Stimulus Repetition Rate Research Articles

Auditory Evoked Potentials in Northern Elephant Seals (<I>Mirounga angustirostris</I>)

Auditory evoked potentials (AEPs) were investigated in northern elephant seals (Mirounga angustirostris) to characterize the responses elicited by different acoustic stimulus types, examine temporal resolving capabilities, and evaluate the potential for using evoked responses to estimate hearing sensitivity. Clicks and tone pips were presented to individual seals to characterize evoked responses to broadand narrowband stimuli. Tone pip trains and sinusoidally amplitude-modulated (SAM) tones were used to determine modulation rate transfer functions (MRTF) of the auditory system and to determine if the magnitude of the envelope-following response (EFR) relative to the stimulus level can be used to estimate hearing thresholds. Click evoked responses were characterized by three early positive peaks (~2.6, 4.4, and 6.1 ms) and a dominant negative peak at 7.2 ms and had average amplitudes of 264 nV (peak-to-peak [pk-pk]) for a corresponding stimulus level of 126 dB re 20 μPa (pk-pk). The use of dissociative drugs for the immobilization of the seals showed no demonstrable effect on the latencies or amplitudes of the click evoked response. Both the rate following response (RFR) and EFR amplitudes were maximal when the stimulus repetition rate or the amplitude modulation rate, respectively, were < 100 Hz. EFR amplitudes at the rate of amplitude modulation tracked near linearly with stimulus level. Thresholds for a 4-kHz SAM tone were estimated to be 45 dB re 20 μPa. Thus, the recording of AEPs is a viable means of studying auditory processes in the northern elephant seal.

New Efficient Stimuli for Evoking Frequency-Specific Auditory Steady-State Responses

ASSR is a promising tool for the objective frequency-specific assessment of hearing thresholds in children. The stimulus generally used for ASSR recording (single amplitude-modulated carrier) only activates a small area on the basilar membrane. Therefore, the response amplitude is low. A stimulus with a broader frequency spectrum can be composed by adding several cosines whose frequency intervals comply with the desired stimulus repetition rate. Compensation of the travelling wave delay on the basilar membrane is possible with a stimulus of this type. Through this, a better synchronization of the neural response can be obtained and, as a result, higher response amplitudes can be expected, particularly for low-frequency stimuli. The additional introduction of a frequency offset enables the use of a q-sample test for the response detection, especially important at 500 Hz. The results of investigations carried out on a large group of normally hearing test subjects have confirmed the efficiency of this stimulus design. The new stimuli lead to significantly improved ASSRs with higher SNRs and thus higher detection rates and shorter detection times.

Transient Suppression of Ipsilateral Primary Somatosensory Cortex during Tactile Finger Stimulation

The whole human primary somatosensory (SI) cortex is activated by contralateral tactile stimuli, whereas its subarea 2 displays neuronal responses also to ipsilateral stimuli. Here we report on a transient deactivation of area 3b of the ipsilateral SI during long-lasting tactile stimulation. We collected functional magnetic resonance imaging data with a 3 T scanner from 10 healthy adult subjects while tactile pulses were delivered at 1, 4, or 10 Hz in 25 s blocks to three right-hand fingers. In the contralateral SI cortex, activation [positive blood oxygenation level-dependent (BOLD) response] outlasted the stimulus blocks by 20 s, with an average duration of 45 s. In contrast, a transient deactivation (negative BOLD response) occurred in the ipsilateral rolandic cortex with an average duration of 18 s. Additional recordings on 10 subjects confirmed that the deactivation was not limited to the right SI but occurred in the SI cortex ipsilateral to the stimulated hand. Moreover, the primary motor cortex (MI) contained voxels that were phasically deactivated in response to both ipsilateral and contralateral touch. These data indicate that unilateral touch of fingers is associated, in addition to the well known activation of the contralateral SI cortex, with deactivation of the ipsilateral SI cortex and of the MI cortex of both hemispheres. The ipsilateral SI deactivation could result from transcallosal inhibition, whereas intracortical SI-MI connections could be responsible for the MI deactivation. The shorter time course of deactivation than activation would agree with stronger decay of inhibitory than EPSP at the applied stimulus repetition rates.

