Abstract
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.
Highlights
The human nervous system changes dynamically with experience
There was a significant reduction of neural activity evoked by the tinnitus frequency (TF) in a left temporal cluster (Fig 4A) for the time interval of 75–120 ms, F (1, 16) = 13.14, p = .002, η2 =
A right orbitofrontal cluster (Fig 4C) in the time window of 95– 122 ms showed a significant reduction of TF evoked neural activity after music exposure (F (1, 16) = 15.91, p = .001, η2 = .50), as well as a right cerebral region with extents towards occipital and temporal areas (Fig 4D) for the time interval of 70–98 ms (F (1, 16) = 13.39, p = .002, η2 = .46)
Summary
The human nervous system changes dynamically with experience. This phenomenon, named plasticity, was first described by James in 1890 [1] and is still of enormous research interest. One prerequisite for the functionality of brain plasticity is the balance between excitatory and inhibitory neural activation [4] If this balance gets disturbed, dysfunctionalities such as developmental intellectual disabilities [5], autism [4] or phantom perceptions [6,7,8] can appear. A syndrome assumed to derive from imbalanced excitation and inhibition in the auditory cortex [9,10,11], is another example of dysfunctional plasticity. It is defined as the perception of a sound without an external source [12]. With a prevalence rate of 10–15% in industrialized countries [15,16] and a 10-year cumulative incidence rate of 12.7% for people aged between 48 and 92 years [17], chronic tinnitus is a quite common, though hardly treatable [18,19] syndrome
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