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H Pratt

Publications and source records attributed to H Pratt.

At least 37 records · Page 2Linked to original sources
PubMed1999

'Oddball' event-related potentials and information processing during REM and non-REM sleep.

Auditory stimuli consisting of the subject's own name and an irrelevant word, counterbalanced in probabilities, were presented to 15 male subjects in the awake state and during natural sleep. Potentials recorded to these stimuli, as well as to clicks presented during sleep in a preceding night, were recorded and compared. Principal component analysis (PCA) was conducted on evoked potentials to distinguish temporally overlapping components, and ANOVA was applied on the eigenvector coefficients. During non-REM sleep a parietal P450, more prominent in stage 2, was observed in addition to the prominent waveform of a K-complex, which was also recorded in response to clicks and consisted of N350, N550 and P1000. During REM sleep, a fronto-central negativity which resembled non-REM N350, a parietal positivity at about 450 ms and a large N700 were detected. ANOVA on PCA coefficients showed a significant effect of verbal-stimulus type (name/irrelevant) on an eigenvector, which included all the components observed during stage 2 sleep. Coefficients during REM sleep showed a significant effect of stimulus probability on an eigenvector consisting of a prominent P450, suggesting a resemblance to the awake P300 component. This could not be demonstrated during non-REM sleep. The results indicate continued evaluation of auditory input salience during sleep, which diminishes during deep sleep, and is replaced by evaluation of stimulus context in a train of stimuli during REM sleep.

Adult↗
PubMed1999

Auditory brain-stem evoked potentials in patients with thyroid and parathyroid dysfunction: adaptation to chronic hormonal dysequilibrium.

Auditory Brainstem Evoked Potentials (ABEPs) and pure tone audiograms were obtained from 24 patients with parathyroid dysfunction (17 hypercalcemia and 7 hypocalcemia) and 12 patients with thyroid dysfunction (6 hyperthyroid and 6 hypothyroid) and from 10 control subjects. ABEPs were characterized by I-V interpeak latency difference at 10/sec click rate and by the effect of increasing stimulus rate to 55/sec. None of the ABEP measures were significantly affected by levels of serum calcium, thyroid hormones or their interactions. Moreover no correlation was found between biochemical and electrophysiological measures. This stability of ABEP measures contrasts with earlier reports on acute effects of calcium and thyroid hormonal levels on auditory brainstem evoked potentials. We propose that chronic calcium or thyroid hormonal homeostatic changes are associated with adaptive mechanisms resulting in normal function of the auditory brainstem.

Adaptation, Biological↗
PubMed1999

Correlations between audiogram and objective hearing tests in sensorineural hearing loss.

Owing to its subjective nature, behavioral pure-tone audiometry often is an unreliable testing method in uncooperative subjects, and assessing the true hearing threshold becomes difficult. In such cases, objective tests are used for hearing-threshold determination (i.e., auditory brainstem evoked potentials [ABEP] and frequency-specific auditory evoked potentials: slow negative response at 10 msec [SN-10]). The purpose of this study was to evaluate the correlation between pure-tone audiogram shape and the predictive accuracy of SN-10 and ABEP in normal controls and in patients suffering from sensorineural hearing loss (SNHL). One-hundred-and-fifty subjects aged 15 to 70, some with normal hearing and the remainder with SNHL, were tested prospectively in a double-blind design. The battery of tests included pure-tone audiometry (air and bone conduction), speech reception threshold, ABEP, and SN-10. Patients with SNHL were divided into four categories according to audiogram shape (i.e., flat, ascending, descending, and all other shapes). The results showed that ABEP predicts behavioral thresholds at 3 kHz and 4 kHz in cases of high-frequency hearing loss. Also demonstrated was that ABEP threshold estimation at 3 kHz was not affected significantly by audiogram contour. A good correlation was observed between SN-10 and psychoacoustic thresholds at 1 kHz, the only exception being the group of subjects with ascending audiogram, in which SN-10 overestimated the hearing threshold.

Acoustic Stimulation↗
PubMed1998

Effects of localized pontine lesions on auditory brain-stem evoked potentials and binaural processing in humans.

