Biomedical subjects

A Sek

Publications and source records attributed to A Sek.

At least 19 recordsLinked to original sources

PubMed2001

Anti-CD16/CD30 bispecific antibody treatment for Hodgkin's disease: role of infusion schedule and costimulation with cytokines.

The natural killer cell-activating anti-CD16/CD30 bispecific monoclonal antibody (BiMAb) had shown efficacy in a Phase I/II trial of refractory Hodgkin's disease (HD). To gain additional information on clinical efficacy and to investigate the effects of different application schedules and the concomitant application of cytokines, we performed a second randomized pilot trial using this BiMAb in patients with refractory HD. Patients received 4 x 25 mg HRS-3/A9 either as a continuous infusion for 4 days or as a 1-h infusion every other day. In case of an objective response, retreatment was attempted after 4 weeks; in case of stable disease (SD), a second course was given after prestimulation with interleukin 2 and followed by granulocyte macrophage colony-stimulating factor s.c. A total of 16 heavily pretreated patients received one to four BiMAb courses. Overall, we observed one complete remission and three partial remissions lasting 5-9 months (three of four of these responses occurred after continuous BiMAb infusion) and four cases of SD for 3 to >6 months. Interleukin 2 pretreatment before the second BiMAb course resulted in a significant increase of circulating natural killer cells in all five patients treated. This coincided with the conversion of two cases of SD into one complete remission and one partial remission. HRS-3/A9-related side effects consisted of mild fever in only six patients. In summary, this second trial confirmed the antitumor efficacy of this BiMAb against HD and the minor toxicity of this BiMAb. Coadministration of cytokines might contribute to an augmented antitumor activity, and additional clinical trials are warranted to optimize this novel treatment modality.

Adult↗

PubMed2001

Acid phosphatase activity in different organs as a marker of acute pancreatitis.

The aim of the experiment was to establish and quantify the changes in the activity of acid phosphatase in the pancreas, liver, spleen and kidneys during the course of experimental pancreatitis. The experiment was carried out on 65 male rats of Wistar strain, whose weight varied from 250 to 350 g. The animals were standard fed. They drank only water 24 hours before operation. The rats were randomly divided into three groups: A--intact animals group which were not operated and were used to mark initial biochemical parameters (15 rats), B--the experimental group of animals which were injected by retrograde way with sodium taurocholate into the common bile-pancreatic duct to induce acute necrotic pancreatitis (50 rats). After laparotomy an injection needle was inserted into the common bile-pancreatic duct via the proximal part of the duodenum (Aho's method). After 2, 6, 12, 24, 48 hours rats were anaesthetised again, and thoracotomy was performed by taking blood for amylase determination from the left ventricle of the heart. Then the animals were given an overdose of ketamine, and the organs were removed during laparotomy and frozen at the temp. of -20 degrees C. Alpha-amylase activity in the blood serum was determined by the enzymatic method. Acid phosphatase activity was assayed by spectrophotometric methods using a substrate which releases 4-methyloumbeliferol reacting with the enzyme. The authors concluded that the activity of membranous fraction of acid phosphatase changed non-specifically over the course of experimentally induced acute pancreatitis in rats, but statistically significant difference was found in the enzyme's activity during different periods of pancreatitis only in the pancreas and in the liver.

Acid Phosphatase↗

PubMed2000

Effects of relative phase and frequency spacing on the detection of three-component amplitude modulation.

These experiments explored the effect of relative modulator phase on the detection of a three-component modulator applied to a 4,000-Hz sinusoidal carrier with a level of 70 dB SPL. The central modulator component had a frequency of 50 Hz, and the two other components had frequencies of 50+/-5, 10, 25, 40, or 45 Hz. Thus, the modulator waveform was always periodic. Each modulator component had the same modulation index, m. The relative phases of the components were chosen to give a variety of modulation waveforms differing in the ratio of maximum to minimum value (max-min) and in crest factor. In experiment 1, modulation detection thresholds were measured by varying m, using an adaptive two-interval forced-choice procedure. Thresholds were found to be independent of relative modulator phase and of the frequency spacing of the components. In experiment 2, detectability (d') of the modulation was measured for several fixed values of m. Detectability was found to be independent of relative modulator phase and of the frequency spacing of the components. The results are not consistent with the idea that modulation detection thresholds are determined by the max-min value or crest factor of the envelope. The results are consistent with a model which assumes that the stimuli are subjected to a nonlinearity, and thresholds are determined by the root-mean-square value (or the mean square value) of the ac component of the envelope, following this nonlinearity. The nonlinearity may partly reflect compression on the basilar membrane, but other nonlinearities may be involved. This model can also explain some aspects of earlier results on the sensitivity to relative modulator phase [E. A. Strickland and N. F. Viemeister, J. Acoust. Soc. Am. 99, 3638-3646 (1996)].

