Biomedical subjects
Publications and source records attributed to L Geurts.
In recent years many technical evolutions have been applied in hearing aids. In this paper differences between analog, programmable and fully digital hearing aids, the basic and supplementary functions of a hearing aid, and some important issues and future directions for digital hearing aids will be mentioned.
In this study the perception of the fundamental frequency (F0) of periodic stimuli by cochlear implant users is investigated. A widely used speech processor is the Continuous Interleaved Sampling (CIS) processor, for which the fundamental frequency appears as temporal fluctuations in the envelopes at the output. Three experiments with four users of the LAURA (Registered trade mark of Philips Hearing Implants, now Cochlear Technology Centre Europe) cochlear implant were carried out to examine the influence of the modulation depth of these envelope fluctuations on pitch discrimination. In the first experiment, the subjects were asked to discriminate between two SAM (sinusoidally amplitude modulated) pulse trains on a single electrode channel differing in modulation frequency ( deltaf = 20%). As expected, the results showed a decrease in the performance for smaller modulation depths. Optimal performance was reached for modulation depths between 20% and 99%, depending on subject, electrode channel, and modulation frequency. In the second experiment, the smallest noticeable difference in F0 of synthetic vowels was measured for three algorithms that differed in the obtained modulation depth at the output: the default CIS strategy, the CIS strategy in which the F0 fluctuations in the envelope were removed (FLAT CIS), and a third CIS strategy, which was especially designed to control and increase the depth of these fluctuations (F0 CIS). In general, performance was poorest for the FLAT CIS strategy, where changes in F0 are only apparent as changes of the average amplitude in the channel outputs. This emphasizes the importance of temporal coding of F0 in the speech envelope for pitch perception. No significantly better results were obtained for the F0 CIS strategy compared to the default CIS strategy, although the latter results in envelope modulation depths at which sub-optimal scores were obtained in some cases of the first experiment. This indicates that less modulation is needed if all channels are stimulated with synchronous F0 fluctuations. This hypothesis is confirmed in a third experiment where subjects performed significantly better in a pitch discrimination task with SAM pulse trains, if three channels were stimulated concurrently, as opposed to only one.
Five post-lingually deafened users of the LAURA cochlear implant were presented with two trains of biphasic pulses applied concurrently to two widely separated channels. They could all discriminate between stimuli where pulses on the two channels were nearly synchronous (inter-channel delay=0.1 ms) and those where there was a longer delay applied to one channel. All showed an asymmetry, being more sensitive when the longer delay was on either the more basal or, depending on the listener, the more apical channel. For four out of the five listeners this asymmetry could be at least partly attributed to one stimulus, with a 0.1-ms delay in either the apical (three listeners) or basal (one listener) channel, sounding markedly different from all other stimuli used in the experiment. Both the overall sensitivity of listeners and the general pattern of results survived the presentation of maskers on intermediate channels, and did not vary markedly with changes in the polarity of the pulses applied to one channel. Although the results varied substantially across listeners, it is concluded that they demonstrate a genuine sensitivity to the relative timing of stimulation applied to discrete populations of auditory nerve fibers.
APEX, an acronym for computer Application for Psycho-Electrical eXperiments, is a user friendly tool used to conduct psychophysical experiments and to investigate new speech coding algorithms with cochlear implant users. Most common psychophysical experiments can be easily programmed and all stimuli can be easily created without any knowledge of computer programing. The pulsatile stimuli are composed off-line using custom-made MATLAB (Registered trademark of The Mathworks, Inc., http://www.mathworks.com) functions and are stored on hard disk or CD ROM. These functions convert either a speech signal into a pulse sequence or generate any sequence of pulses based on the parameters specified by the experimenter. The APEX personal computer (PC) software reads a text file which specifies the experiment and the stimuli, controls the experiment, delivers the stimuli to the subject through a digital signal processor (DSP) board, collects the responses via a computer mouse or a graphics tablet, and writes the results to the same file. At present, the APEX system is implemented for the LAURA (Registered trademark of Philips Hearing Implants) cochlear implant. However, the concept-and many parts of the system-is portable to any other device. Also, psycho-acoustical experiments can be conducted by presenting the stimuli acoustically through a sound card.
A new method to code the speech envelope in continuous interleaved sampling (CIS) processors for cochlear implants is proposed. In this enhanced envelope, the rapid adaptation seen in the response of auditory nerves to sound stimuli is incorporated. Two strategies, one using the standard envelope (CIS) and one using the enhanced envelope (EECIS), were tested perceptually with six postlingually deafened users of the LAURA cochlear implant. The tests included identification of stop consonants in three different vowel contexts and monosyllabic consonant-vowel-consonant (CVC) words. Significant improvements in correct identification scores were observed for stop consonants in intervocalic /a/ context (p = 0.026): average results varied from 46% correct for CIS to 55% for EECIS. This improvement was mainly due to the better transmission of place of articulation. The differences in identification scores for stop consonants in /i/ and /u/ context were not significant. The identification scores for the medial vowels of the CVC words were significantly higher when the EECIS strategy was used: average results increased from 39% correct to 46% correct (p = 0.018). No significant differences were observed between the results for initial and final consonants of the CVC words. The present results demonstrate that the inclusion of the rapid adaptation in the speech processing for cochlear implants can improve speech intelligibility.
The status of speech processing for cochlear implants is reviewed. Points of interest to enhance further speech intelligibility based on improved signal processing, are discussed.
To determine the primary excretory by-products of testosterone (T), 85 microCi [3H]T was administered i.v. to two adult Eld's deer stags. Blood (10 ml) was collected by jugular venipuncture at 0, 5, 10, 15, 30, 45, 60, 90, 120, 150, 180, 240, and 480 min after isotope infusion, and all urine and feces were collected for 96 h after injection. Seventy percent of labeled circulating steroid was conjugated by 30 min postinfusion. The majority (80.4 +/- 3.2%) of T metabolites were excreted into urine, and 95.0 +/- 0.9% of these were conjugated, 95.8 +/- 0.2% being hydrolyzable with glucuronidase. Seven urinary androgen metabolites, including androstanediol (5 alpha-androstan-3 alpha-17 beta-diol and 5 beta-androstan-3 alpha-17 beta-diol), were identified in glucoronidase-hydrolyzed, ether-extracted Eld's deer urine pools after gas chromatography/mass spectrometry. A double-antibody 125I RIA for 5 alpha-androstanediol-3 alpha, 17 beta-diol,17-glucuronide (3 alpha-diol-G) was validated for unprocessed urine. Longitudinal assessments of urine samples collected from 13 stages for 3 yr revealed biological concordance between fluctuations in urinary 3 alpha-diol-G and serum T, as well as seasonal changes in secondary sexual characteristics. Overall correlation between "same-day" matched serum T and urinary 3 alpha-diol-G was 0.58, (n = 6; p < 0.001). Thus, monitoring urinary 3 alpha-diol-G provides a noninvasive, alternative method for characterizing male endocrine interrelationships in an endangered ungulate species.