10The Limbic and Emotional Network

FLoudness is not distress: a dissociable system

One of the most clinically important discoveries in tinnitus neuroscience is that the perception of the phantom sound and the suffering it causes are handled by partly separate brain systems. Patients matched for tinnitus pitch and loudness can differ enormously in handicap, and treatments such as cognitive behavioural therapy can slash distress without changing measured loudness at all [2012].

The structures that turn a neutral signal into an aversive experience belong to the limbic and paralimbic networks — the amygdala, the anterior cingulate cortex (ACC), the insula, the hippocampus and the ventromedial prefrontal cortex (vmPFC). These regions assign emotional value, attentional priority and bodily arousal to sensory events [2010].

TThe amygdala, ACC and insula: tagging the sound as a threat

The amygdala attaches negative valence to stimuli. In bothersome tinnitus it becomes hyper-responsive even to innocuous sound, effectively flagging the phantom percept as something to be feared and watched [2011]. The anterior cingulate cortex performs salience evaluation and conflict monitoring; its activity scales with subjective distress and with the degree of attentional capture by the tinnitus.

The insula integrates interoception — the felt state of the body — with emotion, linking the percept to autonomic arousal (a racing heart, poor sleep, vigilance). Functional connectivity studies show that what predicts handicap is not the loudness signal itself but the strength of coupling between auditory cortex and this limbic cluster [2010].

TRauschecker’s noise-cancellation model

Josef Rauschecker and colleagues proposed that a healthy brain runs a limbic–thalamic noise-cancellation system that subtracts unwanted or self-generated signals before they reach awareness. In their model the vmPFC and nucleus accumbens evaluate the tinnitus signal and, via the thalamic reticular nucleus, gate it out at the medial geniculate body — the auditory thalamus — so it never becomes a conscious, distressing percept [2010].

Chronic, bothersome tinnitus is reframed as a failure of this gate. When the frontostriatal circuit is structurally or functionally compromised, the aberrant signal is no longer cancelled and breaks through. Crucially the model explains why some people with marked hearing loss never notice tinnitus while others with a near-normal audiogram are disabled: the difference lies in the cancellation circuit, not the periphery [2016].

CThe nucleus accumbens as gatekeeper

The nucleus accumbens (ventral striatum) is the model’s central valve. Structural imaging shows grey-matter changes in the vmPFC–accumbens region that distinguish people with tinnitus from controls and, importantly, separate the percept itself from the distress dimension [2012]. The accumbens, richly dopaminergic and serotonergic, is positioned to decide which sensory signals are “relevant” and to drive the descending gate.

This circuit logic has therapeutic teeth. It predicts that interventions which restore top-down frontostriatal control — counselling, CBT, habituation-based sound therapy, and frontal neuromodulation — should reduce suffering even if the peripheral lesion is fixed, which is exactly what the clinical evidence shows [2016].