Biomedical subjects
Publications and source records attributed to R Burkard.
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Single photon emission computed tomography (SPECT) with thallium-201-chloride (201Tl) was used in 22 patients to assess the grade of malignancy of brain tumors. Low- and high-grade malignant gliomas could be well differentiated by calculating the Grade Index (GI), i.e., 201Tl uptake in the tumor area relative to a contralateral brain region. Low-grade gliomas (WHO-grade I-II) usually showed a GI of < 1.5. Tumors classified histologically as high-grade malignant (WHO-grade III-IV) had GI values greater than 1.42 and a mean value of 1.89. Until labelled amino-acid tracers for gamma-cameras become commercially available, thallium-201 brain-SPECT can provide an independent and complementary method to CT/MRI for the differential diagnosis of grading of brain tumors. This simple technique can help to reduce sampling errors during needle biopsies of brain tumors, particularly of high-grade lesions incorrectly graded as low-grade tumors due to inadequate biopsy material. In addition, pre- and post-therapy studies can influence the strategy of therapy itself and allow an early detection of recurrences.
The effect of audiometric configuration on the auditory brain stem response was studied in a large patient sample, and wave I latencies, wave V latencies, and the I-V interwave intervals were compared to those from a previous report. Patients with notched hearing losses showed longer wave V latencies and I-V interwave intervals than those with other audiometric configurations, but the magnitude of the effect was relatively small, and the confidence limit for cochlear diagnosis was essentially the same as that based upon a cochlear hearing loss population without regard to audiometric configuration.
Two experiments concerning the effects of continuous broadband noise on wave V of the click-evoked human BAER are reported. Experiment I compared the effects of broadband masking noise presented ipsilaterally, contralaterally, and biaurally on the BAER to 100-dB pSPL monaurally presented clicks. For noise levels up to 75 dB SPL, contralateral masking had no effect on either the latency or the amplitude of wave V. Ipsilateral and binaural noise levels above 35-45 dB SPL increased wave V latency and decreased its amplitude, and the magnitudes of these effects were similar for ipsilateral and binaural noise conditions. Experiment II compared the effects of broadband masking noise on wave V to 100-dB pSPL clicks presented monaurally to each ear, and binaurally, with the noise being presented to the ear(s) of click presentation. In agreement with experiment I, noise levels above 35-45 dB SPL increased wave V latency and decreased wave V amplitude. There were no significant differences for right versus left ear. Wave V latency did not vary significantly for monaural versus binaural presentation, while wave V amplitude was larger for binaural than monaural presentation modes for all noise-level conditions.
Brain-stem auditory evoked responses (BAERs) were obtained in eight normal-hearing young adults. Stimuli included clicks, noise bursts, and tone bursts. Tone bursts included carrier frequencies of 1, 2, 4, and 8 kHz. All stimuli were presented at 60 dB nHL. BAERs were obtained by presenting stimuli in pseudorandom trains, called maximum length sequences (MLSs). BAERs were recovered by cross correlating the responses with a recovery sequence. MLS-BAERs were obtained with minimum pulse intervals (MPIs) of 6, 4, and 2 ms. Conventional BAERs were also obtained for stimuli presented at a rate of 30 Hz. BAERs were obtained for all stimuli, for both the conventional averaging technique and for the cross-correlation technique. BAERs were observed for MPIs as short as 2 ms for all stimuli. Wave V was the only peak consistently identifiable for these stimuli. For all stimuli, wave V latency increased and wave V amplitude decreased with decreasing MPI. This is the first demonstration of the use of maximum length sequences combined with cross correlation to obtain BAERs to noise burst and tone burst stimuli.
The effects of noiseburst rise time and level on the latency and amplitude of waves i and v of the gerbil brainstem auditory evoked response (BAER) were evaluated. For tonal carriers, changes in rise time produce changes in stimulus spectrum. The use of a noise carrier eliminates such confounding spectral changes. Noiseburst levels included 40, 55, 70 and 85 dB SPL. Noiseburst rise times included 0, 0.5, 1.25 and 2.5 ms. With increasing noiseburst level, there is a decrease in peak latency, an increase in peak amplitude and a small increase in the i-v interval. With increasing noiseburst rise time, there is an increase in peak latency, a decrease in peak amplitude and a small increase in the i-v interval. The slopes of wave i and v latency/intensity functions increase with increasing rise time. The changes in peak latencies are always less than the increase in rise time. The slopes of these functions increase with decreasing noiseburst level and are greatest for the change in rise time from 0 to 0.5 ms. These data demonstrate changes in the BAER with rise time manipulation, even in the absence of stimulus spectral changes. It was hypothesized that changes in the BAER with increasing noiseburst rise time were due to a decrease in the effective amplitude of the stimulus. It was found that this hypothesis does not, in isolation, adequately account for the peak latency and amplitude changes found in the present investigation.