Auditory brainstem responses in the Eastern Screech Owl: An estimate of auditory thresholds

The auditory brainstem response (ABR), a measure of neural synchrony, was used to estimate auditory sensitivity in the eastern screech owl (Megascops asio). The typical screech owl ABR waveform showed two to three prominent peaks occurring within 5 ms of stimulus onset. As sound pressure levels increased, the ABR peak amplitude increased and latency decreased. With an increasing stimulus presentation rate, ABR peak amplitude decreased and latency increased. Generally, changes in the ABR waveform to stimulus intensity and repetition rate are consistent with the pattern found in several avian families. The ABR audiogram shows that screech owls hear best between 1.5 and 6.4 kHz with the most acute sensitivity between 4-5.7 kHz. The shape of the average screech owl ABR audiogram is similar to the shape of the behaviorally measured audiogram of the barn owl, except at the highest frequencies. Our data also show differences in overall auditory sensitivity between the color morphs of screech owls.

The effects of neural synchronization and peripheral compression on the acoustic-reflex threshold

This study investigates the acoustic reflex threshold (ART) dependency on stimulus phase utilizing low-level reflex audiometry [Neumann et al., Audiol. Neuro-Otol. 1, 359-369 (1996)]. The goal is to obtain optimal broadband stimuli for elicitation of the acoustic reflex and to obtain objective determinations of cochlear hearing loss. Three types of tone complexes with different phase characteristics were investigated: A stimulus that compensates for basilar-membrane dispersion, thus causing a large overall neural synchrony (basilar-membrane tone complex-BMTC), the temporally inversed stimulus (iBMTC), and random-phase tone complexes (rTC). The ARTs were measured in eight normal-hearing and six hearing-impaired subjects. Five different conditions of peak amplitude and stimulus repetition rate were used for each stimulus type. The results of the present study suggest that the ART is influenced by at least two different factors: (a) the degree of synchrony of neural activity across frequency, and (b) the fast-acting compression mechanism in the cochlea that is reduced in the case of a sensorineural hearing loss. The results allow a clear distinction of the two subjects groups based on the different ART for the utilized types and conditions of the stimuli. These differences might be useful for objective recruitment detection in clinical diagnostics.

Impact of conventional anesthesia on auditory brainstem responses in mice

Anesthesia is known to affect the auditory brainstem response (ABR) in animals often used in hearing research. This study describes the differences in ABRs between awake and anesthetized FVB/N mice. Intracranial electrodes connected to a head fixation pedestal were used for click-evoked ABR recordings. This pedestal served to immobilize mice, either awake or under anesthesia, in a `free' sound field. The presence of myogenic noise in the awake condition obviously increases recording time. However it is demonstrated that recording times can be significantly reduced by increasing the stimulus repetition rate from 23 up to 80 impulses per second. This causes only a small but significant increase in absolute peak latencies in the awake condition, but has no significant effect on the overall ABR-waveform, nor on the ABR-threshold, nor on the ABR interpeak latencies, nor on the absolute peak latencies in the anesthetized condition. Anesthesia with ketamine/xylazine caused a significant prolongation of ABR-peak latencies and interpeak latencies as well as a significant upward shift (8.0 ± 1.8 dB) of ABR-thresholds as compared to the awake condition. Under anesthesia the measurement accuracy of peak latencies, interpeak latencies and thresholds decreases. In conclusion, the awake condition is preferable for more accurate measurements of ABR characteristics, in spite of the myogenic noise concomitant with this condition.

Neural processing of high-rate auditory stimulation under conditions of various maskers

This study investigated a new auditory evoked-response, G-waves, being the response to tone-pip stimuli delivered at a rate of 40/s. These waves were detected using the Q-sequence deconvolution (QSD) method, which utilizes circular convolution and deconvolution on a quasi-periodic stimulus sequence to recover the transient response to each stimulus in the sequence. Thus, the G-waves recorded to stimuli delivered at 40/s show the transient responses that underlie the “ 40 Hz steady-state response”. Based upon initial data showing differences in masker effects as a function of stimulus repetition rate, we studied several maskers, showing that at the 40/s rate white noise maskers affect the waveforms, whereas music masking at the same intensity does not.

