Temporal Envelope Modulations Research Articles

Psychoacoustic and electroencephalographic responses to changes in amplitude modulation depth and frequency in relation to speech recognition in cochlear implantees

Temporal envelope modulations (TEMs) are one of the most important features that cochlear implant (CI) users rely on to understand speech. Electroencephalographic assessment of TEM encoding could help clinicians to predict speech recognition more objectively, even in patients unable to provide active feedback. The acoustic change complex (ACC) and the auditory steady-state response (ASSR) evoked by low-frequency amplitude-modulated pulse trains can be used to assess TEM encoding with electrical stimulation of individual CI electrodes. In this study, we focused on amplitude modulation detection (AMD) and amplitude modulation frequency discrimination (AMFD) with stimulation of a basal versus an apical electrode. In twelve adult CI users, we (a) assessed behavioral AMFD thresholds and (b) recorded cortical auditory evoked potentials (CAEPs), AMD-ACC, AMFD-ACC, and ASSR in a combined 3-stimulus paradigm. We found that the electrophysiological responses were significantly higher for apical than for basal stimulation. Peak amplitudes of AMFD-ACC were small and (therefore) did not correlate with speech-in-noise recognition. We found significant correlations between speech-in-noise recognition and (a) behavioral AMFD thresholds and (b) AMD-ACC peak amplitudes. AMD and AMFD hold potential to develop a clinically applicable tool for assessing TEM encoding to predict speech recognition in CI users.

Evaluation of phase-locking to parameterized speech envelopes.

Humans rely on the temporal processing ability of the auditory system to perceive speech during everyday communication. The temporal envelope of speech is essential for speech perception, particularly envelope modulations below 20 Hz. In the literature, the neural representation of this speech envelope is usually investigated by recording neural phase-locked responses to speech stimuli. However, these phase-locked responses are not only associated with envelope modulation processing, but also with processing of linguistic information at a higher-order level when speech is comprehended. It is thus difficult to disentangle the responses into components from the acoustic envelope itself and the linguistic structures in speech (such as words, phrases and sentences). Another way to investigate neural modulation processing is to use sinusoidal amplitude-modulated stimuli at different modulation frequencies to obtain the temporal modulation transfer function. However, these transfer functions are considerably variable across modulation frequencies and individual listeners. To tackle the issues of both speech and sinusoidal amplitude-modulated stimuli, the recently introduced Temporal Speech Envelope Tracking (TEMPEST) framework proposed the use of stimuli with a distribution of envelope modulations. The framework aims to assess the brain's capability to process temporal envelopes in different frequency bands using stimuli with speech-like envelope modulations. In this study, we provide a proof-of-concept of the framework using stimuli with modulation frequency bands around the syllable and phoneme rate in natural speech. We evaluated whether the evoked phase-locked neural activity correlates with the speech-weighted modulation transfer function measured using sinusoidal amplitude-modulated stimuli in normal-hearing listeners. Since many studies on modulation processing employ different metrics and comparing their results is difficult, we included different power- and phase-based metrics and investigate how these metrics relate to each other. Results reveal a strong correspondence across listeners between the neural activity evoked by the speech-like stimuli and the activity evoked by the sinusoidal amplitude-modulated stimuli. Furthermore, strong correspondence was also apparent between each metric, facilitating comparisons between studies using different metrics. These findings indicate the potential of the TEMPEST framework to efficiently assess the neural capability to process temporal envelope modulations within a frequency band that is important for speech perception.

Temporal integration contributes to the masking release by amplitude modulation

Modulated maskers produce less amount of masking than unmodulated maskers, an effect referred to as masking release (MR). Both listening in the temporal dips and fast cochlear compression have been suggested as underlying mechanisms. We addressed the role of dip listening by measuring temporal integration in simultaneous masking using Schroeder-phase harmonic complexes (SPHC) with various phase curvatures. In an experiment with six normal-hearing listeners, SPHC masker and pure-tone target stimuli were covaried in duration at a high masker level. The MR increased with stimulus duration, suggesting integration of target information across multiple masker dips. The duration dependence of the MR was predicted by a physiology-inspired model based on the temporal envelope modulation strength in the auditory periphery. The modeling analysis suggested that listeners detect the presence of the target by a reduction in fluctuation strength that results primarily from a decline of F0-based response peaks, an effect known as synchrony capture. The detailed pattern of masked thresholds across various masker phase curvatures was not predicted by the model, suggesting that its phase response does not well fit the human phase response. Overall, temporal integration across neural envelope features associated with the masker dips seems to contribute to the MR with SPHCs.

