Contributions of medial geniculate body subdivisions to the middle latency response

Habituation disorders in auditory middle latency response of persistent postural-perceptual dizziness patients

Objectives

To study habituation disorders in auditory middle latency response (AMLR) to repetitive stimuli of persistent postural-perceptual dizziness (PPPD) patients.

Subjects

Twenty-eight PPPD (10 men and 18 women, mean 59.5 years of age, 26-81 years of age) were enrolled. For comparison, data of 13 definite vestibular migraine (VM) patients (3 men, 10 women, mean age 45.5), 13 definite unilateral Meniere's disease (MD) patients (2 men, 11 women, mean age 50.6), and 8 healthy control (HC) subjects (2 men, 6 women, mean age 37.1) in the previous study were utilized.

Methods

The electrodes were placed on the vertex and the spinal process of the fifth cervical vertebra. Clicks (0.1 msec, 70 dB nHL) were binaurally presented and averaged (800 times). Averaged responses were divided into 4 sets (S1 to S4) according to the temporal order. As peaks, Na, and Pa were identified, and relative Na-Pa amplitudes in S2-S4 to S1 were analyzed.

Results

The mean relative amplitude of PPPD patients showed lack of habituation (potentiation) as shown in VM patients, although the extent of potentiation was weaker than VM. Comparison of relative S4 amplitudes showed significant differences among the 4 groups (p = 0.0013 one-way ANOVA), Multiple comparison revealed significant differences between PPPD and MD (p = 0.0337 Dunnet's test).

Conclusion

PPPD patients showed lack of habituation (potentiation) of Na-Pa amplitude in AMLR to repetitive stimuli. Lack of habituation (potentiation) might be associated with sensory processing disorders in PPPD.

Tonotopic Selectivity in Cats and Humans: Electrophysiology and Psychophysics

We describe a scalp-recorded measure of tonotopic selectivity, the "cortical onset response" (COR) and compare the results between humans and cats. The COR results, in turn, were compared with psychophysical masked-detection thresholds obtained using similar stimuli and obtained from both species. The COR consisted of averaged responses elicited by 50-ms tone-burst probes presented at 1-s intervals against a continuous noise masker. The noise masker had a bandwidth of 1 or 1/8th octave, geometrically centred on 4000 Hz for humans and on 8000 Hz for cats. The probe frequency was either - 0.5, - 0.25, 0, 0.25 or 0.5 octaves re the masker centre frequency. The COR was larger for probe frequencies more distant from the centre frequency of the masker, and this effect was greater for the 1/8th-octave than for the 1-octave masker. This pattern broadly reflected the masked excitation patterns obtained psychophysically with similar stimuli in both species. However, the positive signal-to-noise ratio used to obtain reliable COR measures meant that some aspects of the data differed from those obtained psychophysically, in a way that could be partly explained by the upward spread of the probe's excitation pattern. Our psychophysical measurements also showed that the auditory filter width obtained at 8000 Hz using notched-noise maskers was slightly wider in cat than previous measures from humans. We argue that although conclusions from COR measures differ in some ways from conclusions based on psychophysics, the COR measures provide an objective, noninvasive, valid measure of tonotopic selectivity that does not require training and that may be applied to acoustic and cochlear-implant experiments in humans and laboratory animals.

Longitudinal auditory pathophysiology following mild blast-induced trauma

Blast-induced hearing difficulties affect thousands of veterans and civilians. The long-term impact of even a mild blast exposure on the central auditory system is hypothesized to contribute to lasting behavioral complaints associated with mild blast traumatic brain injury (bTBI). Although recovery from mild blast has been studied separately over brief or long time windows, few, if any, studies have investigated recovery longitudinally over short-term and longer-term (months) time windows. Specifically, many peripheral measures of auditory function either recover or exhibit subclinical deficits, masking deficits in processing complex, real-world stimuli that may recover differently. Thus, examining the acute time course and pattern of neurophysiological impairment using appropriate stimuli is critical to better understanding and intervening in bTBI-induced auditory system impairments. Here, we compared auditory brainstem response, middle-latency auditory-evoked potentials, and envelope following responses. Stimuli were clicks, tone pips, amplitude-modulated tones in quiet and in noise, and speech-like stimuli (iterated rippled noise pitch contours) in adult male rats subjected to mild blast and sham exposure over the course of 2 mo. We found that blast animals demonstrated drastic threshold increases and auditory transmission deficits immediately after blast exposure, followed by substantial recovery during the window of 7-14 days postblast, although with some deficits remaining even after 2 mo. Challenging conditions and speech-like stimuli can better elucidate mild bTBI-induced auditory deficit during this period. Our results suggest multiphasic recovery and therefore potentially different time windows for treatment, and deficits can be best observed using a small battery of sound stimuli.NEW & NOTEWORTHY Few studies on blast-induced hearing deficits go beyond simple sounds and sparsely track postexposure. Therefore, the recovery arc for potential therapies and real-world listening is poorly understood. Evidence suggested multiple recovery phases over 2 mo postexposure. Hearing thresholds largely recovered within 14 days and partially explained recovery. However, midlatency responses, responses to amplitude modulation in noise, and speech-like pitch sweeps exhibited extended changes, implying persistent central auditory deficits and the importance of subclinical threshold shifts.

