Altered intensity coding in the salicylate-overdose animal model of tinnitus.

G Model BIO 3582 No. of Pages 7

BioSystems xxx (2015) xxx–xxx

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Altered intensity coding in the salicylate-overdose animal model of tinnitus Ilynn Wana , Ondrej Pokorab , Tzaiwen Chiuc , Petr Lanskyd , Paul Waifung Poona,* a

Department of Physiology, Medical College, National Cheng Kung University, Tainan, Taiwan Department of Mathematics and Statistics, Masaryk University, Brno, Czech Republic Department of Biological Science and Technology, National Chiao Tung University, Hsinchu, Taiwan d Institute of Physiology, Academy of Sciences of the Czech Republic, Prague, Czech Republic b c

A R T I C L E I N F O

A B S T R A C T

Article history: Received 2 March 2015 Received in revised form 30 June 2015 Accepted 30 June 2015 Available online xxx

Tinnitus is one of the leading disorders of hearing with no effective cure as its pathophysiological mechanisms remain unclear. While the sensitivity to sound is well-known to be affected, exactly how intensity coding per se is altered remains unclear. To address this issue, we used a salicylate-overdose animal model of tinnitus to measure auditory cortical evoked potentials at various stimulus levels, and analyzed on single-trial basis the response strength and its variance for the computation of the lower bound of Fisher information. Based on Fisher information profiles, we compared the precision or efficiency of intensity coding before and after salicylate-treatment. We found that after salicylate treatment, intensity coding was unexpectedly improved, rather than impaired. Also, the improvement varied in a sound-dependent way. The observed changes are likely due to some central compensatory mechanisms that are activated during tinnitus to bring out the full capacity of intensity coding which is expressed only in part under normal conditions. ã 2015 Elsevier Ireland Ltd. All rights reserved.

Keywords: Auditory evoked potential Electrocorticogram Fisher information Salicylate-overdose Tinnitus Rat

1. Introduction Tinnitus is one of the leading disorders of hearing with a high prevalent rate (review see Roberts et al., 2010). In extreme cases, it can be disabling to normal life. Symptoms of this disorder, besides some degree of hearing loss, include: (a) the perception of phantom sound (i.e., in clinical terms, tinnitus); and/or (b) the overly-loud perception of sounds at moderate levels (i.e., in clinical terms, hyperacusis) (Baguley, 2003; Wang et al., 2008). May or may not be related to hyperacusis, a number of electrophysiological recordings from auditory cortex in awake rats showed enhanced responses during experimental tinnitus (e.g., with salicylate overdose or acoustic trauma) (Eggermont, 2013; Lu et al., 2011; Sun et al., 2009; Stolzberg et al., 2012a,b). No effective cure, but only behavioral management is available to treat tinnitus as the underlying pathophysiology still remains unclear (Cazals, 2000; Jastreboff, 2007; Møller, 2007). Research on experimental tinnitus in animals has led to theories of its pathogenesis. The prevailing one is that an initial hearing loss leads to the activation of a central gain control system in order to compensate for the impaired

sensory input (Qiu et al., 1999; Norena, 2011). The key issue is therefore the compensation for an altered intensity coding. One fundamental question in tinnitus regarding the phenomenon of intensity compensation is how the efficiency or precision of intensity coding is altered (i.e., better, no change, or worse). To address this issue, we studied a reversible model of tinnitus that involves salicylate-overdose in rats (Jastreboff et al., 1988; Bauer et al., 1999). Specifically we measured, in their awake state, singletrial auditory evoked responses from the cortex before and after tinnitus-induction. Based on single-trial response strength and its variance, we computed the lower bound of Fisher information at different stimulus levels for an estimation of the coding efficiency of sound intensity (Bethge et al., 2002; Kostal et al., 2013; Roy, 2004). Below is a brief account of Fisher information with relevance to this study. Fisher information is a measure of the amount of information that an observable random variable (in our case, the strength of evoked potential integral, EPI) carries about an unknown parameter (in our case, the stimulus sound pressure level, or SPL). The probability function for EPI, which is also the likelihood function for SPL, is a function f(EPI; SPL), which describes the probability that we would observe a sample of EPI given a known value of SPL. If f(EPI; SPL) is sharply peaked (or large in size) with respect to SPL, that means it contains a lot of

* Corresponding author. http://dx.doi.org/10.1016/j.biosystems.2015.06.010 0303-2647/ ã 2015 Elsevier Ireland Ltd. All rights reserved.

