Biomedical subjects

J J Eggermont

Publications and source records attributed to J J Eggermont.

At least 19 recordsLinked to original sources

PubMed2001

Spontaneous burst-firing in three auditory cortical fields: its relation to local field potentials and its effect on inter-area cross-correlations.

Burst-firing refers to epochs of sharply elevated neural discharge. It has been suggested that correlated firing in different cortical areas in anesthetized animals results from spontaneous burst-firing related to electroencephalogram spindling activity and state of drowsiness. To investigate this, simultaneous recordings of spontaneous firings of neurons in the primary (AI), secondary (AII) and anterior (AAF) fields of the auditory cortex in the lightly anaesthetized cat were obtained. This allowed a study of bursting behavior in the three cortical areas under exactly the same anesthetic state. Burst occurrences were detected using the Poisson-surprise method, and were typically highly synchronized with local field potentials (LFPs) and with burst-firing of other neurons recorded on the same electrode. Burst-firing occurred in 85% of 371 units studied, and in 48 (15%) thereof there were at least 100 bursts per 15 min. Neurons in Al were bursting at a significantly higher rate, but with fewer spikes per burst, than units in AII. The average percentage of the time that a spontaneously firing neuron is in the bursting state is only about 3% (range 0.004, 29%). The average peak cross-correlation coefficients between spikes and LFP triggers were largest for burst-onset spikes, followed by those between all burst spikes and LFP triggers, and smallest when all spikes of the single unit were used in the correlation. This was the case for within- and between-area conditions. Burst-onset times in different auditory fields were not correlated. Thus, the major cause of the observed correlation of spontaneous firing in different cortical areas is not synchronous burst-firing.

Action Potentials↗

PubMed2001

Between sound and perception: reviewing the search for a neural code.

This review investigates the roles of representation, transformation and coding as part of a hierarchical process between sound and perception. This is followed by a survey of how speech sounds and elements thereof are represented in the activity patterns along the auditory pathway. Then the evidence for a place representation of texture features of sound, comprising frequency, periodicity pitch, harmonicity in vowels, and direction and speed of frequency modulation, and for a temporal and synchrony representation of sound contours, comprising onsets, offsets, voice onset time, and low rate amplitude modulation, in auditory cortex is reviewed. Contours mark changes and transitions in sound and auditory cortex appears particularly sensitive to these dynamic aspects of sound. Texture determines which neurons, both cortical and subcortical, are activated by the sound whereas the contours modulate the activity of those neurons. Because contours are temporally represented in the majority of neurons activated by the texture aspects of sound, each of these neurons is part of an ensemble formed by the combination of contour and texture sensitivity. A multiplexed coding of complex sound is proposed whereby the contours set up widespread synchrony across those neurons in all auditory cortical areas that are activated by the texture of sound.

Animals↗

PubMed2000

Moderate noise trauma in juvenile cats results in profound cortical topographic map changes in adulthood.

Cortical topographic map changes have been reported after profound drug-induced hearing loss in neonates, after progressive high-frequency hearing loss, and after mechanically induced lesions in the cochlea of adult animals. The present study demonstrates that exposure of 5-week-old kittens to a loud 6 kHz tone, producing mild to moderate high-frequency hearing loss, induces a profound reorganization of the frequency map in auditory cortex. In the reorganized cortical region, the frequency-tuning curves were of normal sharpness with near normal thresholds. Inhibitory tuning curve bandwidths were similar to those in control animals. Spontaneous activity in the reorganized part of the cortex was significantly increased. In contrast, the strength of the cross-correlation of the spontaneous activity of units recorded on different electrodes was the same in the normal and reorganized part. Minimum first-spike latency was significantly increased in trauma cats, largely for units at the dorsal side of the sampled region. Because most other neural response properties are normal in the reorganized part of cortex, sub-cortical topographic map changes are likely involved in producing the altered cortical topographic maps.

Aging↗

PubMed2000

Noise suppression of transient-evoked otoacoustic emissions. II. Derived narrow-band contributions.