S-Methylcysteine may be a Causal Factor in Monohalomethane Neurotoxicity

S-Methylcysteine (SMC) is formed after exposure to monohalomethanes in rodents as well as in humans. The present study was performed to study whether SMC, directly or indirectly, contributes to the well-known neurotoxicity of monohalomethanes. We have investigated the effects of acute exposure to SMC by means of electrophysiolocal measurements in freshly prepared hippocampal slices and dissociated hippocampal neurons in culture. For longer-term exposures (24 h) we have used organotypic cultures (2 weeks in culture), taking electrophysiologic recordings and assessing membrane integrity with propidium iodide (PI) fluorescence. We found that only high concentrations of SMC (10 −2 M; exposure time 30 min) in freshly isolated slices of adult rats reduce synaptically evoked population spikes in the CA1 region. This effect was at least partially reversible. In organotypic cultures, at 5×10 −5 M after 24 h of exposure, SMC compromises membrane integrity as revealed by PI fluorescence, only in the dentate gyrus, spreading to pyramidal cell layers at 5×10 −4 M. At 5×10 −6 and 2×10 −5 M, under the same experimental conditions, no changes were seen with the PI method, but we recorded increased population spike amplitudes, repetitive discharges and frequency potentiation (at a stimulus repetition rate of 0.05 Hz). Using whole-cell patch clamp in hippocampal dissociated neurons we have found that SMC (applied for approximately 1 s) reduces GABA-induced currents (IC 50=4.4×10 −4 M) without having an effect of its own, acting like a competitive antagonist at GABA A receptors. Our findings are in line with the view that the ability of monohalomethanes to induce the formation of SMC is an important factor for their neurotoxicity, provided that SMC is allowed to act at least for several hours. The effects exerted by SMC seem to be due, at least in part, to its interaction with GABA receptors.

Evaluation of methods for eliciting somatosensory-evoked potentials in the awake, freely moving rat

To standardise the method of eliciting somatosensory-evoked potentials (SEPs), SEPs were generated by electrical stimulation of different stimulus sites and recorded bilaterally from the primary somatosensory cortex (S1) and from midline in awake, freely moving rats. Increasing stimulus intensity enhanced amplitudes of all SEPs. At supramaximal stimulation, SEPs following vibrissae and tail stimulation (V-SEP and Ta-SEP, respectively) but not following trunk stimulation (Tr-SEP), fulfilled our criterion of signal-to-noise ratio ≥4. The first V-SEP component coincided with a stimulus artefact, disqualifying these recordings for a standard stimulation protocol. The Ta-SEP generated stable and reproducible recordings and was considered to be the preferred technique. Early components of the contralateral S1 recorded V-SEP and Tr-SEP occurred at latencies different from the other recordings. Increasing stimulus repetition rate (SRR) decreased amplitudes of all SEPs. At the highest obtainable SRR, the amplitude between the V-SEP second positive and second negative components in all recordings was 70–80% of the amplitude at 0.1 Hz, whereas peak amplitudes of subsequent components and those of the Tr-SEP and Ta-SEP were 20–50%. These results indicate that the different SEP components might be generated by different ascending neural pathways.

Effects of Stimulus Rate and Gender on the Auditory Middle Latency Response

The effects of stimulus rate and gender on the auditory middle latency response (AMLR) waveforms were examined in 20 young adult male and female subjects. Four different repetition rates were presented to subjects (1.1/sec, 4.1/sec, 7.7/ sec, and 11.3/sec). Stimulus repetition rate had a significant effect on Pa latency, Pa amplitude, and Pb amplitude. Pa and Pb amplitudes decreased with increasing the stimulus rate, and Pa latency significantly increased with increasing the stimulus rate. No significant differences were seen on Pb latency or site of recording. Gender had a significant effect on Pa latency and Pa amplitude. Pa latencies were longer in male subjects, and Pa amplitudes were larger in female subjects. Gender did not have a significant effect on the Pb waveform.

Symmetry measures of vestibular evoked myogenic potentials using objective detection criteria.

