A Comprehensive Summary of the Hypothesized Mechanism Behind Tailor-Made Notched Sound Therapy
Tailor-Made Notched Sound Therapy is an umbrella term that subsumes two types of Notched Sound Therapy:
- Notched Music Therapy (also referred to as Tailor-Made Notched Sound Therapy by the researchers who discovered it)
- Notched White Noise Therapy (also referred to as Windowed Sound Therapy by the researchers who discovered it)
Proposed mechanism of Notched Music Therapy:
The observed reductions in tinnitus loudness, annoyance and handicapping as well as the reductions in evoked neural activity appear cumulative, indicating a long-term neuroplastic effect. There is evidence in humans that tinnitus is associated with a relative excitatory-inhibitory cortical neural network dysbalance, at the expense of the inhibitory system.
This loss of inhibition may lead to hyperactivity and/or spontaneous hypersynchrony of a certain cortical neuronal population, which would eventually contribute to the tinnitus perception. By means of our customized music modification, we intended to “re-attract” lateral inhibition to these neurons in order to reverse their maladaptive hyperactivity and/or hypersynchrony. It might be a consequence of this induced reversion that the tinnitus related auditory cortex activity of the target group patients decreased, and that their tinnitus became less loud (and possibly therefore less annoying and less handicapping).
Despite the existence of diseases caused by maladaptive cortical reorganization, the consequences of reorganization can be beneﬁcial. Here, we used knowledge regarding maladaptive cortical reorganization in tinnitus to design a procedure that appears suited to reduce brain activity corresponding to the tinnitus frequency and thus possibly tinnitus perception. Our target notched music introduced a functional deafferentation of auditory neurons corresponding to the eliminated frequency band, and because his frequency band overlapped the individual tinnitus frequency, the notched music no longer stimulated the cortical area corresponding to the tinnitus frequency, although it still excited surrounding neurons. Thus, the neurons, which were not stimulated due to the notch, were presumably actively suppressed via lateral inhibitory inputs originating from surrounding neurons. Alternatively, listening to the target notched music could have induced synaptic and/or cellular plasticity mechanisms. For instance, the deprivation from auditory input in the frequency range of the tinnitus frequency could have caused long-term depression of auditory neurons corresponding to the tinnitus frequency.
The described reversion of maladaptive cortical reorganization by the notched music training would have been initiated by bottom-up neural inputs triggered by the music. However, topdown neural processes also play an essential role in cortical reorganization. In the present study, patients were given the opportunity to listen to their most enjoyable music. It is reasonable to assume that enjoyable music strongly engages attention, and evidently it affects brain functioning. As such, joyful listening to music activates the reward system of the brain and leads to release of dopamine, which plays an important role in cortical reorganization. Thus, a combination of bottom-up and top-down neural processes initiated by the target notched and relished music could provide a basis for the reversion of the putative maladaptive cortical reorganization underlying tinnitus emergence and maintenance in auditory cortex.
Proposed Mechanism of Notched White Noise Therapy (referred to as Windowed Sound Therapy (WST) and Windowed White Noise (WWN) by authors)
The key element of the WST was the auditory stimulation of tinnitus sufferers with masking noise presumably not comprising the tinnitus frequencies, often suggested to be responsible for the exacerbation of the symptom. Jastreboff and Jastreboff stressed the importance of ensuring that the sound-enriched environment provided with the TRT should not contain sounds causing annoyance and exacerbation of the tinnitus sensation. The purpose of TRT is the reduction of the alarm reaction associated with the tinnitus sensation by altering the connections between the limbic and autonomic nervous systems (responsible for the alarm reaction) and the auditory system. Therefore, the observed effectiveness of the WST in reducing tinnitus loudness appears consistent with the neurophysiological model of tinnitus underlying TRT.
An alternative explanation for the decreased tinnitus loudness in patients treated with WST might involve a change in the neural reorganization in the tinnitus region induced by stimulation with noise around, but not inside, the tinnitus pitch region. For instance, it has been hypothesized the overrepresentation of the edge frequencies before the hearing loss region is responsible for the tinnitus sensation. Conversely, tinnitus pitch is often located inside the hearing loss region toward the audiogram’s edge frequencies, a phenomenon having strong analogy with the one described for the occurrence of the Zwicker tone using notched noise. Thus, a conceivable hypothesis is that a stimulation of the edge frequencies, but not the tinnitus region, by the noise frequencies just below the notched region has occurred during the listening of the WWN sound treatment. In addition, activation of the mechanism underlying the Zwicker tone must have occurred in patients listening to the WWN treatment, although none of them reported such an aftereffect, presumably because of the pitch similarity between the Zwicker tone and the tinnitus percepts. One or both of these phenomena might have determined a reorganization of the neural activity of the edge frequencies resulting in the observed longterm reduction of the tinnitus percept in such patients.