Brain mapping of auditory steady-state responses: A broad view of cortical and subcortical sources

Auditory steady-state responses (ASSRs) are evoked brain responses to modulated or repetitive acoustic stimuli. Investigating the underlying neural generators of ASSRs is important to gain in-depth insight into the mechanisms of auditory temporal processing. The aim of this study is to reconstruct an extensive range of neural generators, that is, cortical and subcortical, as well as primary and non-primary ones. This extensive overview of neural generators provides an appropriate basis for studying functional connectivity. To this end, a minimum-norm imaging (MNI) technique is employed. We also present a novel extension to MNI which facilitates source analysis by quantifying the ASSR for each dipole. Results demonstrate that the proposed MNI approach is successful in reconstructing sources located both within (primary) and outside (non-primary) of the auditory cortex (AC). Primary sources are detected in different stimulation conditions (four modulation frequencies and two sides of stimulation), thereby demonstrating the robustness of the approach. This study is one of the first investigations to identify non-primary sources. Moreover, we show that the MNI approach is also capable of reconstructing the subcortical activities of ASSRs. Finally, the results obtained using the MNI approach outperform the group-independent component analysis method on the same data, in terms of detection of sources in the AC, reconstructing the subcortical activities and reducing computational load.

Vestibular Migraine Patients Show Lack of Habituation in Auditory Middle Latency Responses to Repetitive Stimuli: Comparison With Meniere's Disease Patients

Objectives: To compare habituation in auditory middle latency response (AMLR) to repetitive stimuli of vestibular migraine (VM) patients with Meniere's disease (MD) patients and healthy controls (HC) and to assess usefulness of AMLR for diagnosis of VM. Subjects: Thirteen unilateral definite MD patients (2 men, 11 women, mean age 50.6), 13 definite VM patients (3 men, 10 women, mean age 45.5), and 8 HC subjects (2 men, 6 women, mean age 37.1) were enrolled. Methods: The electrodes were placed on the vertex and the spinal process of the fifth cervical vertebra. Binaural click stimulation (0.1 ms, 70 dBnHL) was presented. A total of 800 responses were averaged. Averaged responses were divided into four sets (S1 to S4) according to the temporal order. No, Po, Na, and Pa were identified, and amplitudes and latencies were measured. Results: Concerning latencies, HC subjects showed a tendency of shorter latencies. However, there was no clear effect of repetitive stimulation. Concerning No-Po amplitudes, no significant differences were observed. Raw amplitudes of Na-Pa showed statistically significant differences in S1 and S2 among the groups (p < 0.01 one-way ANOVA). Differences were shown in MD vs. VM and HC vs. VM in S1 (smaller in VM) (p < 0.01 Bonferroni's test) and in MD vs. VM in S2 (smaller in VM) (p < 0.01 Bonferroni test). Relative amplitudes of Na-Pa to S1 showed statistically significant differences in S4 (p < 0.01 one-way ANOVA). Differences were shown in MD vs. VM and HC vs. VM (larger in VM) (p < 0.01 Bonferroni's test). Differences of Na-Pa amplitudes in S2 to S4 from Na-Pa amplitude in S1 were significant in S4 of VM patients (Dunnett's test). Conclusions: VM patients showed lack of habituation (potentiation) of Na-Pa amplitude in AMLR to repetitive stimuli while MD patients and HC subjects showed habituation. Observation of lack of habituation has high diagnostic accuracy for differential diagnosis of VM from MD.

A comparison of auditory evoked potentials to acoustic beats and to binaural beats

The purpose of this study was to compare cortical brain responses evoked by amplitude modulated acoustic beats of 3 and 6 Hz in tones of 250 and 1000 Hz with those evoked by their binaural beats counterparts in unmodulated tones to indicate whether the cortical processes involved differ. Event-related potentials (ERPs) were recorded to 3- and 6-Hz acoustic and binaural beats in 2000 ms duration 250 and 1000 Hz tones presented with approximately 1 s intervals. Latency, amplitude and source current density estimates of ERP components to beats-evoked oscillations were determined and compared across beat types, beat frequencies and base (carrier) frequencies. All stimuli evoked tone-onset components followed by oscillations corresponding to the beat frequency, and a subsequent tone-offset complex. Beats-evoked oscillations were higher in amplitude in response to acoustic than to binaural beats, to 250 than to 1000 Hz base frequency and to 3 Hz than to 6 Hz beat frequency. Sources of the beats-evoked oscillations across all stimulus conditions located mostly to left temporal lobe areas. Differences between estimated sources of potentials to acoustic and binaural beats were not significant. The perceptions of binaural beats involve cortical activity that is not different than acoustic beats in distribution and in the effects of beat- and base frequency, indicating similar cortical processing.

Cortical evoked potentials to an auditory illusion: binaural beats

OBJECTIVE

To define brain activity corresponding to an auditory illusion of 3 and 6Hz binaural beats in 250Hz or 1000Hz base frequencies, and compare it to the sound onset response.

METHODS

Event-Related Potentials (ERPs) were recorded in response to unmodulated tones of 250 or 1000Hz to one ear and 3 or 6Hz higher to the other, creating an illusion of amplitude modulations (beats) of 3Hz and 6Hz, in base frequencies of 250Hz and 1000Hz. Tones were 2000ms in duration and presented with approximately 1s intervals. Latency, amplitude and source current density estimates of ERP components to tone onset and subsequent beats-evoked oscillations were determined and compared across beat frequencies with both base frequencies.

RESULTS

All stimuli evoked tone-onset P(50), N(100) and P(200) components followed by oscillations corresponding to the beat frequency, and a subsequent tone-offset complex. Beats-evoked oscillations were higher in amplitude with the low base frequency and to the low beat frequency. Sources of the beats-evoked oscillations across all stimulus conditions located mostly to left lateral and inferior temporal lobe areas in all stimulus conditions. Onset-evoked components were not different across stimulus conditions; P(50) had significantly different sources than the beats-evoked oscillations; and N(100) and P(200) sources located to the same temporal lobe regions as beats-evoked oscillations, but were bilateral and also included frontal and parietal contributions.

