Symptoms and signs caused by neural plasticity

Transcutaneous Electrical Nerve Stimulation for Treatment of Tinnitus: A Systematic Review and Meta-analysis

OBJECTIVE

To evaluate the treatment efficacy of transcutaneous electrical nerve stimulation (TENS) in patients with tinnitus.

DATA SOURCES

PubMed, Scopus, Web of Science, and Cochrane Library were searched for the following concepts: "Transcutaneous Electric Nerve Stimulation" and "Tinnitus."

STUDY SELECTION

Inclusion: 1) double- or single-blinded randomized controlled trials; 2) double- or single-blinded randomized comparison trials; 3) prospective or retrospective observational studies; and 4) case series. Exclusion: Non-English studies, nonhuman studies, case reports (n ≤ 5), and review articles.

DATA EXTRACTION

Tinnitus Handicap Inventory (THI), the Visual Analog Scale (VAS), and perceived tinnitus suppression after treatment. Additional data collected included tinnitus laterality, duration of symptoms, location of electrode placement, time to follow-up, etiology of tinnitus, and treatment side effects.

DATA SYNTHESIS

The literature search yielded 2941 unique articles. After reviewing 118 full-text articles, 17 studies reporting on 1,215 patients were included for final analysis. Four studies provided data available for meta-analysis of pre- and posttreatment THI and VAS (Cochrane Review Manager). TENS showed significant overall reduction on THI (-7.55 [-10.93 to -4.18], p < 0.0001) and VAS (-0.65 [-0.99 to -0.30], p < 0.0002). Subjective improvement of tinnitus was pooled across 13 studies using meta-analysis of proportions (MedCalc). Tinnitus suppression occurred in 40.0% [28.9-51.7%] patients. Among those who responded, 22.2% [12.2-29.7%] experienced complete suppression and 10.2% [0.2-31.9%] experienced persistent improvement at 3 months.

CONCLUSIONS

TENS represents a safe and feasible treatment option for tinnitus and might be a worthy consideration among the spectrum of interventions developed for tinnitus.

Auditory and visual 3D virtual reality therapy as a new treatment for chronic subjective tinnitus: Results of a randomized controlled trial

BACKGROUND

Subjective tinnitus (ST) is a frequent audiologic condition that still requires effective treatment. This study aimed at evaluating two therapeutic approaches: Virtual Reality (VR) immersion in auditory and visual 3D environments and Cognitive Behaviour Therapy (CBT).

METHODS

This open, randomized and therapeutic equivalence trial used bilateral testing of VR versus CBT. Adult patients displaying unilateral or predominantly unilateral ST, and fulfilling inclusion criteria were included after giving their written informed consent. We measured the different therapeutic effect by comparing the mean scores of validated questionnaires and visual analog scales, pre and post protocol. Equivalence was established if both strategies did not differ for more than a predetermined limit. We used univariate and multivariate analysis adjusted on baseline values to assess treatment efficacy. In addition of this trial, purely exploratory comparison to a waiting list group (WL) was provided.

RESULTS

Between August, 2009 and November, 2011, 148 of 162 screened patients were enrolled (VR n = 61, CBT n = 58, WL n = 29). These groups did not differ at baseline for demographic data. Three month after the end of the treatment, we didn't find any difference between VR and CBT groups either for tinnitus severity (p = 0.99) or tinnitus handicap (p = 0.36).

CONCLUSION

VR appears to be at least as effective as CBT in unilateral ST patients.

A neuroplastic deafferentation hypothesis for bipolar disorder

Bipolar disorder, characterised by extreme cyclical variations in mood between depression and mania, is a common, debilitating and sometimes fatal psychiatric condition with an unclear aetiology. In this paper we propose a hypothesis for the development of bipolar disorder through which neuroplastic changes in response to an index depressive episode leads to the amplification of subthreshold pleasurable stimuli that then drive conversion into a manic state. This ‘pleasure deafferentation hypothesis’ is reached through a discussion of the neuroscientific basis of deafferentation at the level of the neuron and its role in the development of various neurological and psychiatric phenomena before a case for deafferentation as applied to bipolar disorder is justified and its implications discussed.

Are the non-classical auditory pathways involved in autism and PDD?

OBJECTIVE

To test the hypothesis that some of the abnormal sensory perceptions that characterize autism may be explained by an abnormal activation of non-classical (extra-lemniscal) sensory pathways.

