Have you ever noticed that when there is a sudden silence or pause in a conversation or ambient noise, your brain seems to automatically “fill in” the silence with a sound? Many people report hearing a faint ringing or buzzing noise when it becomes very quiet. So what causes this phenomenon? In this article, we’ll explore the science behind why our brains seem to abhor silence and work to fill it in.
The Auditory System and Noise Processing
To understand why we perceive sound in silence, it helps to first understand how our auditory system processes sound in general. Here’s a quick overview:
- Sound waves enter the outer ear and travel through the ear canal to the eardrum, causing it to vibrate.
- These vibrations are transmitted to the middle ear bones (ossicles), which amplify the vibrations before passing them on to the fluid-filled cochlea.
- The cochlea converts the mechanical vibrations into electrical signals that are transmitted to the auditory nerve.
- The auditory nerve carries the signals to the brain, where they are processed and interpreted.
Our ears and auditory pathways are designed to detect very faint sounds – the healthy human ear can detect frequencies between 20 Hz and 20 kHz. But even when there is no actual sound stimulus, the auditory nerve and brain may still produce a weak level of baseline electrical activity. This is known as spontaneous activity or spontaneous firing.
Spontaneous Firing of the Auditory Nerve
Research shows that in complete silence, with no external sound stimulation, the auditory nerve still exhibits a base level of random spontaneous firing. Individual nerve fibers fire at rates of up to 100 discharges per second even without noise input 1. This spontaneous activity is believed to result from random fluctuations in the release of neurotransmitters.
The spontaneous firing may represent a type of built-in “white noise” that could help mask the small random background noises our ears are constantly detecting 2. This steady stream of neural activity also seems to prime the auditory system to be extra sensitive to the faintest of sounds.
The Brain’s Role in Filling in Silence
The spontaneous firing patterns transmitted to the brain during silence are perceived as noise and seem to provide enough stimulation that the auditory cortex doesn’t register a “true” absence of sound 3. However, the brain doesn’t like uncertainty and the unpredictable firing patterns are interpreted as disordered. To make sense of the silence, the brain essentially “fills in” the gaps to create an artificial perception of a more stable sound.
Researchers suggest that this is an evolutionary adaptation designed to keep us alert to our surroundings. The buzzing or ringing sensation motivates us to actively listen for real threats that might be present during periods of calm 4.
Predictive Coding
The brain also relies on past experience and expectations to interpret sensory stimuli using a process called predictive coding. Because we are so used to constant ambient noise in our daily lives, true silence is unexpected, and so the brain tries to add sound to better match our predictions 5.
The Role of Spontaneous Otoacoustic Emissions
There is another noise source that could contribute to the perception of sound where there is none: spontaneous otoacoustic emissions. These are faint noises generated by the inner ear that result from the vibration of hair cells. Spontaneous otoacoustic emissions are usually inaudible, but can become more pronounced in silent environments 6.
Researchers estimate that 38% of normal-hearing adults have emissions strong enough to be detectable during silence. However, there is still debate over whether spontaneous emissions actually contribute to the brain’s filling in of noise during quiet conditions 7.
Changes in Brain Processing of Silence
Interestingly, the way silence is processed in the brain seems to change as we age. One study comparing young and middle-aged subjects found less auditory cortex activation in response to silence in the middle-aged group 8. The researchers propose that with experience, we become more accustomed to silence and it demands less sensory processing.
In contrast, someone with hearing loss may experience exaggerated filling in of noise during silence. The lack of auditory stimulation can trigger the brain to ramp up gain to boost detection of neural activity. This results in the perception of phantom sounds like tinnitus 9.
Advantages of Filling in the Silence
Research suggests that filling in sound actually confers some advantages:
- By activating auditory areas, it prevents the brain from shutting down during prolonged silence, keeping it alert.
- The added background noise may help improve tone discrimination.
- In infants, generating noise spontaneously aids in the maturation of auditory pathways.
So although we may perceive silence as not truly being silent, this feature seems to benefit our auditory system.
External Factors That Contribute to Phantom Noise
In some cases, external factors can make the filling-in phenomenon more noticeable or exaggerated:
Hyperacusis
Individuals with hyperacusis have increased sensitivity to normal environmental sounds. Silence may be perceived as abnormally loud 10.
Tinnitus
Tinnitus patients hear noise even in the absence of any external sound stimulus, which covers any natural noise-filling 11. The tinnitus tone often gets worse in silence.
Noise Exposure
Exposure to loud noise can make silence uncomfortable or unbearable by exacerbating tinnitus 12. Military members and those who frequently attend concerts or work in noisy conditions are most likely to be affected.
Stress and Anxiety
Stress hormones and neurochemical changes associated with anxiety are believed to increase excitability of the auditory cortex, making it more apt to produce noise spontaneously 13.
Ways to Experience True Silence
Most of us don’t get to experience true, deep silence very often. But some evidence indicates that experiencing silence can have positive effects on the brain and mental health:
- Meditation and float tanks: Meditation retreats and floatation tanks aim to eliminate external stimuli completely, which may reset the auditory system and provide a respite from excess noise.
- Noise-cancelling devices: Modern noise-cancelling headphones and earbuds counter ambient noise with an opposite sound wave to create a zone of silence.
- Soundproof rooms: Acoustic engineering can produce special rooms with complete noise dampening useful for scientific and meditative purposes.
These interventions may allow you to experience something closer to true silence and its effects. However, the brain’s tendency to fill in sound likely still remains.
Summary and Conclusions
In summary:
- The auditory system exhibits spontaneous baseline neural activity even when no sound is present.
- This activity is perceived by the brain as noise which seems to fill in periods of silence.
- Filling in sound may have evolved to keep us alert to danger, mask background noise, and prime us for detecting faint stimuli.
- Aging and hearing loss may change our perception of silence.
- True silence is hard to achieve, but may have beneficial effects on brain function.
So in conclusion, the phenomenon of phantom noise during silence seems to be a byproduct of how our auditory system evolved. The brain abhors uncertainty so it constructs sound to make sense of silence. While not completely silent, this fill-in noise likely provides some advantages for sensory processing and keeping us tuned into our surroundings.
References
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10. Anari, M., Axelsson, A., Eliasson, A. & Magnusson, L. (1999). Hypersensitivity to sound: questionnaire data, audiometry and classification. Scandinavian Audiology, 28(4), 219-230.
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13. Popa, L. M., Selejan, O., Scotti, L., Falkai, P., Gruber, O., & Linka, T. (2019). Reading anxiety manifests anxio-depressive pathology and alters ensuing affective perception–evidence from co-registered EEG-fMRI. European archives of psychiatry and clinical neuroscience, 269(1), 79-92.