Auditory steady-state responses in the rabbit

Network resonance and the auditory steady state response

The auditory steady state response (ASSR) arises when periodic sounds evoke stable responses in auditory networks that reflect the acoustic characteristics of the stimuli, such as the amplitude of the sound envelope. Larger for some stimulus rates than others, the ASSR in the human electroencephalogram (EEG) is notably maximal for sounds modulated in amplitude at 40 Hz. To investigate the local circuit underpinnings of the large ASSR to 40 Hz amplitude-modulated (AM) sounds, we acquired skull EEG and local field potential (LFP) recordings from primary auditory cortex (A1) in the rat during the presentation of 20, 30, 40, 50, and 80 Hz AM tones. 40 Hz AM tones elicited the largest ASSR from the EEG acquired above auditory cortex and the LFP acquired from each cortical layer in A1. The large ASSR in the EEG to 40 Hz AM tones was not due to larger instantaneous amplitude of the signals or to greater phase alignment of the LFP across the cortical layers. Instead, it resulted from decreased latency variability (or enhanced temporal consistency) of the 40 Hz response. Statistical models indicate the EEG signal was best predicted by LFPs in either the most superficial or deep cortical layers, suggesting deep layer coordinators of the ASSR. Overall, our results indicate that the recruitment of non-uniform but more temporally consistent responses across A1 layers underlie the larger ASSR to amplitude-modulated tones at 40 Hz.

Depression of auditory cortex excitability by transcranial alternating current stimulation

Transcranial alternating current stimulation (tACS) is a type of noninvasive brain stimulation technique that has been shown to modulate motor, cognitive and memory function. Direct electrophysiological evidence of an interaction between tACS and the auditory cortex excitability has rarely been reported. Different stimulation parameters and areas of tACS may have different influence on the regulatory results. In this study, 11-Hz tACS was applied to the auditory cortex of 12 subjects with normal hearing in order to explore its effects on the auditory steady-state response (ASSR). The results indicate that tACS has an inhibitory effect on 40-Hz ASSR. In addition, EEG source analysis shows that 11-Hz tACS may enhance the activity of the middle temporal gyrus under both sham and real conditions, while the estimated source activity of the posterior cingulate gyrus may be reduced under real condition. The results reveal that tACS applied to the temporal lobe of humans will make the 40-Hz ASSR a tendency to decrease, and help improve the understanding of modulation of tACS-induced auditory cortex excitability changes in humans.

Steady state responses: electrophysiological assessment of sensory function in schizophrenia

Persons with schizophrenia experience subjective sensory anomalies and objective deficits on assessment of sensory function. Such deficits could be produced by abnormal signaling in the sensory pathways and sensory cortex or later stage disturbances in cognitive processing of such inputs. Steady state responses (SSRs) provide a noninvasive method to test the integrity of sensory pathways and oscillatory responses in schizophrenia with minimal task demands. SSRs are electrophysiological responses entrained to the frequency and phase of a periodic stimulus. Patients with schizophrenia exhibit pronounced auditory SSR deficits within the gamma frequency range (35-50 Hz) in response to click trains and amplitude-modulated tones. Visual SSR deficits are also observed, most prominently in the alpha and beta frequency ranges (7-30 Hz) in response to high-contrast, high-luminance stimuli. Visual SSR studies that have used the psychophysical properties of a stimulus to target specific visual pathways predominantly report magnocellular-based deficits in those with schizophrenia. Disruption of both auditory and visual SSRs in schizophrenia are consistent with neuropathological and magnetic resonance imaging evidence of anatomic abnormalities affecting the auditory and visual cortices. Computational models suggest that auditory SSR abnormalities at gamma frequencies could be secondary to gamma-aminobutyric acid-mediated or N-methyl-D-aspartic acid dysregulation. The pathophysiological process in schizophrenia encompasses sensory processing that probably contributes to alterations in subsequent encoding and cognitive processing. The developmental evolution of these abnormalities remains to be characterized.