CONCLUSIONS

Neural activity with slightly different volley frequencies from left and right ear converges and interacts in the central auditory brainstem pathways to generate beats of neural activity to modulate activities in the left temporal lobe, giving rise to the illusion of binaural beats. Cortical potentials recorded to binaural beats are distinct from onset responses.

SIGNIFICANCE

Brain activity corresponding to an auditory illusion of low frequency beats can be recorded from the scalp.

Multiple neural origins of early auditory evoked potential of rat

The multiple-origin hypothesis has been often considered for an unclear neurogenesis of a characteristic wave in various evoked potentials, none of which has been verified so far. Auditory evoked potential (AEP) in the temporal cortex of rodents has typical slow positive/negative (P1/N1) biphasic waves, which are occasionally associated with an additional 2-4-ms earlier small deflection (P0/N0). Despite previous extensive efforts, P0/N0 deflection is still discussed within the multiple-origin hypothesis. In this historical perspective, we hypothesized that observable AEP is an additive mixture of mutually temporally independent signals from different origins, and that the balance of the mixture impacts on the waveform of AEP. We attempted to verify this hypothesis for the first time by independent component analysis (ICA) of epidurally densely mapped AEPs in the primary auditory cortex of rats. The mapping showed that low amplitude AEPs tended to have more P0/N0 deflections in both pentobarbital- and ketamine/xylazine-anesthesia preparations. ICA of these AEP maps suggested that AEP consisted of at least three independent components and that the deflection appeared when subcortical contribution to AEP was equal to or larger than cortical contribution. In epicranially measured evoked potentials, subcortical and cortical contributions are mixed together because distances from electrodes to cortical sources approximate distances to subcortical sources. In such conditions, e.g. in human scalp-recording experiments or routine clinical screenings, our idea is specifically worth considering for the interpretation of signals.

Pb(P1) resonance at 40Hz: Effects of high stimulus rate on auditory middle latency responses (MLRs) explored using deconvolution

OBJECTIVE

In this study, the effects of high stimulus rate on middle latency response (MLR) components P(a) and P(b) (P(1) or P50) were studied using high rate clicks in normal hearing awake subjects were investigated.

METHODS

Five jittered click sequences at different mean rates (24.4, 39.1, 58.6, 78.1, 97.7Hz) were presented to 10 subjects. Overlapping MLRs were deconvolved using the frequency domain continuous loop averaging deconvolution (CLAD) [Ozdamar O., Bohórquez, J., Signal to noise ratio and frequency analysis of continuous loop averaging deconvolution (CLAD) of overlapping evoked potentials. J. Acoust. Soc. Am., 119:429-438, 2006]. In addition conventional auditory transient MLRs at 4.88Hz were obtained using conventional averaging.

RESULTS

P(a) amplitude, latency and waveshape remained fairly constant up to 78.1Hz. P(b) component, however, showed a variable waveshape with latencies covering a wide range (50-70ms) and N(b)-P(b) amplitudes increasing at 39.1 and 58.6Hz and decreasing at other rates.

CONCLUSIONS

Recordings show that both P(a) and P(b) MLR components can be consistently recorded at all rates up to 100Hz. P(b) amplitude shows an increase at around 40Hz showing a resonance at that frequency.

SIGNIFICANCE

The dramatic increase of the P(b) component at around 40Hz may account for the high amplitude of the 40Hz ASSR.

Auditory processing disorders, verbal disfluency, and learning difficulties: a case study

This case study reports the findings of auditory behavioral and electrophysiological measures performed on a graduate student (identified as LN) presenting verbal disfluency and learning difficulties. Results of behavioral audiological testing documented the presence of auditory processing disorders, particularly temporal processing and binaural integration. Electrophysiological test results, including middle latency, late latency and cognitive potentials, revealed that LN's central auditory system processes acoustic stimuli differently to a reference group with normal hearing.

Object-related brain potentials associated with the perceptual segregation of a dichotically embedded pitch

The cortical mechanisms of perceptual segregation of concurrent sound sources were examined, based on binaural detection of interaural timing differences. Auditory event-related potentials were measured from 11 healthy subjects. Binaural stimuli were created by introducing a dichotic delay of 500-ms duration to a narrow frequency region within a broadband noise, and resulted in a perception of a centrally located noise and a right-lateralized pitch (dichotic pitch). In separate listening conditions, subjects actively discriminated and responded to randomly interleaved binaural and control stimuli, or ignored random stimuli while watching silent cartoons. In a third listening condition subjects ignored stimuli presented in homogenous blocks. For all listening conditions, the dichotic pitch stimulus elicited an object-related negativity (ORN) at a latency of about 150-250 ms after stimulus onset. When subjects were required to actively respond to stimuli, the ORN was followed by a P400 wave with a latency of about 320-420 ms. These results support and extend a two-stage model of auditory scene analysis in which acoustic streams are automatically parsed into component sound sources based on source-relevant cues, followed by a controlled process involving identification and generation of a behavioral response.