METHODS

Twenty-one individuals, 18--45 years of age who were diagnosed with autism participated in the study. Sounds (clicks presented at a rate of 40 per second and 65 dB above the normal threshold) were applied through earphones. Electrical stimulation (100 microS rectangular impulses at a rate of 4 per second) was applied through electrodes placed on the skin over the median nerve at the wrist. The participants were asked to match the loudness of the sound with and without the electrical stimulation applied to the median nerve.

RESULTS

Electrical stimulation of the median nerve at the wrist in individuals with autism could change the perception of loudness of sounds presented to one ear through an earphone showing a statistically significant abnormal sensory cross-modal interaction.

DISCUSSION

We interpreted our results to support the hypothesis that some individuals with autism have an abnormal cross-modal interaction between the auditory and the somatosensory systems. Cross-modal interaction between senses such as hearing and the somatosensory system does not occur normally in adults. As only the non-classical (extralemniscal) ascending auditory pathways receive somatosensory input, the presence of cross-modal interaction in autistic individuals is a sign that autism is associated with abnormal involvement of the non-classical auditory pathways, implying that sensory information is processed by different populations of neurons than in non-autistic individuals.

Usefulness of BFB/EMG in facial palsy rehabilitation

OBJECTIVE

To analyze and to compare the recovery and the development of synkinesis in patients with idiopathic facial palsy (Bell's palsy) following treatment with two methods of rehabilitation, kinesitherapy (KT) and biofeedback/EMG (BFB/EMG).

STUDY DESIGN

Retrospective cases--series review.

METHODS

Seventy-four patients with Bell' palsy were clinically evaluated within 1 month from onset of palsy and at 12 months after palsy (House scale and synkinesis evaluation). Electromyography (EMG) and Electroneurography (ENG) were performed about 4 weeks after palsy to better evaluate functional abnormalities due to facial nerve lesion. The patients followed two different protocols for rehabilitation: the first 32 patients were treated with therapeutic exercises performed by therapists (KT group), the latter 42 patients were treated using BFB/EMG methods (BFB group) with inhibition of synkinetic movement as the primary goal.

RESULTS

KT and BFB patients were evaluated for clinical and neurophysiological characteristics before rehabilitative treatment. BFB patients showed better clinical recovery and minor synkinesis than KT patients.

CONCLUSIONS

BFB/EMG seems to be more useful than KT in Bell's palsy treatment. This could be due to the fact that BFB/EMG gives more accurate information than KT on muscle activation with better modulation in voluntary recruitment of motor unit.

Cross-modal interactions of auditory and somatic inputs in the brainstem and midbrain and their imbalance in tinnitus and deafness

PURPOSE

This review outlines the anatomical and functional bases of somatosensory influences on auditory processing in the normal brainstem and midbrain. It then explores how interactions between the auditory and somatosensory system are modified through deafness, and their impact on tinnitus is discussed.

METHOD

Literature review, tract tracing, immunohistochemistry, and in vivo electrophysiological recordings were used.

RESULTS

Somatosensory input originates in the dorsal root ganglia and trigeminal ganglia, and is transmitted directly and indirectly through 2nd-order nuclei to the ventral cochlear nucleus, dorsal cochlear nucleus (DCN), and inferior colliculus. The glutamatergic somatosensory afferents can be segregated from auditory nerve inputs by the type of vesicular glutamate transporters present in their terminals. Electrical stimulation of the somatosensory input results in a complex combination of excitation and inhibition, and alters the rate and timing of responses to acoustic stimulation. Deafness increases the spontaneous rates of those neurons that receive excitatory somatosensory input and results in a greater sensitivity of DCN neurons to trigeminal stimulation.

CONCLUSIONS

Auditory-somatosensory bimodal integration is already present in 1st-order auditory nuclei. The balance of excitation and inhibition elicited by somatosensory input is altered following deafness. The increase in somatosensory influence on auditory neurons when their auditory input is diminished could be due to cross-modal reinnervation or increased synaptic strength, and may contribute to mechanisms underlying somatic tinnitus.

Controversy: Does repetitive transcranial magnetic stimulation/ transcranial direct current stimulation show efficacy in treating tinnitus patients?

