Discussion

The purpose of Experiment 1 was to investigate the cerebral hemispheric asymmetry of language in patients suffering from tinnitus using a dichotic listening procedure. The results showed the expected RE/LH advantage for verbal stimuli (see Bryden, 1988 for a review) in all participants, but in right tinnitus patients. Indeed, right tinnitus patients did not exhibit any lateral difference, as a consequence of an especially low RE/LH performance. On the contrary, left tinnitus patients showed an increased REA compared to control participants, due to a lower LE/RH performance. The analysis performed on side-inversions errors revealed that participants produced more errors of the “RE for LE inversion” type, which appears consistent with the RE/LH advantage for the processing of verbal stimuli. Finally, bilateral tinnitus patients performed the dichotic listening task as accurately as control subjects and did not differ in terms of lateral differences.

On the one hand, the results of this first experiment suggest that the presence of tinnitus might have modified the normal left-hemisphere specialization for language functions, especially in right tinnitus patients where we observed an absence of the RE/LH advantage for language. The increased RE/LH advantage in left tinnitus patients might also reflect a modification in the organization of cerebral functions. Such modifications seem to occur only in the case of unilateral tinnitus, since bilateral tinnitus patients exhibited normal lateral differences. In addition, it seems worth to notice that the presence of tinnitus in both ears did not decrease patients’ overall performance.

On the other hand, our results might be interpreted in terms of interference at the perceptual level caused by the presence of tinnitus. Indeed, the tinnitus signal may act as a noise that may interfere with the processing of the pertinent auditory signal. In such a case, right tinnitus patients may suffer from the fact that tinnitus interferes with the stimulus presented in the right ear, which consequently decreases RE/LH performance and may suppress the expected left-hemisphere advantage for language processing. The opposite effect may be observed in left tinnitus patients, which, in contrast, may decrease LE/RH performance and enhance the left-hemisphere advantage for language. However, the present interpretation in terms of interference at the perceptual level seems to be inconsistent with the fact that bilateral tinnitus patient, whose performance is expected to decrease on both sides, exhibited normal performance in this task. In addition, we controlled for perceptual differences by increasing the intensity of the signal in the tinnitus ear in unilateral tinnitus patients in order to equalize the intelligibility between ears. Thus, it seems unlikely that the decrease in performance in the tinnitus ear was due to a perceptual interference. Therefore, an interpretation in terms of a modification of the organization of cerebral functions seems more appropriate than an interpretation in terms of an interference at the perceptual level.

Experiment 2 was designed to gather further evidence in favor of a modification of cerebral organization in tinnitus patients by investigating hemispheric differences at a level that is not affected by a possible impairment in the perceptual encoding of the stimulus. Thus, we decided to test functional hemispheric asymmetry of language in tinnitus patients in the visual modality. This attempt to investigate hemispheric differences in the visual modality rested on two points. First, we made the hypothesis that tinnitus leads to a reorganization of functions within the auditory language areas. Second, we made the hypothesis that such a reorganization affects cerebral functions at a more general, amodal, level, beyond auditory language areas.