Electrical stimulation of the auditory nerve: II. Effect of stimulus waveshape on single fibre response properties

To investigate the generation of action potentials by electrical stimulation we studied the response of auditory nerve fibres (ANFs) to a variety of stimulus waveforms. Current pulses were presented to longitudinal bipolar scala tympani electrodes implanted in normal and deafened cochleae. Capacitively coupled monophasic current pulses evoked single ANF responses that were more sensitive to one phase (the ‘excitatory’ phase) than the other. Anodic pulses produced a significantly shorter mean latency compared with cathodic pulses, indicating that their site for spike initiation is located more centrally along the ANF. The fine temporal structure of ANF responses to biphasic pulses appeared similar to that evoked by monophasic pulses. An excitatory monophasic pulse evoked a significantly lower threshold than a biphasic current pulse having the same polarity and duration leading phase, i.e. the addition of a second phase leads to an increase in threshold. Increasing the temporal separation of the two phases of a biphasic pulse resulted in a moderate reduction in threshold which approached that of an excitatory monophasic pulse for interphase gaps >100 μs. Greater threshold reductions were observed with narrower current pulses. There was a systematic reduction in threshold with increasing pulse width for biphasic current pulses, reflecting the general charge-dependent properties of ANFs for narrow pulse widths. Chopped biphasic current pulses, which uniformly delivered multiple packets of charge (2×30 μs, 3×20 μs or 6×10 μs) with the same polarity over a 120 μs period, followed by a similar series in the reverse polarity, demonstrated the ability of the neural membrane to integrate sub-threshold packets of charge to achieve depolarisation. Moreover, thresholds for these current pulses were 1.5 dB lower than 60 μs/phase biphasic current pulses with no interphase gap. Finally, stimulation using charge-balanced triphasic and asymmetric current pulses produced systematic changes in threshold and latency consistent with the charge-dependent properties of ANFs. These findings provide insight into the mechanisms underlying the generation of action potentials using electrical stimuli. Moreover, a number of these novel stimuli may have potential clinical application.     

Effects of stimulus manipulation on electrophysiological responses in pediatric cochlear implant users. Part I: duration effects

Discrepancies between electrophysiological and behavioral thresholds in cochlear implant users might be due to differences in stimuli such as the duration and rate of the electrical pulse train. In the present study, we asked: Is there an effect of stimulus duration on electrophysiological responses of the auditory brainstem, thalamo-cortex, and behavioral thresholds? In 5 pediatric cochlear implant users, behavioral thresholds in response to electrical pulse trains at 500 pulses per second (pps) were significantly lower for 40ms than 2ms duration pulse trains. Clear electrically evoked auditory brainstem responses (EABR) and electrically evoked middle latency responses (EMLR) were generated by single electrical pulses and 2, 6, and 10ms pulse trains (500pps) in 5 children. There was a linear decrease in the inter-wave latency between the eV of the EABR and the Na of the EMLR as duration increased. No significant effect of duration was found on eV latency relative to the last pulse in the train or Na latency relative to the onset of the stimuli. Behavioral threshold data is consistent with temporal integration of auditory activity. Electrophysiological data indicates that: (a) recognizable EABR and EMLR waveforms can be recorded in response to electrical pulse trains of up to 10ms; and (b) pulse train stimuli have unique effects on the auditory brainstem compared to thalamo-cortical areas.     

Unanesthetized Auditory Cortex Exhibits Multiple Codes for Gaps in Cochlear Implant Pulse Trains