BACKGROUND

Tinnitus affects 10% of the population, its pathophysiology remains incompletely understood, and treatment is elusive. Functional imaging has demonstrated a relationship between the intensity of tinnitus and the degree of reorganization in the auditory cortex. Experimental studies have further shown that tinnitus is associated with synchronized hyperactivity in the auditory cortex. Therefore, targeted modulation of auditory cortex has been proposed as a new therapeutic approach for chronic tinnitus.

METHODS

Repetitive transcranial magnetic stimulation (rTMS) and transcranial direct current stimulation (tDCS) are noninvasive methods that can modulate cortical activity. These techniques have been applied in different ways in patients with chronic tinnitus. Single sessions of high-frequency rTMS over the temporal cortex have been successful in reducing the intensity of tinnitus during the time of stimulation and could be predictive for treatment outcome of chronic epidural stimulation using implanted electrodes.

RESULTS

Another approach that uses rTMS as a treatment for tinnitus is application of low-frequency rTMS in repeated sessions, to induce a lasting change of neuronal activity in the auditory cortex beyond the duration of stimulation. Beneficial effects of this treatment have been consistently demonstrated in several small controlled studies. However, results are characterized by high interindividual variability and only a moderate decrease of the tinnitus. The role of patient-related (for example, hearing loss, tinnitus duration, age) and stimulation-related (for example, stimulation site, stimulation protocols) factors still remains to be elucidated.

CONCLUSIONS

Even in this early stage of investigation, there is a convincing body of evidence that rTMS represents a promising tool for pathophysiological assessment and therapeutic management of tinnitus. Further development of this technique will depend on a more detailed understanding of the neurobiological effects mediating the benefit of TMS on tinnitus perception. Moreover clinical studies with larger sample sizes and longer follow-up periods are needed.

Tinnitus and Neural Plasticity of the Brain

OBJECTIVE

To describe the current ideas about the manifestations of neural plasticity in generating tinnitus.

DATA SOURCES

Recently published source articles were identified using MEDLINE, PubMed, and Cochrane Library according to the key words mentioned below.

STUDY SELECTION

Review articles and controlled trials were particularly selected.

DATA EXTRACTION

Data were selected systematically, scaled on validity and comparability.

CONCLUSION

An altered afferent input to the auditory pathway may be the initiator of a complex sequence of events, finally resulting in the generation of tinnitus at the central level of the auditory nervous system. The effects of neural plasticity can generally be divided into early modifications and modifications with a later onset. The unmasking of dormant synapses, diminishing of (surround) inhibition and initiation of generation of new connections through axonal sprouting are early manifestations of neural plasticity, resulting in lateral spread of neural activity and development of hyperexcitability regions in the central nervous system. The remodeling process of tonotopic receptive fields within auditory pathway structures (dorsal cochlear nucleus, inferior colliculus, and the auditory cortex) are late manifestations of neural plasticity. The modulation of tinnitus by stimulating somatosensory or visual systems in some people with tinnitus might be explained via the generation of tinnitus following the nonclassical pathway. The similarities between the pathophysiological processes of phantom pain sensations and tinnitus have stimulated the theory that chronic tinnitus is an auditory phantom perception.

Long-Term Evaluation of Treatment of Chronic, Therapeutically Refractory Tinnitus by Neurostimulation

OBJECTIVE

Long-term evaluation of treatment of chronic, therapeutically refractory tinnitus by means of chronic electrical stimulation of the vestibulocochlear nerve.

PATIENTS

Inclusion criteria were severe, chronic, therapeutically refractory, unilateral tinnitus and severe hearing loss at the ipsilateral site. Out of 6 patients, 4 patients were selected for long-term evaluation. Two patients were not evaluated because of premature dropout.

MATERIAL AND METHODS

A stimulation electrode was placed around the vestibulocochlear nerve through a retrosigmoid approach and connected to a subcutaneously positioned pulse generator via an extension cable. Follow-up was performed 3 months and 42.5 months after implantation. Three measures for treatment outcome were used. First, effect sizes were determined by means of the total Tinnitus Handicap Inventory (THI) score using Cohen's formula. Second, general and tinnitus-specific audiometric tests were performed in on and off conditions of the neurostimulation system. Third, recordings were noted for tinnitus severity and treatment success on a visual analogue scale.