Transient-evoked otoacoustic emissions (TEOAEs) were decomposed into cochlear place specific components using high-pass noise suppression. This was performed using high-pass filtered noise with cut-off frequencies between 0.7 and 5.6 kHz in 0.5-octave steps. Subtraction of the TEOAEs obtained in the presence of two high-pass noise suppressors with 0.5-octave difference in their cut-off frequencies, f(A) and f(B), should theoretically result in TEOAE components with frequencies between f(A) and f(B). The reconstructed wide-band emission power spectrum obtained by summing the narrow-band emission power spectra, was nearly identical to the power spectrum of the original wide-band emission. This suggests that no phase-cancellation occurs and that the individual narrow-band TEOAEs are uncorrelated, and thus that their generators are potentially independent. About 66% of the derived narrow-band emissions had spectral components that extended below the cut-off frequency of the lower high-pass noise filter. These tail components were interpreted as resulting from high-frequency side suppression of the high-pass noise on the click emission and potentially distortion product components from the TEOAE.

Acoustic Stimulation↗

PubMed2000

Noise suppression of transient-evoked otoacoustic emissions. I. A comparison with the non-linear method.

A new method to record transient-evoked otoacoustic emissions (TEOAEs) is introduced. Click stimuli were presented both with and without a simultaneously presented wide-band noise burst. Subtraction of the recorded signal evoked by the noise burst plus click from the signal evoked by the click alone, cancelled the eardrum reflection components of the response and resulted in a measure of the emission. This was used to obtain the TEOAEs from 21 subjects for peak click stimulus levels of 48-66 dB SPL. The root-mean-square (RMS) level of the noise burst was set 10 dB higher than the peak click level, and resulted in suppression of the TEOAE by up to 20 dB. The TEOAE waveforms obtained by the new method were compared to those obtained with Kemp's non-linear method, and were indistinguishable in 20 of the 21 subjects. On basis of the emission spectra, they were indistinguishable in 18 out of 21 subjects. The latencies of narrow-band filtered components from the TEOAEs obtained with the two methods were also similar. This suggests that this noise-suppression method produces similar results as Kemp's non-linear method with the advantage that emission components with very short latencies can be obtained.

Acoustic Stimulation↗

PubMed2000

Neuronal responses in cat primary auditory cortex to natural and altered species-specific calls.

We investigated how natural and morphed cat vocalizations are represented in primary auditory cortex (AI). About 40% of the neurons showed time-locked responses to major peaks in the vocalization envelope, 60% only responded at the onset. Simultaneously recorded multi-unit (MU) activity of these peak-tracking neurons on separate electrodes was significantly more synchronous during stimulation than under silence. Thus, the representation of the vocalizations is likely synchronously distributed across the cortex. The sum of the responses to the low and high frequency part of the meow, with the boundary at 2.5 kHz, was larger than the neuronal response to the natural meow itself, suggesting that strong lateral inhibition is shaping the response to the natural meow. In this sense, the neurons are combination-sensitive. The frequency-tuning properties and the response to amplitude-modulated tones of the MU recordings can explain the responses to natural, and temporally and spectrally altered vocalizations. Analysis of the mutual information in the firing rate suggests that the activity of at least 95 recording sites in AI would be needed to reliably distinguish between the nine different vocalizations. This suggests that a distributed representation based on temporal stimulus aspects may be more efficient than one based on firing rate.

Acoustic Stimulation↗

PubMed2000

Maturation of human central auditory system activity: evidence from multi-channel evoked potentials.