Effects of cochlear-implant pulse rate and inter-channel timing on channel interactions and thresholds

Interactions among the multiple channels of a cochlear prosthesis limit the number of channels of information that can be transmitted to the brain. This study explored the influence on channel interactions of electrical pulse rates and temporal offsets between channels. Anesthetized guinea pigs were implanted with 2-channel scala-tympani electrode arrays, and spike activity was recorded from the auditory cortex. Channel interactions were quantified as the reduction of the threshold for pulse-train stimulation of the apical channel by sub-threshold stimulation of the basal channel. Pulse rates were 254 or 4069 pulses per second (pps) per channel. Maximum threshold reductions averaged 9.6 dB when channels were stimulated simultaneously. Among nonsimultaneous conditions, threshold reductions at the 254-pps rate were entirely eliminated by a 1966-micros inter-channel offset. When offsets were only 41 to 123 micros, however, maximum threshold shifts averaged 3.1 dB, which was comparable to the dynamic ranges of cortical neurons in this experimental preparation. Threshold reductions at 4069 pps averaged up to 1.3 dB greater than at 254 pps, which raises some concern in regard to high-pulse-rate speech processors. Thresholds for various paired-pulse stimuli, pulse rates, and pulse-train durations were measured to test possible mechanisms of temporal integration.

Representation of concurrent acoustic objects in primary auditory cortex

Auditory scene analysis involves the simultaneous grouping and parsing of acoustic data into separate mental representations (i.e., objects). Over two experiments, we examined the sequence of neural processes underlying concurrent sound segregation by means of recording of human middle latency auditory evoked responses. Participants were presented with complex sounds comprising several harmonics, one of which could be mistuned such that it was not an integer multiple of the fundamental frequency. In both experiments, Na (approximately 22 ms) and Pa (approximately 32 ms) waves were reliably generated for all classes of stimuli. For stimuli with a fundamental frequency of 200 Hz, the mean Pa amplitude was significantly larger when the third harmonic was mistuned by 16% of its original value, relative to when it was tuned. The enhanced Pa amplitude was related to an increased likelihood in reporting the presence of concurrent auditory objects. Our results are consistent with a low-level stage of auditory scene analysis in which acoustic properties such as mistuning act as preattentive segregation cues that can subsequently lead to the perception of multiple auditory objects.

Electrophysiological correlates of azimuth and elevation cues for sound localization in human middle latency auditory evoked potentials

OBJECTIVE

To study, in humans, the effects of sound source azimuth and elevation on primary auditory cortex binaural activity associated with sound localization.

DESIGN

Middle Latency Auditory Evoked Potentials (MLAEPs) were recorded from three channels, in response to alternating polarity clicks, presented at a rate of 5/sec, at nine virtual spatial locations with different azimuths and elevations. Equivalent dipoles of Binaural Interaction Components (BICs) of MLAEPs were derived from 15 normally and symmetrically hearing adults by subtracting the response to binaural clicks at each spatial location from the algebraic sum of responses to stimulation of each ear alone. The amplified potentials were averaged over 4000 repetitions using a dwell time of 78 micro sec/address/channel. Variations in magnitudes, latencies and orientations of the dipole equivalents of cortical activity were noted in response to the nine spatial locations.

RESULTS

Middle-latency BICs included six major components corresponding in latency to the vertex-neck recorded components of MLAEP. A significant decrease of equivalent dipole magnitude was observed for two of the components: Pa2 in response to clicks in the backward positions (medium and no elevation); and Nb in response to clicks in the back and front positions (medium and no elevation) in the midsagittal plane. In the coronal plane, Pa2 equivalent dipole magnitude significantly decreased in response to right-horizontal (no elevation) clicks. Significant effects on equivalent dipole latencies of Pa2 were found for backward positions (no elevation) in the midsagittal plane. No significant effects on Pa2 and Nb equivalent dipole orientations were found across stimulus conditions.

CONCLUSIONS

The changes in equivalent dipole magnitudes and latencies of MLAEP BICs across stimulus conditions may reflect spectral tuning in binaural primary auditory cortex neurons processing the frequency cues for sound localization.

Effects of noise and cue enhancement on neural responses to speech in auditory midbrain, thalamus and cortex

Speech perception depends on the auditory system's ability to extract relevant acoustic features from competing background noise. Despite widespread acknowledgement that noise exacerbates this process, little is known about the neurophysiologic mechanisms underlying the encoding of speech in noise. Moreover, the relative contribution of different brain nuclei to these processes has not been fully established. To address these issues, aggregate neural responses were recorded from within the inferior colliculus, medial geniculate body and over primary auditory cortex of anesthetized guinea pigs to a synthetic vowel-consonant-vowel syllable /ada/ in quiet and in noise. In noise the onset response to the stop consonant /d/ was reduced or eliminated at each level, to the greatest degree in primary auditory cortex. Acoustic cue enhancements characteristic of 'clear' speech (lengthening the stop gap duration and increasing the intensity of the release burst) improved the neurophysiologic representation of the consonant at each level, especially at the cortex. Finally, the neural encoding of the vowel segment was evident at subcortical levels only, and was more resistant to noise than encoding of the dynamic portion of the consonant (release burst and formant transition). This experiment sheds light on which speech-sound elements are poorly represented in noise and demonstrates how acoustic modifications to the speech signal can improve neural responses in a normal auditory system. Implications for understanding neurophysiologic auditory signal processing in children with perceptual impairments and the design of efficient perceptual training strategies are also discussed.