Vestibular evoked myogenic potentials (VEMP) has been put forward as a test to evaluate the symmetry of saccular function. In the present study, the symmetry of VEMP was evaluated in 23 healthy subjects using automatic analysis. In response to binaural clicks with a stimulus repetition rate of 4/s all subjects revealed significant VEMP on both sides. The ipsilateral response to monaural clicks was similar to the response to binaural clicks. Although there were large interindividual variations in amplitude, there were only small variations in latency. VEMP measurements were also assessed using different stimulus rates. At the higher stimulus rates there was a decrease in the VEMP amplitude but there were only small changes in the latency. These findings might suggest that amplitude measures are more likely than latency measures to reveal small vestibular lesions.

Habituation and sensitization of aggression in bullfrogs (Rana catesbeiana): testing the dual-process theory of habituation.

The aggressive response of male bullfrogs (Rana catesbeiana) habituates with repeated broadcasts of acoustic stimuli simulating a new territorial neighbor. The effects of stimulus repetition rate and stimulus intensity on bullfrog aggressive responses were tested in a field experiment designed to test the assumptions of a dual-process theory of habituation. Synthetic advertisement calls were broadcast at 2 repetition rates and 2 intensities in a factorial design. Bullfrogs were more aggressive at the higher stimulus intensity at both repetition rates. Aggressive responses habituated more slowly at the higher stimulus intensity and slower repetition rate compared with other treatments. Several biotic and abiotic factors had small or negligible effects on aggressive responses. Although consistent with the operation of 2 opposing processes, habituation and sensitization, the data provide only partial support for the assumptions of dual-process theory.

Repeatability of the intensity dependence of cortical auditory evoked potentials in the assessment of cortical information processing.

Data on repeatability and optimal settings are needed when studying the influence of drugs on the intensity dependence of auditory evoked cortical potentials (IDAP). IDAP was recorded at intervals of 1, 2, and 24 h at two centers in 22 healthy volunteers. Settings were modified to compare fixed versus randomly varied stimulus repetition rate, as well as 30 Hz and 100 Hz low pass filters. Repeatability was assessed for different intervals and different settings. Group means did not differ between centers, the 2-h or 24-h retest, or when using different settings. We observed an order effect for the 1 h retest. Fixed repetition rate and the 30 Hz filter improved repeatability with still high intraindividual variability. IDAP group means can be compared between centers for retest intervals of 2 h and 24 h and different settings. Variability is too large to compare individuals.

Different cortical organization of visceral and somatic sensation in humans.

Sensory stimuli from the visceral domain exhibit perceptual characteristics different from stimuli applied to the body surface. Compared with somatosensation there is not much known about the cortical projection and functional organization of visceral sensation in humans. In this study, we determined the cortical areas activated by non-painful electrical stimulation of visceral afferents in the distal oesophagus, and somatosensory afferents in the median nerve and the lip in seven healthy volunteers using whole-head magnetoencephalography. Stimulation of somatosensory afferents elicited short-latency responses (approximately 20-60 ms) in the primary somatosensory cortex (SI) contralateral (median nerve) or bilateral (lip) to the stimulated side, and long-latency responses (approximately 60-160 ms) bilaterally in the second somatosensory cortex (SII). In contrast, stimulation of visceral oesophageal afferents did not evoke discernible responses in SI but well reproducible bilateral SII responses (approximately 70-190 ms) in close vicinity to long-latency SII responses following median nerve and lip stimuli. Psychophysically, temporal discrimination of successive stimuli became worse with increasing stimulus repetition rates (0.25 Hz, 0.5 Hz, 1 Hz, 2 Hz) only for visceral oesophageal, but not for somatosensory median nerve stimuli. Correspondingly, amplitudes of the first cortical response to oesophageal stimulation emerging in the SII cortex declined with increasing stimulus repetition rates whereas the earliest cortical response elicited by median nerve stimuli (20 ms SI response) remained unaffected by the stimulus frequency. Our results indicate that visceral afferents from the oesophagus primarily project to the SII cortex and, unlike somatosensory afferents, lack a significant SI representation. We propose that this cortical projection pattern forms the neurophysiological basis of the low temporal and spatial resolution of conscious visceral sensation.