Auditory Threshold↗

PubMed2000

Detection of quasitrapezoidal frequency and amplitude modulation.

It has been proposed that the detection of frequency modulation (FM) of sinusoidal carriers can be mediated by two mechanisms; a place mechanism based on FM-induced amplitude modulation (AM) in the excitation pattern, and a temporal mechanism based on phase locking in the auditory nerve. The temporal mechanism appears to be "sluggish" and does not play a role for FM rates above about 10 Hz. It also does not play a role for high carrier frequencies (above about 5 kHz). This experiment provided a further test of the hypothesis that the effectiveness of the temporal mechanism depends upon the time spent close to frequency extremes during the modulation cycle. Psychometric functions for the detection of AM and FM were measured for two carrier frequencies, 1 and 6 kHz. The modulation waveform was quasitrapezoidal. Within each modulation period, P, a time Tss was spent at each extreme of frequency or amplitude. The transitions between the extremes, with duration Ttrans had the form of a half-cycle of a cosine function. The modulation rate was 2, 5, 10, or 20 Hz, giving values of P of 500, 200, 100, and 50 ms. TSS varied from 0 ms (sinusoidal modulation) up to 160, 80, 40, or 20 ms, for rates of 2, 5, 10, and 20 Hz, respectively. The detectability of AM was not greatly affected by modulation rate or by the value of TSS, except for a slight improvement with increasing TSS for the lowest modulation rates; this was true for both carrier frequencies. For FM of the 6-kHz carrier, the pattern of results was similar to that found for AM, which is consistent with an excitation-pattern model of FM detection. For FM of the 1-kHz carrier, performance improved markedly with increasing TSS, especially for the lower FM rates; there was no change in performance with TSS for the 20-Hz modulation rate. The results are consistent with the idea that detection of FM of a 1-kHz carrier is partly mediated by a sluggish temporal mechanism. That mechanism benefits from greater time spent at frequency extremes of the modulation cycle for rates up to 10 Hz.

Auditory Perception↗

PubMed1999

Enrichment polymerase chain reaction for the detection of Ki-ras mutations: relevance of Taq polymerase error rate, initial DNA copy number, and reaction conditions on the emergence of false-positive mutant bands.

Screening for oncogene mutations as a marker for malignancy can be a powerful tool for the early diagnosis of cancer. The enrichment polymerase chain reaction (PCR) is a sensitive method for the detection of low-frequency mutations in small samples. However, false-positive results, caused by methodological errors, may have severe clinical implications. When applied to the detection of Ki-ras mutations in pancreatic secretions, the assay sensitivity is limited to approximately 1:1400. Our investigation of Ki-ras mutations in blood samples from patients with pancreatic carcinoma revealed PCR bands presumably derived from mutant Ki-ras in samples from healthy volunteers, while all blood samples of the patients with pancreatic carcinomas showed a wild-type band pattern. Mathematical modeling of the PCR reaction reveals that the rate of false positive PCR results depends on the initial amount of DNA, the Taq polymerase error rate, the number of PCR reaction cycles, reaction efficiency and the restriction endonuclease chosen. The overall error rate of false positive results of the enrichment PCR can be reduced to the square of the rate of a single-step analysis if repeated amplifications of the same DNA specimen show an identical result.

False Positive Reactions↗

PubMed1999

Discrimination of frequency steps linked by glides of various durations.