OBJECTIVE: The purpose of this study was to evaluate central auditory system maturation based on detailed data from multi-electrode recordings of long-latency auditory evoked potentials (AEPs). METHODS: AEPs were measured at 30 scalp-electrode locations from 118 subjects between 5 and 20 years of age. Analyses focused on age-related latency and amplitude changes in the P1, N1b, P2, and N2 peaks of the AEPs generated by a brief train of clicks presented to the left ear. RESULTS: Substantial and unexpected changes that extend well into adolescence were found for both the amplitude and latency of the AEP components. While the maturational changes in latency followed a pattern of gradual change, amplitude changes tended to be more abrupt and step-like. Age-related latency decreases were largest for the P1 and N1b peaks. In contrast, P2 latency did not change significantly and the N2 peak increased in latency as a function of age. Abrupt changes in P1, P1-N1b, and N2 peak amplitude (also RMS amplitude) were observed around age 10 at the lateral electrode locations C3 and C4, but not at the midline electrodes Cz and Fz. These changes in amplitude coincided with a sharp increase and plateau in AEP peak and RMS amplitude variability from 9 to 11 years of age. CONCLUSIONS: These analyses demonstrated that the observed pattern of AEP maturation depends on the scalp location at which the responses are recorded. The distinct maturational time courses observed for individual AEP peaks support a model of AEP generation in which activity originates from two or more at least partly independent central nervous system pathways. A striking parallel was observed between previously reported maturational changes in auditory cortex synaptic density and, in particular, the age-related changes in P1 amplitude. The results indicate that some areas of the brain activated by sound stimulation have a maturational time course that extends into adolescence. Maturation of certain auditory processing skills such as speech recognition in noise also has a prolonged time course. This raises the possibility that the emergence of adult-like auditory processing skills may be governed by the same maturing neural processes that affect AEP latency and amplitude.

Adolescent↗

PubMed2000

Spontaneous firing activity of cortical neurons in adult cats with reorganized tonotopic map following pure-tone trauma.

We hypothesized that moderate sensorineural hearing loss resulting from acoustic trauma would cause (i) a change in the cortical tonotopic map, (ii) an increase in spontaneous activity in the reorganized region and (iii) increased inter-neuronal synchrony within the reorganized part of the cortex. Five kittens were exposed to a 126 dB sound pressure limit tone of 6 kHz for 1 h at both 5 and 6 weeks of age. Recordings were performed 7-16 weeks after the exposure. Auditory brainstem response thresholds for frequencies above 12 kHz were increased by 30 dB on average relative to those in normal cats. Tonotopic maps in the primary auditory cortex were reorganized in such a way that the area normally tuned to frequencies of 10-40 kHz was now entirely tuned to 10 kHz. Spontaneous firing rates were significantly higher in reorganized areas than in normal areas. In order to test for changes in inter-neuronal synchrony, cross-correlation analysis was done on 225 single-unit pairs recorded in the traumatized cats. For the single- and dual-electrode pairs there was no significant difference in peak cross-correlation coefficients for the firings of simultaneously recorded cells between normal and reorganized areas. However, the percentage of correlations that differed significantly from zero was higher in the reorganized area than in the normal area. This suggests a potential correlation between cortical reorganization, increased spontaneous firing rate and inter-neuronal synchrony that might be related to tinnitus found in high-frequency hearing loss induced by acoustic trauma.

Acoustic Stimulation↗

PubMed2000

Sound-induced synchronization of neural activity between and within three auditory cortical areas.

Neural synchrony within and between auditory cortical fields is evaluated with respect to its potential role in feature binding and in the coding of tone and noise sound pressure level. Simultaneous recordings were made in 24 cats with either two electrodes in primary auditory cortex (AI) and one in anterior auditory field (AAF) or one electrode each in AI, AAF, and secondary auditory cortex. Cross-correlograms (CCHs) for 1-ms binwidth were calculated for tone pips, noise bursts, and silence (i.e., poststimulus) as a function of intensity level. Across stimuli and intensity levels the total percentage of significant stimulus onset CCHs was 62% and that of significant poststimulus CCHs was 58% of 1,868 pairs calculated for each condition. The cross-correlation coefficient to stimulus onsets was higher for single-electrode pairs than for dual-electrode pairs and higher for noise bursts compared with tone pips. The onset correlation for single-electrode pairs was only marginally larger than the poststimulus correlation. For pairs from electrodes across area boundaries, the onset correlations were a factor 3-4 higher than the poststimulus correlations. The within-AI dual-electrode peak correlation was higher than that across areas, especially for spontaneous conditions. Correlation strengths for between area pairs were independent of the difference in characteristic frequency (CF), thereby providing a mechanism of feature binding for broadband sounds. For noise-burst stimulation, the onset correlation for between area pairs was independent of stimulus intensity regardless the difference in CF. In contrast, for tone-pip stimulation a significant dependence on intensity level of the peak correlation strength was found for pairs involving AI and/or AAF with CF difference less than one octave. Across all areas, driven rate, between-area peak correlation strength, or a combination of the two did not predict stimulus intensity. However, between-area peak correlation strength performs better than firing rate to decide if a stimulus is present or absent.