Cochlear implant listeners receive auditory stimulation through amplitude-modulated electric pulse trains. Auditory nerve studies in animals demonstrate qualitatively different patterns of firing elicited by low versus high pulse rates, suggesting that stimulus pulse rate might influence the transmission of temporal information through the auditory pathway. We tested in awake guinea pigs the temporal acuity of auditory cortical neurons for gaps in cochlear implant pulse trains. Consistent with results using anesthetized conditions, temporal acuity improved with increasing pulse rates. Unlike the anesthetized condition, however, cortical neurons responded in the awake state to multiple distinct features of the gap-containing pulse trains, with the dominant features varying with stimulus pulse rate. Responses to the onset of the trailing pulse train (Trail-ON) provided the most sensitive gap detection at 1,017 and 4,069 pulse-per-second (pps) rates, particularly for short (25 ms) leading pulse trains. In contrast, under conditions of 254 pps rate and long (200 ms) leading pulse trains, a sizeable fraction of units demonstrated greater temporal acuity in the form of robust responses to the offsets of the leading pulse train (Lead-OFF). Finally, TONIC responses exhibited decrements in firing rate during gaps, but were rarely the most sensitive feature. Unlike results from anesthetized conditions, temporal acuity of the most sensitive units was nearly as sharp for brief as for long leading bursts. The differences in stimulus coding across pulse rates likely originate from pulse rate-dependent variations in adaptation in the auditory nerve. Two marked differences from responses to acoustic stimulation were: first, Trail-ON responses to 4,069 pps trains encoded substantially shorter gaps than have been observed with acoustic stimuli; and second, the Lead-OFF gap coding seen for <15 ms gaps in 254 pps stimuli is not seen in responses to sounds. The current results may help to explain why moderate pulse rates around 1,000 pps are favored by many cochlear implant listeners.     

Effects of pulse rate on thresholds and loudness of biphasic and alternating monophasic pulse trains in electrical hearing

Detection thresholds and most comfortable loudnesses (MCLs) were determined as a function of pulse rate for standard biphasic pulse trains (BP) and for anodic and cathodic monophasic phases alternating at fixed intervals (ALT-m). Three different phase durations were examined.

With a 100-μs phase duration, thresholds for the ALT-m stimulus were substantially (up to 12 dB) lower than for the BP stimuli at relatively low rates (200 pps), but were similar to the BP thresholds at high rates (1000 pps). Thresholds for BP pulse trains decreased monotonically with increasing rate, whereas the function for ALT-m waveforms was non-monotonic with a maximum between 400 and 1000 pps. These trends occurred for three different cochlear implant devices, different electrode configurations, and, generally, for different phase durations (10.8, 25, and 100 μs/phase). However, at the shorter phase durations, thresholds remained lower for the ALT-m stimulus, even at 5000 pps, the highest rate studied.

Dynamic ranges of the BP pulse trains increased with increasing rate, irrespective of the phase duration under test, but for the ALT-m stimuli this was only true at the shorter phase durations tested. At a 100-μs phase duration, dynamic ranges for the ALT-m waveforms did not differ significantly as a function of rate.

The results confirm previous reports that delaying charge recovery, in this case by switching from a BP to an ALT-m wave shape, can substantially reduce thresholds [Van Wieringen, A., Carlyon, R.P., Laneau, J., Wouters, J., 2005. Effects of waveform shape on human sensitivity to electrical stimulation of the inner ear. Hear. Res. 200, 73–86; Carlyon, R.P., van Wieringen, A., Deeks, J.M., Long, C.J., Lyzenga, J, Wouters, J., 2005. Effect of inter-phase gap on the sensitivity of cochlear implant users to electrical stimulation. Hear. Res. 205, 210–224]. However, at high pulse rates, this advantage only occurs at short phase durations. In addition, we show that the complex interaction between the effects of pulse shape, rate, and phase duration on thresholds can be captured by the simple linear model described by Carlyon et al.     

Responses of chopper units in the ventral cochlear nucleus of the anaesthetised guinea pig to clicks-in-noise and click trains