RESULTS

All 4 patients reported successful treatment with neurostimulation. The effect size after 3 months was 0.7, indicating an average effect, while the effect size measured during long-term follow-up was 1.75, indicating a substantial effect with major clinical implications. No changes in hearing level for both ears were measured. The neurostimulation system did not change the tinnitus pitch in any of the patients, and resulted in a minimal reduction of tinnitus loudness in only 2 patients. In all 4 patients the original tinnitus sound was replaced by another, pleasantly perceived sound. The average VAS score of perceived tinnitus severity was reduced from 8 to 3.25. The average VAS score for treatment success was 7.25.

CONCLUSIONS

The long-term follow-up of neurostimulation treatment for chronic tinnitus shows promising results. Long-term results were better than those determined after a 3-month follow-up. In all patients the tinnitus was replaced by another sound, which was perceived as pleasant. Further studies are needed before accepting neurostimulation as a treatment modality for chronic, therapeutically refractory tinnitus.

Concepts of nerve regeneration and repair applied to brachial plexus reconstruction

Brachial plexus injury is a serious condition that usually affects young adults. Progress in brachial plexus repair is intimately related to peripheral nerve surgery, and depends on clinical and experimental studies. We review the rat brachial plexus as an experimental model, together with its behavioral evaluation. Techniques to repair nerves, such as neurolysis, nerve coaptation, nerve grafting, nerve transfer, fascicular transfer, direct muscle neurotization, and end-to-side neurorraphy, are discussed in light of the authors' experimental studies. Intradural repair of the brachial plexus by graft implants into the spinal cord and motor rootlet transfer offer new possibilities in brachial plexus reconstruction. The clinical experience of intradural repair is presented. Surgical planning in root rupture or avulsion is proposed. In total avulsion, the authors are in favor of the reconstruction of thoraco-brachial and abdomino-antebrachial grasping, and on the transfer of the brachialis muscle to the wrist extensors if it is reinnervated. Surgical treatment of painful conditions and new drugs are also discussed.

Consequences of suppressing thoughts about tinnitus and the effects of cognitive distraction on brain activity in tinnitus patients

Tinnitus is the perception of sound in the absence of any appropriate external stimulus. Based on the clinical observation that tinnitus patients may distract themselves from their sounds, we performed an experimental test on the effects of suppressing thoughts about tinnitus with 45 tinnitus patients, to systematically evaluate the immediate consequences of suppressing thought vs. attending to tinnitus. Suppression instructions tended to lead to a subsequent decrease in tinnitus-related thoughts, whereas attention to tinnitus resulted in an increase in such thoughts. No effects were seen in a control group who neither suppressed nor attended to their tinnitus. In an independent positron emission tomography study of cerebral blood flow with 8 patients we found that silent backward counting ('serial sevens test') led to a decrease in neural activity in auditory cortex, as well as perceived decrease of tinnitus loudness and annoyance. Thus, distraction that altered the tinnitus experience seemed to attenuate auditory cortex activity.

Neuronavigierte repetitive transkranielle Magnetstimulation (rTMS)

BACKGROUND AND OBJECTIVE

Idiopathic tinnitus is a frequent and debilitating disorder of largely unknown pathophysiology. Focal brain activation in the auditory cortex has recently been demonstrated in chronic tinnitus. Low-frequency rTMS can reduce cortical hyperexcitability.

PATIENTS AND METHODS

In 12 patients with chronic tinnitus, fusion of [18F]deoxyglucose-PET and structural MRI (T1, MPRAGE) scans allowed the area of increased metabolic activity in the auditory cortex to be exactly identified; this area was selected as the target for rTMS. A neuronavigational system adapted for TMS positioning enabled the relative positions of the figure-8 coil and the target area to be monitored. Repetitive TMS (110% motor threshold; 1 Hz; 2000 stimuli per day over 5 days) was performed using a placebo-controlled crossover design. A sham coil system was used for the placebo stimulation. Treatment outcome was assessed with a specific tinnitus questionnaire (Goebel and Hiller).

RESULTS

In all 12 patients an asymmetrically increased metabolic activation of the gyrus of Heschl was detected. The tinnitus score was significantly improved after 5 days of active rTMS, an effect not seen after placebo stimulation.

CONCLUSION

These preliminary results show that neuronavigated rTMS may improve our understanding and treatment of chronic tinnitus.

