cupular behaviour, results in considerable changes in the response characteristics in the low frequency region. An attempt has been made to quantify the influence of the cupular time constant T1 and the adaptation time constant Ta on the amplitude and phase characteristics of the transfer function, which represents the human VOR system. The results of this theoretical study will be employed in the following paper.

Acoustic Stimulation↗
PubMed1983

Frequency response of the human vestibulo-ocular reflex system at low frequencies: the effect of adaptation. II. Experimental investigation.

The frequency response of the human vestibulo-ocular reflex (VOR) system was investigated by determining its amplitude and phase characteristics. The angular head velocity and the resulting angular eye velocity were measured in 7 healthy subjects using a rotation room. Rotations about a vertical axis were carried out at frequencies ranging from 0.0025 to 0.1 Hz; peak head velocity was 50 degrees/s. From the experimental data, the gain and the phase relationship were calculated. The results appear to be in agreement with the frequency response of the theoretical transfer function which represents the VOR system. This is based on a second order differential equation for cupular behaviour, modified by an adaptation term. According to the experimental phase characteristic the cupular restoration time constant T1 was 16.6 s, and the adaptation time constant was 114 s. Linearity of the VOR system, both for amplitude and phase, was demonstrated at frequencies of 0.05 and 0.005 Hz.

Acoustic Stimulation↗
PubMed1983

Response of the human vestibulo-ocular reflex system to constant angular acceleration. I. Theoretical study.

The response of the human vestibulo-ocular reflex system to a constant angular acceleration is calculated using a second order model with an adaptation term. After first reaching a maximum the peracceleratory response declines. When the stimulus duration is long the decay is mainly governed by the adaptation time constant Ta, which enables to reliably estimate this time constant. In the postacceleratory period of constant velocity there is a reversal in response. The magnitude and the time course of the per- and postacceleratory response are calculated for various values of the cupular time constant T1, the adaptation time constant Ta, and the stimulus duration, thus enabling their influence to be assessed.

Acceleration↗
PubMed1983

Response of the human vestibulo-ocular reflex system to constant angular acceleration. II. Experimental investigation.

The vestibulo-ocular reflex (VOR) response to a constant angular acceleration was investigated in a small group of subjects by varying the stimulus amplitude and the stimulus duration. Various parameters of the response were determined and compared with the results of the theoretical study. This form of stimulation yields a rather high value for the adaptation time constant Ta, which perhaps indicates that there is an adaptation gain of less than unity. This supposition is supported by finding of non-linearity within the VOR system at high stimulus amplitudes.

Acceleration↗
PubMed1981

Thresholds for the perception of rotation: variability, psychometric curves, and comparison with hearing thresholds.

Psychometric curves were determined for the perception of rotation about a vertical axis. If a logarithmic scale is used, an integrated Gaussian curve gives a reasonable description of the psychometric curve. This curve is characterized by two values: the 50% point, which is defined as the threshold, and the s.d. The s.d. of the vestibular psychometric curve is about 10 dB which is 4 times larger than the s.d. found for the auditory system. The distribution of vestibular thresholds in healthy subjects obtained by Clark and Stewart (1969) is plotted on a logarithmic acceleration scale; it can fitted by a Gaussian function. The s.d. of this function is 5.5 dB, a value which is similar to that found for auditory thresholds. From this it is concluded that the inter-individual variability of the detection threshold for the vestibular system is the same as the inter-individual variability of the detection for the auditory system.

Acoustic Stimulation↗
PubMed1981

The threshold of perception of angular acceleration as a function of duration.

The threshold for rotation about the yaw axis was determined for constant acceleration stimuli as a function of their duration in the range from 3 to 25 s. From the torsion-swing model the following theoretical equation can be derived: alpha thr = C/[1--exp(-ts/tau 1)], where alpha thr = acceleration amplitude at threshold, ts = duration of the acceleration, tau 1 = time constant, C = threshold for very long stimuli. According to this formula the Mulder product (i.e. the product of the threshold acceleration amplitude and the duration of the stimulus) is constant for durations up to 0.3 tau 1. The best fit of this theoretical function to the somatosensory data is found for tau 1 = 14.5 s, and C = 0.220/s2. The time within the Mulder product is constant (about 5 s) is doubtless due to the mechanics of the semicircular canals. For the oculogyral data a lower value of tau 1 is found. We do not have any explanation for this lower value.

Acceleration↗