Auditory brainstem responses (ABRs) have been measured with clicks, clicks masked by noise, click trains and pseudorandom maximum length sequences (MLS) of clicks. To investigate the neuronal populations contributing to the ABR under these stimulation conditions, we measured the extracellular responses of ventral cochlear nucleus (VCN) units in the urethane-anaesthetised guinea pig. We studied 23 chopper, 7 primary-like and 7 onset units. This report focuses on the responses from chopper units. The probability of discharge for chopper units increased with increasing click level reaching nearly 100% in many units, over a range of about 20–30 dB. Following each response to a click there was a 5–10 ms suppression of the spontaneous or noise evoked activity. As the level of the noise was increased over a range of 20–30 dB, the response to the clicks gradually decreased leading to a complete abolition of the click response at high noise levels. In a few units, low level noise produced a facilitation of the response to single clicks. In response to constant level equally spaced click trains, discharge probability increased with increasing minimum pulse interval (MPI), approaching 100% for MPIs of 4–8 ms in some units. The recovery afforded by the gaps in the MLS train often resulted in higher discharge probability for MLS than click trains with the same MPI, while response probabilities for MLS and click trains were similar when compared at equivalent average click rates. At short MPIs (0.5 and 1.0 ms), peri stimulus time histograms in response to click trains resembled those to best frequency (BF) tones and noisebursts, with chopping peaks unrelated to unit BF. VCN units show highly synchronised and reliable responses to click trains, MLS trains and clicks masked by noise. The decrease in discharge rate and increase in latency of chopper units with decreasing click level, increasing click rate and increasing masker level parallel the peak amplitude and latency changes observed in the auditory brainstem response.     

Signal processing in brainstem auditory neurons which receive giant endings (calyces of Held) in the medial nucleus of the trapezoid body of the cat

In the medial nucleus of the trapezoid body (MNTB), each principal neuron receives one large axonal ending (a calyx of Held) and many small endings. In this same region, microelectrode recordings show unusual 'unit' waveforms which have two components separated by about 0.5 ms. We show that the first component (C1) of such a waveform corresponds to a spike from the calyx of Held and that the second component (C2) corresponds to a spike from the MNTB principal neuron. There are two kinds of evidence for these correspondences. First, electrical stimulation of calyciferous axons in the contralateral trapezoid body evokes C1 spikes with latencies of 0.1-0.3 ms. These latencies are too short for there to be an intervening synapse and are consistent with C1 being a presynaptic spike. Second, shocks in the lateral superior olive (which receives projections from MNTB principal-neurons) evoke 'A' spikes in the MNTB which can be shown by their waveshapes and mutual refractoriness with C2 spikes to result from antidromic activation of the neurons producing C2 spikes. Spontaneous and sound-evoked responses in dozens of cats anesthetized by barbiturates or Ketamine always had a C2 spike following each C1 spike. This implies that there is normally one-to-one spike transmission from the calyx of Held input to the MNTB principal neuron output. The small endings on MNTB principal neurons are also capable of evoking spikes. Electric shocks (and in one case, sound), evoked long latency (1-3 ms) 'LC2' spikes, which (by mutual refractoriness and waveshape) are from the same neural elements as C2 and 'A' spikes. Since LC2 spikes are not preceded by C1 spikes, LC2 spikes must be mediated by small axonal endings on MNTB principal neurons. We found some evidence of inhibition, possibly recurrent inhibition, in MNTB principal neurons. In a few neurons, sound or shocks inhibited 'A' spikes or LC2 spikes. In some cases, after each C2 spike, LC2 spikes were blocked or reduced in amplitude for several milliseconds. Our data firmly establish that there is fast, secure spike transmission from calyces of Held to MNTB principal neurons and suggest that under some circumstances there is additional signal processing in MNTB principal neurons.     

Effect of noise exposure on gap detection in rats

The effects of intense (110–120 dB) noise exposure (broadband noise for one hour) on temporal resolution was estimated in rats by measuring the behavioural gap detection threshold (GDT). Changes in GDT after 120 dB noise exposure were compared with changes in the threshold and amplitude of middle latency responses (MLR) recorded in response to tone stimuli. GDT values increased from 1.6 to 4.3 or 7.8 ms after exposure to 110 or 115 dB SPL, respectively; GDT recovered to pre-exposure values in 3–7 days. Three main types of noise-induced changes were observed after 120 dB SPL exposure: (I) GDT changes similar to those following noise exposure to 115 dB SPL and maximal hearing threshold shifts (TSs) at high frequencies of about 45 dB; (II) more pronounced changes in GDT (up to 60 ms) with maximal hearing threshold shifts of about 65 dB and (III) a lack of reliable responses to gap during the first weeks post-exposure with maximal hearing threshold shifts of about 80 dB. An increased GDT was present two months after noise exposure in animals with types II and III post-exposure changes; enhanced MLR amplitudes were also found in most of these in the first post-exposure week. The pronounced deficit in gap detection in some rats after 120 dB SPL noise exposure may signal the presence of a noise-induced tinnitus.     