Specific plasticity responses to unilaterally decreased or increased hearing intensity in the adult cochlear nucleus and beyond

Variations of sensory activation in strength and pattern are known to affect structure and function of the mammalian brain. Whereas such malleability is readily granted to forebrain structures at early developmental stages, acceptance of experience-dependent structural plasticity has been slow for the adult brainstem. Over the past years we have identified consequences of cochlear ablation, noise trauma, or electrical intracochlear stimulation on neurons and circuitry of the auditory brainstem of the adult rat. We found that loss of sensory activation as well as a substitution for it entail specific molecular, ultrastructural, and morphological changes to central auditory neurons. Here, we make a first attempt to compare these different patterns of central remodeling. We tentatively suggest that after hearing loss or intracochlear stimulation responses of the central neural network in the adult brainstem suit the concept of functional adaptation.

Changes in glycine immunoreactivity in the rat superior olivary complex following deafness

The balance between inhibitory and excitatory amino acid neurotransmitters contributes to the control of normal functioning of the auditory brainstem. Changes in the level of neuronal activity within the auditory brainstem pathways influence the balance between inhibition and excitation. Activity-dependent plasticity in the auditory pathways can be studied by creating a large decrease in activity through peripheral deafening. Deafness-related decreases in GABA have previously been shown in the inferior colliculus. However, glycine is a more prevalent inhibitory transmitter in the mature superior olivary complex (SOC). The present study therefore examined if there were deafness-related changes in glycine in the SOC using postembedding immunocytochemistry. Animals were bilaterally deafened by an intrascalar injection of neomycin. Five nuclei in the SOC, the lateral superior olive (LSO), superior paraolivary nucleus (SPoN), and the medial, lateral, and ventral nuclei of the trapezoid body (MNTB, LNTB, and VNTB) were examined 14 days following the deafening and compared to normal hearing age-matched controls. The LSO and SPoN were divided into high and low frequency regions. The number of glycine immunoreactive puncta on the somata of principal cells showed significant decreases in all regions assessed, with changes ranging from 50% in the VNTB to 23% in the LSO.

The Prevalence and Onset of Gaze Modulation of Tinnitus and Increased Sensitivity to Noise After Translabyrinthine Vestibular Schwannoma Excision

OBJECTIVE

To determine the prevalence and timing of onset of gaze-modulated tinnitus and increased sensitivity to noise in patients who had undergone translabyrinthine excision of a vestibular schwannoma.

STUDY DESIGN

Retrospective questionnaire study.

SETTING

University hospital departments of audiology and neurotology.

PATIENTS

A cohort of 359 patients who had undergone translabyrinthine excision of a vestibular schwannoma in the period 1997 to 2003.

INTERVENTION

Translabyrinthine excision of a unilateral sporadic vestibular schwannoma.

MAIN OUTCOME MEASURES

Patient reports and visual analogue scale measures of the timing of onset of gaze-modulated tinnitus and the presence, timing of onset, and persistence of increased sensitivity to noise after surgery.

RESULTS

Completed questionnaires were returned by 275 patients (77%), of whom 132 (48%) were men and 143 (52%) were women. Preoperative tinnitus was reported in 150 patients (55%). In 124 of these 150 (83%) the tinnitus persisted, and in 26 of 150 (17%) it abated. Of the 125 patients without preoperative tinnitus, 43 (34%) developed it postoperatively. In 167 (61%) patients of the total group of 275, postoperative tinnitus was reported. Gaze-modulated tinnitus was reported in 53 patients (19%). Somatic-evoked or -modulated tinnitus was reported in 38 patients (14%). In response to the question, "Did you notice being extra sensitive to noise after your operation?," 138 patients (50%) replied that they did. In 111 patients, this persisted. The onset of the modulation of tinnitus was distributed throughout the postoperative period, whereas the onset of increased sensitivity to noise was overwhelmingly in the first 2 months after surgery.

CONCLUSION

Gaze modulation of tinnitus after vestibular schwannoma removal was identified in 19% of patients in this series. The onset data did not convincingly argue for any specific mechanism. The prevalence of increased sensitivity to noise is surprising and may represent central hyperacusis in response to unilateral deafferentation of the auditory system.