Please cite this article in press as: I. Wan, et al., Altered intensity coding in the salicylate-overdose animal model of tinnitus, BioSystems (2015), http://dx.doi.org/10.1016/j.biosystems.2015.06.010


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I. Wan et al. / BioSystems xxx (2015) xxx–xxx

information about SPL, since it is easy to predict the correct value of SPL given the EPI data. Otherwise, if the function is flat (or small in size), it would need more samples of EPI to estimate the correct SPL, and hence the EPI data has less information about SPL (please see Supplementary Text 1 for details).

amount to induce behavioral tinnitus in rats (Jastreboff and Sasaki, 1994; Wu et al., 2003). None of our rats that received SA injections showed signs of distress nor died prematurely.

2. Materials and methods

A week after surgery, animals were put in the sound-attenuated room (IAC, USA) to first adapt to the environment for 2 days (5 h/ day). Evoked response recordings were done in the next 8–14 days. During the first 3–5 days, either no drug treatment or only the injection of the vehicle was given (i.e., the pre-drug controls). In the subsequent 5 days SA was applied daily through a single intraperitoneal injection. To assess auditory sensitivities, sounds at various levels were used to evoke auditory responses from the cortex under passive listening conditions.

2.1. Animals Ten adult Sprague Dawley rats (6 weeks old, 250 g b.w.) were obtained from the Animal Center of National Cheng Kung University (NCKU). Experimental procedures have been approved by the Animal Ethics Committee, NCKU.

2.5. Experimental paradigm

2.2. Recording electrodes 2.6. Acoustic stimulation Bipolar recording electrodes custom-made of Teflon-coated silver wires (125 mm ID, 175 mm OD; A-M Systems, USA) were soldered at one end to miniature strip male-sockets (model 520200; A-M Systems, USA). The recording electrodes contain an active lead and two reference leads which were connected to the same contact point on the miniature socket. The active lead was stripped free of Teflon-coating at the cortical end for about 1 mm and curled before compressed into a circular pad of about 0.5 mm in diameter. One of the reference leads again bare at the free end was bent into a circular hook 1.5 mm in size embracing the active electrode pad. The other reference lead was made into a pad shape but slightly larger in size (for physical details please see Supplementary Fig. 1). This configuration of recording electrode, particularly with the dual references leads, was found to be more stable than the conventional single reference lead. Supplementry material related to this article found, in the online version, at http://dx.doi.org/10.1016/j.biosystems.2015.06.010. 2.3. Neurosurgery Animals first went through aseptic surgery to implant the recording electrodes over the auditory cortices. Rats were under general anesthesia (sodium pentobarbital, Sigma; 50 mg/kg, i.p.). Body temperature was maintained at 39 C using a regulated heating pad. The brain was surgically exposed by first resecting the overlying tissues and then drilling opened the skull with a high speed drill. Two holes (3 mm wide) were made with one immediately medial to the primary auditory cortex, midway between the bregma and lambda landmarks and another on the frontal skull. The recording electrodes were inserted through the skull opening to rest over the dural surface right on top of the primary auditory cortex. One of reference electrodes was placed around the active electrode, and another at the frontal skull opening according to the conventional evoked potential recordings (Shaw, 1993). Dental cement (GC Fuji Plus, Tokyo) was applied to cover the electrodes and to fix the miniature connector socket on the dorsal skull. To facilitate fixation, absolute alcohol was applied to dehydrate the skull surface before cement application. The skin was finally sutured (fine silk, 6/0) to close the wound. To promote healing, antibiotic cream (Neomycin, Wyeth) was applied topically over the wound and animals checked daily during recovery in the post-operative week.