The neurophysiology of auditory perception: from single units to evoked potentials

Evoked electric potential and magnetic field studies have the immense benefit that they can be conducted in awake, behaving humans and can be directly correlated with aspects of perception. As such, they are powerful objective indicators of perceptual properties. However, given a set of evoked potential and/or evoked field waveforms and their source locations, obtained for an exhaustive set of stimuli and stimulus contrasts, is it possible to determine blindly, i.e. predict, what the stimuli or stimulus contrasts were? If this can be done with some success, then a useful amount of information resides in scalp-recorded activity for, e.g., the study of auditory speech processing. In this review, we compare neural representations based on single-unit and evoked response activity for vowels and consonant-vowel phonemes with distinctions in formant glides and voice onset time. We conclude that temporal aspects of evoked responses can track some of the dominant response features present in single-unit activity. However, N1 morphology does not reliably predict phonetic identification of stimuli varying in voice onset time, and the reported appearance of a double-peak onset response in aggregate recordings from the auditory cortex does not indicate a cortical correlate of the perception of voicelessness. This suggests that temporal aspects of single-unit population activity are likely not inclusive enough for representation of categorical perception boundaries. In contrast to population activity based on single-unit recording, the ability to accurately localize the sources of scalp-evoked activity is one of the bottlenecks in obtaining an accessible neurophysiological substrate of perception. Attaining this is one of the requisites to arrive at the prospect of blind determination of stimuli on the basis of evoked responses. At the current sophistication level of recording and analysis, evoked responses remain in the realm of extremely sensitive objective indicators of stimulus change or stimulus differences. As such, they are signs of perceptual activity, but not comprehensive representations thereof.

Auditory thalamocortical synaptic transmission in vitro

To facilitate an understanding of auditory thalamocortical mechanisms, we have developed a mouse brain-slice preparation with a functional connection between the ventral division of the medial geniculate (MGv) and the primary auditory cortex (ACx). Here we present the basic characteristics of the slice in terms of physiology (intracellular and extracellular recordings, including current source density analysis), pharmacology (including glutamate receptor involvement), and anatomy (gross anatomy, Nissl, parvalbumin immunocytochemistry, and tract tracing with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate). Thalamocortical transmission in this preparation (the "primary" slice) involves both alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid/kainate and N-methyl-D-aspartate-type glutamate receptors that appear to mediate monosynaptic inputs to layers 3-4 of ACx. MGv stimulation also initiates disynaptic inhibitory postsynaptic potentials and longer-duration intracortical, polysynaptic activity. Important differences between responses elicited by MGv versus conventional columnar ("on-beam") stimulation emphasize the necessity of thalamic activation to infer thalamocortical mechanisms. We also introduce a second slice preparation, the "shell" slice, obtained from the brain region immediately ventral to the primary slice, that may contain a nonprimary thalamocortical pathway to temporal cortex. In the shell slice, stimulation of the thalamus or the region immediately ventral to it appears to produce fast activation of synapses in cortical layer 1 followed by robust intracortical polysynaptic activity. The layer 1 responses may result from orthodromic activation of nonprimary thalamocortical pathways; however, a plausible alternative could involve antidromic activation of corticotectal neurons and their layer 1 collaterals. The primary and shell slices will provide useful tools to investigate mechanisms of information processing in the ACx.

Acute effect of nicotine on non-smokers: II. MLRs and 40-Hz responses

This paper is the second in a series of three investigating the role of cholinergic mechanisms in the auditory system by assessing the acute effects of nicotine, an acetylcholinomimetic drug, on aggregate responses within the auditory pathway. In a single-blind procedure, auditory responses were obtained from 20 normal-hearing, non-smokers (10 male) under two conditions (nicotine, placebo). The effects of nicotine on central, mesogenous responses of the auditory system (middle latency and 40-Hz responses) are described in this second paper. Results indicated that transdermal administration of nicotine to non-smokers does significantly affect the central, neural transmission of acoustic information. Na-Pa amplitude and Nb latency of the middle latency response and latency measures of the 40-Hz response were acutely altered by the presence of nicotine.

Acute effect of nicotine on non-smokers: III. LLRs and EEGs

This paper is the last in a series of three investigating the role of cholinergic mechanisms in the auditory system by assessing the acute effects of nicotine, an acetylcholinomimetic drug, on aggregate responses within the auditory pathway. In a single-blind procedure, auditory responses were obtained from 20 normal-hearing, non-smokers (10 male) under two conditions (nicotine, placebo). The effects of nicotine on long-latency responses of the auditory system and on electroencephalograms are described in this paper. Results indicated that transdermal administration of nicotine to non-smokers significantly affects the afferent and efferent transmission of acoustic information, as well as enhancing cortical activation. Long-latency response amplitudes and electroencephalogram activity (dominant power and frequencies) were altered by acute doses of transdermal nicotine.

Cochlear implant thresholds: comparison of middle latency responses with psychophysical and cortical-spike-activity thresholds

The electrically evoked middle latency response (EMLR) is a potentially useful measure of activation of the auditory system by a cochlear prosthesis. The present study compared cochlear prosthesis thresholds determined using EMLR with thresholds determined for psychophysical detection and for spike activity in cortical neurons. In systemically deafened guinea pigs, the difference between EMLR and psychophysical threshold level varied, with differences ranging from -4.6 dB (EMLR threshold more sensitive) to +10.7 dB (psychophysical threshold more sensitive) across animals and phase durations. Threshold differences between EMLR and auditory cortex neural spike responses were similar in magnitude and range (-6 to +15 dB) to those seen for EMLR vs. psychophysical thresholds. These ranges are comparable to the behavioral operating range for a given condition. In 3 of 12 subjects, the EMLR was absent for some or all electrode configurations tested, even at levels well above the threshold for psychophysical detection or cortical neuronal response. These results suggest that neither the EMLR thresholds nor cortical neuronal spike thresholds are an adequate substitute for psychophysical measures of threshold. While not sufficient for use in place of psychophysical measures, EMLR threshold level is strongly correlated with psychophysical threshold level across subjects (R(2)=0.82). Interestingly, plots of thresholds vs. phase duration were roughly parallel for psychophysical and EMLR thresholds, in contrast to the divergence of psychophysical and more peripheral (e.g., electrically evoked auditory brainstem response) evoked neural threshold vs. phase duration functions.