Comparison between local field potentials and unit cluster activity in primary auditory cortex and anterior auditory field in the cat

Multi-unit (MU) activity and local field potentials (LFP) were simultaneously recorded from 161 sites in the middle cortical layers of the primary auditory cortex (AI) and the anterior auditory field (AAF) in 51 cats. Responses were obtained for frequencies between 625 Hz and 40 kHz, at intensities from 75 dB SPL to threshold. We compared the response properties of MU activity and LFP triggers, in terms of characteristic frequency (CF), threshold at CF, minimum latency and frequency tuning-curve bandwidth 20 dB above threshold. On average, thresholds at CF were significantly lower for LFP events than those for MU spikes (4.6 dB for AI, and 3 dB for AAF). Minimum latencies were significantly shorter for LFP events than for MU spikes (1.5 ms in AI, and 1.7 ms in AAF). Frequency tuning curves were significantly broader for LFP events than those for MU spikes (1.0 octave in AI, and 1.3 octaves in AAF). In contrast, the CF was not significantly different between LFP events and MU spikes. The LFP results indicate that cortical neurons receive convergent sub-cortical inputs from a broad frequency range. The sharper tuning curves for MU activity compared to those of LFP events are likely the result of intracortical inhibitory processes.     

Brainstem auditory evoked response in the ferret (Mustela putorius)

The effect of click intensity, repetition rate and binaural interaction on the brainstem auditory evoked response (BAER) was examined in sixteen pigmented adult male ferrets. Potentials were recorded from platinum needle electrodes inserted over the vertex and left and right mastoids. Square waves, 100 microseconds in duration, were transduced by earphones enclosed in an assembly designed to fit securely over the ferret's external ear. The BAER in the ferret consists of four prominent vertex-positive peaks (P1-P4) and a fifth peak of smaller amplitude and more variable latency. The mean latencies of P1-P4 at 104 dB peak SPL were 0.96, 1.83, 2.75 and 3.62 ms. Reducing intensity over a 70 dB range resulted in a reduction in amplitude and a corresponding increase in latency ranging between 0.57 and 0.67 ms. Also, increasing click repetition rate resulted in a reduction in amplitude and an increase in latency. With intensity fixed at 104 dB peak SPL comparison of latencies at 10 and 50/s showed a mean increase of 20, 50, 60 and 80 microseconds for P1-P4, respectively. The effect of binaural interaction on the BAER was examined using the procedure of Dobie and Berlin (1979); the response evoked by binaural stimulation was subtracted from the summed left and right monaural responses to obtain a binaural interaction component. Binaural interaction in the ferret gave rise to a distinct vertex-negative wave with a latency similar to P4. An increase in click intensity over a 70 dB range resulted in a monotonic increase in amplitude and a decrease in latency of the binaural interaction component.     

Changes Across Time in the Temporal Responses of Auditory Nerve Fibers Stimulated by Electric Pulse Trains

Most auditory prostheses use modulated electric pulse trains to excite the auditory nerve. There are, however, scant data regarding the effects of pulse trains on auditory nerve fiber (ANF) responses across the duration of such stimuli. We examined how temporal ANF properties changed with level and pulse rate across 300-ms pulse trains. Four measures were examined: (1) first-spike latency, (2) interspike interval (ISI), (3) vector strength (VS), and (4) Fano factor (FF, an index of the temporal variability of responsiveness). Data were obtained using 250-, 1,000-, and 5,000-pulse/s stimuli. First-spike latency decreased with increasing spike rate, with relatively small decrements observed for 5,000-pulse/s trains, presumably reflecting integration. ISIs to low-rate (250 pulse/s) trains were strongly locked to the stimuli, whereas ISIs evoked with 5,000-pulse/s trains were dominated by refractory and adaptation effects. Across time, VS decreased for low-rate trains but not for 5,000-pulse/s stimuli. At relatively high spike rates (>200 spike/s), VS values for 5,000-pulse/s trains were lower than those obtained with 250-pulse/s stimuli (even after accounting for the smaller periods of the 5,000-pulse/s stimuli), indicating a desynchronizing effect of high-rate stimuli. FF measures also indicated a desynchronizing effect of high-rate trains. Across a wide range of response rates, FF underwent relatively fast increases (i.e., within 100 ms) for 5,000-pulse/s stimuli. With a few exceptions, ISI, VS, and FF measures approached asymptotic values within the 300-ms duration of the low- and high-rate trains. These findings may have implications for designs of cochlear implant stimulus protocols, understanding electrically evoked compound action potentials, and interpretation of neural measures obtained at central nuclei, which depend on understanding the output of the auditory nerve.     

