Andrea Simpson

Improvements in speech perception with an experimental nonlinear frequency compression hearing device.

The performance of an experimental frequency compression hearing device was evaluated using tests of speech understanding in quiet. The device compressed frequencies above a programmable cut-off, resulting in those parts of the input signal being shifted to lower frequencies. Below the cut-off, signals were amplified without frequency shifting. Subjects were experienced hearing aid users with moderate-to-severe sensorineural hearing loss and sloping audiograms. Their recognition of monosyllabic words was tested using the experimental device in comparison with conventional hearing aids. Of the 17 subjects, eight showed a significant score improvement (p < 0.05), whereas one subject showed a significant score decrease. Some of the improvements may have resulted from the better audibility provided in the high frequencies by the experimental device in comparison with the conventional aids. However, a subsequent study found that increasing the high-frequency gain in the conventional aids did not produce equivalent perceptual benefits.

Frequency-compression outcomes in listeners with steeply sloping audiograms

Previous investigation of an experimental, wearable frequency-compression hearing aid revealed improvements in speech perception for a group of listeners with moderately sloping audiograms (Simpson et al, 2005). In the frequency-compression hearing aid, high frequencies (above 1600 Hz) were amplified in addition to being lowered in frequency. Lower frequencies were amplified without frequency shifting. In the present study, an identical frequency-compression scheme was evaluated in a group of seven subjects, all of whom had steeply sloping hearing losses. No significant differences in group mean scores were found between the frequency-compression device and a conventional hearing instrument for understanding speech in quiet. Testing in noise showed improvements for the frequency-compression scheme for only one of the five subjects tested. Subjectively, all but one of the subjects preferred the sound quality of the conventional hearing instruments. In conclusion, the experimental frequency-compression scheme provided only limited benefit to these listeners with steeply sloping hearing losses. Sumario Las investigaciones previas sobre un auxiliar auditivo experimental con compresión de frecuencia revelaron una mejor percepción del lenguaje en un grupo de oyentes con audiogramas de pendiente moderada (Simpson y col., 2005). En el auxiliar auditivo con compresión de frecuencia, las frecuencias agudas (por encima de 1600Hz) se amplificaron además de ser reducidas en frecuencia. Las frecuencias más graves fueron amplificadas sin cambio en la frecuencia. En el presente estudio, un esquema similar de compresión de frecuencia fue evaluado en un grupo de 7 sujetos, todos los cuales tenían hipoacusia con pendientes abruptas. No se encontraron diferencias significativas en los puntajes medios de grupo para entender el lenguaje en silencio entre el dispositivo con compresión de frecuencia y el instrumento convencional. La evaluación en ruido mostró mejoría para el esquema de compresión de la frecuencia en sólo uno de los cinco sujetos evaluados. Subjetivamente, todos menos uno de los sujetos, prefirieron la calidad del sonido de los instrumentos auditivos convencionales. En conclusión, el esquema experimental de compresión de frecuencia aportó sólo un beneficio limitado para los oyentes con hipoacusias de pendiente abrupta.

Electro-Acoustic Stimulation

For simultaneous acoustic and electric stimulation to be perceived as complementary, it may be beneficial for hearing aids and cochlear implants (CI) to be adjusted to provide compatible pitch sensations. To this end, estimates of the pitch perceived for a set of acoustic and electric stimuli were obtained from 14 CI users who had usable low-frequency hearing, either in the non-implanted ear or in both ears. The subjects assigned numerical pitch estimates to each of 5 acoustic pure tones and 5 single-electrode electric pulse trains. On average, the acoustic frequency that corresponded in pitch to stimulation on the most apical electrode was approximately 480 Hz. This was about 1 octave lower than the frequency expected from Greenwood’s frequency-place function applied to estimates of the electrode insertion angle based on X-ray images. Furthermore, evidence was found suggesting that pitch decreased with increasing duration of CI use. Pitch estimates from 5 subjects who completed the experiment before experiencing any other sounds through their CI were generally close to the values expected from a recently published frequency map for the cochlear spiral ganglion. Taken together, these findings suggest that some perceptual adaptation may occur that would compensate in part for the apparent mismatch between the intracochlear position of the electrodes and the acoustic frequencies assigned to them in the sound processor.

Benefits of audibility for listeners with severe high-frequency hearing loss.

A consonant identification test was carried out with 10 hearing-impaired listeners under various low-pass filter conditions. Subjects were also tested for cochlear dead regions with the TEN test. All subjects had moderate-to-severe high-frequency hearing losses. Consonant recognition was tested under conditions in which the speech signals were highly audible to subjects for frequencies up to the low-pass filter cut-off. Extensive dead regions were found for one subject with the TEN test. The remaining subjects may have had dead regions above 3 kHz, because of the severity of their hearing losses, but these could not be demonstrated with the TEN test. Average consonant scores for the subject group improved significantly (p<0.05) with increasing audibility of high-frequency components of the speech signal. There were no cases of speech perception being reduced with increasing bandwidth. Nine of the subjects showed improvements in scores with increasing audibility, whereas the remaining subject showed little change in scores. For this subject, speech perception results were consistent with the TEN test findings. In general, the results suggest that listeners with severe high-frequency losses are often able to make some use of high-frequency speech cues if these cues can be made audible.

Comparison of two frequency-to-electrode maps for acoustic-electric stimulation

The fitting of a cochlear implant together with aided residual hearing was evaluated by means of matching frequency and/or perceived pitch between acoustic and electric modalities. Five cochlear implant users with the Nucleus® Freedom™ electrode array with residual acoustic hearing participated. Psychophysical procedures were used to create a map in which the implant was programmed to provide the listener with high-frequency information only above the frequency at which acoustic hearing was no longer considered useful. This was compared to a second map which provided the full frequency range. Listeners wore each map for a number of weeks before speech recognition was measured in quiet and noise. Post-operatively across subjects, average hearing thresholds worsened by 27 dB. However, cochlear implantation provided superior recognition of speech compared to pre-operative scores, with the best results found when subjects were wearing their hearing aids together with the implant. No significant differences were found between the two maps on speech tests when subjects were wearing their implant together with hearing aid/s. In conclusion, the combination of a cochlear implant together with hearing aid/s was effective at providing speech perception benefits for the listeners of the current study, regardless of the frequency-to-electrode allocation selected.