Discussion

This third experiment was aimed at testing the effect of an external noise presented to one ear only on the performance of the dichotic and visual laterality tasks of Experiment 1 and Experiment 2. We reasoned that simulating tinnitus in normal participants would allow us to study the effect of tinnitus over time on the reorganization of cerebral functions in tinnitus patients. Indeed, if cerebral hemisphere asymmetry differs in tinnitus-simulated participants compared to tinnitus patients, evidence would have been provided for a reorganization of auditory-verbal cerebral functions over time in tinnitus patients.

The results of the dichotic task demonstrated the expected RE/LH advantage for verbal auditory stimuli in all participants. Indeed, both LSIM and RSIM participants exhibited a LH advantage.

The results of the visual laterality task revealed that tinnitus-simulated participants were more accurate for RVF/LH presentations than for LVF/RH presentations, but for word stimuli only. This result is consistent with the well-documented LH advantage for words. We did not expect similar patterns of lateral differences for pseudo-words, since lateral differences may depend upon the stimulus morphological structure that was not examined and controlled in the present experiment (see Koenig, Wetzel, & Caramazza, 1992). Finally, no difference was observed between left- and right-simulated participants on the LH advantage for words.

Overall, results of Experiment 3 reveal that the presence of an external noise presented to one ear that simulated an unilateral tinnitus did not seem to modify the normal LH advantage for auditory-verbal and visuo-verbal stimuli. This was true whatever the stimulated ear. This is an important finding, since this suggests that lateral differences reflect hemispheric specialization, that is, differences occurring at a central level of processing, rather than differences occurring at the entry, peripheral level.

It seems reasonable to propose that a more complex auditory stimulus (e.g., an identifiable environment sound, a language-like sound, or a word) could have indeed interfered with central processes and consequently modified lateral differences. But this is apparently not the case with a tinnitus-like sound.