Effects of stimulus manipulation on electrophysiological responses of pediatric cochlear implant users. Part II: rate effects

Electrophysiological thresholds do not accurately predict behavioral thresholds in pediatric cochlear implant users possibly due to differences in rate and duration of pulse presentation. We asked: (1) Is there an effect of rate of stimulus presentation on the electrophysiological responses of the auditory brainstem and thalamo-cortex? and (2) can the relationship between electrophysiological and behavioral thresholds be improved by using the same rate of pulse presentation? Behavioral and electrophysiological (EABR and EMLR) responses were elicited for 14 children to single electrical pulses and pulse trains of 2ms ranging in rate from 500 to 3600 pulses per second (pps). Low rate (500pps) pulse trains resulted in an increase in EABR wave eIII amplitude and a decrease in wave eV amplitude. Further rate increases resulted in smaller EABR wave amplitudes. EMLR amplitudes were unaffected by increases in rate as were EABR and EMLR latencies. Behavioral thresholds decreased with increasing rate, however, there was no associated reduction in electrophysiological thresholds. Correlation between behavioral and electrophysiological thresholds did not improve by using the same rate of electrical pulse stimulation. Results suggest: (1) Higher rates of electrical pulse presentation increase the potential for neural adaptation in the auditory brainstem and (2) using the same rate of electrical pulse presentation does not improve the ability of EABR and EMLR thresholds to predict behavioral thresholds.     

Effects of interaural intensity and time disparity on transient evoked otoacoustic emissions

Monaural and binaural 11/s, 65 dB pe SPL clicks with interaural time and intensity disparities known to affect central auditory processing were used to study contralateral suppression of transient evoked otoacoustic emissions (TEOAEs) in 10 subjects (20 ears). Psychophysical assessment of sound lateralization induced by the same stimuli was also conducted. TEOAEs were recorded to monaural (ipsilateral to the OAE recording probe) and to binaural clicks when clicks to the contralateral ear were synchronous and symmetrical in intensity, or, in the binaural intensity disparity conditions, synchronous but 10 dB higher or 10 dB lower in the ear contralateral to the OAE recording probe. When interaural time disparities were studied, the clicks to the contralateral ear were of the same intensity throughout, but 400 μs earlier or 400 μs later than to the ear with the probe. The TEOAE components at 13–15.8 ms showed suppression, relative to monaural responses, under all binaural conditions. This contralateral suppression did not correlate with the psychophysical findings. Suppression effects were more pronounced with binaural disparity than with binaurally symmetrical clicks. Thus, although contralateral click intensity was the same with time disparities, suppression was paradoxically enhanced compared to the binaurally symmetrical stimulation. To explain these results we propose that two factors are involved in TEOAE suppression with binaural clicks: (1) contralateral intensity and (2) interaural disparity (time or intensity). The latency of the suppressions observed, the effect of interaural disparity on these suppressions, coupled with the anatomical origin of the crossed efferent fibers and the disparity sensitivity of the superior olivary complex (SOC), all suggest SOC involvement in these TEOAE suppressions.     

Long-term study after perforation of the round window. Animal experiments using electric response audiometry

The round-window membrane of the inner ear of the guinea pig was perforated with a platinum wire under ketamine-xylazine anaesthesia. The latency times of waves I and V (Jewett) increased to 0.6 ms at 100 dB click HL stimulus loudness. The interpeak latencies did not change (4.0-4.2 ms). At 60 dB CHL stimulus loudness, no responses were discernible. Closure of the membrane damage by adhesive fibrin tissue had no effect on the auditory nerve potentials or the brain-stem responses. Normal latency times of waves I-V were seen 7 days after perforation. There was no difference between the animals with repaired and unrepaired membrane damage. We observed spontaneous healing of the round-window membrane 7 days after perforation, and a normal organ of Corti.     

