Enhancing Inhibition-Induced Plasticity in Tinnitus – Spectral Energy Contrasts in Tailor-Made Notched Music Matter

Chronic tinnitus seems to be caused by reduced inhibition among frequency selective neurons in the auditory cortex. One possibility to reduce tinnitus perception is to induce inhibition onto over-activated neurons representing the tinnitus frequency via tailor-made notched music (TMNM). Since lateral inhibition is modifiable by spectral energy contrasts, the question arises if the effects of inhibition-induced plasticity can be enhanced by introducing increased spectral energy contrasts (ISEC) in TMNM. Eighteen participants suffering from chronic tonal tinnitus, pseudo randomly assigned to either a classical TMNM or an ISEC-TMNM group, listened to notched music for three hours on three consecutive days. The music was filtered for both groups by introducing a notch filter centered at the individual tinnitus frequency. For the ISEC-TMNM group a frequency bandwidth of 3/8 octaves on each side of the notch was amplified, additionally, by about 20 dB. Before and after each music exposure, participants rated their subjectively perceived tinnitus loudness on a visual analog scale. During the magnetoencephalographic recordings, participants were stimulated with either a reference tone of 500 Hz or a test tone with a carrier frequency representing the individual tinnitus pitch. Perceived tinnitus loudness was significantly reduced after TMNM exposure, though TMNM type did not influence the loudness ratings. Tinnitus related neural activity in the N1m time window and in the so called tinnitus network comprising temporal, parietal and frontal regions was reduced after TMNM exposure. The ISEC-TMNM group revealed even enhanced inhibition-induced plasticity in a temporal and a frontal cortical area. Overall, inhibition of tinnitus related neural activity could be strengthened in people affected with tinnitus by increasing spectral energy contrast in TMNM, confirming the concepts of inhibition-induced plasticity via TMNM and spectral energy contrasts

Complex-Tone Pitch Discrimination in Listeners With Sensorineural Hearing Loss

Physiological studies have shown that noise-induced sensorineural hearing loss (SNHL) enhances the amplitude of envelope coding in auditory-nerve fibers. As pitch coding of unresolved complex tones is assumed to rely on temporal envelope coding mechanisms, this study investigated pitch-discrimination performance in listeners with SNHL. Pitch-discrimination thresholds were obtained for 14 normal-hearing (NH) and 10 hearing-impaired (HI) listeners for sine-phase (SP) and random-phase (RP) complex tones. When all harmonics were unresolved, the HI listeners performed, on average, worse than NH listeners in the RP condition but similarly to NH listeners in the SP condition. The increase in pitch-discrimination performance for the SP relative to the RP condition ( F 0 DL ratio) was significantly larger in the HI as compared with the NH listeners. Cochlear compression and auditory-filter bandwidths were estimated in the same listeners. The estimated reduction of cochlear compression was significantly correlated with the increase in the F 0 DL ratio, while no correlation was found with filter bandwidth. The effects of degraded frequency selectivity and loss of compression were considered in a simplified peripheral model as potential factors in envelope enhancement. The model revealed that reducing cochlear compression significantly enhanced the envelope of an unresolved SP complex tone, while not affecting the envelope of a RP complex tone. This envelope enhancement in the SP condition was significantly correlated with the increased pitch-discrimination performance for the SP relative to the RP condition in the HI listeners

Acoustic Signal Encoding in Children with Auditory Processing Disorders

Auditory perception has been shown to be a problem for some children with diagnosed learning, language, reading, or attention disorders. Evaluation of discrimination abilities, as part of an auditory processing test battery, has been recommended but few commercial tools are available for the audiologist to accomplish this task. Few studies have investigated signal feature encoding with children at risk for an auditory processing disorder (APD). The purpose of this project was to investigate signal encoding abilities in children suspected of having APD. School-aged children, part of a clinical population referred for assessment of their auditory processing skills, participated in the project. To assess signal encoding abilities, an adaptive procedure with feedback was combined with a three alternative forced choice task and presented with graphics in a game-like format. A series of five studies was designed to represent the spectral, level, and temporal features of sound. The series included evaluation of frequency resolution, frequency discrimination, intensity discrimination, temporal resolution and temporal integration. Results demonstrated that some children in the clinical population have difficulty accurately and efficiently encoding acoustic signal features. Poor performance varied on an individual and group basis across signal encoding tasks but most listeners demonstrated difficulty with spectral and temporal encoding. Elevated and outlying thresholds were not restricted only to the children receiving an APD diagnosis. In addition to the threshold values, trial-by-trial data provided qualitative information about the nature of the poor performance and assisted in differentiating poor signal encoders from children that were inattentive