Acoustic Stimulation↗

PubMed2000

Neural responses in primary auditory cortex mimic psychophysical, across-frequency-channel, gap-detection thresholds.

Responses of single- and multi-units in primary auditory cortex were recorded for gap-in-noise stimuli for different durations of the leading noise burst. Both firing rate and inter-spike interval representations were evaluated. The minimum detectable gap decreased in exponential fashion with the duration of the leading burst to reach an asymptote for durations of 100 ms. Despite the fact that leading and trailing noise bursts had the same frequency content, the dependence on leading burst duration was correlated with psychophysical estimates of across frequency channel (different frequency content of leading and trailing burst) gap thresholds in humans. The duration of the leading burst plus that of the gap was represented in the all-order inter-spike interval histograms for cortical neurons. The recovery functions for cortical neurons could be modeled on basis of fast synaptic depression and after-hyperpolarization produced by the onset response to the leading noise burst. This suggests that the minimum gap representation in the firing pattern of neurons in primary auditory cortex, and minimum gap detection in behavioral tasks is largely determined by properties intrinsic to those, or potentially subcortical, cells.

Acoustic Stimulation↗

PubMed1999

The magnitude and phase of temporal modulation transfer functions in cat auditory cortex.

Temporal modulation transfer functions (tMTFs) in response to periodic click trains are presented for simultaneous recordings from primary auditory cortex, anterior auditory field, and secondary auditory cortex in 21 cats. The multiunit records could be separated in to 215 single-unit spike trains that allowed a reliable estimate of a group delay, which represents the cumulative delay for responses to repetitive stimuli. For approximately two-thirds of the 215 single units the group delay was within 7.5 msec of the response latency to the first clicks in the trains. For the remaining units, the group delay was on average approximately 14 msec higher, and this may result from differences in synaptic properties. These findings were similar in the three cortical areas studied. The findings are modeled based on presynaptic facilitation and depression and pyramidal cell calcium kinetics, and a quantitative description of the magnitude of the tMTF was obtained that resulted in substantially shorter depression time constants (20 msec) than reported for visual cortex (300 msec). A small amount (0-5.5%) of facilitation that decayed with a time constant of 60 msec was obtained. Auditory cortical cells apparently have much faster recovery mechanisms than visual cortical cells. This allows for the ability of the auditory cortex to reliably track the rhythms that occur in natural sounds.

Acoustic Stimulation↗

PubMed1999

Effects of acute pure tone induced hearing loss on response properties in three auditory cortical fields in cat.

In this study, we assessed the changes in spontaneous activity and frequency tuning by simultaneous recording of multi-units and local field potentials in primary auditory cortex (AI), anterior auditory field (AAF) and secondary auditory cortex (AII) of cats before and immediately after 30 min exposure to a loud (93 123 dB SPL) pure tone. The average difference of the pure tone and the characteristic frequency (CF) was less than one octave for 70% of the recordings. We found that the mean threshold at CF increased significantly in AI and in AAF but not in AII. The mean CF for units in AI decreased significantly, whereas no significant effect was noted in AAF and AII. The mean frequency-tuning curve bandwidth decreased significantly in AII. Spontaneous activity increased significantly in AI, did not change in AAF, and decreased significantly in AII. Inter-area neural synchrony was not affected. Multi-unit response areas were usually similarly affected as local field potentials based response areas because the 'damaged area', defined as the response surface before minus the surface after the trauma, was very similar. This suggests that the damage reflects peripheral activity changes. Enhancement of frequency response areas around CF, but at least one octave below the frequency of the traumatizing tone, was found most frequently in AAF and suggests a reduction of inhibition likely as a result of the peripheral hearing loss.