Auditory processing across the sleep-wake cycle: simultaneous EEG and fMRI monitoring in humans

We combined fMRI and EEG recording to study the neurophysiological responses associated with auditory stimulation across the sleep-wake cycle. We found that presentation of auditory stimuli produces bilateral activation in auditory cortex, thalamus, and caudate during both wakefulness and nonrapid eye movement (NREM) sleep. However, the left parietal and, bilaterally, the prefrontal and cingulate cortices and the thalamus were less activated during NREM sleep compared to wakefulness. These areas may play a role in the further processing of sensory information required to achieve conscious perception during wakefulness. Finally, during NREM sleep, the left amygdala and the left prefrontal cortex were more activated by stimuli having special affective significance than by neutral stimuli. These data suggests that the sleeping brain can process auditory stimuli and detect meaningful events.

Neural architecture of the rat medial geniculate body

The rat medial geniculate body was subdivided using Nissl preparations to establish nuclear boundaries, with Golgi-Cox impregnations to identify projection and local circuit neurons, and in fiber stained material to delineate the fiber tracts and their distribution. Three divisions were recognized (ventral, dorsal and medial): the first two had subdivisions. The ventral division had lateral and medial parts. The main cell type had bushy tufted dendrites which, with the afferent axons, formed fibrodendritic laminae oriented from dorso-lateral to ventro-medial; such laminae were not as regular medially, in the ovoid nucleus. The dorsal division contained several nuclei (dorsal superficial, dorsal, deep dorsal, suprageniculate, and ventrolateral) and neurons with radiating or bushy dendrites; the nuclear subdivisions differed in the concentration of one cell type or another, and in packing density. A laminar organization was present only in the dorsal superficial nucleus. Medial division neurons were heterogeneous in size and shape, ranging from tiny cells to magnocellular neurons; the various cell types intermingled. so that no further subdivision could be made. This parcellation scheme was consistent with, and supported by, the findings from plastic embedded or fiber stained material. There were very few small neurons with locally ramifying axons and which could perform an intrinsic role like that of Golgi type II cells. Their rarity was consistent with the small number of such profiles in plastic embedded or Nissl material and the few GABAergic medial geniculate body neurons seen in prior immunocytochemical work. While similar neuronal types and nuclear subdivisions are recognized in the rat and cat, there may be major interspecific differences with regard to interneuronal organization in the auditory thalamus whose functional correlates are unknown.

Effects of chronic ethanol ingestion on mid-latency auditory evoked potentials depend on length of exposure

We hypothesized that chronic ethanol ingestion is associated with modifications in components of mid-latency auditory evoked potentials (MAEPs). To test this, male Long Evans rats were administered 10% ethanol in drinking water as the sole fluid source for 3, 6, or 9 months. MAEPs were obtained and compared to age-matched control groups. MAEPs were obtained from additional rats after 4 weeks of abstinence. Data were obtained for varying frequencies (4, 8, 16, 24, 32 kHz) and intensities (65, 75, 85 dB SPL). Three months of ethanol exposure was associated with increased latencies and amplitudes of Na and Pa. MAEP components recovered and returned to control values after 4 weeks' abstinence following 3 months of EtOH exposure. Few significant differences were observed in the ethanol-treated or abstinent group after 6 months' exposure. However, 9 months of ethanol exposure revealed a significant increase in latencies and decrease in amplitudes of both Na and Pa components. After 4 weeks of abstinence, the Na and Pa component peak latencies appeared earlier than age-matched controls. The Na and Pa peak amplitudes were slightly greater than the ethanol-treated group; however, they did not recover to control values. These findings suggest that chronic ethanol consumption may produce time-dependent structural and/or neurochemical alterations in substrates for cortical information processing, which may be irreversible. In the present paradigm, this irreversibility may occur after 6 or more months of ethanol intake, and may be detected with the use of MAEPs.

Effect of stimulus level and frequency on ABR and MLR binaural interaction in human neonates

Auditory brainstem (ABR) and middle latency (MLR) responses were evoked by click and tone-burst stimuli from human neonates. Electrophysiologic evidence of binaural interaction was measured by subtracting waveforms obtained for binaural stimulus conditions from waveforms obtained for the sum of right ear monaural and left ear monaural stimulus conditions. The effects of stimulus level and stimulus frequency on binaural interaction were evaluated by measuring the number, latency and amplitude of components found in the derived binaural interaction waveform, that is, binaural interaction components (BIC). BICs were more prevalent in the latency range of ABRs than for MLRs. Click and tonal stimuli were equally effective for deriving ABR-BICs, while tone-bursts were somewhat less effective than clicks for deriving MLR-BICs. Stimulus-response dependencies for ABR and MLR component latencies were apparent in monaural, binaural and binaural interaction waveforms. Normalized amplitudes for BICs showed that low-frequency tone-burst stimuli resulted in the largest values compared to click and high-frequency tonal stimuli. Comparison of these results with published results from adults demonstrated immaturity of binaural interaction in neonates.

