2.3.3.a Introduction

Tinnitus consists in a phantom auditory perception in the absence of any corresponding external acoustic stimulus. It is estimated that nearly 10 % of the population (Coles, 1996) experience tinnitus. Like any continuous or repeated sound, the tinnitus signal normally loses its relevance as a consequence of an adaptation of the Central Nervous System (CNS) that is called habituation. Actually, although most tinnitus patients (75 %) make a successful adaptation to the presence of these phantom sounds, for those who fail to adapt, tinnitus may become a source of significant disability, leading to a depressive state or even to suicide (Lewis, Stephens, & McKenna, 1994).

Most researchers agree with the idea that tinnitus could be the consequence of aberrant neural activity within the auditory pathways that is processed as a sound in auditory centers. Moreover, because tinnitus is often associated with hearing loss that can be related with damage to hair cells, it has been thought to be due to a modification of neural discharge patterns in the inner ear (Hazell, 1996; Jastreboff, 1990; Jastreboff & Jastreboff, 2000). However, some clinical studies have shown that most tinnitus patients who undergo eighth nerve section continue to experience their tinnitus postsurgically (House & Brackman, 1981), which supports the idea that the neural generators of at least some forms of tinnitus may lie more centrally. Some recent studies showed that cochlear damage could result in significant functional reorganization in central auditory centers. Indeed Kaltenbach and Afman (2000) demonstrated that exposure to intense sound, one of the most common cause of chronic tinnitus, induced an increase in spontaneous activity in the dorsal cochlear nucleus (DCN) in the hamster, suggesting that the DCN might be signaling the presence of a tone in the absence of acoustic stimulation. Moreover, Giraud et al. (1999), studying a rare form of tinnitus that is elicited by changing gaze direction, reported that tinnitus increased regional cerebral blood flow in temporo-parietal auditory association areas. Finally, tinnitus has been shown to be related to plastic modifications in the primary auditory cortex (A1). Indeed, Mühlnickel, Elbert, Taub, and Flor (1998), using the magnetoencephalographic technique, reported tonotopic modifications in A1, with an increase of the cerebral substrate devoted to the processing of sounds in the range of tinnitus frequency.

Because cerebral imagery techniques indicated abnormal activations in cortical structures and evidence for functional reorganization in tinnitus patients, we decided, in Experiment 1, to test whether modifications can also be demonstrated at the level of functional hemispheric organization using dichotic listening (see Kimura, 1967).

The dichotic listening technique that consists in presenting a different stimulus to each ear simultaneously has been extensively used to test hemispheric specialization for language functions. The logic of this technique can be summarized as follows: Because auditory information is projected both to the contralateral and the ipsilateral hemisphere, with a predominant contralateral projection, presenting information to both ears simultaneously “extinguishes” the ipsilateral projection, which allows us to send information to each hemisphere separately. Hence, the technique can be used to reveal hemispheric specialization and, more specifically, left-hemisphere (i.e., right-ear) dominance for language functions in right-handed people.

We reasoned that in tinnitus patients, tinnitus in the right ear might be predominantly “processed” by the left hemisphere, whereas tinnitus in the left ear might be predominantly “processed” by the right hemisphere. Therefore, in the case of dichotic presentation of verbal stimuli, left-hemisphere verbal processing might be impaired in right-handed patients with right tinnitus, leading to a reduced right-ear advantage (REA). On the contrary, patients with left tinnitus should exhibit an increased REA. The presence of tinnitus in both ears should lead to a normal REA, although the overall performance is expected to be lower than in normal participants. However, it remains unknown whether the above expected modifications of the REA in tinnitus patients reflect differences in hemispheric specialization for language functions, or the consequence of an interference between the processing of language stimuli and the processing of the tinnitus signal at the perceptual level.

Because we aimed at using the dichotic technique to investigate modifications in the organization of cerebral functions, it was critical to control for differences of performance due to the effects of tinnitus at the perceptual level, that is, for a possible reduction of intelligibility caused by the presence of tinnitus. In order to control for such differences, we compensated the perceptual difference between left- and right-ear stimuli by increasing the intensity of the stimulus in the tinnitus ear as needed for each patient.

In the present paper, we also wanted to investigate hemispheric differences for the processing of visual verbal stimuli in tinnitus patients, using the divided visual field technique. The first aim of this additional experiment (Experiment 2) was to determine whether the modification of cerebral organization expected in tinnitus patients could be observed at a more central, amodal (i.e., non exclusively auditory), level. The second aim was to investigate modifications in the organization of cerebral functions in tinnitus patients using a perceptual task where tinnitus cannot compete at a perceptual level with the stimulus to process.

Eventually, in Experiment 3, we decided to “simulate tinnitus” in healthy controls and to test tinnitus-simulated participants both in a dichotic and a divided visual field tasks. We reasoned that, if lateral differences observed in tinnitus patients in the dichotic task are the consequence of a reorganization of cerebral functions, and therefore differ from normal, lateral differences observed in tinnitus-simulated participants should be different from those observed in tinnitus patients, and should only reflect perceptual differences due to the presence of an interfering stimulus. Moreover, in the visual task, we did not expect any modification in lateral differences in tinnitus-simulated participants compared to normal (i.e., non-simulated) participants. Therefore, any abnormal pattern of visual differences in tinnitus patients would strengthen the interpretation of a reorganization of cerebral functions due to tinnitus.