Acute Disease↗

PubMed1999

Neural correlates of gap detection in three auditory cortical fields in the Cat.

Neural correlates of gap detection in three auditory cortical fields in the cat. Mimimum detectable gaps in noise in humans are independent of the position of the gap, whereas in cat primary auditory cortex (AI) they are position dependent. The position dependence in other cortical areas is not known and may resolve this contrast. This study presents minimum detectable gap-in-noise values for which single-unit (SU), multiunit (MU) recordings and local field potentials (LFPs) show an onset response to the noise after the gap. The gap, which varied in duration between 5 and 70 ms, was preceded by a noise burst of either 5 ms (early gap) or 500 ms (late gap) duration. In 10 cats, simultaneous recordings were made with one electrode each in AI, anterior auditory field (AAF), and secondary auditory cortex (AII). In nine additional cats, two electrodes were inserted in AI and one in AAF. Minimum detectable gaps based on SU, MU, or LFP data in each cortical area were the same. In addition, very similar minimum early-gap values were found in all three areas (means, 36.1-41.7 ms). The minimum late-gap values were also similar in AI and AII (means, 11.1 and 11.7 ms), whereas AAF showed significantly larger minimum late-gap durations (mean 21.5 ms). For intensities >35 dB SPL, distributions of minimum early-gap durations in AAF and AII had modal values at approximately 45 ms. In AI, the distribution was more uniform. Distributions for minimum late-gap duration were skewed toward low values (mode at 5 ms), but high values (</=60 ms) were found infrequently as well. A small fraction of units showed a response after the gap only for early-gap durations <20 ms. In AI and AII, the mean minimum early- and late-gap durations decreased significantly with increase in the neuron's characteristic frequency (CF), whereas the lower boundary for the minimum early gap was CF independent. The findings suggest that human within-perceptual-channel gap detection, showing no dependence of the minimum detectable gap on the duration of the leading noise burst, likely is based on the lower envelope of the distribution of neural minimum gap values of units in AI and AAF. In contrast, across-perceptual-channel gap detection, which shows a decreasing minimum detectable gap with increasing duration of the leading noise burst, likely is based on the comparison of ON responses from populations of neurons that converge on units in AII.

Acoustic Stimulation↗

PubMed1999

Prolonged deafness limits auditory system developmental plasticity: evidence from an evoked potentials study in children with cochlear implants.

The use of cochlear implants to restore hearing in profoundly deaf children is increasing, with a trend toward earlier implantation. However, little is known about how auditory deprivation and subsequent implant use affects the maturing central auditory system. Previously reported results indicate that stimulation of the auditory system by a cochlear implant is sufficient to restore at least some aspects of central auditory pathway maturation, as reflected by age-related changes in the auditory evoked potentials. We review animal and human studies on sensory deprivation and report new results based on longitudinal evoked potentials data recorded from two individuals. Analyses show that age-related changes in the EPs may asymptote at levels very different from those found in the adult normal-hearing population. These results suggest that maturation of at least some aspects of central auditory system activity is limited by the onset and duration of the period of deafness prior to implantation.

Adult↗

PubMed1998

Is there a neural code?

Rate coding and temporal coding are two extremes of the neural coding process. The concept of a stationary state corresponds to the information processing approach that views the brain as a decision maker, adopts rate coding as its main strategy and endorses the single- or few neuron approach. If information derived from sensory stimulation is used to continuously update the brain's internal representation of the world, then neural codes may change with time through learning. As a consequence, the same spike sequence may be interpreted differently (or evoke a different behavior) later in the day. This non-stationary viewpoint is embodied in the representational model of brain function that stresses learning and plasticity and employs temporal coding in neural assemblies. We argue that the switching between quasi-stable brain states as a result of learning is more relevant than the neuronal patterns, and the correlations between them, that are found during stationary states. The neural code likely resides in the activity patterns that cause this state-switching.