Tinnitus as a plastic phenomenon and its possible neural underpinnings in the dorsal cochlear nucleus

Tinnitus displays many features suggestive of plastic changes in the nervous system. These can be categorized based on the types of manipulations that induce them. We have categorized the various forms of plasticity that characterize tinnitus and searched for their neural underpinnings in the dorsal cochlear nucleus (DCN). This structure has been implicated as a possible site for the generation of tinnitus-producing signals owing to its tendency to become hyperactive following exposure to tinnitus inducing agents such as intense sound and cisplatin. In this paper, we review the many forms of plasticity that have been uncovered in anatomical, physiological and neurochemical studies of the DCN. Some of these plastic changes have been observed as consequences of peripheral injury or as fluctuations in the behavior and chemical activities of DCN neurons, while others can be induced by stimulation of auditory or even non-auditory structures. We show that many parallels can be drawn between the various forms of plasticity displayed by tinnitus and the various forms of neural plasticity which have been defined in the DCN. These parallels lend further support to the hypothesis that the DCN is an important site for the generation and modulation of tinnitus-producing signals.

Magnetic and electrical stimulation of the auditory cortex for intractable tinnitus

✓ Tinnitus is a distressing symptom that affects up to 15% of the population for whom no satisfactory treatment exists. The authors present a novel surgical approach for the treatment of intractable tinnitus, based on cortical stimulation of the auditory cortex.

Tinnitus can be considered an auditory phantom phenomenon similar to deafferentation pain, which is observed in the somatosensory system. Tinnitus is accompanied by a change in the tonotopic map of the auditory cortex. Furthermore, there is a highly positive association between the subjective intensity of the tinnitus and the amount of shift in tinnitus frequency in the auditory cortex, that is, the amount of cortical reorganization. This cortical reorganization can be demonstrated by functional magnetic resonance (fMR) imaging.

Transcranial magnetic stimulation (TMS) is a noninvasive method of activating or deactivating focal areas of the human brain. Linked to a navigation system that is guided by fMR images of the auditory system, TMS can suppress areas of cortical plasticity. If it is successful in suppressing a patient's tinnitus, this focal and temporary effect can be perpetualized by implanting a cortical electrode.

A neuronavigation-based auditory fMR imaging-guided TMS session was performed in a patient who suffered from tinnitus due to a cochlear nerve lesion. Complete suppression of the tinnitus was obtained. At a later time an extradural electrode was implanted with the guidance of auditory fMR imaging navigation. Postoperatively, the patient's tinnitus disappeared and remains absent 10 months later.

Focal extradural electrical stimulation of the primary auditory cortex at the area of cortical plasticity is capable of suppressing contralateral tinnitus completely. Transcranial magnetic stimulation may be an ideal method for noninvasive studies of surgical candidates in whom stimulating electrodes might be implanted for tinnitus suppression.

Pathophysiology of tinnitus

Tinnitus is not a single entity but a rather diverse group of disorders. Despite symptoms that indicate the ear is the site of the pathology, there is strong evidence that most forms of severe tinnitus are caused by functional changes in the central nervous system. The changes are induced through expression of neural plasticity, some of which may have been caused initially by abnormalities in the ear or the auditory nerve. The involvement of the nonclassical ascending auditory pathway with its subcortical connections to limbic structures (the amygdala) may explain some of the symptoms of some forms of tinnitus including hyperacusis and affective disorders, such as phonophobia and depression, which often accompany severe tinnitus.

Expanding the biological basis of tinnitus: crossmodal origins and the role of neuroplasticity

Tinnitus is most often initiated by modality specific otopathologic disturbances affecting peripheral and central auditory pathways. However, there is growing evidence indicating that the anatomical location generating tinnitus occurs at sites different from the initial pathology. Support for this notion is found in individuals where tinnitus can be triggered or modulated by inputs from other sensory modalities or sensorimotor systems (somatosensory, somatomotor, visual-motor). The use of functional imaging methods combined with psychophysics, detailed physical examinations and questionnaire-based assessments has reinforced and validated these observations. Available data suggest that tinnitus-related crossmodal interactions are more common than previously anticipated. This communication reviews these advancements and suggests that a relatively broad multimodal network of neurons is involved in generating and sustaining the tinnitus perception in some forms of the disorder. Also implicated as part of the tinnitus experience are interactions within large-scale neural networks subserving attention, cognition, and emotion. Incorporating this knowledge into contemporary psychophysiological models will help facilitate the conceptualization of this phantom perception in a more comprehensive manner.

