Off-responses in the auditory system in relation to the signal end phase and neuronal characteristic frequency

Development and validation of a computational method to predict unintended auditory brainstem response during transcranial ultrasound neuromodulation in mice

BACKGROUND

Transcranial ultrasound stimulation (TUS) is a promising noninvasive neuromodulation modality. The inadvertent and unpredictable activation of the auditory system in response to TUS obfuscates the interpretation of non-auditory neuromodulatory responses.

OBJECTIVE

The objective was to develop and validate a computational metric to quantify the susceptibility to unintended auditory brainstem response (ABR) in mice premised on time frequency analyses of TUS signals and auditory sensitivity.

METHODS

Ultrasound pulses with varying amplitudes, pulse repetition frequencies (PRFs), envelope smoothing profiles, and sinusoidal modulation frequencies were selected. Each pulse's time-varying frequency spectrum was differentiated across time, weighted by the mouse hearing sensitivity, then summed across frequencies. The resulting time-varying function, computationally predicting the ABR, was validated against experimental ABR in mice during TUS with the corresponding pulse.

RESULTS

There was a significant correlation between experimental ABRs and the computational predictions for 19 TUS signals (R2 = 0.97).

CONCLUSIONS

To reduce ABR in mice during in vivo TUS studies, 1) reduce the amplitude of a rectangular continuous wave envelope, 2) increase the rise/fall times of a smoothed continuous wave envelope, and/or 3) change the PRF and/or duty cycle of a rectangular or sinusoidal pulsed wave to reduce the gap between pulses and increase the rise/fall time of the overall envelope. This metric can aid researchers performing in vivo mouse studies in selecting TUS signal parameters that minimize unintended ABR. The methods for developing this metric can be adapted to other animal models.

Offset auditory brainstem response (ABR) amplitude in bottlenose dolphins

Although commonly recorded as onset responses, the auditory brainstem response (ABR) can also be elicited at stimulus offset. The offset ABR has not been extensively investigated in marine mammals. Three normal hearing (NH) and three hearing impaired (HI) dolphins were assessed while fully submerged in sea water. Stimulus spectrum, level, rise/fall time (RFT), and plateau duration were manipulated. Onset and offset ABR amplitude were quantified as the rms voltage 1-7 ms following stimulus onset or offset, respectively. For the same stimulus conditions, onset and offset responses were often larger for NH than HI dolphins, and offset responses were typically smaller than onset responses. For the level series, offset response amplitude typically increased with increasing stimulus level, although offset responses were not 3 dB above the noisefloor for 113-kHz tonebursts. Increasing RFT decreased onset and offset response amplitude. For the 40-kHz tonebursts, a RFT of 32 μs produced a large amplitude offset ABR in NH dolphins. Offset responses for 113-kHz tonebursts were 3 dB above the noisefloor at the shortest RFTs. Offset responses were largest for 4 ms duration stimuli (likely due to overlapping onset and offset response analysis windows), but otherwise, offset responses changed little with increasing duration.

Spectral summation and facilitation in on- and off-responses for optimized representation of communication calls in mouse inferior colliculus

Selectivity for processing of species-specific vocalizations and communication sounds has often been associated with the auditory cortex. The midbrain inferior colliculus, however, is the first center in the auditory pathways of mammals integrating acoustic information processed in separate nuclei and channels in the brainstem and, therefore, could significantly contribute to enhance the perception of species' communication sounds. Here, we used natural wriggling calls of mouse pups, which communicate need for maternal care to adult females, and further 15 synthesized sounds to test the hypothesis that neurons in the central nucleus of the inferior colliculus of adult females optimize their response rates for reproduction of the three main harmonics (formants) of wriggling calls. The results confirmed the hypothesis showing that average response rates, as recorded extracellularly from single units, were highest and spectral facilitation most effective for both onset and offset responses to the call and call models with three resolved frequencies according to critical bands in perception. In addition, the general on- and/or off-response enhancement in almost half the investigated 122 neurons favors not only perception of single calls but also of vocalization rhythm. In summary, our study provides strong evidence that critical-band resolved frequency components within a communication sound increase the probability of its perception by boosting the signal-to-noise ratio of neural response rates within the inferior colliculus for at least 20% (our criterion for facilitation). These mechanisms, including enhancement of rhythm coding, are generally favorable to processing of other animal and human vocalizations, including formants of speech sounds.