Possible overlapping potentials of the auditory P50 in humans: factor analysis of middle latency auditory evoked potentials

The auditory P50 in humans may consist of overlapping potentials. To test this hypothesis, we manipulated the conditions of stimulus discrimination and motor response difficulty and evaluated the data by factor analysis. Twenty right-handed males (mean age 27 years) performed the following 4 tasks: (1) a counting task, (2) an easy Go, No-Go task, (3) a difficult Go, No-Go task, and (4) a choice reaction task. Middle latency auditory evoked potentials were obtained with 100 times summation triggered by the onset of the auditory stimulus. Four factors were extracted by factor analysis for a 0-100 ms time period. Factor 1, the maximum factor loading at 91 ms, corresponded to N1, and factor 4, the maximum factor loading at 23 ms, appeared to correspond to P30. The latency of the maximum factor loading in factor 2 was adjacent to that in factor 3, the latency of factor 2 being 12 ms earlier than that of factor 3. Factor 2 and factor 3 latencies were approximately 55 ms which corresponded to the P50. Factor 3 started rising at the point that factor 2 reached the maximum factor loading, and the factor score demonstrated a significant group difference only when analyzed by motor response criteria. These results suggest that the P50 in humans consists of overlapping potentials and that a part of the potential might relate to a motor response process.

Effects of voluntary movements on early auditory brain responses

It has not been clear whether or not early information processing in the human auditory cortex is altered by voluntary movements. We report a movement-related, complex event-related potential consisting of relatively long-lasting amplitude and phase perturbations induced in an ongoing auditory steady-state response (SSR) by brief self-paced finger movements. Our results suggest that processing in the auditory cortex during the first 50-100 ms after stimulus delivery is affected before, during, and after voluntary movements, beginning with a 1- to 2-ms delay in the SSR wave form starting 1-2 s before the movement.

Middle latency auditory evoked potentials to tones of different frequency

The scalp distributions of middle latency auditory evoked potentials (MAEPs) elicited by tone bursts of 250 and 4000 Hz were compared in two experiments. Na (19.9 ms), Pa (29.8 ms), and Pb (51.4 ms) components elicited by tones of either frequency had fronto-central distributions, whereas the Nb component (38.4 ms) was maximal at parietal sites. Although the distributions of MAEP components varied as a function of the ear of stimulation, no significant differences were found as a function of tone frequency. The results are consistent with suggestions that MAEPs reflect activation of non-tonotopically organized generators.

Middle latency response in children with learning disabilities: preliminary findings

The objective of this investigation was to determine if auditory middle latency responses (MLR) obtained from children with learning disabilities (LD) differ from those obtained from children without LD. Simultaneous recordings of auditory brainstem and middle latency responses were obtained in both vertex-ipsilateral (V-I) and vertex-contralateral derivations (V-C) in 22 children (11 LD and 11 normal) in the age range of eight to twelve years whose peripheral hearing was within normal limits to bilateral. The results indicate that for specific recording conditions, the latencies of middle latency responses differ significantly between children with LD and a normal group of children.

Cellular basis of temporal synaptic signalling: an in vitro electrophysiological study in rat auditory thalamus

1. The cellular mechanisms underlying temporal synaptic signalling of tectothalamic pathways were investigated in rat medial geniculate body (MGB) maintained in vitro. Stimulation of the brachium of the inferior colliculus elicited either a short latency, single- (or dual-) spike or a long latency (10-80 ms) burst in MGB neurones. The delayed burst response was found in most non-lemniscal or caudodorsal MGB (MGd) neurones, whereas single-spike units were mainly seen in the lemniscal ventral MGB (MGv). Population latency analysis revealed that the overall relay time of tectothalamic transmission is approximately 50 ms, with at least two excitation peaks occurring around 8 and 15 ms, respectively. 2. Intracellular recordings showed that the delayed burst responses in MGd neurones were mediated by an EPSP-triggered low threshold spike (LTS). Small variations in either the membrane voltage or in EPSP amplitude induced significant shifts of LTS latency. 3. Compared with MGv cells, MGd neurones exhibited a more negative resting membrane potential and a prolonged EPSP; they lacked an apparent hyperpolarization-activated inward rectifier (Ih). These factors seem to lead collectively to a dominant occurrence of long latency burst response in the MGd. In the majority of single-spiking MGv cells that expressed a clear Ih, application of Cs+ consistently hyperpolarized the cell, which transformed a single-spike synaptic response into an EPSP-LTS burst or a subthreshold EPSP. 4. Taken together, these data suggest that the monosynaptic tectothalamic pathways are capable of introducing a ventrodorsal gradient in auditory response time. This synaptic activity pattern is probably dominantly regulated by a set of membrane conductances expressed endogenously in thalamocortical neurones.

Chronic ethanol ingestion alters parameters of mid-latency auditory evoked potentials in male rats

The purpose of this study was to determine the effects of chronic ethanol ingestion on components of mid-latency auditory evoked potentials (MAEPs). Male Sprague-Dawley rats were administered 10% ethanol in drinking water for 10 months. MAEPs were obtained and compared to age-matched controls provided tap water. Data were obtained for varying frequencies (4, 8, 16, 24, and 32 kHz) and intensities (65, 75, and 85 dB sound pressure level). Ethanol treatment was associated with increased latencies, as well as decreased amplitudes of Na and Pa. The effects were most prominent for MAEP component Pa, but also appear for component Na. We suggest that chronic alcohol consumption induces structural and/or neurochemical alterations in substrates for cortical information processing.

