Reorganization of auditory cortex in tinnitus

Abstract

Magnetic source imaging was used to determine whether tonotopy in auditory cortex of individuals with tinnitus diverges from normative functional organization. Ten tinnitus subjects and 15 healthy controls were exposed to four sets of tones while magnetoencephalographic recordings were obtained from the two cortical hemispheres in sequence. A marked shift of the cortical representation of the tinnitus frequency into an area adjacent to the expected tonotopic location was observed. The Euclidean distance of the tinnitus frequency from the trajectory of the tonotopic map was 5.3 mm (SD = 3.1) compared with a distance of 2.5 mm (SD = 1.3) of a corresponding frequency in the healthy controls (t = 3.13, P < 0.01). In addition, a strong positive correlation was found between the subjective strength of the tinnitus and the amount of cortical reorganization (r = 0.82, P < 0.01). These results demonstrate that tinnitus is related to plastic alterations in auditory cortex. Similarities between these data and the previous demonstrations that phantom limb pain is highly correlated with cortical reorganization suggest that tinnitus may be an auditory phantom phenomenon.

Figures

Figure 1

A typical example of the tonotopic map is shown for a left ear tinnitus (Upper) and a control subject (Lower). Equivalent current dipoles elicited by auditory stimulation at the three standard and the tinnitus frequency in the tinnitus subject and the four standard frequencies in the healthy control are superimposed onto an axial slice of Brodman’s area 41 of the right hemisphere. The line in the upper portion of the figure shows the trajectory of the dipole locations of the three standard tones (circles). The triangle (Upper) represents the location of the tinnitus frequency (6,000 Hz in this case). Note that the trajectory of the dipole locations of the four standard frequencies in the healthy control subject (circles, Lower) is linear, whereas the dipole of the affected frequency in the tinnitus subject diverges from the linear trajectory established by the three standard frequencies. The location of T4 as well as the scale of measurement are marked.

Figure 2

Scatterplot of amount of subjective tinnitus strength and deviation of the tinnitus frequency from the tonotopic map in the contralateral hemisphere. A measure of deviation of the tinnitus frequency was obtained by determining the Euclidean distance between the trajectory of the standard tones and the location of the tinnitus frequency in tinnitus subjects or the corresponding comparison frequency in control subjects (see text). The tinnitus strength was assessed by the MTI. Greater subjective tinnitus strength was related to larger deviations from the trajectory of the standard frequencies. This figure suggests that there might not be a linear relationship between tinnitus and reorganization but rather a bimodal distribution of tinnitus sufferers with and without map distortions. The size of the sample studied is too small to clarify this point.