The auditory system

In the human brain, the auditory system is known as ‘the amplifier’.

When you hear something new or are interpreting the sounds in your daily life, the auditory system guides this interpretation, determining the sensory qualities of these sounds, and deciding whether a response is required.

The human auditory system is found bilaterally in the brain, first discovered in the early 21^st century through a collection of imaging investigations that aimed to discern the discrete location of where sound is processed in the brain. Despite not forming part of the 7 major networks, the auditory system remains critical to assigning attention to auditory information and guides several core processes, including:

• Discriminating auditory information
• Interpreting incoming speech
• Determining volume, frequency, and onset time of auditory information
• Decoding melodies
• Interpreting grammar and inflexions in speech
• Establishing relationships between sounds and our daily environment

The auditory system is a multilayered brain system primarily found close to Heschl’s Gyrus in the temporal lobe. Its anatomical boundaries have long been described in three separate regional zones – the 'core', 'belt', and 'parabelt' regions^1,2 . Early animal investigations cemented our understanding of how sounds are processed in the brain, with a tonotopic organization preserved in most mammalian species. This can be thought of like a xylophone, with low pitches represented on the far left of the keys, and high pitches represented on the far right. Hitting the keys with a blunted mallet produces a softer sound, whereas a metallic mallet produces a louder intensity. Using this metaphor, different frequencies, acoustics, and sound intensities are processed in adjacent cortical areas along the length of the auditory system in order to maintain efficiency of sound discrimination².

But sounds do not often come in isolation. They come in association with the rapidly changing world around us – for example when watching television, or walking down the sidewalk. As such, the auditory system must be able to efficiently process the sounds we hear in our daily lives, and then communicate with other brain networks to make sense of what these sounds mean – are they expected, caring, or even potentially dangerous? Through inter-network communication the auditory system enables us to take acoustic stimuli and put them into perspective of what they mean to us.

For instance, imagine sitting at a restaurant and having a waiter approach to take your order. In the background, music plays to set the environment of the restaurant which is already being processed in your auditory system as non-threatening musical information. When the waiter asks for your order, adjacent areas of the auditory system activate, now required to process language at a different tone, pitch, and cadence to the background music. Additionally, perceiving human speech requires complex behaviours like inflexion, syntax, and forming accurate responses, and so by recruiting networks such as the central executive and language networks, the hierarchical nature of the auditory system is able to filter through the background music, focus on processing language, and then convey these more complex cues and response formation to adjacent brain networks (central executive and language, respectively)³.

The auditory system includes functional areas, known as parcels, in the frontal, parietal, and temporal lobes of the brain, as well as a key area of the supplementary motor area – the 'supplementary and cingulate eye field' (SCEF). Most of the sound-receptive parcels are isolated to the temporal lobe, whereas parcels in other lobes are more responsible for directing attention to salient sounds, determining the components of language described above, and other more complicated auditory information^3-5.

As the core system is responsible for processing sounds and auditory input to the brain, loss of function in the auditory system is typically related to hearing deficits or manifestations of abnormal and false auditory information⁶ .

Temporal lobe tumors are commonly associated with unilateral hearing loss, as the auditory system is increasingly damaged or warped by the mass effect of the tumor⁷. This sort of structural damage and hearing loss is also a symptom of disorders which affect white and grey matter density, such as age-related atrophy and frontotemporal dementia^8,9 .

Further, disfluent speech – more commonly known as 'stutter' has been shown to be related to deficits in specifically the left auditory system. Affecting over 5% of children between the ages of 2 and 4, a randomized control trial by Kikuchi et al. demonstrated that individuals with disfluent speech have a reorganized auditory system and abnormal auditory gating. 'Error-signals' occur in these individuals during speech processing, causing the primary symptom of repetition in specific syllables¹⁰ .

With regard to mental illness, the auditory system’s direct role in sound and speech processing can contribute to the symptoms of numerous neuropsychiatric disorders. These include hearing deficits in autism¹¹, auditory hallucination in schizophrenia¹² , and unpleasant background sounds (auditory aura) in migraine and frontal lobe epilepsy^13,14 .

During early evolution, the auditory system played a core role in alerting of danger and potential threats in the surrounding environment. However, as our species has evolved, so too has our utilization of auditory information, such as in complex communication strategies and in the context of melodies and music. To accommodate this, throughout evolution the human temporal lobe developed a more intricate folding pattern, and expanded to improve the surface area of our auditory system^15-17.

In order to effectively process the various types of auditory information we receive throughout our daily lives; the auditory system has key integrations with adjacent networks in the brain. Specifically, these connections include:

• The language system for language processing
• The central executive network for interpreting complex sounds
• The visual system for reorienting towards sounds
• The ventral attention network for alerting us during unexpected auditory information

While each brain network has key characteristics and processes, it is important to remember that every network modulates the interaction of other networks. None exist in a vacuum. Many connections in one network will activate connections in another network, and the auditory system serves as just one node for perceiving the world around us.

Though the auditory system has been an integral part of the human brain throughout our evolution, we are still uncovering connections between the auditory system and other brain networks. These discoveries will continue to give insight into how the auditory system processes sounds and speech in our brain, as well as how damage to the auditory system affects the various qualities of audition.