OBJECTIVES AND METHODS: Four sets of measurements were obtained from 11 patients (44-80 years old) with small, localized pontine lesions due to vascular disease: (1) Monaural auditory brain-stem evoked potentials (ABEPs; peaks I to VI); (2) Binaural ABEPs processed for their binaural interaction components (BICs) in the latency range of peaks IV to VI; (3) magnetic resonance imaging (MRI) of the brain-stem; and (4) psychoacoustics of interaural time disparity measures of binaural localization. ABEPs and BICs were analyzed for peak latencies and interpeak latency differences. Three-channel Lissajous' trajectories (3-CLTs) were derived for ABEPs and BICs and the latencies and orientations of the equivalent dipoles of ABEP and BICs were inferred from them. RESULTS: Intercomponent latency measures of monaurally evoked ABEPs were abnormal in only 3 of the 11 patients. Consistent correlations between sites of lesion and neurophysiological abnormality were obtained in 9 of the 11 patients using 3-CLT measures of BICs. Six of the 11 patients had absence of one or more BIC components. Seven of the 11 had BICs orientation abnormality and 3 had latency abnormalities. Trapezoid body (TB) lesions (6 patients) were associated with an absent (two patients with ventral-caudal lesions) or abnormal (one patient with ventral-rostral lesions) dipole orientation of the first component (at the time of ABEPs IV), and sparing of this component with midline ventral TB lesions (two patients). A deviant orientation of the second BICs component (at the time of ABEPs V) was observed with ventral TB lesions. Psychoacoustic lateralization in these patients was biased toward the center. Rostral lateral lemniscus (LL) lesions (3 patients) were associated with absent (one patient) or abnormal (two patients) orientation of the third BICs component (at the time of ABEPs VI); and a side-biased lateralization with behavioral testing. CONCLUSIONS: These results indicate that: (1) the BICs component occurring at the time of ABEPs peak IV is dependent on ventral-caudal TB integrity; (2) the ventral TB contributes to the BICs component at the time of ABEPs peak V; and (3) the rostral LL is a contributing generator of the BICs component occurring at the time of ABEP peak VI.

Adolescent↗
PubMed1998

Evidence for efferent effects on early components of the human auditory brain-stem evoked potentials.

OBJECTIVES AND METHODS: Auditory brain-stem evoked potentials (ABEPs) were recorded from 10 normal hearing subjects in response to rarefaction clicks, presented at a rate of 11/s. Stimuli were binaurally symmetrical and isochronic at 75 dB peSPL or with interaural time disparities (ITDs) of +/-0.4 ms, or intensity disparities (IIDs) of +/-10 dB. Potentials were recorded from vertex-neck, as well as from 3 orthonormally positioned differential derivations. The amplified potentials were averaged over 8000 repetitions using a dwell time of 20 micros/address/channel. The effects of contralateral stimulation on neural responses of the peripheral auditory system were obtained by subtracting the binaural response from the algebraic sum of responses to left and right monaural stimuli. From the 3 orthonormal derivations, 3-channel Lissajous' trajectories (3-CLTs) to the various stimulus conditions and difference waveforms were derived. RESULTS: The results corroborated earlier studies on binaural interaction components (BICs), which include 3 major components corresponding in latency to the vertex-mastoid peaks IV-VI of ABEP. In addition, the binaural difference waveforms included 3 earlier, low-amplitude components. Latency correspondence and comparison of difference waveform and ABEP 3-CLTs indicated that the first and third early difference waveform components corresponded to the negative peaks following I and III, respectively, of the vertex-neck ABEP to binaural clicks. CONCLUSIONS: These results indicate that early ABEP peaks, generated peripheral to binaural convergence, may be affected by contralateral stimulation. These contralateral effects were in a pattern compatible with suppression. most probably by efferents of the olivo-cochlear bundle.

Acoustic Stimulation↗
PubMed1998

Contralaterally evoked transient otoacoustic emissions.