Electrical and physiological changes during short-term and chronic electrical stimulation of the normal cochlea

In the electrical stimulation (ES) of auditory pathways, the type of stimulus and the electrode/tissue interface are critical parameters for the safety and efficacy of the protocol. In this study the influence of alternate pulses, applied between round window and vertex electrodes in chronically implanted guinea pigs, and maintained during 1 and 25 daily periods of 2 h (short-term and long-term experiments, respectively), was investigated. ES consisted of monophasic current pulses of ±70 μA and 300 μs in duration at a rate of 167/s, with alternate polarity. Compound Action Potential (CAP) audiograms, amplitudes and latencies of click-evoked CAPs, amplitudes and latencies of electrically-evoked auditory responses (EARs), and electrode impedances, were measured periodically outside or during the ES periods. Short-term ES induced no change in CAP thresholds, amplitude and latency in response to clicks at 80 dB above normal threshold, but induced a slight latency increase and amplitude decrease of the EAR, correlated with an exponential decay of the electrode impedance. On a long-term basis, CAP audiograms and latencies did not change significantly, whereas CAP amplitudes and electrode impedances increased, in correlation with each other. In control guinea pigs receiving no ES, the same CAP amplitude and impedance changes were observed over the same long-term period. The EAR and CAP changes can be explained by a variation of the electrical impedance of the electrode/tissue interface. This is possibly due to a change in electrotytes around the electrode under the influence of the ES for the short-term variation, and to an electrode encapsulation by fibrous tissue independent of the ES for the long-term change. In itself, and under the conditions of this experiment, the ES demonstrated no adverse effects on the auditory function and can be safely used for inner-ear exploration.     

Temporal Coding by Cochlear Nucleus Bushy Cells in DBA/2J Mice with Early Onset Hearing Loss

The bushy cells of the anterior ventral cochlear nucleus (AVCN) preserve or improve the temporal coding of sound information arriving from auditory nerve fibers (ANF). The critical cellular mechanisms entailed in this process include the specialized nerve terminals, the endbulbs of Held, and the membrane conductance configuration of the bushy cell. In one strain of mice (DBA/2J), an early-onset hearing loss can cause a reduction in neurotransmitter release probability, and a smaller and slower spontaneous miniature excitatory postsynaptic current (EPSC) at the endbulb synapse. In the present study, by using a brain slice preparation, we tested the hypothesis that these changes in synaptic transmission would degrade the transmission of timing information from the ANF to the AVCN bushy neuron. We show that the electrical excitability of bushy cells in hearing-impaired old DBA mice was different from that in young, normal-hearing DBA mice. We found an increase in the action potential (AP) firing threshold with current injection; a larger AP afterhyperpolarization; and an increase in the number of spikes produced by large depolarizing currents. We also tested the temporal precision of bushy cell responses to high-frequency stimulation of the ANF. The standard deviation of spikes (spike jitter) produced by ANF-evoked excitatory postsynaptic potentials (EPSPs) was largely unaffected in old DBA mice. However, spike entrainment during a 100-Hz volley of EPSPs was significantly reduced. This was not a limitation of the ability of bushy cells to fire APs at this stimulus frequency, because entrainment to trains of current pulses was unaffected. Moreover, the decrease in entrainment is not attributable to increased synaptic depression. Surprisingly, the spike latency was 0.46 ms shorter in old DBA mice, and was apparently attributable to a faster conduction velocity, since the evoked excitatory postsynaptic current (EPSC) latency was shorter in old DBA mice as well. We also tested the contribution of the low-voltage-activated K⁺ conductance (g _KLV) on the spike latency by using dynamic clamp. Alteration in g _KLV had little effect on the spike latency. To test whether these changes in DBA mice were simply a result of continued postnatal maturation, we repeated the experiments in CBA mice, a strain that shows normal hearing thresholds through this age range. CBA mice exhibited no reduction in entrainment or increased spike jitter with age. We conclude that the ability of AVCN bushy neurons to reliably follow ANF EPSPs is compromised in a frequency-dependent fashion in hearing-impaired mice. This effect can be best explained by an increase in spike threshold.     