Receptor cell habituation in the A1 auditory receptor of four noctuoid moths.

Moths of both sexes of Empyreuma affinis (=pugione) and Syntomeida epilais (Arctiidae, Ctenuchinae), Maenas jussiae (Arctiidae, Arctiinae) and Spodoptera frugiperda (Noctuidae, Amphipyrinae) were studied. Spike activity in the A1 cell was recorded using a stainless-steel hook electrode from the tympanic nerve in the mesothorax. Acoustic stimuli consisting of 25 and 100 ms pulses at the best frequency for the species and at intensities that evoke A1 cell saturation response were used at repetition rates of 0.5 and 5 Hz for 100 ms stimuli, and between 2 and 20 Hz for 25 ms stimuli. Stimuli at a repetition rate corresponding to a duty cycle of 5 % (25 ms at 2 Hz and 100 ms at 0.5 Hz) did not evoke monotonic changes in the responses of the A1 cell. With 25 ms pulses, rates above 5 Hz evoked an exponential decrease in the number of spikes and an increase in the latency of the responses of all the 37 specimens tested. The response duration showed no apparent change with stimulus repetition rates even at the highest duty cycle used (50 %), i.e. 25 ms at 20 Hz and 100 ms at 5 Hz. The higher the rate of stimulus repetition, the more marked were the changes in the A1 cell responses. In 16 of 17 preparations from two species, habituation had no effect on the adaptation rate in each response, while in seven of eight specimens of another species, the adaptation rate decreased with stimulus repetition. These results, and those from another mechanoreceptor cell, indicate that receptor cell adaptation (changes evoked in the response by a stimulus of constant intensity) and habituation (changes in the responses due to stimulus repetition rate) are two distinctive phenomena. The A1 cell in its habituated state showed an increase in its response to incremental increases in stimulus intensity of 10 dB. This result supports the idea that receptor cell habituation does not seem to be due to fatigue, i.e. to a temporary loss of the ability to respond to stimulation induced in a sensory receptor by continued stimulation.

Temporal integration of pain from electrical stimulation of the skin

The threshold of sensation and the threshold of pain in response to electrical stimulation (impulses of 1 msec duration) of the skin on the forearm or hand in individuals without pain were compared with the thresholds of individuals with chronic pain in the range 1 to 100 pulses per second repetition rate. The threshold of sensation in patients without pain was little affected by the repetition rate of the stimulation within the range studied, and the threshold for pain decreased exponentially with increasing repetition rate. In individuals with chronic pain the threshold of sensation was similar to that of individuals without pain over the entire range of stimulus repetition rates studied, but the threshold ofpain in patients with pain was lower and less affected by the stimulus rate than it was in the individuals without pain, thus closer to the threshold of sensation. [Neural Res 1997; 19: 481-488J

Rate and adaptation effects on the auditory evoked brainstem response in human newborns and adults

Auditory evoked brainstem response (ABR) latencies increased and amplitudes decreased with increasing stimulus repetition rate for human newborns and adults. The wave V latency increases were larger for newborns than adults. The wave V amplitude decreases were smaller for newborns than adults. These differences could not be explained by developmental differences in frequency responsivity. The transition from the unadapted to the fully adapted response was less rapid in newborns than adults at short (=10 ms) inter stimulus intervals (ISIs). At longer ISIs (=20 ms) there were no developmental differences in the transition to the fully adapted response. The newborn transition occurred in a two stage process. The rapid initial stage observed in adults and newborns was complete by about 40 ms. A second slower stage was observed only in newborns although it has been observed in adults in other studies (Weatherby and Hecox, 1982; Lightfoot, 1991; Lasky et al., 1996). These effects were replicated at different stimulus intensities. After the termination of stimulation the return to the wave V unadapted response took nearly 500 ms in newborns. Neither the newborn nor the adult data can be explained by forward masking of one click on the next click. These results indicate human developmental differences in adaptation to repetitive auditory stimulation at the level of the brainstem.