Experiments were performed in which brain-stem auditory-evoked responses (BAERs) were elicited by two types of pseudorandom pulse trains: maximum length sequences (MLS) and Legendre sequences (LGS). In experiment 1, each pulse sequence was presented at 50 dB nHL with minimum pulse intervals varying from 1 to 10 ms. Wave V latency increased and wave V amplitude decreased with decreasing minimum pulse intervals, with no significant effect of the type of sequence (MLS vs LGS), and no significant interaction between sequence and minimum pulse interval in terms of wave V amplitude or latency. In a second experiment, the minimum pulse interval was held constant at 4 ms, while MLS and LGS levels were varied from 20 to 60 dB nHL. With increasing click intensity, there is a decrease in wave V latency and an increase in wave V amplitude. There was no significant effect of type of sequence (LGS vs MLS) or interaction between type of sequence and stimulus intensity for wave V amplitude or latency. Despite the obvious violation of the assumptions (linearity and stationarity) underlying the application of maximum length sequence analysis and Legendre sequence analysis, both techniques produced reliable responses remarkably similar in morphology to evoked responses obtained by conventional averaging. The results of these experiments support the possibility that analysis methods based on pseudorandom pulse sequences may prove more efficient in data collection and provide a more thorough description of the electrophysiologic behavior of the auditory system compared to conventional averaging.
The effects of masking noise on wave V of the brain-stem auditory-evoked response (BAER) obtained to pseudorandom pulse sequences are evaluated in two experiments. In the first experiment, the level of broadband noise was covaried with minimum pulse interval (rate) using maximum length sequence analysis (MLSA). Both increasing noise level and decreasing minimum pulse interval decrease wave V amplitude and increase wave V latency. A nonadditivity of rate and noise level was observed such that, at the shortest interpulse intervals, simultaneous background noise produced virtually no latency change and minimal amplitude change, for the noise levels tested. In a second experiment, high-pass masking was performed to assess the feasibility of derived-band techniques using maximum length sequence analysis (MLSA) and to compare the frequency regions responsible for the BAER using MLSA versus conventional averaging. Results of experiment 2 showed that reliable responses across high-pass masker cutoff frequency could be obtained in normal-hearing listeners. The frequency specificity of the MLSA-based responses was nearly identical to that obtained by conventional averaging, although both amplitude and latency of wave V were affected by the high-pass masker cutoff and minimum pulse interval values. These studies suggest that the neuronal populations and frequency regions responsible for the BAER are virtually the same for MLSA and conventional averaging.
Two experiments were performed that evaluated the effects of ipsilateral-direct broadband noise maskers on the gerbil brain-stem auditory-evoked response (BAER) to click stimuli. In experiment 1, clicks were presented at 27 Hz at levels including 70, 80, 90, and 100 dB pSPL. Noise conditions included a no-noise control, and included noise levels varying in 10-dB increments from 20 dB SPL to a maximum noise level of 50, 60, 70, and 80 dB SPL for click levels of 70, 80, 90, and 100 dB pSPL, respectively. Gerbil BAER peaks were labeled with small roman numerals to distinguish them from human BAER peaks. The dependent variables included waves i and v latencies and amplitudes. Peak latencies increased and peak amplitudes decreased with decreasing click level and increasing noise level. To a first approximation, peak latencies and amplitudes showed changes with increasing noise level that were similar across click level. With increasing click level, there was little or no effect on the i-v interval. There was an increase in the i-v interval with increasing noise level. In experiment 2, click level was held constant at 90 dB pSPL, and click rates included 15, 40, 65, and 90 Hz. For each click rate, noise conditions included a no-noise control, and noise levels included 20, 30, 40, 50, 60, and 70 dB SPL. With increasing click rate and noise level, there was an increase in peak latencies, an increase in the i-v interval, and a decrease in peak amplitudes. The magnitude of peak latency and amplitude shifts with increasing click rate was dependent on noise level. Specifically, the magnitude of rate-dependent changes decreased with increasing level of broadband noise. These data are compared to human BAER experiments, and are found to be in fundamental agreement.