Chirp-evoked potentials in the awake and anesthetized rat. A procedure to assess changes in cortical oscillatory activity

Steady-state potentials are oscillatory responses generated by rhythmic stimulation of a sensory pathway. The frequency of the response, which follows the frequency of stimulation and potentially indicates the preferential working frequency of the auditory neural network, is maximal at a stimulus rate of 40 Hz for auditory stimuli in humans, but may be different in other species. Our aim was to explore the responses to different frequencies in the rat. The stimulus was a tone modulated in amplitude by a sinusoid with linearly-increasing frequency from 1 to 250 Hz ("chirp"). Time-frequency transforms were used for response analysis in 12 animals, awake and under ketamine/xylazine anesthesia. We studied whether the responses were due to increases in amplitude or to phase-locking phenomena, using single-sweep time-frequency transforms and inter-trial phase analysis. A progressive decrease in the amplitude of the response was observed from the maximal values (around 15 Hz) up to the limit of the test (250 Hz). The high-frequency component was mainly due to phase-locking phenomena with a smaller amplitude contribution. Under anesthesia, the amplitude and phase-locking of lower frequencies (under 100 Hz) decreased, while the phase-locking over 200 Hz increased. In conclusion, amplitude-modulation following responses differ between humans and rats in response range and frequency of maximal amplitude. Anesthesia with ketamine/xylazine modifies differentially the amplitude and the phase-locking of the responses. These findings should be taken into account when assessing the changes in cortical oscillatory activity related to different drugs, in healthy rodents and in animal models of neurodegenerative diseases.

Normative Auditory Brainstem Response Data for Hearing Threshold in the Rabbit

In an experimental study, we determined the physiological hearing threshold of the rabbit in order to use these data as normative values for further experimental investigations. The aim was to use different acoustic stimuli (click and tone-pip stimuli) with different frequency spectra for air and bone conduction (BC) in order to obtain further information about the optimal form of stimulus when recording auditory evoked potentials in the rabbit. For the investigation, we used 46 female New Zealand rabbits weighing 3.2-4.4 kg and aged 6 months. The equipment used to record brainstem auditory evoked potentials was the Nicolet Viking IV P System (Nicolet Biomedical, Inc.). In accordance with the experimental set-up, the measurements took place under intubation anesthesia, with a total of four repeat measurements performed on each ear at different times. Tone-pip and click stimuli with varying intensities of stimulus, transmitted via air conduction and BC, were applied. The I-IV waves proved the most stable for both stimulus modalities. They were registrable in 98.7% of cases, whereas only 30.2% of the V waves could be recorded. Values averaged from all measurements made throughout the study yielded a potential threshold of 34.8 dB peak equivalent (p.e.) SPL for the click stimulus, 13.8 dB p.e. SPL for the tone-pip stimulus at 8 kHz and 34.2 dB p.e. SPL for the click stimulus transmitted via BC. With regard to latencies, the results indicated a good reproducibility through different stimuli with acceptable standard deviations. The values for physiological hearing threshold obtained here can serve as normative data in subsequent experimental animal studies.

Effects of isoflurane on auditory middle latency (MLRs) and steady-state (SSRs) responses recorded from the temporal cortex of the rat

Auditory steady-state responses (SSRs) are believed to result from superimposition of middle latency responses (MLRs) evoked by individual stimuli during repetitive stimulation. Our previous studies showed that besides linear addition of MLRs, other phenomena, mainly related to the adaptive properties of neural sources, interact in a complex way to generate the SSRs recorded from the temporal cortex of awake rats. The aim of this study was to evaluate the effects of the inhalational general anesthetic, isoflurane, on MLRs and SSRs at several repetition rates (30-60 Hz) recorded from the temporal cortex of rats. Auditory evoked potentials were obtained by means of epidural electrodes in the awake condition and during anesthesia at three isoflurane concentrations (0.38, 0.76 and 1.13 vol.% in oxygen). MLR latency significantly increased during anesthesia in a concentration-dependent manner, while MLR amplitude, even when significantly attenuated with respect to the mean awake baseline value, failed to correlate with isoflurane concentration. SSRs decreased in amplitude and increased in phase during anesthesia in a concentration-dependent manner and the anesthetic-induced decrease of SSR amplitude appeared to be higher than the corresponding MLR attenuation. SSR prediction curves synthesized by linear addition of MLRs failed to predict SSRs in both amplitude and phase. Moreover, phase discrepancies proved to be higher during anesthesia. Our results suggest that MLRs and SSRs recorded from the temporal cortex of the rat exhibit differential sensitivity to isoflurane and that isoflurane could enhance the role of rate-dependent effects in SSR generation.