Contralaterally evoked transient otoacoustic emissions (CETOAEs) were recorded from 10 normal-hearing young adults (20 ears) in response to monaural, 11/s, 65 dB pe SPL clicks to the ear contralateral to the microphone probe. A burst of CETOAEs was observed 12-22 ms (average peak at 18.5 ms) after the contralateral click, and its mean level was -7.3 dB pe SPL, 4 dB above the averaged noise level. The frequency content of CETOAEs included a prominence around 1 kHz. In 40% of the ears examined CETOAEs were 3 dB or more above noise level in both replications of records from the same ear. To explain these results CETOAEs are suggested to reflect mechanical events induced by the crossed efferent system in the cochlea that was contralateral to the stimulated ear. The latency of the contralateral responses suggests that they may be related to the contralateral suppression effect observed with binaural stimulation. The latency of the response, coupled with the anatomical origin of the crossed efferent system at the superior olivary complex, suggest its involvement in the contralateral CETOAEs reported here.

Acoustic Stimulation↗
PubMed1998

Binaural masking level difference in human binaural interaction components.

OBJECTIVE: The purpose of this study was to compare the effects of monaural and binaural broadband masking noise on binaural interaction components (BICs) of the human auditory brain stem evoked potentials (ABEPs). DESIGN: The BICs of the human ABEPs were studied by subtracting the potentials to binaural clicks from the algebraic sum of monaurally evoked potentials to clicks alone or to clicks with ipsilateral monaural or binaural broadband masking noise. Alternating polarity, 11/sec clicks were presented at 65 dB nHL, and noise was presented at 45 dB nHL. Analysis included peak-to-prestimulus baseline amplitudes and latencies of BICs' peaks and troughs from the vertex-mastoid (A) and vertex-neck (Z) channels. In addition, 3-channel Lissajous' trajectory (3-CLT) analysis, estimating the single, centrally located dipole equivalent of surface activity, was performed on data recorded from three orthogonally positioned electrode pairs. 3-CLT measures included apex latency, amplitude, and orientation, as well as planar segment duration, size, shape, and orientation. RESULTS: All BICs 3-CLTs included five main components (labeled BdI, BdII, BdIII, BeI, and BeII). In general, apex latencies were longer with masking noise. However, BdII and BeI apex latencies were shorter with binaural than with ipsilateral monaural masking noise. Apex amplitude and planar segment size of component BeI, as well as P1 peak amplitude in BICs of the Z-channel records, were larger with binaural than with monaural noise. No significant difference between the monaural and binaural noise conditions was found in durations, shapes, and orientations of planar segments of BICs 3-CLT, nor in peak latency of BICs in the A- and Z-channel records. CONCLUSIONS: We suggest that these effects on the latency and amplitude of BICs reflect binaural processing in the human brain stem. In particular, the larger amplitudes and shorter latencies of P1 and BeI with binaural than with ipsilateral monaural masking may be associated with the psychophysical effect of binaural masking level difference.

Adolescent↗
PubMed1997

Multichannel wavelet-type decomposition of evoked potentials: model-based recognition of generator activity.

Scalp recording of electrical events allows the evaluation of human cerebral function, but contributions of the specific brain structures generating the recorded activity are ambiguous. This problem is ill-posed and cannot be solved without physiological constraints based on the spatio-temporal characteristics of the generators' activity. In our model-based analysis of evoked potentials for the purpose of generator activity detection, multichannel scalp-recorded signals are decomposed into a combination of wavelets, each of which can describe the neural mass coherent activity of cell assemblies. Elimination of contributions of specific generators and/or distributed background activity can produce physiologically motivated time-frequency filtering. The decomposition and filtering procedures are demonstrated by three examples; simulation of the surface manifestation of known intracranial generators; decomposition and reconstruction of auditory brainstem evoked potentials which reflect the differences among generators of these potentials; and cognitive components of evoked potentials which are diminished in the averaged recording but are clearly detected in single-trial signals.

Algorithms↗
PubMed1997

Evidence for separate processing in the human brainstem of interaural intensity and temporal disparities for sound lateralization.