Thresholds were measured for detecting steps in frequency linked by glides of various durations. The goals were to assess the relative importance of place and temporal information for this task, and to determine whether there is a mechanism for detecting dynamic frequency changes per se, as opposed to comparing the initial and final frequencies of the stimuli. Subjects discriminated a 500-ms sinusoid of constant frequency from a sinusoid with three parts: an initial part with constant frequency, a downward frequency glide, and a final part with constant frequency. The overall duration was 500 ms, and the glide duration was varied from 5 to 500 ms. In one special case, the portion of the stimuli when a glide might occur was replaced by a brief silent interval. The center frequency was fixed at 0.5, 1, 2, 4, or 6 kHz (condition 1), or varied randomly from one stimulus to the next over a 4-ERB range around the nominal center frequency (condition 2). The randomization impaired performance, but thresholds remained lower than the best that could be achieved by monitoring either the initial or final frequency of the stimuli. Condition 3 was like condition 2, but for each stimulus a glide in level was added at the time when a frequency glide might occur, so the initial and final levels differed; the glides in level varied randomly in extent and direction from one stimulus to the next over the range +/- 20 dB. This impaired performance, but thresholds remained lower than the best that could be achieved by monitoring changes in excitation level on only one side of the excitation pattern. Excitation-pattern models of frequency discrimination predict that thresholds should not vary across center frequency when expressed as the change in ERB number, delta E. For all conditions, delta E values increased at 6 kHz, suggesting a role for temporal information at lower frequencies. The increase was smallest for the longest glide duration, consistent with a greater relative role of place information when there was no steady state portion. Performance was better when a brief glide was present than when no glide was present, but worsened with increasing glide duration. The results were fitted well by a model based on the assumption that information from the steady parts of the stimuli (perhaps extracted mainly using temporal information) was combined with information from the glides (perhaps extracted mainly using place information).

Auditory Threshold↗

PubMed1999

Modulation masking produced by beating modulators.

This study examined whether "modulation masking" could be produced by temporal similarity of the probe and masker envelopes, even when the masker envelope did not contain a spectral component close to the probe frequency. Both masker and probe amplitude modulation were applied to a single 4-kHz sinusoidal or narrow-band noise carrier with a level of 70 dB SPL. The threshold for detecting 5-Hz probe modulation was affected by the presence of a pair of masker modulators beating at a 5-Hz rate (40 and 45 Hz, 50 and 55 Hz, or 60 and 65 Hz). The threshold was dependent on the phase of the probe modulation relative to the beat cycle of the masker modulators; the threshold elevation was greatest (12-15 dB for the sinusoidal carrier and 9-11 dB for the noise carrier, expressed as 20 log m) when the peak amplitude of the probe modulation coincided with a peak in the beat cycle. The maximum threshold elevation of the 5-Hz probe produced by the beating masker modulators was 7-12 dB greater than that produced by the individual components of the masker modulators. The threshold elevation produced by the beating masker modulators was 2-10 dB greater for 5-Hz probe modulation than for 3- or 7-Hz probe modulation. These results cannot be explained in terms of the spectra of the envelopes of the stimuli, as the beating masker modulators did not produce a 5-Hz component in the spectra of the envelopes. The threshold for detecting 5-Hz probe modulation in the presence of 5-Hz masker modulation varied with the relative phase of the probe and masker modulation. The pattern of results was similar to that found with the beating two-component modulators, except that thresholds were highest when the masker and probe were 180 degrees out of phase. The results are consistent with the idea that nonlinearities within the auditory system introduce distortion in the internal representation of the envelopes of the stimuli. In the case of two-component beating modulators, a weak component is introduced at the beat rate, and it has an amplitude minimum when the beat cycle is at its maximum. The results could be fitted well using two models, one based on the concept of a sliding temporal integrator and one based on the concept of a modulation filter bank.

Auditory Perception↗

PubMed1998

Discrimination of frequency glides with superimposed random glides in level.