Anesthesia and the electrophysiology of auditory consciousness

Empirical work is reviewed which correlates the presence or absence of various parts of the auditory evoked potential with the disappearance and reemergence of auditory sensation during induction of and recovery from anesthesia. As a result, the hypothesis is generated that the electrophysiological correlate of auditory sensation is whatever neural activity generates the middle latency waves of the auditory evoked potential. This activity occurs from 20 to 80 ms poststimulus in the primary and secondary areas of the auditory cortex. Evidence is presented suggesting that earlier or later waves in the auditory evoked potential do not covary with auditory sensation (as opposed to auditory perception) and it is therefore suggested that they are possibly not the electrophysiological correlates of sensation.

Auditory steady-state responses to click trains from the rat temporal cortex

In order to investigate the mechanisms underlying the generation of steady-state responses (SSRs), auditory evoked potentials elicited by click trains presented at several stimulation rates (30, 40, 50, 60 Hz) were recorded in 7 awake rats by means of epidural electrodes placed over the temporal cortex. Mean amplitude-rate function calculated on the recorded responses appeared almost flat and showed the maximum value at 50 Hz, while mean phases showed a linear increase when increasing the stimulation rate. In each rat, predictions of the recorded responses at 30, 40, 50 and 60 Hz were synthesized by superimposing middle-latency auditory evoked potentials (MAEPs) at suitable time intervals at each rate. Mean amplitudes calculated on the predicted curves decreased linearly when increasing the stimulation rate and appeared higher in comparison to those obtained from the recorded SSRs. Predicted phases showed a linear increase when increasing the stimulation rate and were leading with respect to corresponding phase values calculated for recorded SSRs. Our findings indicate that the MAEP superimposition mechanism does not adequately predict the generation of temporal recorded SSRs in rats. This was explained by admitting that phenomena related to the recovery cycle and, to a lesser extent, to rate-dependent facilitating effects come into play.

Generation of human auditory steady-state responses (SSRs). I: Stimulus rate effects

Auditory evoked responses were recorded in 16 normally hearing subjects in order to investigate the mechanisms underlying the generation of the 40 Hz steady-state response (SSR). In the first part of our study, auditory potentials were evoked by 0.1 ms clicks presented at 105 dB p.e. SPL with repetition rates of 7.9 (to obtain middle latency response, MLR), 20, 30, 40, 50, 60 Hz. In each subject predictions of the responses recorded at stimulus repetition rates of 30, 40, 50, 60 Hz were synthesized by superimposing MLRs at suitable time intervals. The calculated mean amplitude/rate and phase/rate functions behaved similarly for the recorded and predicted curves, showing the highest amplitude at 40 Hz and a linear increase of phase values when increasing the stimulus rate. Nevertheless the synthetic curves closely predicted amplitude and phase values of the recorded responses only at 40 Hz. At frequencies below 40 Hz, the mean amplitude of the predicted curve was lower than that of the recorded one while at frequencies above 40 Hz the mean amplitude was higher. Predicted phase values were found lagging at 30 Hz, and leading at 50 Hz and 60 Hz in comparison to phase values calculated on the recorded responses. Our findings suggest that a model based on the linear addition of transient MLRs is not able to adequately predict steady-state responses at stimulus rates other than at 40 Hz. Other mechanisms related to the recovery cycle of the activated system come into play in the steady-state response generation causing a decrease in amplitude and an increase in phase lag when increasing the stimulus repetition rate.

