You don’t have to be a neuroscientist to know that the human brain is a pretty miraculous piece of machinery. Not only does it keep us alive, but it makes us who we are, seven billion exemplars of quirky individuals scattered across the face of the globe.
But the brain is far more than simply the exquisite marriage of biology and psychology. It is also perhaps the finest surveillance system ever to exist in nature. Every second of every day, your brain is monitoring and responding to a seemingly infinite array of internal and external stimuli, from ambient and internal temperature to the location and positioning of your body in space.
Of all these complex and captivating neurological systems that keep us alive, aware, and functioning, it is perhaps the auditory system that is among the most sophisticated and awe-inspiring. Many of its attributes and mechanisms, though, remain largely a mystery.
Nevertheless, neuroscientists continue to strive to unlock the secrets of the human auditory system and recent years have brought with them a number of stunning discoveries. In the process, neuroscience is providing vitally important lessons on what it means not only for us humans to hear but also to listen.
Cutting Through the Noise
New evidence suggests that a significant amount of sensory processing occurs in the deepest and most primitive areas of the brain, such as the thalamus and the basal ganglia. Research into auditory processing, in particular, illustrates the complex interplay of cortical, midbrain, and deep brain structures in helping humans not only to hear (a process that automatically happens as sound is perceived by the ear) but also to listen (an active process in which effort is made to absorb and understand the sounds being perceived) (1, 2, 3). Indeed, neurological studies suggest that the thalamic reticular nucleus (TRN), which coils around the thalamus, functions as a sort of gatekeeper of sensory stimuli, communicating with other areas of the brain to shape auditory reception and enable appropriate responses to such auditory stimuli.
Selective Attention
The elegance of the auditory system manifests principally in the protective and utilitarian functions the system serves.
More specifically, the filtering processes of the auditory system enable the brain to automatically “dim” irrelevant stimuli while “spotlighting” that which requires attention. This is why you can typically follow a conversation in a noisy room with relative ease. It’s also why mothers typically awaken quickly even from a deep sleep when their infant begins to cry. Studies have shown that, in women, the sound of an infant’s cry immediately interrupts brain activity associated with rest, galvanizing the women for caregiving action (4, 5). What this means is that even when you’re not aware of it, your nervous system is always on the alert, monitoring ambient noise for signs that danger is at hand or a loved one is in need.
Thus, even when you are not consciously focusing your attention on specific sounds while endeavoring to ignore irrelevant background noise, your brain is already doing such heavy lifting for you. Indeed, the evidence suggests that hearing in general and listening, in particular, require the ear, the auditory midbrain, the deep brain, and the cortical forebrain to work together to inhibit some auditory stimuli while amplifying others. This realization has helped to explain, in part, the mechanisms of some forms of age-related hearing loss.
Researchers have found that, as we age, brain synapses become less responsive to neurotransmitters responsible for inhibiting auditory stimuli. This, in turn, means that deep brain and midbrain structures begin to lose their efficacy as “gatekeepers” serving to filter out or “dim” auditory inputs that are unimportant or non-urgent (6, 7). Researchers suspect that this is why age-related hearing loss most often manifests as an increased difficulty in following conversations in noisy environments. The auditory system’s capacity for attentional focusing is reduced, making it harder for older adults to differentiate and process speech as opposed to background noise.
The Effort To Listen
As we’ve seen, hearing involves a feedback loop that requires the coordination of a wide array of neurological structures to receive, filter, transmit, and process auditory stimuli. That means that hearing is, necessarily, a process that can demand a tremendous amount of energy. This is particularly true when your goal isn’t simply to receive sound but to truly absorb and understand it, to listen as well as to hear (8, 9, 10).
Research has shown that cognitive load, as well as listening effort, can be exacerbated by a number of factors. In addition to persons with age-related hearing loss, individuals with hearing impairments, second-language speakers, and those who are exposed to significant background noise are likely to face a considerable increase in listening effort and cognitive demand (11, 12, 13, 14, 15, 16, 17, 18). The result is a reduction in cognitive performance, task-based performance, sound differentiation and attribution, and speech recognition and semantic processing. In other words, the harder you have to work to hear and listen, the less likely you are to be able to do so with ease and accuracy.
There is, however, good news. The same research that confirms the effort required to hear and listen, particularly in complex conditions, such as noisy environments or in the face of hearing impairment, also confirms that the reduction in background noise can decrease listening effort and correlated cognitive load. By externally filtering as much of the nonessential auditory stimuli your brain receives, its auditory structures will be better able to do their important work of filtering out what does not matter and focusing instead on what does. This, in turn, will also free up your mind to carry on other important cognitive tasks.
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Sources:
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