To deliver far field acoustic stimuli, an electrically-shielded speaker (Pioneer SE-77) was positioned at the ceiling of the soundtreated room over-looking the animal container from a distance of 190 cm. Two kinds of acoustic stimuli were used: (a) short tone bursts of different frequencies (1, 4, 10 and 16 kHz, 10 ms long, 2.5 ms linear rise /fall time); (b) transient sounds (single click, 0.01 ms rectangular pulse; or in some cases, a 10 ms long clicktrain, 2 ms inter-click intervals). Acoustic stimuli were presented in random order across sessions. Within each session the same sound was repeated at a fixed intensity level at randomized intervals (0.3–0.7 s). Across sessions, sounds were presented at randomized levels from 20 to 60 dB SPL at 5 dB steps. The exact stimulus level was calibrated with a microphone (B&K 4149) placed at the site of the animal. 2.7. Electrophysiological recording The post-operative rat was put into a top-opened Perspex container (40 cm wide) inside the sound-attenuated room (IAC, USA). The rat was allowed to move freely while the miniature socket on its head was connected through flexible low-noise cables (New England Electric Wire Corp.) to a pre-amplifier (WPI, Duo 773). Electrocorticogram signals were amplified (10,000; PARC 5113) and band-pass filtered (0.1–3 kHz) before fed to an interface (TDT system II), captured on-line and stored in a computer for offline analysis. Each response trial was collected at 100 kHz (16 bit) within a 100 msec peri-stimulus time window (25 ms pre- and 75 ms post-stimulus onset). An off-line sub-sampling of the raw evoked signal was down to 2 kHz to expedite data analysis. 2.8. Confirmation of recording locations At the end of recording session, animal was sacrificed with a pentobarbital overdose (100 mg/kg, i.p.) and head removed and put in fixative (4% formaldehyde). A week later the skull was reopened to examine the placement of the recording electrodes. Electrodes were typically placed over the core auditory area according to the standard rat stereotaxic atlas (Paxinos and Watson, 1998), though it also covered to various degrees the nearby presumably auditory belt areas (Barth et al., 1993). 2.9. Data analyses

2.4. Salicylate preparation and dosage Sodium salicylate (SA; Sigma, USA) was dissolved in a buffer solution (PBS; pH 7.4, 0.5 gm/10 ml). A moderate dosage (250 mg/ kg) was chosen to be sufficiently effective on the one hand but not lethal on the other. This dose represents about twice the minimal

1. Data preprocessing: because of recording from awake animals, noisy interference from muscles (EMG) or movement artifacts could contaminate the evoked responses (as they are much larger in amplitude). Contaminated trials, typically with oversized values, needed to be excluded. First, for each animal,




we calculated the power of evoked responses on single trial basis in terms of its root-mean-square (RMS) value. RMS histogram was generated to determine the outliers. The estimated upper bound (typically 2 SD) was then taken as the rejection criterion for excluding oversized trials. 2. Construction of the response-level function: after data preprocessing, a post-stimulus onset time window of 28 ms long (corresponded to 8–35 ms after subtracting air transmission time of 6 ms from speaker to animal) was used to compute the mean evoked response from the cortex (Shaw, 1993, 1995). We then characterized the strength of evoked potential in terms of its integral or EPI (Devanne et al., 2002). This procedure involved computing the mean pre-stimulus baseline EEG level, before subtracting this level from the mean EP waveform. Full-wave rectification of the averaged evoked potential signal followed by summation within a given time window gave the EPI (in mVms). A plot of the stimulus level against EP integral (EPI) gave the response-level function which was further fitted with a sigmoid curve (Prism 3.0, GraphPad or with a custom-made MATLAB program). Figs. 1–3 (left panels) show examples of the responses in the control and salicylate-treated conditions. 3. Single-trial evoked responses extraction: for some animals, likely due to a good-placement of the electrode in proximity with the cortical tissues, the evoked responses were of sufficiently large signal-to-noise ratios (discernible even among the ongoing EEG background in the raw signal). For these animals, we applied an adaptive filter (Lam et al., 1994; Qiu et al., 2002) to further extract the EP from the noise background. With single-trial EPIs extracted similar to the way described above, the inter-trial variance of EPI was calculated for the session (which contained 90 trials), followed by Fisher information analysis as described below. 4. Lower bound of Fisher information:

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Fig. 2. Averaged response-level function of auditory evoked potential obtained from 10 rats before (Control) and after Salicylate (SA) treatment (showing mean and SD). (A, B) Large open circles mark roughly the threshold in the response-level functions; (C) Boltzmann sigmoid fitted curves in panels (A and B) overlaid to show the lowering of response threshold after SA treatment; additional open circles on the right mark the response maximum of the response functions. In (C), the control response function (dark tracing) is artificially multiplied (blue dashed tracing) to match response maximum with the SA function (red tracing). Note the SAenhancement (disparity between the red and blue dashed tracings) cannot be accounted for by a simple gain increment in the control function.

Fig. 1. Superimposed EP time waveforms in one animal in response to an intensity series of 16 kHz tone bursts (stimulus marked by the blue trapezoid signal at the top). Each intensity session was randomly chosen between 20 and 55 dB SPL, at 5 dB intervals. Each response tracing represents the average of 90 trials, with red and green marking EPs at the extreme stimulus levels. Good signal-to-noise ratio of EP is reflected by the relatively flat pre-stimulus response time waveform. Note after salicylate (SA) treatment (right panel) the clear increase in response amplitude with minimal change in the shape of waveform. For the comparison with responses to tones of other frequencies, please see Supplementary Fig. 2.



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4. Results 2 1 dEðYÞ J 2 ðxÞ ¼ VarðYÞ dx where x is stimulus level, Var(Y) is the inter-trial variance function of EPI, E(Y) is the expected probability function of EPI (for more mathematical details, please refer to Supplementary text 1). The J2 profiles so derived are shown in Supplementary Fig. 2 (right panels). Statistical analyses: all statistics were done with a commercially available package (Prism 3.0, GraphPad). Nonlinear least square fits were done with a statistical package written in free software R. The response-level functions of the 10 animals were expressed as mean SD. Statistical analysis between the salicylate-treated group and the control was performed using repeated measures ANOVA (with variables of SA and sound level), with statistical significance set at p < 0.01. For animals with datasets of good signal-to-noise ratios, a non-parametric statistics (Wilcoxon signed-rank test) was applied with statistical significance set at p < 0.05. Supplementry material related to this article found, in the online version, at http://dx.doi.org/10.1016/j.biosystems.2015.06.010.

Depending on animals, the auditory evoked responses could be of very good signal to noise ratio as shown in Fig. 1. Here the average EP showed clear incremental changes from the control (pre-drug) to the salicylate-treated (post-drug) conditions, with minimal changes in the shape of the response waveform. From the total of 10 rats studied, 3 of them reached the criterion of good signal-to-noise ratio, and showed significant correspondence with the four-parameter logistic regression function (see Supplementary Text 1). Their results are presented below on individual basis. First, because averaged evoked responses, in contrast to singletrial responses, can tolerate a lower signal-to-noise ratio, the results from all 10 rats (regardless of signal-to-noise ratio) are presented as averaged results across animals. This is just to show the change in response-level function for the population. The averaged EPI showed a clear enhancement in the strength of response after salicylate treatment, compared with the pre-drug control (Fig. 2). EPI responses were stronger across all sound intensities tested, reflecting in the response-level function as a higher response saturation level, and with a steeper slope. The interactions between the two variables (drug and sound intensity) are significant according to two-way ANOVA. The effects of drug and intensity on EPI are therefore assessed separately. The drug

Fig. 3. Two sessions of auditory evoked response data to 10 kHz tone bursts (top and bottom rows) obtained in one animal showing the salicylate (SA) effect. Each session has 90 single-trial EPs extracted with the adaptive filter. Left panels: each ‘+’ symbol represents a recorded value of the EPI at the corresponding stimulus sound level. Solid line represents the mean EPI fitted by four-parameter logistic function (accompanied by two flanking dashed lines showing the SD). Right panels: same data as in the left panels, showing the comparison of mean EPI (solid line), its SD (dashed line) and the bell-shaped lower bound Fisher information J2 profile. To facilitate comparison, all EPI values are displayed with the same y-scale (left-hand side) while J2 has its own y-scale (right hand side).