Generation of human auditory steady-state responses (SSRs). II: Addition of responses to individual stimuli

In order to investigate the generation of the 40 Hz steady-state response (SSR), auditory potentials evoked by clicks were recorded in 16 healthy subjects in two stimulating conditions. Firstly, repetition rates of 7.9 and 40 Hz were used to obtain individual middle latency responses (MLRs) and 40 Hz-SSRs, respectively. In the second condition, eight click trains were presented at a 40 Hz repetition rate and an inter-train interval of 126 ms. We extracted from the whole train response: (1) the response-segment taking place after the last click of the train (last click response, LCR), (2) a modified LCR (mLCR) obtained by clearing the LCR from the amplitude enhancement due to the overlapping of the responses to the clicks preceding the last within the stimulus train. In comparison to MLRs, the most relevant feature of the evoked activity following the last click of the train (LCRs, mLCRs) was the appearance in the 50-110 ms latency range of one (in 11 subjects) or two (in 2 subjects) additional positive-negative deflections having the same periodicity as that of MLR waves. The grand average (GA) of the 40 Hz-SSRs was compared with three predictions synthesized by superimposing: (1) the GA of MLRs, (2) the GA of LCRs, (3) the GA of mLCRs. Both the MLR and mLCR predictions reproduced the recorded signal in amplitude while the LCR prediction amplitude resulted almost twice that of the 40 Hz-SSR. With regard to the phase, the MLR, LCR and mLCR closely predicted the recorded signal. Our findings confirm the effectiveness of the linear addition mechanism in the generation of the 40 Hz-SSR. However the responses to individual stimuli within the 40 Hz-SSR differ from MLRs because of additional periodic activity. These results suggest that phenomena related to the resonant frequency of the activated system may play a role in the mechanisms which interact to generate the 40 Hz-SSR.

Effect of natural sleep on auditory steady state responses in adult subjects with normal hearing

Auditory 40-Hz steady state response (SSR), auditory brainstem response (ABR), and middle-latency response (MLR) were recorded in 12 healthy adult females with normal hearing while awake and asleep. The responses were recorded with 500-Hz tone pips at 55 dBnHL. Synthesized SSR were made by superimposing the recorded ABR and MLR waveforms (ABR-MLR), and their amplitudes were compared with those of the actually recorded SSR. In the waking state, the ratio of the mean amplitude of recorded SSR to that of synthesized SSR was 0.819, whereas in the sleeping state it decreased to 0.522, a statistically significant difference. The results indicate that the SSR can be predicted from the linear superimposition of ABR and MLR in the waking state, but not in the sleeping state.

Temporal integration and oscillatory responses of the human auditory cortex revealed by evoked magnetic fields to click trains

We recorded neuromagnetic evoked responses from the right auditory cortex of 7 healthy adults with a 24-channel planar SQUID gradiometer. The stimuli were 200-ms click trains presented at rates of 40, 80, 160 and 320 Hz, with interstimulus intervals (ISIs) of 1 and 4 s. The transient N100m response to the train onset depended on the click rate: the peak latency shortened to the same extent as the interval between successive clicks decreased in trains with rates from 40 Hz to 320 Hz. The N100m amplitude increased simultaneously, saturating at rates of 160-320 Hz. The mean N100m latency was slightly longer with the 1-s than with the 4-s ISI for all click rates. The systematic changes of the N100m amplitude and latency according to click rate demonstrate the importance of temporal integration for N100m generation, and imply an integration time of 20-25 ms. The 20- and 40-Hz click trains also elicited oscillatory 40-Hz responses 80-250 ms after the train onset. The 40-Hz responses were more resistant than N100m to changes of the ISI, and their sources slightly differed from those of N100m. These two responses evidently reflect different aspects of auditory processing.

Steady-state auditory evoked potentials (SSAEPs) in the rabbit. Contribution of the inferior colliculus

Steady-state auditory evoked potentials (SSAEPs) were recorded in rabbits with both surface and depth electrodes. Surface recording from the bregma provided the largest and most typical SSAEPs as compared to other surface locations when a stimulus rate of 50 Hz was used. The medial geniculate body (MGB) showed no potential corresponding to the surface SSAEP. On the other hand, the latency of SSAEP in the inferior colliculus (IC) corresponded closely to that of the surface potential. Furthermore, the amplitude of the IC potential tended to become large with the stimulus rate of 50 Hz as compared with transient stimuli. Although other auditory nuclei in the brain-stem, the ventral nucleus of the lateral lemniscus, the trapezoid body and the auditory nerve responded to transient stimuli with an amplitude larger than that of the IC, no amplification occurred with 50 Hz stimuli in these nuclei. These findings suggest that the IC contributes to the generation of SSAEP to a great extent.