Binaural interaction and the effects of stimulus intensity and repetition rate in human auditory brain-stem

Binaural interaction (BI) components in brain-stem auditory evoked potential (BAEP) and their changes with stimulus intensity and repetition rate were examined in human adult. Seven BI components were identified, which occurred between the latency range of 5 and 11 ms and coincided consistently with the latency range of BAEP waves IV–VII. Waves DV and DVII, occurring at the downslopes of BAEP-waves V and VII, respectively, were the two most prominent and reproducible BI components. Wave DVII existed consistently at high, moderate and, in most cases, low stimulus intensities, suggesting that this component is neurogenic although acoustic cross-talk may account for a part of its waveform at high stimulus intensities. The latencies of all BI components increased as a function of decreasing stimulus intensity, while the interpeak intervals, especially DV–DVII, were essentially constant at different intensity levels. The amplitudes of BI components decreased slightly with decreasing intensity. As click repetition rate increased, BI wave latencies and interpeak intervals increased slightly and amplitudes decreased slightly. When repetition rate increased to above 20/s, BI components became poorly differentiated. Lower repetition rates, e.g. 10/s, are therefore preferred for routine derivation of the BI. The changes in the latency and amplitude of BI components with stimulus intensity and repetition rate were associated or concomitant with those of the corresponding BAEP components in monaural and binaural potentials. In view of the concomitant relationship between BI and BAEP latency, we designate BI components in association with the corresponding BAEP components.

Binaural interaction and the effects of stimulus intensity and repetition rate in human auditory brain-stem

Binaural interaction (BI) components in brain-stem auditory evoked potential (BAEP) and their changes with stimulus intensity and repetition rate were examined in human adult. Seven BI components were identified, which occurred between the latency range of 5 and 11 ms and coincided consistently with the latency range of BAEP waves IV–VII. Waves DV and DVII, occurring at the downslopes of BAEP-waves V and VII, respectively, were the two most prominent and reproducible BI components. Wave DVII existed consistently at high, moderate and, in most cases, low stimulus intensities, suggesting that this component is neurogenic although acoustic cross-talk may account for a part of its waveform at high stimulus intensities. The latencies of all BI components increased as a function of decreasing stimulus intensity, while the interpeak intervals, especially DV–DVII, were essentially constant at different intensity levels. The amplitudes of BI components decreased slightly with decreasing intensity. As click repetition rate increased, BI wave latencies and interpeak intervals increased slightly and amplitudes decreased slightly. When repetition rate increased to above 20/s, BI components became poorly differentiated. Lower repetition rates, e.g. 10/s, are therefore preferred for routine derivation of the BI. The changes in the latency and amplitude of BI components with stimulus intensity and repetition rate were associated or concomitant with those of the corresponding BAEP components in monaural and binaural potentials. In view of the concomitant relationship between BI and BAEP latency, we designate BI components in association with the corresponding BAEP components.

Functional GABAergic Synaptic Connection in Neonatal Mouse Barrel Cortex

Intracortical inhibition is crucial to proper functioning of the mature neocortex, yet, paradoxically, is reported to be rare or absent in the neonatal animal. We reexamined this issue by recording whole-cell postsynaptic currents (PSCs) of barrel cortex neurons in thalamocortical brain slices from neonatal mice. Monosynaptic, excitatory thalamocortical responses were elicited in layers V/VI neurons as early as postnatal day 0 (P0, the first 24 hr after birth) and in presumptive layer IV as early as P2. At very low stimulation frequencies, the monosynaptic response was invariably followed by a prolonged (up to 1 sec) synaptic barrage, which fatigued at stimulus repetition rates of 2/min or higher. This barrage consisted of postsynaptic responses to spiking activity in neighboring cortical cells, because (1) it could also be evoked by intracortical stimulation in coronal slices and (2) it was abolished by antagonists to NMDA receptors (NMDARs), even when NMDARs on the recorded cell were under a voltage-dependent block. Some of the larger polysynaptic events changed polarity at a negative reversal potential and were blocked by GABAA receptor (GABAAR) antagonists, with a concurrent enhancement of the extracellular field potential, indicating that they were GABAAR- mediated, CI-dependent inhibitory PSCs (IPSCs). We conclude that a network of functional intracortical GABAAR-mediated synaptic connections exists from the earliest postnatal ages, although it gives rise to responses that differ from mature IPSCs in reversal potential and latency.