Letteratura scientifica selezionata sul tema "Dichotic multiple assr"

Auditory steady-state responses (ASSRs) recorded with simultaneous presentation of multiple tones modulated from 77–105 Hz exhibit frequency specificity and can be acquired with monotic or dichotic stimulation. This study examined the frequency specificity and dichotic characteristics of 40 Hz ASSRs recorded with amplitude-modulated (AM) probe tones and unmodulated (UM) or AM interfering tones in 27 normal-hearing adults. The effects on ASSR amplitudes of monotically or dichotically presented interfering tones of various frequency, modulation depth, and modulation rate were studied. Significant decreases in ASSR amplitudes occurred when the UM interfering tone was monotic, higher in frequency, and approximately within an octave of the probe tone. ASSR amplitudes were also reduced when the AM interfering tone was monotic and modulated at a lower depth and was an octave above the probe tone. Probe and interfering AM tones modulated at different rates produced similar reductions in amplitude for ASSRs acquired with monotic and dichotic stimulation. The findings of this study contribute to clarifying the carrier and temporal envelope interactions between tonal stimuli. Description of the effects of these stimulus parameters on 40 Hz ASSRs can benefit clinical applications of this technique, including evaluating auditory function in adults not capable of participating in behavioral audiometric tests. Las respuestas auditivas de estado estable (ASSR) registradas con una presentación simultánea de múltiples tonos modulados desde 77 a 105 Hz, exhiben especificidad frecuencial y puede ser adquiridas con estimulación monótica o dicótica. Este estudio examinó la especificidad frecuencial y las características dicóticas de las ASSR de 40 Hz registradas con sondas de prueba de amplitud modulada (AM) y con tonos de interferencia no modulados (UM), en 27 sujetos adultos normoyentes. Se estudiaron los efectos sobre las amplitudes de las ASSR producto de tonos de interferencia presentados en forma monótica o dicótica, caracterizados por diferentes frecuencias, profundidades de modulación y tasa de modulación. Ocurrieron reducciones significativas en la amplitud de las ASSR cuando el tono de interferencia UM era monótico, de frecuencia mayor, y aproximadamente dentro de una octava del tono de prueba. Las amplitudes de las ASSR también se redujeron cuando el tono de interferencia de AM era monótico y modulado a una profundidad menor y a una octava por encima del tono de prueba. Los tonos de prueba y de interferencia con AM, modulados a diferentes tasas, produjeron reducciones similares en la amplitud de las ASSR, cuando se lograban con estimulación monótica o dicótica. Los hallazgos de este estudio contribuyen a aclarar las interacciones con la envolvente temporal y la del portador, entre estímulos tonales. La descripción de los efectos de estos parámetros del estímulo sobre las ASSR de 40 Hz pueden beneficiar la aplicación clínica de esta técnica, incluyendo la evaluación de funciones auditivas en adultos no capaces de participar en pruebas audiométricas conductuales.

The accuracy of dichotic multiple frequency auditory steady state in predicting pure-tone thresholds at 0.5, 1, 2, and 4.0 kHz compared to an ABR protocol (click and tone burst at 0.5 kHz) were explored in a group of 25 hearing-impaired subjects across the degree and configuration spectrum. Mean steady state thresholds were within 14, 18, 15, and 14 dB of the pure tones at 0.5, 1, 2, and 4 kHz, compared to the tone-burst ABR at 0.5 kHz pure-tone difference of 24 dB, and a click-evoked pure-tone (2–4 kHz) difference of 9 dB. Recording time for the steady state protocol was 28 minutes (+/-11) compared to 24 minutes (+/- 9) of the ABR protocol. Degree of loss had a significant effect on steady state; configuration of hearing loss had a limited effect. Mf ASSR predicted thresholds with relative accuracy although some configurations showed discrepancies for low-frequency estimates.

