AMPLITUDE-MODULATION FOLLOWING RESPONSE ( AMFR) - AN OBJECTIVE FREQUENCY-SPECIFIC AUDIOLOGY DIAGNOSTIC-TOOL

Background: The objective evaluation of the hearing threshold below Ik Hz by means of early auditory potentials leads to problems caused by t he discrepancy between the required steep rise edge of stimulus and th e frequency specificity of the potentials. Therefore the aim of this s tudy was to evaluate the potential role of the AMFR as a diagnostic to ol for the assessment of hearing below 1 kHz. Method: The threshold of the AMFR was compared to the behavioral threshold in 13 normal hearin g subjects and 46 patients with hearing loss. The stimulus used was an amplitude-modulated tone at the carrier frequencies of 0.5 and 1 kHz, a modulation frequency of 40 Hz; the modulation depth was 80%. The in troduction of an empiric detectibility criterion based on spectral ana lysis of the response curve allowed the investigator to minimize the e xamination time and objectify the interpretation of the response. Addi tional investigations by means of highpass-masking took place to estim ate the frequency specificity of the AMFR. Results: The results show a good correspondence of the AMFR threshold to the behavioral threshold . Depending on hearing loss the mean values of differences between AMF R threshold and behavioral threshold are 3 dB-13 dB. The degree of cor respondence was highest in the patients with the most severe hearing l oss. An influence of underlying cause of the hearing loss could not be found. Additionally the frequency specificity of the response potenti al was proven with high-pass masking in normal hearing subjects. Maski ng with cut-off frequencies above the carrier frequencies had no influ ence on the response while masking at the carrier frequency resulted i n a strong reduction of the response curve. Conclusions: The results s how that the 40 Hz-AMFR is a suitable method for the objective frequen cy-specific assessment of hearing in adults. Problems in the clinical use of the AMFR are caused by the long investigation time and the depe ndence of the potentials on the state of wakeness.