Tinnitus, the perception of noise in the ears or head when there is no external source, affects millions of Americans. For some, the symptoms are intermittent, but for others, they are constant, with ringing, buzzing, hissing or humming sounds continuously playing a private symphony in the sufferer’s head.
Although the noises are not real, the debilitating and life-altering effects can be, but researchers from the University of Michigan Medical School claim to be a step closer to understanding what is going on inside these “unquiet brains.”
According to their report, published in the Journal of Neuroscience, they have identified that the process called stimulus-timing dependent multisensory plasticity is altered in animals with tinnitus.
They claim this plasticity is “exquisitely sensitive” to the timing of signals to a key area of the brain.
The dorsal cochlear nucleus is where signals from the auditory nerve enter the brain. Neurons in this area integrate this auditory information with other sensory signals, such as touch. In tinnitus, when the sounds from the ear are reduced, the signals from the somatosensory nerves in the face and neck – which relate to touch – are amplified.
Prof. Susan Shore, senior author on the paper, explains:
“It’s as if the signals are compensating for the lost auditory input, but they overcompensate and end up making everything noisy.”
And while this may explain why some tinnitus sufferers can change the volume and pitch of the sounds they hear by clenching their teeth or moving their head and neck, this is not the complete picture of tinnitus.
Prof. Shore explains that the precise timing of these signals in relation to one another changes the nervous system’s plasticity.
For the study, Prof. Shore and colleagues exposed guinea pigs to a narrowband noise that produced a temporary elevation of auditory brainstem response thresholds.
Tinnitus was measured by a gap-induced prepulse inhibition of the acoustic startle reflex.
Tinnitus commonly occurs with hearing loss, but it can follow head and neck trauma, such as after a car accident. It is also associated with exposure to loud noises and is the top cause of disability among veterans and members of the armed forces following exposure to blasts in war zones.
Yet, no one knows why some people develop the condition while others do not, even when exposed to the same levels of noise.
Similarly with the guinea pigs, only half of the animals developed tinnitus, and the researchers found that those that did not, had fewer changes in the multisensory plasticity than those who suffered the condition.
Prof. Shore and her team are now developing a device that combines sound and electrical stimulation of the face and neck in order to return the neural activity to normal.
“If we get the timing right, we believe we can decrease the firing rates of neurons at the tinnitus frequency, and target those with hyperactivity.”
However, she notes that treatments will have to be customized to each patient and delivered regularly.