Animals↗

PubMed1998

Salicylate and quinine selectively increase spontaneous firing rates in secondary auditory cortex.

This study presents firing rates for simultaneously recorded spontaneous and stimulus driven multi-unit activity in primary auditory cortex (AI), anterior auditory field (AAF) and secondary auditory cortex (AII) in cats before and after application of salicylate or quinine. From 21 cats, in three cortical areas simultaneously, a total of 1533 multi-unit files were obtained. The data suggest (1) that both salicylate and quinine significantly increase spontaneous firing rates in AII, whereas in AI and AAF both quinine and salicylate reduced the spontaneous rate; (2) the effect of both drugs was to increase spontaneous rates for recording sites with high characteristic frequency (CF) and a tendency to decrease them for low CF sites; (3) the mean stimulus driven firing rates were not affected by either drug except for a decrease produced by quinine in AI; (4) changes in driven firing rate were positively correlated with changes in spontaneous firing rates. This suggests that tinnitus inducing agents selectively increase spontaneous firing rates in the extralemniscal pathway.

Acoustic Stimulation↗

PubMed1998

Estimating cochlear filter response properties from distortion product otoacoustic emission (DPOAE) phase delay measurements in normal hearing human adults.

This study examined cochlear filter response properties derived from f1- and f2-sweep phase delay difference measures in 60 normal hearing human adults. Seven different f2 frequencies ranging from 1.1 to 9.2 kHz were presented (f2/f1 ratios of 1.1-1.3). F2 intensity level was varied in 5 dB steps from 30 to 50 dB SPL (the level of f1 was 15 dB above the level of f2). DPOAE delay estimates in a f2-sweep paradigm are longer than in a f1-sweep paradigm at the same frequency and intensity. This indicates that the f2-sweep DPOAE phase delay is composed of a greater proportion of the filter response time at the site of DPOAE generation than the f1-sweep delay. This proportion was isolated by subtracting f1-sweep DPOAE delays from f2-sweep delays at similar f2 frequencies and intensities. Under the assumption of linearity and minimum phase the impulse response of the filter at each f2 stimulus level was calculated from the mean phase delay difference. Frequency response properties were calculated by Fourier transformation of the impulse response at each f2 frequency and intensity. High frequency low intensity impulse responses had longer response times and narrower frequency bandwidths than low frequency high intensity responses. The Q10dB values of DPOAE derived tuning curves ranged from 2.4 (1.5 kHz) to 7.3 (8.5 kHz).

Acoustic Stimulation↗

PubMed1998

The effects of sensory hearing loss on cochlear filter times estimated from auditory brainstem response latencies.

Derived-band auditory brainstem responses (ABRs) were obtained in 43 normal-hearing and 80 cochlear hearing-impaired individuals using clicks and high-pass noise masking. The response times across the cochlea [the latency difference between wave V's of the 5.7- and 1.4-kHz center frequency (CF) derived bands] were calculated for five levels of click stimulation ranging from 53 to 93 dB p.-p.e. SPL (23 to 63 dB nHL) in 10-dB steps. Cochlear response times appeared to shorten significantly with hearing loss, especially when the average pure tone (1 to 8 kHz) hearing loss exceeded 30 dB. Examination of derived-band latencies indicates that this shortening is due to a dramatic decrease of wave V latency in the lower CF derived band. Estimates of cochlear filter times in terms of the number of periods to maximum response (Nmax) were calculated from derived-band latencies corrected for gender-dependent cochlear transport and neural conduction times. Nmax decreased as a function of hearing loss, especially for the low CF derived bands. The functions were similar for both males and females. These results are consistent with broader cochlear tuning due to peripheral hearing loss. Estimating filter response times from ABR latencies enhances objective noninvasive diagnosis and allows delineation of the differential effects of pathology on the underlying cochlear mechanisms involved in cochlear transport and filter build-up times.

Acoustic Stimulation↗