Six young adult Sprague-Dawley rats were unilaterally cochleotomized, Brain-stem auditory-evoked responses (BAERs) to clicks and to 1-, 2-, 4-, 8- and 16-kHz tone bursts were obtained. In addition, response thresholds were estimated before and after ossicular disruption in the noncochleotomized ear of 4 animals. With increasing tone burst frequency, there was a decrease in BAER peak latencies as well as a decrease in threshold. With increasing click and tone burst intensity, there was a decrease in peak latencies and an increase in peak amplitudes. BAER peak latency/intensity functions to click stimuli ranged from -.013 to -.018 ms/dB. With increasing tone burst frequency there was a decrease in the slope of the latency/intensity function. Following ossicular disruption, BAER thresholds to clicks were elevated by an average of 49 dB. Threshold shifts to tone burst stimuli were smallest for 1- and 2-kHz tone bursts (35-36 dB) and increased with increasing frequency up to a maximum of 65 dB for 16-kHz tone bursts.
Three experiments evaluating the effects of various stimulus manipulations on the click-evoked gerbil brain-stem auditory-evoked response (BAER) are reported. In experiment 1, click polarity and level were covaried. With increasing click level, there is a parallel decrease in the latency of the first five BAER peaks (i-v) and an increase in BAER peak amplitudes. Mean wave i amplitude was greater for rarefaction than condensation clicks at high click levels; mean wave v amplitude was greater for condensation clicks at higher click levels. Experiment 2 covaried click rate and polarity. The latency of the BAER peaks increased with increasing click repetition rate. This rate-dependent latency increase was greater for the later BAER peaks, resulting in an increase in the i-v interval with increasing click rate. As rate increased, the amplitudes of waves i and v decreased monotonically, whereas the amplitudes of waves ii-iv were largely uninfluenced by click rate. As in experiment 1, mean wave i amplitude was greater for rarefaction clicks, whereas mean wave v amplitude was greater for condensation clicks. The magnitude of these polarity dependencies on waves i and v amplitude decreased with increasing click rate. Experiment 3 evaluated the effects of click polarity on BAERs to high-intensity (100 dB pSPL) clicks presented at a rate of 10 Hz. In eight of ten gerbils evaluated, wave i amplitude was greater to rarefaction clicks, and, in all ten animals, wave v amplitude was greater to condensation clicks. The effects of click level and rate on BAER peak amplitudes, latencies, and interwave intervals are reminiscent of stimulus dependencies reported for the human BAER. The effects of click polarity on the amplitudes of waves i and v of the gerbil BAER have also been reported for the human BAER.
Two experiments concerning the effects of masking noise on the gerbil brain-stem auditory-evoked response (BAER) are reported. Experiment 1 evaluated the effects of broadband masking noise on the BAER obtained to condensation and rarefaction clicks. With increasing noise level, there was an increase in BAER peak latencies, an increase in the i-v interval, and a decrease in peak amplitudes. Experiment 2 evaluated the effects of high-pass masking noise on the BAER obtained to condensation and rarefaction clicks. Both high-pass responses and derived-band responses were evaluated. For high-pass responses, with decreasing masker cutoff frequency, there was an increase in BAER peak latencies, a decrease in the i-v interval, and a decrease in peak amplitudes. For derived-band responses, with decreasing derived-band frequency, there was an increase in peak latencies and a decrease in the i-v interval. A comparison of wave i and wave v amplitudes across derived-band frequency demonstrates a greater contribution of high-frequency cochlear regions to wave i than wave v. Small, insignificant, effects of click polarity on BAER peak amplitudes were observed. These trends were in the direction seen in a companion paper [R. Burkard and H. F. Voigt, J. Acoust. Soc. Am. 85, 2514-2525 (1989)] and were, in general, reduced by the presence of broadband or high-pass maskers.
The effects of broadband noise on the brain-stem auditory evoked response (BAER) are reported for two experiments. Experiment 1 used a high-pass subtractive-masking technique and covaried derived bandwidth and continuous broadband noise level. Comparison of responses to half-octave wide derived bands in the presence of within-band noise showed that wave V latency changes were greater than could be explained on the basis of shifts in the cochlear region responsible for generating the response. The magnitude of within-band noise-induced wave V latency shift was independent of the frequency separation of the masker cutoffs. In experiment 2 the effects of noise level and rate on waves I, III, and V of the BAER were evaluated. Peak latencies increased and peak amplitudes decreased with increasing noise level and rate. Higher noise levels and rates produced an increased central (I-V) conduction time in which the wave III-V increase was greater than the wave I-III increase. Together, these results are most consistent with the hypothesis that a nonplace, central auditory mechanism produces most of the noise-induced latency shifts in normal-hearing adults.