The dichotic presentation of two sinusoids with a slight difference in frequency elicits subjective fluctuations called binaural beat (BB). BBs provide a classic example of binaural interaction considered to result from neural interaction in the central auditory pathway that receives input from both ears. To explore the cortical representation of the fluctuation of BB, we recorded magnetic fields evoked by slow BB of 4.00 or 6.66 Hz in nine normal subjects. The fields showed small amplitudes; however, they were strong enough to be distinguished from the noise accompanying the recordings. Spectral analyses of the magnetic fields recorded on single channels revealed that the responses evoked by BBs contained a specific spectral component of BB frequency, and the magnetic fields were confirmed to represent an auditory steady-state response (ASSR) to BB. The analyses of spatial distribution of BB-synchronized responses and minimum-norm current estimates revealed multiple BB ASSR sources in the parietal and frontal cortices in addition to the temporal areas, including auditory cortices. The phase of synchronized waveforms showed great variability, suggesting that BB ASSR does not represent changing interaural phase differences (IPD) per se, but instead it reflects a higher-order cognitive process corresponding to subjective fluctuations of BB. Our findings confirm that the activity of the human cerebral cortex can be synchronized with slow BB by using information on the IPD.

This study investigated the use of the dichotic multiple frequency ASSR technique for characterising normal hearing in a group of infants. A descriptive research design was implemented to describe ASSR thresholds obtained in 10 normal hearing infant ears (3 male, 2 female participants) between the age of 3 and 8 weeks. Normal hearing was controlled for by conducting a DPOAE screening test on all ears and ensuring no risk factors for hearing loss were present. Results indicated mean ASSR thresholds at 0.5, 1, 2, and 4 kHz to vary between 30 and 37 dB, ±8 - 11 dB within a range of 20 - 50 dB HL. Eighteen percent of ASSR thresholds were obtained at 20 dB, 45% were obtained at 30 dB, and 38% were obtained at elevated levels of 40 and 50 dB. The recorded dichotic multiple frequency ASSR thresholds for infants with normal hearing were within the mild to moderate hearing loss range which makes differentiating between le severe degrees of hearing loss and normal hearing difficult. Until future research has been conducted, caution must be practiced when interpreting ASSR thresholds below 60 dB in young infants and additional techniques such as the ABR must be used to cross-check results.

The Auditory Steady State Response (ASSR) procedure has been established as a frequency specific, objective audiologic measure, which can provide reliable thresholds to within 10 dB of the behavioral thresholds. In order for ASSR to find its place in the existing framework of audiometric procedures, the full potential of the procedure needs to be explored. The aim of this study was to determine the accuracy of monotic ASSR in estimating hearing thresholds in a group of 15 normal hearing subjects and 15 hearing-impaired subjects. A comparative research design was implemented. Indicating that results obtained in the study was compared to relevant literature where dichotic multiple ASSR was implemented. This was done in order to ascertain ASSR’s capabilities with regard to stimulus presentation methods. Monotic single ASSR predicted behavioural thresholds in the normal hearing subjects within an average of 24 dB across the frequency range (0.5, 1, 2&4 kHz). In the hearing-impaired group, ASSR thresholds more closely resembled behavioural thresholds, with an average difference of 18 dB, which is consistent with recent literature. The literature suggests that better prediction of behavioural thresholds will occur with greater degrees of hearing loss, due to recruitment. The focus in this group also centered on the accurate prediction of the configuration of the hearing loss. It was found that ASSR could reasonably accurately predict the configuration of the hearing loss. In the last instance, monotic single and dichotic multiple ASSR were compared with regard to threshold estimation and prediction of configuration of the hearing loss in the hearing-impaired group. Little difference was reported between the two techniques with regard to the estimation of thresholds in both the normal hearing and hearing impaired groups. In conclusion it was established that monotic ASSR could predict behavioural thresholds of varying degrees and configurations of hearing loss in normal and hearing-impaired subjects with a reasonable amount of accuracy. At this stage, however, more research is required to establish the clinical validity of the procedure, before it is routinely included within an objective test battery.
Dissertation (M (Communication Pathology))--University of Pretoria, 2005.
Speech-Language Pathology and Audiology
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