Sound lateralization can be induced by interaural intensity disparities (IIDs) or by interaural temporal disparities (ITDs). The purpose of this study was to indicate whether IIDs and ITDs are processed by the same central units that detect interaural disparity in timing of afferent activity. If sound lateralization to intensity and time cues was determined by the same afferent latency disparity detectors in the brainstem, lateralization would be the same, regardless of whether latency disparity was induced by IIDs or ITDs. Moreover, the disparity detectors, and thus their dipole equivalents, would be the same for equal lateralizations, whether induced by IIDs or ITDs. Auditory brainstem evoked potentials (ABEPs) were recorded in response to monaural and binaural clicks, with a variety of IIDs and ITDs. Peak II (proximal auditory nerve activity), peak III (input to the superior olivary complex), and binaural interaction components (BICs) BeI and BeII (binaurally activated upper pons) were identified and their latencies measured. The psychophysical lateralization of the clicks (in cm from vertex) was also measured in response to the same binaural stimuli. The correlations between interaural afferent latency disparities (difference in corresponding peak latencies originating in each ear) and psychophysical click lateralization were calculated. Similarly, the correlations with click lateralization of the BICs equivalent dipole latency as well as orientation change (relative to symmetrical clicks) were determined. A strong correlation with lateralization was found for peaks II and III latency disparities, with steeper slopes for IIDs than for ITDs. Moreover, binaural activity across the same lateralizations differed between IIDs and ITDs. These results, therefore, indicate that interaural time and intensity cues are processed by separate systems in the brainstem, both at the afferent convergence level and after interaural disparities are determined.

Acoustic Stimulation↗
PubMed1997

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 micros earlier or 400 micros 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.

Acoustic Stimulation↗
PubMed1997

Effects of auditory/visual and lexical/non-lexical comparisons on event-related potentials in a memory-scanning task.

A late parietal positivity (P3) and behavioural measures were studied during performance of a two-item memory-scanning task. Stimuli were digits presented as memorized items in one modality (auditory or visual) while the following probe, also a digit, was presented in the same or the other modality. In a separate set of experiments, P3 and behaviour were similarly studied using only visual stimuli that were either lexical (digits) or non-lexical (novel fonts with the same contours as the digits) to which subjects assigned numerical values. Reaction times (RTs) and P3 latencies were prolonged to non-lexical compared to lexical stimuli. Although RTs were longer to auditory than to visual stimuli, P3 latencies to memorized items were prolonged in response to visually compared to auditorily presented memorized items, and were further prolonged when preceding visual probes. P3 amplitudes were smaller to auditory than to visual stimuli, and were smaller for the second memorized item when lexical/non-lexical comparisons were involved. The most striking finding was scalp distribution variations indicating changes in relative contributions of brain structures involved in processing memorized items, according to the probes that followed. These findings are compatible, in general, with a phonological memorization, but they suggest that the process is modified by memorizing the item in the same terms as the expected probe that follows.

Adult↗
PubMed1997

Modeling and estimation of single evoked brain potential components.

In this paper, we present a novel approach to solving the single-trial evoked-potential estimation problem. Recognizing that different components of an evoked potential complex may originate from different functional brain sites and can be distinguished according to their respective latencies and amplitudes, we propose an estimation approach based on identification of evoked potential components on a single-trial basis. The estimation process is performed in two stages: first, an average evoked potential is calculated and decomposed into a set of components, with each component serving as a subtemplate for the next stage; then, the single measurement is parametrically modeled by a superposition of an emulated ongoing electroencephalographic activity and a linear combination of latency and amplitude-corrected component templates. Once optimized, the model provides the two assumed signal contributions, namely the ongoing brain activity and the single evoked brain response. The estimator's performance is analyzed analytically and via simulation, verifying its capability to extract single components at low signal-to-noise ratios typical of evoked potential data. Finally, two applications are presented, demonstrating the improved analysis capabilities gained by using the proposed approach. The first application deals with movement related brain potentials, where a change of the single evoked response due to external loading is detected. The second application involves cognitive event-related brain potentials, where a dynamic change of two overlapping components throughout the experimental session is detected and tracked.

Brain↗
PubMed1997

Motor evoked potentials in the preoperative and postoperative assessment of normal pressure hydrocephalus.