These experiments were designed to test the hypothesis that glides in frequency are detected and discriminated by monitoring changes in excitation level on the low-frequency side of the excitation pattern. Thresholds were measured for detecting an increase in the extent of a frequency glide, for various standard extents (transition spans). The center frequency of each stimulus was roved, to prevent subjects from using the start or endpoint frequencies of the stimuli as cues. The level was either fixed at 70 dB SPL, or changed linearly in dB/s by an amount that varied randomly in extent and direction, keeping the level at the midpoint of the glide at 70 dB SPL. These random changes in level were intended to disrupt cues based on monitoring changes in excitation level on one side of the excitation pattern. For some conditions, performance was too good to be explained by subjects monitoring the start or endpoint frequencies of the stimuli. Performance was also too good to be explained in terms of the discrimination of changes in excitation level on one side of the excitation pattern. Thresholds, expressed as a proportion of the equivalent rectangular bandwidth (ERB) of the auditory filter, did not vary greatly with center frequency (0.5, 2, or 6 kHz), suggesting that discrimination did not depend strongly on information derived from phase locking. Glide duration (50 or 400 ms) and glide direction (upward or downward) also had little effect. Thresholds increased with increasing standard transition span, when that span was increased beyond 0.5 ERB. It is concluded that changes in glide extent per se can be discriminated, but this is not done by monitoring just one side of the excitation pattern.

Analysis of Variance↗

PubMed1998

Restriction digest PCR (RD-PCR) for the analysis of gene mutations. Application to Ki-ras.

The Kirsten-ras (onco)gene codes for a GTP-binding membrane protein that is involved in signal transduction. Activated ras triggers a cascade of protein-phosphorylations that ultimately lead to cell proliferation. Ras-mutations are the main cause for adenocarcinomas of the pancreas besides some mutations in the tumor suppressor gene p53 and the c-erbB-2 oncogene. The site of ras mutations in pancreatic cancer is restricted to codon 12 that normally encodes a glycine. For analysis of codon-12 mutations, DNA is extracted from cells in pancreatic fluid and amplified by PCR. Because most of these cells originate from normal tissue with only a few tumor cells in the fluid, "enrichment PCR" must be utilized: In a first round of the PCR, ras sequences from all cells are amplified. By utilizing an appropriate restriction enzyme, wild-type sequences can be digested and the remaining fragments containing mutated sequences be amplified again. An artificial restriction site must be introduced by the 5'primer (...GGA CCT GGT...) for an enzyme (BstNI) (5'CC!WGG 3') to differentiate between wild-type sequence (...GGA GCT GGT...) (during amplification, the G is replaced by a C) and mutated sequences (_...GGA GCT (GTT), (CGT), (CCT), etc.). The necessary manipulations pose a considerable risk for contamination for the second round of the PCR procedure. Therefore, we considered whether it would be feasible to perform the restriction digest simultaneously with the first PCR reaction, and avoiding the second round altogether. The results of our experiments demonstrate that one tumor cell in 1000 normal cells can be determined readily, paralleling the results with the original two step-assay. The restriction enzyme used to enrich mutated sequences is stable long enough to be included into the PCR procedure. By this, wild-type sequence amplicons are digested while they are formed and mutated sequences can be enriched selectively.

Base Sequence↗

PubMed1996

Detection of frequency modulation at low modulation rates: evidence for a mechanism based on phase locking.

These experiments tested the hypothesis that detection of frequency modulation (FM) at very low rates depends mainly on temporal information (phase locking to the carrier) for carriers below about 5 kHz, whereas FM detection at higher rates (10 Hz and above) depends mainly on changes in the excitation pattern (a "place" mechanism). In experiment 1, thresholds for detecting FM were measured for a wide range of carrier frequencies (0.25-6 kHz) for modulation rates, fm, of 2, 5, 10, and 20 Hz. Thresholds were determined when FM only was present and when the carriers in both intervals of a forced-choice trial were amplitude modulated at the same rate as the FM with a modulation index of 0.333. The phase of the amplitude modulation (AM) relative to the FM was randomly selected on each trial, in order to disrupt cues for FM detection based on changes in the excitation pattern. For carrier frequencies up to 4 kHz, the deleterious effect of the added AM increased with increasing fm. For the 6-kHz carrier, the deleterious effect was independent of fm. In experiment 2, psychometric functions were measured for detecting combined FM and AM of a 1-kHz carrier, with fm = 2 Hz, as a function of the relative phase of the modulators. The modulation depths for AM and FM were chosen so that each would be equally detectable if presented alone. This was done both in quiet and in the presence of noise designed to mask either the lower or the upper side of the excitation pattern. In contrast to earlier results obtained with fm = 10 Hz [Moore and Sek, J. Acoust. Soc. Am. 96, 741-751 (1994)], only small effects of relative modulator phase were found. Experiment 3, was similar to experiment 2, except that all measurements were done in quiet, and carrier frequencies of 0.25, 1.0, and 6.0 kHz were used. There were no effects of relative modulator phase for the 0.25-kHz carrier, small effects for the 1-kHz carrier, and large effects for the 6-kHz carrier. The pattern of results is consistent with the hypothesis that both temporal and place mechanisms are involved in FM detection. The temporal mechanism dominates for carriers below about 4 kHz, and for very low modulation rates. The place mechanism dominates for high carrier frequencies, and for lower carrier frequencies when stimuli are frequency modulated at high rates.

