Neural plasticity in tinnitus.

Two distinctly different kinds of tinnitus occur: objective and subjective tinnitus. Objective tinnitus is caused by sounds generated in the body while subjective tinnitus is caused by abnormal neural activity that is not evoked by sound. This chapter discusses subjective tinnitus. Subjective tinnitus has many forms. In most forms of tinnitus the anatomical location of the physiological abnormality is in the central nervous system, although the sensation is often referred to one ear or both ears. The cause of most forms of subjective tinnitus is the changes that have occurred as a result of expression of neural plasticity, thus a form of reprogramming of the brain that is not to the benefit of the individual person. Tinnitus often occurs together with hearing loss, indicating that the expression of neural plasticity has been evoked by deprivation of input. Tinnitus is often accompanied by hyperacusis, and sometimes phonophobia and depression, indicating altered processing of auditory information or rerouting of information. Several studies have brought evidence that some forms of tinnitus are associated with an abnormal involvement of the nonclassical (extralemniscal, diffuse, or polysensory) auditory pathways that bypass the primary auditory cerebral cortex and provide subcortical connections to limbic structures among others. There is no general treatment for tinnitus, but there are several treatments that can alleviate or reduce the tinnitus in some patients.