The additivity of forward masking and repetitive stimulation effects on wave V of the brain-stem auditory evoked response (BAER) was investigated. The effects of repetitive stimulation were evaluated for a stimulus train (called the adaptation series), with a 12.5-ms within-train interclick interval. The forward masker was a 100-ms, 80-dB SPL broadband noise with forward-masker intervals ranging from 12.5-87.5 ms. Forward masking and repetitive stimulation increased the latency of wave V of the BAER. The combined forward masking/adaptation series produced less wave V latency shift than the summed individual effects. Forward masking reduced wave V amplitude at brief forward masker intervals, while repetitive stimulation did not affect wave V amplitude. Wave V amplitude was decreased for the combined forward masking/adaptation series, and the time course of amplitude recovery of the combination was prolonged compared to the forward masking alone condition. The nonadditivity of forward masking and rate effects on wave V latency is similar to that found for repetitive stimulation and simultaneous masking [Burkard and Hecox, J. Acoust. Soc. Am. 74, 1204-1213 (1983)]. These findings are consistent with the position that forward masking and rate effects on wave V latency are produced by overlapping mechanisms.
Threshold tuning curves were obtained from neurons in the cerebellar auditory area of the cat. The threshold of the brainstem auditory evoked response was also measured in each animal as a function of sound frequency in order to monitor the overall frequency sensitivity of the auditory periphery. Cerebellar auditory neurons responded to sound stimuli with little discrimination for the sound frequency. The values of Q10dB (a measure of the sharpness of tuning) were less than 2 for most of the neurons in this study. There was no significant difference in the sharpness of tuning for neurons in the various layers of the cerebellar auditory area. Electrophysiological mapping showed that the frequency sensitivity of single neurons did not appear to vary as a function of location within the cerebellar auditory area which includes lobules VI and VII of Larsell. Broad tuning was observed in long-latency (greater than 11 ms) neurons which responded to binaural sound stimuli as well as in short-latency (less than 6 ms) neurons which only responded to monaural sound stimuli. Within each animal, tuning curves of single cerebellar neurons were essentially superimposable onto each other and matched well with the overall frequency sensitivity of the animal as shown by brainstem auditory evoked response. Since the frequency tuning of these neurons appeared to reflect the overall frequency sensitivity of the auditory periphery, auditory neurons in the posterior vermis may receive inputs that involve convergence and integration from the entire length of the cochlea.
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The sound pressure level (SPL) of an acoustic transient can be quantified in several ways. The SPL value obtained is dependent on measurement procedure, in addition to signal and transducer characteristics. The acoustic spectrum of a signal shows sound pressure as a function of frequency. The acoustic spectrum can be determined by the use of analog filtering or by Fourier transformation. A constant electrical signal can produce different acoustic spectra due to varying transfer functions across transducers. Signal center frequency, rise/fall time and plateau influence acoustic spectrum. Recording parameters, such as constant bandwidth versus logarithmic bandwidth filtering, or the time domain windowing function used prior to Fourier transformation, also influence the acoustic spectrum.
Brainstem evoked responses were obtained to 60-dB nHL clicks and click pairs presented at 27/s. Paired-click intervals ranged from 9.8 to 0.5 ms. 6 normal-hearing young adults served as subjects. Derived responses were obtained by digital response subtraction of single-click from paired-click responses. Wave V was observed to unsubtracted click 2 of the click pair at paired-click intervals down to 4 ms. Wave V of the derived (subtracted) click 2 response was observed for paired-click intervals down to 1 ms. To assess additivity, the click 1 response of the click pair was compared to the single-click response, and the unsubtracted click 2 response was compared to the derived click 2 response. At longer paired-click intervals, wave V latency was greater for click 1 of the click pair than for the single-click response. This latency difference decreases at shorter paired-click intervals and is attributed to adaptation of click 1 by click 2 of the previous pair. Wave V amplitudes for single-click and paired-click click 1 responses were not significantly different at any paired-click interval. The latency and amplitude for unsubtracted click 2 and derived click 2 responses were not significantly different at any paired-click interval. It is concluded that the responses to click 1 and click 2 of the paired clicks are additive, at least at very brief paired-click intervals.