2.3.3.e General Discussion

The purpose of our study was to test the possible effect of tinnitus, a phantom auditory perception, on the organization of cerebral functions.

In Experiment 1, we tested the hemispheric organization of language using a dichotic task in tinnitus and healthy participants. The results showed the expected RE/LH advantage for verbal stimuli in all participants, except for right tinnitus patients who exhibited no interhemispheric differences. This result appeared to be due to a particularly low performance on RE/LH trials. Thus, our data suggest that the presence of tinnitus might have modified the organization of cerebral functions associated with auditory language processes.

In Experiment 2, we investigated the hemispheric organization of language functions using a divided visual field task. Our aim was to determine whether the cerebral reorganization of functions observed in right tinnitus patients in Experiment 1 could be observed at a higher, amodal level. The results revealed a RVF/LH advantage for word processing in all participants. This suggests that the reorganization of cerebral functions involved by the presence of tinnitus might concern the auditory modality exclusively, and did not extend to other modalities.

Finally, in Experiment 3, we tested healthy participants submitted to a tinnitus simulation using the same dichotic and divided visual field tasks as the ones used in Experiment 1 and 2. We aimed at verifying whether the presence of a simulated tinnitus was sufficient to elicit a modification of the expected ear and visual field advantages for the processing of verbal stimuli. The results showed the expected RE/LH advantage for the processing of auditory-verbal stimuli and the expected RVF/LH advantage for the processing of visuo-verbal stimuli in all tinnitus-simulated participants.

Taken together, our data suggest that a reorganization of the cerebral functions involved in language processes may be caused by the continuous presence of tinnitus. Such plastic changes may have occurred in right tinnitus patients as a consequence of the deleterious effect of the tinnitus signal sent directly to the language dominant left hemisphere. These plastic changes are consistent with the study of Mühlnickel et al. (1998) who reported modifications of the primary auditory cortex tonotopy in tinnitus patients using the magnetoencephalographic technique. Moreover, other studies have brought evidence for tinnitus-related brain abnormalities (Giraud et al, 1999; Lockwood et al., 1998; Meikle, Griest, Press, & Stewart, 1992; Melcher, Sigalovsky, Guinan, & Levine, 2000; Mirz et al., 1999), suggesting that cortical reorganization may occur over time in response to the continuous perception of tinnitus.

It is worth noticing that these plastic changes take time to occur. In our study, tinnitus participants had been affected by tinnitus between 10 months and 27 years. In contrast, tinnitus-simulated participants underwent a “tinnitus-like simulation” for approximately one hour only. This is apparently not enough to involve modifications in the organization of cerebral functions. The presence of a normal pattern of lateral differences in tinnitus-simulated participants also suggests that the presence of an interfering noise cannot by itself engender modifications in lateral differences. This observation is consistent with the hypothesis of an effect of real tinnitus occurring at a higher (cortical) level.

Finally, although we controlled for parameters that could have influenced the performance of the participants in the dichotic and divided-visual tasks (tinnitus patients with a relatively small hearing loss only were selected, and compensation of differences in intelligibility between both ears was provided when necessary), it remains possible to interpret our data in terms of an attention effect. Actually, some studies have found evidence of perturbations affecting attention mechanisms in tinnitus patients (Cuny, Norena, Koenig, Bougeant, & Chéry-Croze, 2002; Jacobson et al., 1996). It has been hypothesized that such perturbations may explain the persistence of tinnitus and the absence of habituation (Andersson, 2002; Jastreboff & Jastreboff, 2000). However, the precise way attention may intervene remains unknown. On the one hand, attention may be automatically driven by the tinnitus. Indeed, attention may be too easily engaged toward the tinnitus ear, which would make the tinnitus particularly disturbing. In contrast, patients may have difficulty to disengage attention from the tinnitus ear, which would be just as disturbing. On the other hand, tinnitus patients may learn to orientate deliberately their attention toward their healthy ear in order to avoid the tinnitus. Using such a strategy, unilateral tinnitus patients may exhibit a better performance for stimuli presented to the healthy ear in the dichotic task. This could explain why right tinnitus patients did not exhibit the expected RE/LH advantage for word stimuli and why left tinnitus patients showed an enhanced RE/LH advantage.

However, this interpretation seems questionable since such a strategy should also have been observed in tinnitus-simulated participants. However, this effect did not appear in tinnitus-simulated participants in Experiment 3. Thus, an interpretation of our results in terms of modifications in the organization of auditory-verbal cerebral functions seems more appropriate.

To conclude, our findings provide evidence in favor of the hypothesis that the aberrant auditory signal responsible for tinnitus perception may induce a cerebral reorganization of language functions. This reorganization seems to be limited to auditory-verbal functions. Further studies should be undertaken to confirm and better document this reorganization, using different techniques and different patients groups with various durations of tinnitus experience.