The neural basis of Charles Bonnet hallucinations: a hypothesis

OBJECTIVES

To describe the hallucinations occurring as a result of a macular hole in each eye and to investigate the neural basis.

METHODS

Psychophysical observations including sketches of the hallucinations calibrated for size.

RESULTS

All the hallucinations were of the geometric (patterned) type and lasted for only a few days.

CONCLUSIONS

The observations can be explained on the basis of a "deafferentation" model, which is described in some detail. It is proposed that the hallucinations result from activation of the "blobs" of area V1 and the "stripes" of area V2 in the visual cortex. A theory is proposed to account for the disappearance of the hallucinations by a "filling in" mechanism.

Gaze-evoked tinnitus following acoustic neuroma resection: a de-afferentation plasticity phenomenon?

A review of the prevalence of gaze-evoked tinnitus in patients who have undergone acoustic neuroma resection was undertaken. The relationship of gaze-evoked tinnitus to tumour size, pre-existing tinnitus, side of the lesion or direction of eye gaze was also analysed. The design was a retrospective review of patients who underwent total unilateral translabyrinthine vestibular schwannoma resection by the senior author. Patients 1 year and 5 years following surgery were assessed. Results were obtained by either direct questioning in a clinical setting or via questionnaire. Data was collected on 36 subjects at an average of 14.6 months postoperatively (range 8-20 months) and on 32 subjects at an average 62 months postoperatively (range 56-68 months). Of the first group, 13 had evidence of gaze-evoked tinnitus (36%) compared with six (19%) of the second. The presence of preoperative tinnitus was a significant variable in univariate and multivariate analyses of the presence of postoperative tinnitus and postoperative gaze-evoked tinnitus. In a reduced model multivariate logistic regression using the two groups combined, the significant independent risk factors for gaze-evoked postoperative tinnitus were female sex (P = 0.046, odds ratio = 4.3), right side surgery (P = 0.02, odds ratio = 5.5) and pre-existing tinnitus (P = 0.008, odds ratio = 24.2). Gaze-evoked tinnitus is more common than previously reported in patients undergoing vestibular schwannoma resection. The underlying pathophysiology remains unclear but neural plasticity following unilateral de-afferentation remains a viable theory. The reduced prevalence over time suggests that this phenomenon may represent an active reinnervation process within the brain, which stabilizes with time. The significance of pre-existing tinnitus suggests that mechanisms of neural reorganization modify an existing physiological pathway. Further research is required into the longevity of this phenomenon, preferably with longitudinal studies.

Brain imaging of the effects of lidocaine on tinnitus

Using a single-blind placebo-controlled design, we mapped lidocaine related changes in neural activity, measured by regional cerebral blood flow (rCBF) with (15)O-H(2)O positron emission tomography. Intravenous lidocaine produced both increases and decreases in the loudness of tinnitus. The change in tinnitus loudness was associated with a statistically significant change in neural activity in the right temporal lobe in auditory association cortex. Decreases in tinnitus loudness resulted in larger changes in rCBF than increases. The unilateral activation pattern associated with tinnitus, in contrast with the bilateral activation produced by a real sound, suggests that tinnitus originates in the central auditory system rather than the cochlea. In addition, generalized lidocaine effects were found in the basal ganglia, thalamus, and a region spanning the Rolandic fissure.

The non-classical auditory pathways are involved in hearing in children but not in adults

Auditory information ascends through the brainstem to the cerebral cortices in two parallel pathways, known as the classical and the non-classical ascending auditory pathways. The importance of the non-classical auditory pathway for hearing in humans is unknown but its subcortical connection to limbic structures may be important in tinnitus. In this study we show evidence that non-classical pathways are involved in loudness perception in young individuals but not in adults. We used the fact that some neurons in the non-classical auditory pathways receive somatosensory input and we determined the effect on loudness perception of monaural sounds from electrical stimulation of the median nerve at the wrist. Stimulation of the somatosensory system had the greatest effect on loudness perception in the youngest children that we studied (7-8 years) and the effect was minimal for individuals above 20 years of age. The effect was an increase in loudness in 20 of the 40 individuals we studied and a decrease in 4 individuals; 16 experienced no noticeable change in loudness during somatosensory stimulation.