Auditory Perception↗

PubMed1996

Detection of auditory "events" based on amplitude and frequency modulation.

These experiments examined the ability of subjects to detect auditory "events" composed of a brief modulation in the temporal center of an otherwise steady sinusoid. In experiment 1, psychometric fluctuations were measured for detecting either amplitude modulation (AM) or frequency modulation (FM) composed of a single cycle of a raised-cosine function (either positive going or negative going); the modulation frequency was 10 Hz, so the event lasted for 100 ms. Then, psychometric functions were measured for stimuli with both AM and FM, using pairs of values of AM and FM that were equally detectable; pairs were always modulated in the same direction, so a positive amplitude excursion went together with a positive frequency excursion. Performance was compared with the "reference" detectability that would be predicted from the optimal combination of independent sources of information. When the AM and FM were synchronous, detectability was better than the reference detectability. When the FM was delayed by 100 ms relative to the AM, detectability was mostly equal to or less than the reference detectability. The better performance with synchronous AM and FM is consistent with an explanation based on an excitation-pattern model. Experiment 2 was similar to experiment 1, except that the event was a single cycle of sinusoidal modulation starting at 0 degree phase or 180 degrees phase. When the AM and FM were synchronous and in phase, detectability was better than the reference detectability. When the AM was delayed by 200 ms relative to the FM or had opposite starting phase, detectability was close to or below the reference detectability. However, for the case where the delay was 100 ms and the modulation had the same starting phase for AM and FM, detectability was better than the reference detectability. A control experiment using two successive cycles of either AM or FM showed a similar, but slightly smaller, effect. It appears that detectability can be enhanced when two successive events form a regular temporal pattern.

Auditory Perception↗

PubMed1995

Effects of carrier frequency, modulation rate, and modulation waveform on the detection of modulation and the discrimination of modulation type (amplitude modulation versus frequency modulation).

Initially, psychometric functions were measured for the detection of amplitude modulation (AM) or frequency modulation (FM), using a two-alternative forced-choice (2AFC) task. Carrier frequencies were 125, 1000, and 6000 Hz, and modulation rates were 2, 5, and 10 Hz. For the two lower carrier frequencies, FM detection tended to be best at the lowest modulation rate while AM detection was best at the highest rate. For the 6000-Hz carrier, both AM and FM detection tended to be poorest at the lowest modulation rate. Then, pairs of values of AM and FM were selected that would be equally detectable, and psychometric functions were measured for the discrimination of AM from FM, again in a 2AFC task. For carrier frequencies of 125 and 1000 Hz, the ability to discriminate AM from FM was always poorest at the highest modulation rate (10 Hz); at this rate some subjects were essentially unable to discriminate AM from FM when the detectability of the modulation was relatively low (d' of 1.16 and below). For a modulation rate of 2 Hz, and when the detectability of the modulation was moderate (d' up to about 2), some subjects discriminated the type of modulation rate varied across subjects, but there was still a trend for poorer discrimination of modulation type at the highest modulation rate. It is suggested that FM detection at a 10-Hz modulation rate is based largely on changes in excitation level for all carrier frequencies. For a 2-Hz modulation rate, and for the two lowest carrier frequencies, an extra mechanism, possibly based on phase locking, may play a role in the detection and discrimination of FM. This mechanism may be ineffective at modulation rates above about 5 Hz because the stimuli spend insufficient time at frequency extremes. To check on this, psychometric functions were measured for the detection of FM and AM using quasitrapezoidal modulation with a rate of five periods per second and carriers of 250, 1000, and 6000 Hz. This produced improvements in performance relative to that obtained with 5-Hz sinusoidal modulation and, for the two lower carrier frequencies only, the improvements were markedly greater for FM than for AM detection. This is consistent with the idea that the use of of phase-locking information depends on the time that the stimuli spend at frequency extremes.

