Listening to loud music can make it harder to discriminate sounds - even when the hearing threshold is normal.
The problem concerns the vividness of sounds, rather than their perceived volume - and it can't be detected by the usual hearing test in which subjects are examined in an otherwise silent environment using a series of individual tones.
"It can be said that listening to music at high volumes burdens the nerves of the brain and auditory system and can cause a decline in the ability to discriminate sounds, even if the usual hearing test results are normal and the subject is unaware of any changes," says Dr Hidehiko Okamoto of Japan's National Institute for Physiological Sciences.
The research group examined the brain's response to sound using the biomagnetism measurement device magnetoencephalography (MEG), which measures brain activity without needing any reponse from the subject.
They tested two groups of 13 young adults, one which had regularly listened to music at full blast, and another which hadn't. Subjects listened to a sound of a specific frequency contained in background noises while watching a movie.
The team found that the group which had listened regularly to portable music players found it much harder to dissociate a sound from background noises. It's not a problem that can be detected though a standard hearing test, which gave the same results for both groups.
Okamoto says that people should be aware of the risks of turning up the volume.
"It would be better to suppress environmental noises by using devices such as noise cancellers instead of turning up the volume when enjoying a mobile music player in a noisy place," he says.