effect on EPI is significant (Mann–Whitney test, p < 0.0001) and so is the intensity effect (Friedman test with post-hoc Dunn’s multiple comparison test). The shape of response-level functions (i.e., monotonic curve), in either the pre- or post-drug conditions is also similar to what has been reported in the literature (Bancroft et al., 1991; Boettcher et al., 1989; Gerken et al., 2001). Findings are in general consistent with increased EP responses on salicylatetreated awake rats as reported previously (Yang et al., 2007). This population picture has nevertheless no implication on coding efficiency which depends on the results of single-trial responses. Second, single-trial responses were successfully extracted from 3 animals (rat-3, 9 and 10) for Fisher information analysis. Recorded samples from the 3 rats consisted of 140 datasets; each appeared similar to those shown in Fig. 3 (left panels). The mean EPI response function in each dataset was analyzed as a fourparameter logistic regression function (details please see Supplementary Text 1) and the r2 (or adjusted r2) criterion was checked. Only datasets with r2 > 0.50 were used for testing the null

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hypothesis that salicylate treatment does not affect the parameters of the mean response function, and for the analysis of the J2. As a result, only a total of 45 control and 60 salicylate-treated sessions were analyzed subsequently. Each session consisted of repeated measurements (n = 80–100 trials) at fixed intensity levels with randomized inter-stimulus intervals (0.3–0.7 s). Across different sessions, sounds were presented at a fixed level (chosen randomly between 20 and 60 dB SPL, at 5 dB intervals). The four-parameter logistic regression function was applied to the mean response function for interpreting its parameters (see Fig. 3 right panels). The four-parameter logistic function fits used for datasets chosen for the present analysis had r2 values falling between 0.50 and 0.98, with a median of 0.89. Surely, the response function could be fitted by other functions (e.g., spline) with better goodness of fit. That alternative approach was not used due to the difficulty in analytically tracking the dependence with the stimulus. Here, we rather followed the empirical observation of the approximate logistic dependence of the response on the stimulus, which is more

Fig. 4. Results from one animal showing the response-level functions (top row) and the Fisher information (FI) lower bound (J2) profiles (bottom row). The stimulus type (tone bursts of different frequencies in Hz, or clicks) is marked at the top of each panel. Control condition: (blue dashed tracings); salicylate-treated (red tracings). In both panels, data from different daily sessions are marked by corresponding numerals against the tracings. For curve fitting, please see Fig. 3 and Supplementary Text 1.



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robust than other advanced statistical approach that would exceed the scope of this study. In general, we found that our data could be better analyzed by the present method of functional analysis. Detailed results from another animal (rat-3) are shown here (Fig. 4, upper panels showing the EPI level functions and lower panels the J2 profiles) and similar results from the other two (rat- 9, and 10) are presented in the Supplementary Figs. 3 and 4. We noted the large response variations across sessions, even from the same animal, which made the averaging across sessions hard to justify. Furthermore, across sessions, the functional response of the fourparameter logistic functions does not necessarily have the same parametric form. Despite these large response variations across sessions (as expected from awake preparations), some parameters of the response functions on different daily sessions showed salicylate effects consistent with the averaged EPI profile of the population. In many cases, the Fisher information (J2) profiles revealed a general trend of increase, in the form of a rise in peak J2 value and/or a broadening of the dynamic range (responsive intensity range). Among the various types of stimuli we used (1, 4, 10, 16 kHz tones and clicks), statistically significant changes (p < 0.05) were found for high frequency tones (e.g., 16 kHz) and to a lesser degree for click (p < 0.05 or p = 0.09) (see Table 1 for more details). In most cases, the changes in J2 profile after salicylate treatment are consistent with an increase in the efficiency or precision of intensity coding. Supplementry material related to this article found, in the online version, at http://dx.doi.org/10.1016/j.biosystems.2015.06.010. Supplementry material related to this article found, in the online version, at http://dx.doi.org/10.1016/j.biosystems.2015.06.010. 5. Conclusions and discussion The principal finding of this study is that during the experimental tinnitus, there is an increase in the efficiency or precision of intensity coding. Furthermore, the increase in J2 is not the same for different sounds (apart from daily variations with identical stimuli, as expected from such behavioral paradigms of passive listening). First, for tones of different frequencies, high frequency tones (16 kHz in particular) showed significant changes of the responselevel function (including an elevated saturation level, a larger intensity range, and a steeper slope) compared with tones of lower frequencies. This frequency-biased effect is consistent with the