Auditory Perception↗

PubMed1995

Frequency discrimination as a function of frequency, measured in several ways.

Frequency discrimination was measured for a wide range of center frequencies (0.25-8 kHz) using three different tasks. In the first (difference limens for frequency, DLFs) subjects were required to indicate which of two successive tone pulses was higher in frequency. In the second (difference limens for change, DLCs), two successive pairs of tone pulses were presented; one pair had the same frequency and the other pair differed in frequency. Subjects were required to indicate which pair differed in frequency. In the third (frequency-modulation difference limens, FMDLs), subjects were required to indicate which of two successive tone pulses was frequency modulated. Modulation rates were 2, 5, or 10 Hz. For frequencies up to 2 kHz, DLFs and DLCs were small (less than 0.6% of the center frequency) and were similar to one another. For frequencies of 4 kHz and above, both DLFs and DLCs increased markedly, but the increase was greater for DLFs. Thus the worsening of performance at high frequencies is greater when subjects are required to indicate the direction of a frequency change than when they just have to detect any change. FMDLs, when expressed relative to the carrier frequency, varied much less with frequency than DLFs or DLCs. At 2 kHz and below, FMDLs were larger than DLFs or DLCs. Above 4 kHz, FMDLs were smaller than DLFs or DLCs. At 2 kHz and below, FMDLs usually worsened with increasing modulation frequency. Above 4 kHz, FMDLs improved with increasing modulation frequency.(ABSTRACT TRUNCATED AT 250 WORDS)

Auditory Perception↗

PubMed1995

Modulation discrimination interference for narrow-band noise modulators.

The discrimination of the depth of amplitude modulation of a signal carrier frequency can be disrupted by the presence of other modulated carriers (maskers), an effect called modulation discrimination interference (MDI). This paper examines whether MDI is influenced by the similarity in the envelope pattern of the signal and masker. A narrow-band noise (centered at 10 Hz) was used as the signal modulator. The first experiment used masker modulators that were narrow-band noises identical in spectral characteristics to the signal modulator. The masker modulators were either identical to the signal modulator, negatively correlated with it, or uncorrelated with it. The amount of MDI was similar for all three cases. In experiment 2, the masker was sinusoidally modulated at rates varying from 2 to 64 Hz. The results showed a broad tuning for modulation rate, comparable to that found for sinusoidal modulation of the signal. The maximum amount of MDI produced by the sinusoidally modulated masker was similar to that produced by the noise-modulated maskers when modulation depths were expressed as their root-mean-square values. It is concluded that similarity of the moment-by-moment envelope pattern of the signal and masker modulators plays only a minor role in MDI, although similarity in modulation rate has some influence.

Auditory Perception↗

PubMed1994

The critical modulation frequency and its relationship to auditory filtering at low frequencies.

If the thresholds for detecting sinusoidal amplitude or frequency modulation of a sinusoidal carrier with frequency fc are expressed in terms of the respective modulation indices, m and beta, the ratio beta/m decreases as the modulation frequency increases, and approaches an asymptotic value of unity. The modulation frequency at which the ratio first becomes unity is called the critical modulation frequency (CMF). It has been suggested that the CMF is reached when the spectral sidebands in the stimulus first become detectable and that the CMF corresponds to half the value of the critical bandwidth (CB) at fc. In this paper it is demonstrated that the CMF is confounded as a measure of frequency selectivity at low frequencies, since, for modulation frequencies around the CMF, the sideband that is most detectable changes with fc. For values of fc above 250 Hz, the lower sideband is most detectable. For values of fc below 200 Hz, the upper sideband is most detectable. These findings can account for the fact that the CMF flattens off at low carrier frequencies, reaching an asymptotic value of about 40 Hz, whereas the auditory filter bandwidth continues to decrease down to very low center frequencies.

Acoustic Stimulation↗

PubMed1994

Detection of mixed modulation using correlated and uncorrelated noise modulators.

