@Lisa88 Well, I'm no research guru, but will try my best to answer.
Most researchers cite one year as the dividing line between acute and chronic tinnitus. I've never read that different tinnitus etiologies might effect brain plasticity and subsequent chronicity. And, so, your finding is very interesting! What book/article are you reading? Some types of tinnitus--e.g., resulting from middle ear pathologies like ETD--will be attenuated or even cured upon
surgery.
They are still looking into different brain responses and time frames for different causes of tinnitus, i.e. stress vs acoustic trauma vs gradual hearing loss
Oh, sorry. I just reread your post and saw I didn't answer it correctly. I see you're also talking about age-related tinnitus or presbycusis compared with other tinnitus etiologies. Unfortunately, I've never read about brain plasticity and this type of tinnitus. But I presume there's a point where the hair loss (and perhaps a stress trigger) becomes too great for the auditory cortex to ignore and thus it starts the "filling in" mechanism, which some researchers believe causes the tinnitus sounds.
From my reading, chronic tinnitus becomes less amenable to treatment--in part--because more brain areas become involved over time. The maladaptive plasticity that starts in the brain stem (DCN), inferior colliculus, and auditory cortex recruits other areas and more connections are formed among brain areas. Brain imaging studies, such fMRI, look at this inter connectivity.
Because of increasing connectivity, neuromodulation techniques--especially rTMS--become less effective the longer a person has tinnitus. I need to find my research on this, but I believe this technique begins to loose efficacy after one year and is not efficacious after four years. But researchers are currently developing new protocols--utilizing more brain areas--as a method to counteract the maladaptive plasticity that tinnitus creates in our brain, which increases over time.
That said, habituation may occur at any time with chronic tinnitus--even after many years. The noise may go away completely--unless you seek it--or it may no longer become an issue. This is because tinnitus awareness and tinnitus disturbance occupy different--though somewhat overlapping--brain networks. This is what Dr. Hubbard, among others, talk about when they discuss habituation. Your brain still has the tinnitus--but it no longer affects you.
That said, I've been impressed with Retigabine's ability to attenuate chronic tinnitus. That thread has caused me to revise my formerly less-than-optimistic belief that chronic tinnitus cannot be cured. Perhaps, it can be cured by potassium channel modulators like Retigabine and Autifony's new drug. Of course, "cured" may really be "maintenance." But that's fine too. Taking a pill or two a day--with acceptable side effects--is a small sacrifice to be noise free.