Humans and other animals gradually learn what sounds or other sensory cues in their surroundings are meaningful or potentially threatening. Via a process known as habituation, they gradually learn to ignore non-threatening stimuli so that they can focus on new, unknown, and more significant events happening in their environment. This innate sensory filtering process prevents the nervous system from becoming overwhelmed by the flow of sensory information that animals and humans are constantly exposed to during waking hours.
Disruptions in habituation can lead to a hypersensitivity to sounds or other sensory stimuli, which is experienced by many autistic people, as well as some individuals with other neurodevelopmental and neuropsychiatric conditions.
In the brains of mammals, sounds are primarily processed in a region known as the primary auditory cortex (A1). Predictions and decisions related to sensory stimuli, on the other hand, are made with the support of the orbitofrontal cortex (OFC), a region in the frontal part of the brain.
Researchers at University of North Carolina at Chapel Hill recently carried out a study involving mice investigating how a repeated exposure to sounds influences neural activity in the A1 and the OFC. Their findings, published in Nature Neuroscience, offer new insight into how the brain predicts and filters out non-threatening sensory inputs that have been repeatedly encountered before.
“Habituation is a crucial sensory filtering mechanism whose dysregulation can lead to a continuously intense world in disorders with hypersensitivity,” wrote Hiroaki Tsukano, Michellee M. Garcia, and their colleagues. “Although habituation is often termed the simplest form of learning, its circuit mechanisms remain elusive. Conventional peripheral explanations fail to fully account for long-term habituation in complex sensory environments, leading to theories proposing top-down regulation.”
Testing competing sound habituation theories
As part of their study, the researchers set out to test two different theoretical explanations of how the brain supports habituation. The first, dubbed predictive filtering theory, suggests that as the brain experiences the same sound several times, it learns to predict the occurrence of this sound. This is hypothesized to occur when higher brain areas (such as the OFC) send signals to the regions responsible for processing sensory stimuli, prompting them to cancel out expected sounds.
The second theory tested by the researchers, dubbed novelty amplification theory, suggests that new stimuli result in additional brain activity, which gradually fades over time and after further exposure. If this theory is valid, repeated sounds should produce weaker responses in the brain, as the novelty-related brain activity attenuates.
To evaluate the two theories, the researchers exposed adult mice to sound sequences daily, while recording activity in the A1 region of their brain. In some trials, they also temporarily deactivated the OFC and looked at whether this influenced activity in the A1.
Interestingly, they found that turning off the OFC reversed habituation processes, which confirms the role of this brain region in the filtering of expected sounds. The data they collected suggests that the OFC sends signals that gradually increase during exposure to sounds, suppressing activity in the A1.
“After daily sound exposure, neural habituation in the primary auditory cortex (A1) was reversed by inactivating the OFC,” wrote the authors. “Top-down projections from the OFC, but not other frontal areas, carried predictive signals that grew with daily sound experience and suppressed A1 via somatostatin-expressing inhibitory neurons. Thus, prediction signals from the OFC cancel out anticipated stimuli by generating their ‘negative images’ in sensory cortices.”
Refining the present understanding of sound hypersensitivity
The researchers’ observations suggest that habituation is not merely a gradual reduction in brain responses to familiar sounds, but it is actively supported by OFC predictions. If validated in other studies involving other mammals and humans, these observations could potentially improve the current understanding of sensory filtering processes, as well as instances in which these processes are disrupted.
The recent efforts by this research team might also contribute to the understanding of the hypersensitivity often associated with autism spectrum disorder (ASD). In the future, this could potentially help to develop new promising therapeutic strategies aimed at facilitating habituation processes or reducing people’s hypersensitivity to sensory stimuli, which can be very distressing and debilitating.