This article is concerned with the mechanisms underlying the detection of amplitude modulation (AM), frequency modulation (FM), and mixed modulation (MM), i.e., simultaneously occurring AM and FM. In a previous study [B. C. J. Moore and A. Sek, J. Acoust. Soc. Am. 92, 3119-3131 (1992)], psychometric functions were measured for the detection of AM alone and FM alone, using a 10-Hz sinusoidal modulator and a 1-kHz carrier frequency. Detectability was then measured for combined AM and FM, with modulation depths selected so that each type of modulation would be equally detectable if presented alone. The detectability of the MM was better than would be predicted if the two types of modulation were coded completely independently. This study examined the possibility that the good detectability of MM was caused by the fact that the AM and the FM were correlated, so that each was predictable from the other. The design was similar to that of our earlier study, but the 10-Hz sinusoidal modulator was replaced by a narrow-band noise modulator. In the MM conditions, the modulators for AM and FM were either strongly positively correlated or essentially uncorrelated. In experiment 1, the waveforms of the noise modulators were fixed throughout the experiment (frozen noise). In experiment 2, the waveforms of the noise modulators were chosen independently for each trial. In both experiments, for both correlated and uncorrelated modulators, the detectability of the MM was better than would be predicted if the two types of modulation were coded completely independently.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗

PubMed1994

Discrimination of modulation type (amplitude modulation or frequency modulation) with and without background noise.

These experiments compare the ability to detect amplitude modulation (AM) and frequency modulation (FM) with the ability to discriminate AM from FM, using 10-Hz sinusoidal modulation of a 1000-Hz carrier. Initially, psychometric functions were measured for the detection of AM and FM alone, using a two-alternative forced-choice (2AFC) task. In experiment 1, pairs of values of AM and FM were selected that would be equally detectable, and psychometric functions were measured for the discrimination of AM from FM, again in a 2AFC task. Values of d' for discriminating AM from FM were always lower than the values of d' for detection of the AM or FM. When the detectability of the AM and FM was low (d' = 0.66), two subjects were essentially unable to discriminate AM from FM. This was true both for stimuli presented in quiet and for stimuli presented with continuous noise chosen to mask either the lower or the upper side of the excitation pattern. In experiment 2, subjects were again required to discriminate AM from FM, but the AM depth was fixed within a block of trials, while the FM depth was varied across trials. The discriminability of AM from FM did not show distinct minima at specific FM depths. Again, this was true both for stimuli presented in quiet and for stimuli presented with continuous noise chosen to mask either the lower or the upper side of the excitation pattern. This result suggests that the discrimination of AM from FM was not based on monitoring just one side of the excitation pattern of the carrier.(ABSTRACT TRUNCATED AT 250 WORDS)

Auditory Perception↗

PubMed1994

Effects of carrier frequency and background noise on the detection of mixed modulation.

This article is concerned with the mechanisms underlying the detection of amplitude modulation (AM), frequency modulation (FM), and mixed modulation (MM), i.e., simultaneously occurring AM and FM. In a previous study [B. C. J. Moore and A. Sek, J. Acoust. Soc. Am. 92, 3119-3131 (1992)], psychometric functions were measured for the detection of AM alone and FM alone, using a 10-Hz modulation rate and a 1-kHz carrier frequency. Detectability was then measured for combined AM and FM, with modulation depths selected so that each type of modulation would be equally detectable if presented alone. The detectability of the combined AM and FM was better than would be predicted if the two types of modulation were coded completely independently. Significant effects of relative modulator phase were found when detectability was relatively high, but these effects were not correctly predicted by either of two excitation-pattern models considered. The first experiment reported here was similar to the earlier experiment, but performance was compared for carrier frequencies of 1 and 6 kHz; at the latter frequency, neural synchrony to the stimulus fine structure (phase locking) does not occur. The results at both carrier frequencies were similar to those of our earlier experiment, suggesting that the presence or absence of phase-locking information plays little role in the detection of MM. The second experiment was again similar, but bands of noise were used to mask selectively either the upper or lower side of the excitation pattern of the modulated carrier. The phase effects in this case were in the direction predicted by excitation pattern models. The overall pattern of the results could be predicted reasonably well using a multichannel excitation pattern model based on the assumption that listeners use an unweighted sum of decision variables across all suprathreshold channels with a positive signal-to-noise ratio.

Auditory Perception↗