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Figure: Shutterstock/Lucia Fox
The U.S. Centers for Disease Control and Prevention estimates that nearly 15 percent of the general public—over 45 million Americans—experience some form of tinnitus. Roughly 20 million struggle with burdensome chronic tinnitus, while 2 million experience extreme and debilitating effects of the condition. Tinnitus is the leading service-related disability among U.S. veterans, with 9.7 percent of all veterans receiving service-related disability compensation. An estimated one in five high schoolers suffer from tinnitus.
That being so, and despite extensive research efforts as manifested by the plethora of literature on the subject, the mechanism contributing to tinnitus continues to evade us. This may be related to the insistence of neuroscience to search for proximate explanations based on mechanisms, but developing a full biological explanation may require an evolutionary approach to understand the origin and function of tinnitus.
Figure: Sylvester Fernandes, MBBS, LLB
This paradigm proposes that (a) tinnitus may have an evolutionary basis and (b) instantiates at a “nagging center” with a “halting problem” possibly in the thalamic regions; and that (c) tinnitus and hearing have separate paths (the former initiates centrally and the latter peripherally) that compete for attention at the consciousness level (Otolaryngol [Sunnyvale]. 2015;5:205 http://bit.ly/2dxCo5Q).
Some indicators of an evolutionary status are:
1. In the Heller and Bergman study, 75 of 80 normal hearing adults experienced tinnitus in quiet surroundings (Ann Otol Rhinol Laryngol. 1953;62[1]:73 http://bit.ly/2feHn7v). Considering such a high proportion, which is also seen in similar studies, including one with a placebo suggestion, the possibility that tinnitus may lie in our evolved cognitive architecture cannot be ruled out.
2. Tinnitus provokes a high level of alertness or vigilance (Acta Otolaryngol Suppl. 2006;[556]:39 http://bit.ly/2feEvYj). The iterative sound creates an atmosphere of present-centeredness that may have survival value for an organism by forcing the recruitment of a broad network of task-related neural resources. The triggered limbic and autonomic events may be such (“fight or flight”) responses. The oft-noted association of tinnitus with the limbic/autonomic response is thus easily explained.
3. Tinnitus is significantly comorbid with other afflictions like anxiety, depression, chronic pain, sleep disorders, and addiction, which also have an evolutionary basis and similar prefrontal-limbic neural path.
4. The experimental animal models support the fact that tinnitus exists in our common ancestor species, at least as far back as rodents. Tinnitus may be a uniform trait in mammals and possibly pre-mammals, potentially emanating from the need for predator vigilance to survive. The human characteristics of language and narrative, the tendency to attribute causes to events in the world, and perhaps the ability to experience emotions like awe make tinnitus a concern for some individuals.
5. Another path that may point to the ancient and primal aspect of tinnitus and its evolution is its rhythmic quality (Otolaryngol [Sunnyvale]. 2016;6:265 http://bit.ly/2evemai). Rhythm is fundamental to the nervous system, which is a fairly primitively evolved system that abounds in rhythms, including the heartbeat (pulsatile) and the rhythmic oscillations of electrical potentials in the brain (non-pulsatile) that are also found in numerous species of mammals https://en.wikipedia.org/wiki/Mammal.
Due to the eons of time involved, this evolution cannot be subjected to falsifiability. This may not matter here, as this is a historical hypothesis about the causes of traits in current populations.
Such essentially heuristic behaviours, which were quite adaptive during the earlier parts of human evolutionary history, are no longer adaptive, given the current environments in which we find ourselves (mismatch) and are hence considered (medical) neuroses (Otolaryngol. 2015 http://bit.ly/2dxCo5Q). Neuroses also occur over a continuous spectrum, and, like emotions, are not easily subject to voluntary cortical control and may only be subject to change through therapy (e.g., cognitive behavioural therapy or CBT).
The cognitive component of tinnitus is essentially the remnant of the type 1 error (false-positive) response, which was etched into our constitution and provided a more reliable interpretation necessary for the survival of our ancestors when a predator clue emerged. Imagine an ancestor interpreting an unfamiliar sound as nonthreatening (a false-negative or type 2 response). Not many such interpreters would survive and reproduce. Having got out of (the perceived) harm's way pronto, the ancestor is now subject to (negative) thinking to involve the identity of the supposed predator (fear) and methods of deceit/escape, etc. Persistence of this thinking fosters anxiety and depression. An established hypervigilant state may reduce the cognitive capacity to perform tasks that require voluntary, conscious, effortful, and strategic control. Extinction of this basic response is the aim of CBT.
Tinnitus is annoying and unmanageable only when the limbic and autonomic systems are recruited. The measured loudness of tinnitus is maximally within 30 to 40 dBs of threshold, which by itself is not significantly loud, but the associated limbic and autonomic system recruitments provoke anxiety and depression at a subcortical level. CBT is helpful as it attempts to alter the subcortical response but it does not affect the acoustic component of tinnitus, which is primarily a prelimbic component similar to negative automatic thoughts in depression.
Emotional stimuli dependent on the Darwinian hierarchy for survival have the highest priority in this system. In other words, evolutionarily, the brain is always on high alert for perceived threats. Significant neural mechanisms have thus evolved to ensure survival. Recruitment of these mechanisms in a particular situation denotes a survival instinct and corroboration of that particular situation by the organism. The emotional salience of tinnitus is undoubted.
It is plausible that there is a persistent pre-tinnitus activity (innate evolutionary) filtered into awareness as “tinnitus” when there is malfunction in the central executive blocking mechanism (basal ganglia). Such occurrences have been reported (Neurosurgery. 2012;70[2]:398 http://bit.ly/2f8w9Cp;J Neurosurg. 2013;118[1]:192 http://bit.ly/2f8yVro).
Volumetric and other techniques confirm the involvement of non-auditory areas in the region of the cortico-limbic areas. Tinnitus imaging studies report reduction in cortical matter (Int Tinnitus J. 2014;19[1]:10 http://bit.ly/2feEMKO). Similar reduction is reported in addiction studies and usually interpreted in this circumstance as a reduction of prefrontal control of the limbic areas, leading to the excessive “craving” in addiction.
The following points with regard to CBT are worth noting:
- The rationale for CBT from an evolutionary standpoint (type I error) is sound.
- Tinnitus is exacerbated by stress, anxiety, fear, depression, insomnia, pain relational conflict, etc. CBT can simultaneously attend to these, thus reducing the “total limbic load” and contribute a pragmatically valid solution.
- Of the available psychological therapies, CBT is the most clinically evaluated and generally regarded as one of the most effective treatments for tinnitus.
It is proposed here that tinnitus is a maladaptive evolutionary trait that resulted from phylogenetic inertia in humans. Among modern men, it constitutes a mismatch between a slow-evolving organism and a changing environment.
Such evolutionary and instinctive behaviors are not subject to voluntary control. To gain control, a cognition (conditioning) mechanism must be summoned. This can be difficult but nevertheless possible for most individuals, as this stratum is usually occupied by institutionalized (core beliefs) teaching that is difficult to eradicate until effective and logical cognition reins in.