Distinct neural firing mechanisms to tonal stimuli offset in the inferior colliculus of mice in vivo

Offset neurons, which fire at the termination of sound, likely encode sound duration and serve to process temporal information. Offset neurons are found in most ascending auditory nuclei; however, the neural mechanisms that evoke offset responses are not well understood. In this study, we examined offset neural responses to tonal stimuli in the inferior colliculus (IC) in vivo with extracellular and intracellular recording techniques in mice. Based on peristimulus time histogram (PSTH) patterns, we classified extracellular offset responses into four types: Offset, Onset-Offset, Onset-Sustained-Offset and Inhibition-Offset types. Moreover, using in vivo whole-cell recording techniques, we found that offset responses were generated in most cells through the excitatory and inhibitory synaptic inputs. However, in a small number of cells, the offset responses were generated as a rebound to hyperpolarization during tonal stimulation. Many offset neurons fired robustly at a preferred duration of tonal stimulus, which corresponded with the timing of rich excitatory synaptic inputs. We concluded that most IC offset neurons encode the termination of the tone stimulus by responding to inherited ascending synaptic information, which is tuned to sound duration. The remainder generates offset spikes de novo through a post-inhibitory rebound mechanism.

OFF responses in the auditory thalamus of the guinea pig

ON and OFF auditory responses were examined in the medial geniculate body (MGB) of the guinea pig. Single- and multiunit recordings were carried out on 12 anesthetized animals, and noise-burst or pure-tone stimuli were applied to the ear contralateral to the recording hemisphere. One hundred and thirty-five OFF or ON-OFF neurons and 160 ON neurons were studied, and the tuning curves of 21 ON-OFF or OFF neurons were examined from various nuclei of the MGB. The mean minimum threshold of the OFF responses (40.8 +/- 20.0 dB SPL, mean +/- SD; range: 0-80 dB SPL) was significantly higher than that of the ON responses (28.5 +/- 17.6 dB SPL, range: 0-60 dB SPL; n = 17, P < 0.001). Of 10 ON-OFF neurons that showed identifiable tuning frequencies for both ON and OFF responses, 7 showed a higher OFF than ON best frequency (BF), 2 showed the same BF for both ON and OFF, and only 1 showed a slightly lower OFF than ON BF. Most OFF responses sampled from the borders of the ventral (MGv) and the rostromedial (MGrm) nuclei of the MGB showed single-peaked tuning curves, similar to those of the ON responses in the MGv. The neurons located in the shell (MGs) and dorsal (MGd) nuclei of the MGB showed complicated-either multi-peaked or broad-tuning curves. All OFF responses showed long-duration-selectivity for acoustic stimuli: the mean half-maximum duration was 116.5 +/- 114.8 ms (n = 19, range: 27-411 ms). The latencies of 135 OFF responses were studied in various divisions of the MGB. The ventral border region of MGv showed the shortest latency, followed by the dorsal border region of the MGv, the MGrm, and the caudomedial nucleus (MGcm) of the MGB. The posterior nucleus of the thalamus (Po), the MGd, and the MGs showed much longer mean latencies of >30 ms (P < 0.05 compared with the border regions of the MGv, ANOVA), with Po showing the greatest mean latency of 60.3 ms and the greatest deviation of 25.5 ms). The latency of the OFF response (29.0 +/- 14.0 ms, n = 135) was significantly greater than that of the ON response (15.6 +/- 9.6 ms, n = 160, P < 0.001). The present results provide valuable information about the threshold, frequency tuning characteristics, minimal response latency, and duration selectivity of OFF neurons in the auditory thalamus.

Human offset auditory brainstem response: effects of stimulus acoustic ringing and rise-fall time

Offset auditory brainstem response (ABR) traditionally has been thought to be an artifactual response elicited by stimulus acoustic ringing. Additionally, offset ABR's sensitivity to stimulus rise-fall time has been associated with concurrent changes in acoustic ringing. The present study tested the validity of offset ABR by recording the response in 40 young, normal-hearing adults using tone burst stimuli with varying degrees of acoustic ringing and various rise-fall times. Stimuli were computer-generated 10-ms tone bursts of 500 and 2000 Hz. In Experiment 1, offset ABR was recorded using stimuli with no acoustic ringing, normal ringing, and excessive ringing. Rise-fall time was held constant at 0.5 ms. In Experiment 2, rise-fall time was manipulated in a stimulus with no ringing. In Experiment 3, only rise time was manipulated in a no-ringing stimulus, while fall time was held constant at 0.5 ms. Reliable offset ABRs were recorded for all degrees of acoustic ringing, including the "no-ringing' condition. Offset ABR was sensitive to rise and fall times, and was elicited best with a 500-Hz stimulus. The results indicate that offset ABR is a real response and not an artifact produced by acoustic ringing.

Different phase sensitivity of low- and high-frequency neurons from the cat's cochlear nucleus

When tested with two-tone signals while varying the phase value (omega 2) of the second harmonic, the low-frequency neurons (CFs from 0.8 to 5 kHz) showed one maximum and one minimum of the response value over the omega 2-range of 360 degrees. The high-frequency neurons (CFs from 11.5 to 38.5 kHz) showed two maxima and two minima under the same conditions. In addition, the high-frequency neurons, unlike the low-frequency ones, showed no difference in response to click series of opposite polarities. The data are considered in connection with processes of excitation in the low- and high-frequency regions of the cochlea.