Motor evoked potentials and central motor conduction time (CMCT) were examined from both upper and lower limbs in patients with normal pressure hydrocephalus to find a predictor for the success of shunting procedures. The hypotheses that walking disturbances are due to pyramidal tract compression as well as the possibility that the upper limbs are affected subclinically in these patients were also studied. The study suggests that the walking disturbances are not the result of a major pyramidal tract dysfunction but probably involve the sensorimotor integration leading to normal gait. Furthermore, CMCT measured with electromagnetic motor stimulation can help in selecting the patients that will benefit from shunting. The study does not provide electrophysiological evidence of upper limb involvement in normal pressure hydrocephalus.

Adult↗
PubMed1997

Evidence for primary auditory cortex involvement in the echo suppression precedence effect: a 3CLT study.

An echo lagging shortly after a source and arising from another direction perceptually blends with the source, and the location of the fused 'source-echo' is dominated by the source location (the Precedence Effect). The neural substrates underlying the echo localization suppression are ambiguous. We recently suggested an auditory evoked potentials correlate of binaural echo lateralization suppression. A significant and specific reduction in binaural peak amplitude and area of the echo-evoked middle-latency component Pa was observed. The binaural echo-Pa suppression depended on echo lag and correlated with the psychophysical echo lateralization suppression. In this study, the echo-Pa generators were analyzed with 3CLT spatio-temporal analysis, in order to suggest the neural substrates involved in echo lateralization suppression. 3CLT enables reliable identification of components, based on rigid geometrical properties. The results suggest that the Pa1 subcomponent of Pa, associated with primary auditory cortex activity, fully accounts for the echo-Pa suppression. This physiological indication for primary auditory cortex involvement in the precedence effect is the first in humans.

Acoustic Stimulation↗
PubMed1996

Evidence for spatio-topic organization of binaural processing in the human brainstem.

Three-channel Lissajous' trajectories (3-CLT) of the binaural interaction (BI) in auditory brainstem evoked potentials (ABEP) were derived from 13 normally and symmetrically hearing adults by subtracting the response to binaural clicks from the algebraic sum of monaural responses. ABEPs were recorded from four channels, three of them orthonormal to each other, in response to alternating polarity clicks, presented at a rate of 11/s with interaural time differences (ITD) of 0.2, 0.4 and 1.0 ms and an intensity of 65 dB nHL, or isochronic to both ears with interaural intensity differences (IIDs) of 5, 10 and 15 dB (65 dB nHL +/- 2.5, 5.0 and 7.5 dB, respectively). All 3-CLTs included 6 planar segments (labeled BdI, BdII, BdIII, BeI, BeII and Bf). Amplitudes of 3-CLT BI components were not significantly affected by increasing ITDs and IIDs, but latencies of all components increased significantly. The most remarkable finding was a significant change in apex orientations of BeI and BeII of the BI 3-CLT across stimulus conditions. The changes in BeI and BeII apex orientations, across stimulus conditions, may reflect differences in the anatomical representation of activity evoked by differently lateralized sounds. We suggest that this may indicate spatio-topic organization in the human brainstem.

Acoustic Stimulation↗
PubMed1996

Effects of exposure to lead on selected biochemical and haematological variables.

Blood and urine samples were taken from 34 persons occupationally exposed to lead and from 56 non-exposed control persons and blood lead and haemoglobin concentrations, red blood cell count, erythrocyte glutathione peroxidase (GSH-peroxidase) and acetylcholinesterase (AChE), and urinary delta-aminolevulinic acid were determined. Blood lead concentrations of the lead-exposed subjects were within the range of generally accepted as safe for occupationally-exposed adults in many countries (i.e. below 50 micrograms Pb/dl blood). Yet, significant dose-dependent elevations were found in erythrocyte GSH-peroxidase and urinary delta-aminolevulinic acid. The urinary delta-aminolevulinic acid concentration of lead-exposed smokers was significantly elevated over that of lead-exposed non-smokers. Smoking did not effect the urinary delta-aminolevulinic acid concentration of control persons. In addition, a statistically significantly lower red blood cell count was observed in the lead-exposed group. Our results indicate that the above described safety standard for blood lead concentrations in occupationally exposed adults, although generally accepted, needs revision.

Acetylcholinesterase↗