Influence of stimulus intensity on AEP components in the 80- to 200-millisecond latency range

The influence of the intensity of passively perceived 1-kHz tones on the latencies and amplitudes of AEP components in the 80- to 200-ms latency range recorded from frontal, central and temporal electrode locations was investigated in healthy adult subjects. At stimulus intensities from 30 to 70 dB SL the latencies of the frontally and centrally recorded N100 and P175 waves decreased, their amplitudes increased. At stimulus intensities from 70 to 90 dB the latencies of N100 and P175 increased, N100 amplitude declined and P175 amplitude increased at a slower rate than between 30 and 70 dB. The latency of the N140 wave recorded at temporal electrode locations decreased markedly with increasing stimulus intensity.     

Influence of Stimulus Intensity on AEP Components in the 80- to 200-Millisecond Latency Range

The influence of the intensity of passively perceived 1-kHz tones on the latencies and amplitudes of AEP components in the 80- to 200-ms latency range recorded from frontal, central and temporal electrode locations was investigated in healthy adult subjects. At stimulus intensities from 30 to 70 dB SL the latencies of the frontally and centrally recorded N100 and P175 waves decreased, their amplitudes increased. At stimulus intensities from 70 to 90 dB the latencies of N100 and P175 increased, N100 amplitude declined and P175 amplitude increased at a slower rate than between 30 and 70 dB. The latency of the N140 wave recorded at temporal electrode locations decreased markedly with increasing stimulus intensity.     

Noise impairment in the guinea pig. II. Changes of single unit responses in the inferior colliculus

Spontaneous and evoked activity of neurons in the inferior colliculus of guinea pigs was recorded before and after exposure to noise (continuous or intermittent white noise, 115 dB SPL for 30 min). A single unit was investigated in each animal, and its activity was monitored for several hours.Exposure to noise elevated the threshold of the tip of the tuning curve, resulting in a broadening of the tuning curve. Threshold elevation at the characteristic frequency was greater after exposure to intermittent noise (200 ms noise and 200 ms pause), reaching values of 22.8 ± 3.7 dB ($\text{x}\text{?}\text{± S.E.}$) than it was after exposure to continuous noise (threshold elevation of 13.1 ± 1.7 dB). The average threshold shift was 17.1 ± 2 dB.Neither the shape of the poststimulus histograms nor the slope of the spike-intensity curves changed with the noise exposure. The total number of spikes during the response was, however, reduced, and the reduction was in proportion to the threshold elevation. Monaural noise exposure had no effect on the neuronal activity evoked by stimulation of the opposite, nonexposed ear. The latencies of responses recorded after exposure to noise were also longer than the latencies at the same absolute intensity recorded before the exposure. Thus the latencies during the original pre-exposure and acquired postexposure thresholds were practically identical.     

Distributed representation of spectral and temporal information in rat primary auditory cortex.

Modulations of amplitude and frequency are common features of natural sounds, and are prominent in behaviorally important communication sounds. The mammalian auditory cortex is known to contain representations of these important stimulus parameters. This study describes the distributed representations of tone frequency and modulation rate in the rat primary auditory cortex (A1). Detailed maps of auditory cortex responses to single tones and tone trains were constructed from recordings from 50-60 microelectrode penetrations introduced into each hemisphere. Recorded data demonstrated that the cortex uses a distributed coding strategy to represent both spectral and temporal information in the rat, as in other species. Just as spectral information is encoded in the firing patterns of neurons tuned to different frequencies, temporal information appears to be encoded using a set of filters covering a range of behaviorally important repetition rates. Although the average A1 repetition rate transfer function (RRTF) was low-pass with a sharp drop-off in evoked spikes per tone above 9 pulses per second (pps), individual RRTFs exhibited significant structure between 4 and 10 pps, including substantial facilitation or depression to tones presented at specific rates. No organized topography of these temporal filters could be determined.