The effects of background noise and test subject on the perceived amount of bass in phase-modified harmonic complex tones

Äänenvärin havaitseminen liittyy läheisesti äänen tuottamiin suhteellisiin tasoihin simpu- kassa eri taajuuskaistoilla, joita kutsutaan kriittisiksi kaistoiksi. Äänen magnitudispektri määrittää sen taajuuskomponenttien suhteelliset voimakkuudet ja vaihespektri niiden suhteelliset vaiheet. Äänenväri siis riippuu usein pelkästään magnitudispektristä. Tutki- mustulokset ovat kuitenkin osoittaneet, että tietyn tyyppisten äänien äänenväriä voidaan muuttaa myös pelkästään vaihespektriä muuttamalla. Tämän lisäksi aiempi tutkimus on osoittanut, että muuttamalla harmonisen äänen vaihespektriä tietyllä tavalla havaittu bassokkuus muuttuu. Tällaiset äänet ovat siis ’vaiheherkkiä’. Kyseisessä tutkimuksessa käytettiin kahta tällaista vaihemuokattua ääntä, joista toisessa taajuuskomponenttien välillä oli -90 asteen ja toisessa 90 asteen vaihe- ero, ja perustaajuuskomponentti oli molemmissa kosinivaiheessa. Tutkimus osoitti, että suurin bassokkuusero havaitaan matalilla perustaajuuksilla ja se vastaa keskimäärin 2 – 4 dB:n vahvistusta magnitudispektrissä matalilla taajuuksilla. Tämä ilmiön suuruus riippui kuitenkin huomattavasti testihenkilöstä. Lisäksi huomattiin, että bassokkuuserot ovat helpompia kuulla taustakohinan kanssa. Tämän työn tavoitteena oli tutkia edelleen taustakohinan merkitystä ja yksilöllisiä eroja tällaisten vaiheherkkien äänien bassokkuuden havaitsemisessa. Kaksi formaalia kuuntelu- koetta järjestettiin käyttäen kuulokkeita. Ensiksi tutkittiin taustakohinan vaikutusta kyseisten äänien bassokkuuserojen kuulemiseen olettaen, että nämä erot ovat kuultavissa äänekkyyseroina. Tulokset viittaavat, että taustakohinan tason nousun vaikutus testiäänien äänekkyyseroon ei ole tilastollisesti merkittävä, mutta on lähellä merkittävyyden rajaa ja trendi on nähtävissä äänekkyyseron kasvulle. Lisäksi nähdään, että kyseisten vaiheherkkien äänien yleinen äänekkyys laskee kun taustakohinan tasoa voimistetaan. Toiseksi tutkittiin sitä, minkä vaihespektrin omaavan äänen eri ihmiset kuulevat bassokkaimpana. Tulokset osoittavat, että testihenkilöt eroavat siinä, minkä vaihespektrin omaavan äänen he kuulevat bassokkaimpana, ja että tämä ero on tilastollisesti merkittävä.The perception of timbre is closely related to the relative levels produced by a sound in each frequency band, called ‘critical band’, in the cochlea. The magnitude spectrum defines the relative levels and phase spectrum the relative phases of the frequency components in a complex sound. Thus, the timbre of sound depends often only on the magnitude spectrum. However, several studies have shown that the timbre of certain complex sounds can be affected by modifying only the phase spectrum. Moreover, a recent study has shown that with certain modifications of only the phase spectrum of a ‘phase-sensitive’ harmonic complex tone, the perceived level of bass changes. That experiment was conducted using two synthetic harmonic complex tones in which adjacent frequency components have a phase-shift of -90◦ and 90◦, respectively, and the fundamental component is in cosine-phase. The greatest difference in perceived level of bass was found at the fundamental frequency of 50 Hz and it corresponds to a 2 – 4-dB amplification of the magnitude spectrum at low frequencies. However, this effect was reported to vary substantially between individuals. Moreover, the differences were found to be easier to detect in the presence of background noise. The aim of this thesis was to investigate further the roles of background noise and the individual in the perceived level of bass in the phase-sensitive tones. Two formal listening tests were conducted accordingly using headphones. Firstly, the effect of background noise on the discrimination of the phase-sensitive tones based on the perceived level of bass was studied. The effect of increasing background noise level on the perceived loudness difference was found not to be statistically significant, but a trend could be seen towards increasing loudness difference. Additionally, the results indicate that the overall perceived loudness of the test tones decreases with increasing level of background noise. Secondly, an experiment was conducted to find the preferred value of the constant phase shift between adjacent components that produces a tone with the perceptually loudest bass for different individuals. The results show that individuals hear the phase spectrum required to produce the perception of the loudest bass statistically significantly differently from each other