Whole-head mapping of middle-latency auditory evoked magnetic fields

We recorded middle-latency auditory evoked magnetic fields from 9 healthy subjects with a 122-channel whole-head SQUID gradiometer. The stimuli were click triplets, 2.5 msec in total duration, delivered alternately to the two ears once every 333 msec. Contralateral clicks elicited P30m responses in 16 and P50m responses in 12 out of 18 hemispheres studied; ipsilateral clicks did so in 7 and 13 hemispheres, respectively. The field patterns were satisfactorily explained by current dipoles in 16 and 4 hemispheres for contra- and ipsilateral P30m, and in 4 and 10 hemispheres for contra- and ipsilateral P50m. The peak latencies of P30m and P50m were not affected by stimulation side. The results show that middle-latency auditory evoked responses receive a strong contribution from auditory cortical structures, and that differences of input latency to cortical auditory areas, evaluated from MLAEF latencies, do not explain the latency differences seen in late auditory evoked fields to contralateral vs. ipsilateral stimulation.

Recovery of hypoglycaemia-associated compromised cerebral function after a short interval of euglycaemia in insulin-dependent diabetic patients

To test the hypothesis that compromised cerebral function, induced by recurrent hypoglycaemic episodes, may recover after a short interval of euglycaemia, we examined electrophysiological activity and symptom awareness during two sequential euglycaemic-hypoglycaemic clamp studies in 11 insulin-dependent diabetic patients without any signs of peripheral or autonomic neuropathy. Neurophysiological testing and evaluation of hypoglycaemic symptoms were performed at stable glycaemic plateaus of 5.6, 3.3, 2.2, and 1.7 mmol/l. The first clamp study was preceded by 3 short-term hypoglycaemic episodes, whereas the second clamp study followed a 2 day interval of strict euglycaemia. The latter caused a recovery of electrophysiological activity, which was demonstrated by recovery of delays of the middle latency auditory evoked potentials (latency shift of the P(a) component, MANOVA, P < 0.01). Reversal of hypoglycaemic symptom unawareness involved the overall symptom perception (MANOVA, P < 0.04), as well as the autonomic symptoms of heart pounding (P < 0.05) and sweating (P < 0.05). We conclude that the previously reported impaired cerebral function, occurring as a consequence of repetitive hypoglycaemic episodes, may recover after a single euglycaemic interval.

Binaural response patterns in subdivisions of the medial geniculate body

Auditory evoked potentials (AEPs) to binaural click stimulation were examined in the ventral (MGv) and caudomedial (MGcm) subdivisions of the medial geniculate body (MG) in guinea pigs. Binaural stimulation caused a decrease in amplitude for the response component recorded from the MGv, but an increase in amplitude for the AEP component recorded from the MGcm. Findings suggest that the evoked responses recorded from MGv and MGcm are functionally distinct. The inhibitory binaural response (BR) pattern seen in MGv was similar to that of the middle latency response (MLR) component recorded over the temporal cortex, while the additive BR pattern typical of the MGcm was similar to that of the surface midline MLR component. Furthermore, these data imply that the binaural response patterns seen in the primary and non-primary auditory cortex may be processed and encoded at the thalamic level. It is concluded that the distinct BR patterns noted for the two MG subdivisions reflect the predominant type of binaurally responsive neurons within the respective pathways.

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.

Reticular formation influences on primary and non-primary auditory pathways as reflected by the middle latency response

Ongoing studies are aimed at identifying the neural pathways responsible for the middle latency response (MLR). These studies involve the analysis of surface and intracranial potentials following pharmacologic inactivation (with lidocaine) of discrete regions of the guinea pig brain. Previous investigations have shown that MLR surface waves recorded over the temporal lobe originate from pathways anatomically and functionally distinct from those that generate MLR waves recorded over the midline, and that both primary and non-primary auditory thalamo-cortical pathways contribute to the guinea pig MLR. The present investigation examines the role of the mesencephalic reticular formation (mRF) in the MLR generating system. Inactivation of the mRF was associated with disruption of the midline response. These waves have been shown to reflect activity from non-primary subdivisions of the thalamo-cortical pathway. Components recorded over the temporal lobe were also affected, consisting of amplitude reduction and latency prolongation without changes in response morphology. Changes in temporal MLR components with mRF inactivation were smaller than those associated with direct inactivation of primary and non-primary subdivisions of the medial geniculate body. These findings indicate that mRF input is essential for normal generation of those components of the MLR thought to reflect both primary and non-primary auditory pathway activity.

Binaural stimulation reveals functional differences between midline and temporal components of the middle latency response in guinea pigs

Two morphologically distinct auditory middle latency response (MLR) wave forms can be recorded from the surface of the guinea pig brain. The temporal response is recorded from the temporal lobe contralateral to the stimulus ear, and the midline response is recorded over the posterior midline. Experimental evidence suggests that different neural generators contribute to the two responses. Furthermore, it appears that the temporal response principally reflects activity of the primary auditory pathway while the midline response reflects non-primary pathways. Although it is known that neurons throughout the auditory pathway exhibit distinct binaural interaction (BI) properties, thus far there have been no systematic attempts to differentiate the MLR wave forms in response to binaural stimulation. The purpose of this study was to determine if binaural click stimulation could functionally differentiate the midline and temporal MLR responses in the guinea pig. Binaural click stimulation caused a significant decrease in temporal MLR peak amplitudes, and a significant increase in midline MLR amplitudes. The fact that different BI patterns were observed suggests that the two MLR components are functionally distinct. The data further support the hypothesis that the midline and temporal MLR in guinea pigs reflect different neural generators and pathways.