Auditory-somatosensory bimodal stimulation desynchronizes brain circuitry to reduce tinnitus in guinea pigs and humans

Abstract

The dorsal cochlear nucleus is the first site of multisensory convergence in mammalian auditory pathways. Principal output neurons, the fusiform cells, integrate auditory nerve inputs from the cochlea with somatosensory inputs from the head and neck. In previous work, we developed a guinea pig model of tinnitus induced by noise exposure and showed that the fusiform cells in these animals exhibited increased spontaneous activity and cross-unit synchrony, which are physiological correlates of tinnitus. We delivered repeated bimodal auditory-somatosensory stimulation to the dorsal cochlear nucleus of guinea pigs with tinnitus, choosing a stimulus interval known to induce long-term depression (LTD). Twenty minutes per day of LTD-inducing bimodal (but not unimodal) stimulation reduced physiological and behavioral evidence of tinnitus in the guinea pigs after 25 days. Next, we applied the same bimodal treatment to 20 human subjects with tinnitus using a double-blinded, sham-controlled, crossover study. Twenty-eight days of LTD-inducing bimodal stimulation reduced tinnitus loudness and intrusiveness. Unimodal auditory stimulation did not deliver either benefit. Bimodal auditory-somatosensory stimulation that induces LTD in the dorsal cochlear nucleus may hold promise for suppressing chronic tinnitus, which reduces quality of life for millions of tinnitus sufferers worldwide.

Fig. 1. STDP regulates synchrony in fusiform cells of the guinea pig DCN

(A) Spontaneous activity (SA) was recorded across the fusiform-cell (FC) population in ?? guinea pigs for 150s, followed by 60s (5 Hz) of bimodal stimulation (BIS) with bimodal intervals (BI) from −20 to + 20ms. Spontaneous activity was recorded again 15 minutes after BIS for 150 sec. (B) Synchrony was assessed by cross-correlations (x-corr) of spikes in FC pairs (FC1, FC2). Spontaneous activity (SA) of FCs shows Poisson-distributions in interspike interval histograms (ISIH). Synchronous unit-pairs are defined by threshold cross-correlation coefficients (x-corr coef) of 4 SD (dashed line). (C) In one representative FC unit-pair, BI = −10 ms (auditory preceding somatosensory stimulus by 10 ms) reduced the peak x-corr coef (top panel), whereas BI = 10 ms (somatosensory preceding auditory stimulus by 10 ms) increased the peak x-corr coef 15 minutes after BIS (bottom panel). (D) Changes in peak x-corr coef for the FC unit pair in panel C are plotted as a function of BI (learning rule). (E) In a different FC unit pair, BI = 10 increased peak x-corr coef (top panel), whereas BI = 10 ms decreased peak x-corr coef 15 minutes after BIS (bottom panel). (F) For the FC unit pair in panel E changes in x-corr coef after BIS were opposite to that for the FC unit pair in panel D.

Fig. 2. STDP shifts towards LTP in guinea pigs with tinnitus

(A) Increased mean cross-correlation coefficient (x-corr; weighted by the proportion of synchronous unit-pairs) and (B) increased mean spontaneous activity (SA) compared to the normal-hearing (N) and exposed-but-no-tinnitus (ENT) groups of animals. * P< 0.05; data shown are mean ± SEM. Spontaneous activity for the N, ENT, and ET groups was 116, 93, 167 unit-pairs for x-corr, and 106, 387, 478 units, respectively. (C, D) A shift in the proportion of learning rules towards Hebbian-like (Heb; x-axis) and LTP (y-axis) in the ET group for (C) synchrony and (D) spontaneous activity (SA). (E, F) LTD-LTP index (total magnitude of LTP i.e. green area under curve relative to total magnitude of LTD i.e. blue area above curve of learning rules (E inset) is increased in the ET group for (E) synchrony and (F) spontaneous activity (SA).

Fig. 3. Targeted bimodal stimulation suppresses synchrony and spontaneous activity in fusiform cells of guinea pigs

(A) Probability of synchrony (x-corr) or spontaneous activity (SA) suppression as a function of bimodal interval. Probability is computed by proportion of unit-pairs (total n = 159) or units (n = 251) showing decreased x-corr or SA at a given bimodal interval. A probability of 0.5 indicates an equal number of units showing increased or decreased x-corr or SA. The highest probability of suppression occurs for the −10 and −5 ms intervals (error bar = confidence interval for binomial proportion). The −5ms interval was chosen for the treatment. (B) The distributions of suppression vs enhancement of synchrony are compared for the −5 ms bimodal interval, unimodal somatosensory (Uni Som), or unimodal auditory stimulation (Uni Aud). The bimodal stimulus clearly suppressed synchrony whereas unimodal stimuli showed little deviation from zero. (C) Similar to synchrony, the bimodal stimulus suppressed SA, whereas the unimodal stimulus showed little deviation from zero (Bar = 2% bin; shaded curve is fitted by Spline Interpolant).

Fig. 4. LTD-induction reduces synchrony and spontaneous activity and reduces tinnitus in guinea pigs

(A) Four representative animals (one from each group) show increased normalized startles after noise-exposure (pre- to post-exposure) indicating tinnitus (left ordinate), quantified as the TI (right ordinate). After LTD-induction by application of a bimodal auditory-somatosensory stimulus to the fusiform cells (ET-treat), there was a reduction in TI in the treated animal (ET-treat). Sham-treated (ET-sham; sedative only), auditory stimulus only (ET-audio), and somatosensory stimulus only (ET-som) animals either showed no reduction in TI or worsened TI. (B) Mean TI was significantly reduced in the ET-treat group at the treated frequency (On-Freq; 8 kHz) but not untreated frequencies (Off-Freq; 12 and 16 kHz). TI was not significantly reduced in the ET-sham, ET-audio, or ET-som groups. Pink horizontal bar indicates the 95% confidence interval (CI) for the ET-sham group. (C) The weighted mean cross-correlation coefficient (x-corr) for fusiform cells (at best frequencies within the TI bandwidth) is plotted as a function of ΔTI (116, 36, 35, 106 unit-pairs for ET-sham, ET-audio, ET-som, and ET-treat groups, respectively). Grey area indicates the range of x-corr for non-exposed animals. Reduction in synchrony significantly correlated with TI reduction. (D) Spontaneous activity (SA) plotted as a function of ΔTI (446, 204, 202, 696 units). Reduction in SA significantly correlated with TI reduction. Data shown are mean ± SEM.

Fig. 5

Outline of cross-over design for the human study.

Fig. 6. Bimodal treatment results in reduced tinnitus loudness and reduced TFI scores in human patients

(A) Mean loudness by group. Group 1 (n=10) received the active treatment first; group 2 (n=10) received the sham treatment first. (B) Mean changes (relative to baseline) in loudness matching for each condition. (C) Mean TFI changes (relative to baseline) for groups 1 and 2. (D) Mean changes (relative to baseline) in TFI scores. Error bars are SEM.