Table 1 Statistical comparison of parameters of J2 profile between the Control and Salicylate-treated groups (n = 3 rats) showing p values—Wilcoxon signed-rank test. The column headings denote parameters which correspond to the fitted fourparameter logistic function f(x) (see Supplementary Text 1 for details). Specifically, minimum is the lowest fitted value of the EP integral; it is the parameter a of the response function f(x); range is the parameter b; maximum is the sum (a + b) and it models the saturation value of EP integral; midpoint is parameter d and it is the sound pressure level at which the EP integral is equal to the mean of the minimum and maximum; slope is the first derivative of the response function with respect to the sound pressure level evaluated at the midpoint, mathematically it is expressed as bc/4. In simple words, consider a simple representation of the response function by 3 line segments: a horizontal line at the minimum, a tangent to f(x) at the midpoint, and a horizontal line at the maximum. Threshold is the sound pressure level at which the minimum horizontal line joins the (slanted) tangent, i.e., the intersection of the first two segments. Mathematically the threshold is given by formula (d (2/c)). Sound

Minimum

Maximum

Range

Slope

Midpoint

Threshold

1 kHz 4 kHz 10 kHz 16 kHz Click

0.100 0.562 0.159 0.423 0.074

0.100 0.428 0.133 0.005 0.047

0.100 0.492 0.732 0.026 0.090

0.100 0.147 0.026 0.001 0.037

0.999 0.118 0.100 0.189 0.110

0.100 0.875 0.324 0.110 0.074

known high-pitch perception of tinnitus in the salicylate-overdose animal model (Jastreboff and Sasaki, 1994). The effect on the response-level function to click stimulation is almost as strong as the 16 kHz tone. In the generation of evoked responses, clicks are known to be more powerful than tone bursts. This is due to that fact that measurable evoked responses depend on the synchrony of activities across neural elements (i.e., the more synchronized is the evoked activity, the bigger the measured EPI). Increased neural synchrony has long been indicated in studies of experimental tinnitus based on spike activities. Increased neural synchrony, as speculated earlier, reflects an effect of activating the central gain control mechanisms. The same effect could also account for the apparent lowering of the response threshold seen in the population picture. Hearing loss, in terms of a suppressed brainstem evoked response to sound, has been reported in the same animal model of tinnitus (Yang at al., 2007). The cortical responding elements apparently are capable of compensating for the loss of hearing up to a certain level (e.g., 10–30 dB) through, among other mechanisms, the possible activation of the auditory efferent system (Groff and Liberman, 2003). In behavioral studies of this kind, animals were allowed to move freely in the observation chamber during the passive-listening experiment. Uncontrolled factors that might influence the results include the orientation of the ears of the animal to the speaker (hence affecting the sound level reaching the ear drums), apart from the attention level. Salicylate overdose is known to cause abdominal discomfort in some animals (Puel and Guitton, 2007). This could easily affect vigilance; hence the sensitivity to sound stimuli can also be affected. Another finding of this study that stands out from the literature is the auditory responses (specifically EPI) were detected at very low sound intensities (down to 20 dB SPL). In previous studies on auditory cortical response in tinnitus models, the response metric used was typically the peak amplitude (Lu et al., 2011; Sun et al., 2009; Stolzberg et al., 2012a). This measurement requires not only a good signal to noise ratio in the evoked waveform, but also clear peaks and valleys. Datasets in our awake preparation, the average evoked potentials obtained at low stimulus intensities often showed vaguely identifiable peaks and valleys. This situation became worse with single-trial responses. The alternative response metric we used is EP integral (EPI) which does not assume any peak or valley in the waveform. Such difference in the analysis methods, among other differences in experimentation, could lead to our finding of lower thresholds. Consistent with this, single unit responses recorded at the inferior colliculus under the anesthetized condition (from the same laboratory with identical stimulus calibration and delivery system) revealed pure tone thresholds down to 20 dB SPL (Poon and Chen, 1992). With different stimuli, thresholds would also be different. For example in this study, clicks showed a response threshold lower than that of tone bursts. A single-unit study on ferret auditory cortex also showed that the contrast gain can be altered in a stimulus-dependent manner in changing intensity coding (Rabinowitz et al., 2011). It is therefore not too surprising that in our awake preparation (when vigilance is considered) the EPI response threshold could be lowered after salicylate-treatment, when compared with behavioral thresholds obtained in normal rats (Kelly and Masterton, 1977). Moreover, the cortical response may not necessarily correspond to the behavioral sensitivity (review see Eggermont, 2013). We need to point out that due to the technical difficulty in getting good signal-to-noise ratio in single trial EPs from all 10 rats we had studied, the conclusion drawn here was based on a limited sample size (n = 3 rats). Therefore our conclusion must be viewed with caution and would certainly need more experiments for its consolidation. Finally, our finding that intensity coding turning more efficient during tinnitus could provide new perspective of clinically




