Mice with chronic stress appeared to become less sensitive to a range of 'mid-level' sounds between about 50dB and 70dB. Credit: Buena Vista Images via Getty ImagesShareWhile stress is an unfortunate fact of modern life, scientists are still learning how it affects us. Chronic stress, in particular, has been linked with many negative health outcomesand a new study published February 11 in PLOS Biology finds that in mice, at least, it can also affect the way in which the brain processes sound.The teams work investigated the effect of chronic stress on mices perception of sounds as being loud or quiet. The paper describes how two groups of mice were subjected to a number of noises, some loud and some soft. The mice were taught to choose one source of water in response to the quiet noises, and an alternative source in response to loud noises. Some mice were then placed in a state of chronic stress by repeated exposure to a mildly stressful situation (in this case, being restrained for half an hour).Both groups then played a variety of noises. For very loud and very quiet noises, the responses of the stressed and non-stressed mice were largely identical. However, as study co-author Jennifer Resnik explains to Popular Science, the mice experiencing chronic stress appeared to become less sensitive to a range of mid-level sounds between about 50dB and 70dB, reporting these sounds as quiet more often than their non-stressed peers. (A level of 50dB is roughly equivalent to quiet conversation or rainfall, while 70dB is comparable to a vacuum cleaner operating in the same room.)Physiological and behavioral evidence of stress. Left: Schematics of two-photon imaging during baseline and repetitive stress conditions. In repetitive stress sessions, the mice were placed in a 50 ml tube for 30 min to achieve mild stress. The imaging session started directly after the restraint. Individual cells were tracked over imaging days. Shown are examples of 2 imaging planes on day 1 and day 9 (scale bar, 50 m) and the noise-evoked responses of 3 exemplar cells (mean SE). Credit: Bisharat G et al., 2025, PLOS Biology, CC-BY 4.0 So does the stress affect the mices actual ability to hear? Or does it change the way in which their brains process the signals sent to it from their ears? Resnik explains that the answer seems to be the latter: while stress does not change the actual signal sent to a mouses brain by its ears in response to a given noise, it does seem to change how the animal perceives that noise.Resnik says that this effect is caused by the fact that stress induces an increased level of background noise within the brain. In the stressed mice, she explains, we observed an increase in baseline (spontaneous) activity in excitatory cells in the auditory cortex.In a brain without this increased baseline activity, a signal from the ears is relatively strong; in a stressed brain, however, theres less of a difference between the level of the signal and the ambient level of neural activity. The result, Resnik says, [is that] the brain becomes less sensitive to mid-level sounds. You can think of it this way: when youre under chronic stress, your kids might need to speak louder to get your attention.Very loud noises, however, appear to be able to break through this barrier: Responses to high sound levels appear to be preserved, says Resnik. No matter what, the brain still reacts when a sound is loud enough. So if your kids shout, youll hear them just as clearly, regardless of stress.Beyond its loudness, the actual nature of a noise doesnt appear to matter: Resnick says that chronic stress alters auditory processing in a way that primarily affects sensitivity to sound intensity, rather than particular frequencies or sound types. So the effect is the same whether its your kids speaking, the doorbell ringing or the bass frequencies from a passing cars stereo. All that matters is how loud the noise is.Resnik says that the implications of this work could reshape our understanding of how stress affects us: We often think of chronic stress as primarily influencing complex cognitive functions like decision-making and learning. However, our findings show that it also affects much more fundamental processes, such as the perception of sound. This raises an intriguing question: How much of our daily experience is shaped by chronic stress in ways we dont even realize?There are also many other questions raised by the studys findings. Resnik says that one particularly important subject of research is developing a better understanding of how long the effects of chronic stress can linger after the actual source of stress has been removed. The mice in this study, for instance, were not studied while actively stressed, but the stress they had experienced nevertheless had a material effect on their hearing. With regard to this particular effect, she says, Our current findings suggest that it persists for several days. However, she says, We are now conducting further experiments to determine whether these changes have longer-term consequences.Similarly, the way in which such effects take hold also remain the topic of ongoing research. A single exposure to stress had little impact, but as stress became chronic, we observed increasingly pronounced changes in auditory cortex activity. Does the effect continue to intensify as levels of stress increase? Resnik says this remains an open question.Either way, Resnik says, the creeping way in which the effects of stress take hold suggests its effects can be as subtle as they are pernicious. This gradual progression has important implications for daily life, she says. It suggests that prolonged stress doesnt just affect how we feelit can subtly alter how we perceive the world around us.