managing this undesirable phantom sensation by physiological manipulations that desensitize response to loud sounds. For instance, measures such as continuous exposure to some noise, like those used in tinnitus retraining therapy, may be employed (Jastreboff, 2007). Acknowledgements Supported by grants from NCKU,Ministry of Science and Technology, Taiwan, grants 99-2320-B-006-020,100-2923-006001, 101-2911-I-006-511; as well as by grants from Czech Science Foundation, P303/11/J005, P304/12/1342, 15-08066S. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biosystems.2015.06.010. References Baguley, D.M., 2003. Hyperacusis. J. R. Soc. Med. 96, 582–585. Bancroft, B.R., Boettcher, F.A., Salvi, R.J., Wu, J., 1991. Effects of noise and salicylate on auditory evoked-response thresholds in the chinchilla. Hear Res. 54, 20–28. Barth, D.S., Kithas, J., Di, S., 1993. Anatomic organization of evoked potentials in rat parietotemporal cortex: somatosensory and auditory responses. J. Neurophysiol. 69, 1837–1849. Bauer, C.A., Brozoski, T.J., Rojas, R., Boley, J., Wyder, M., 1999. Behavioral model of chronic tinnitus in rats. Otolaryngol. Head Neck Surg. 121, 457–462. Bethge, M., Rotermund, D., Pawelzik, K., 2002. Optimal short-term population coding: when Fisher information fails. Neural Comput. 14, 2317–2351. Boettcher, F.A., Bancroft, B.R., Salvi, R.J., Henderson, D., 1989. Effects of sodium salicylate on evoked-response measures of hearing. Hear Res. 42, 129–141. Cazals, Y., 2000. Auditory sensori-neural alterations induced by salicylate. Prog. Neurobiol. 62, 583–631. Devanne, H., Cohen, L.G., Kouchtir-Devanne, N., Capaday, C., 2002. Integrated motor cortical control of task-related muscles during pointing in humans. J. Neurophysiol. 87, 3006–3017. Eggermont, Hearing loss, J.J., 2013. hyperacusis, or tinnitus: what is modeled in animal research? Hear Res. 295, 140–149. Gerken, G.M., Hesse, P.S., Wiorkowski, J.J., 2001. Auditory evoked responses in control subjects and in patients with problem tinnitus. Hear Res. 157, 52–64. Groff, J.A., Liberman, M.C., 2003. Modulation of cochlear afferent response by the lateral olivocochlear system: activation via electrical stimulation of the inferior colliculus. J. Neurophysiol. 90, 3178–3200. Jastreboff, P.J., 2007. Tinnitus retraining therapy. Prog. Brain Res. 166, 415–423. Jastreboff, P.J., Sasaki, C.T., 1994. An animal model of tinnitus: a decade of development. Am. J. Otol. 15, 19–27.

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