Time‐Resolved Second‐Order Coherence Characterization of Broadband Metallic Nanolasers

AbstractHigh‐bandwidth metallic coaxial nanolasers are of high interest to investigate laser physics such as thresholdless coherence transitions, and have a large variety of promising applications enabled by their ultrasmall size and large spectral bandwidth. Optical coherence properties are commonly characterized in Hanbury‐Brown and Twiss experiments. However, those are difficult to perform in broadband lasers when the coherence time is an order of magnitude shorter than the temporal resolution of the single‐photon detectors, thus requiring significant spectral filtering. This paper demonstrates a new approach in investigating the temporal dynamics of the photon statistics associated with the nanolaser emission, obtained without the requirement of spectral filtering. While optically pumping the nanolasers with nanosecond pulses, time‐resolved second‐order coherence properties are evaluated over the time duration of the pump pulse. Coherence transitions from thermal emission to lasing are observed in the gathered time‐resolved photon statistics, linked to the temporal change in optical power of the nanosecond pump pulses. As nanolasers show better performance for the pulsed pumping scheme, the temporal envelope modulation of these pulses results in varying degrees of coherence within the nanolaser pulse envelope. This approach can also be readily applied to characterize a large variety of broadband lasers.

Spontaneous Mouse Behavior in Presence of Dissonance and Acoustic Roughness.

According to a novel hypothesis (Arnal et al., 2015, Current Biology 25:2051–2056), auditory roughness, or temporal envelope modulations between 30 and 150 Hz, are present in both natural and artificial human alarm signals, which boosts the detection of these alarms in various tasks. These results also shed new light on the unpleasantness of dissonant sounds to humans, which builds upon the high level of roughness present in such sounds. However, it is not clear whether this hypothesis also applies to other species, such as rodents. In particular, whether consonant/dissonant chords, and particularly whether auditory roughness, can trigger unpleasant sensations in mice remains unknown. Using an autonomous behavioral system, which allows the monitoring of mouse behavior over a period of weeks, we observed that C57Bl6J mice did not show any preference for consonant chords. In addition, we found that mice showed a preference for rough sounds over sounds having amplitude modulations in their temporal envelope outside the “rough” range. These results suggest that some emotional features carried by the acoustic temporal envelope are likely to be species-specific.

Electrophysiological assessment of temporal envelope processing in cochlear implant users

Cochlear-implant (CI) users rely on temporal envelope modulations (TEMs) to understand speech, and clinical outcomes depend on the accuracy with which these TEMs are encoded by the electrically-stimulated neural ensembles. Non-invasive EEG measures of this encoding could help clinicians identify and disable electrodes that evoke poor neural responses so as to improve CI outcomes. However, recording EEG during CI stimulation reveals huge stimulation artifacts that are up to orders of magnitude larger than the neural response. Here we used a custom-built EEG system having an exceptionally high sample rate to accurately measure the artefact, which we then removed using linear interpolation so as to reveal the neural response during continuous electrical stimulation. In ten adult CI users, we measured the 40-Hz electrically evoked auditory steady-state response (eASSR) and electrically evoked auditory change complex (eACC) to amplitude-modulated 900-pulses-per-second pulse trains, stimulated in monopolar mode (i.e. the clinical default), and at different modulation depths. We successfully measured artifact-free 40-Hz eASSRs and eACCs. Moreover, we found that the 40-Hz eASSR, in contrast to the eACC, showed substantial responses even at shallow modulation depths. We argue that the 40-Hz eASSR is a clinically feasible objective measure to assess TEM encoding in CI users.

Spectral-Temporal Trade-Off in Vocoded Sentence Recognition: Effects of Age, Hearing Thresholds, and Working Memory.

Cochlear implant (CI) signal processing degrades the spectral components of speech. This requires CI users to rely primarily on temporal cues, specifically, amplitude modulations within the temporal envelope, to recognize speech. Auditory temporal processing ability for envelope modulations worsens with advancing age, which may put older CI users at a disadvantage compared with younger users. To evaluate how potential age-related limitations for processing temporal envelope modulations impact spectrally degraded sentence recognition, noise-vocoded sentences were presented to younger and older normal-hearing listeners in quiet. Envelope modulation rates were varied from 10 to 500 Hz by adjusting the low-pass filter cutoff frequency (LPF). The goal of this study was to evaluate if age impacts recognition of noise-vocoded speech and if this age-related limitation existed for a specific range of envelope modulation rates. Noise-vocoded sentence recognition in quiet was measured as a function of number of spectral channels (4, 6, 8, and 12 channels) and LPF (10, 20, 50, 75, 150, 375, and 500 Hz) in 15 younger normal-hearing listeners and 15 older near-normal-hearing listeners. Hearing thresholds and working memory were assessed to determine the extent to which these factors were related to recognition of noise-vocoded sentences. Younger listeners achieved significantly higher sentence recognition scores than older listeners overall. Performance improved in both groups as the number of spectral channels and LPF increased. As the number of spectral channels increased, the differences in sentence recognition scores between groups decreased. A spectral-temporal trade-off was observed in both groups in which performance in the 8- and 12-channel conditions plateaued with lower-frequency amplitude modulations compared with the 4- and 6-channel conditions. There was no interaction between age group and LPF, suggesting that both groups obtained similar improvements in performance with increasing LPF. The lack of an interaction between age and LPF may be due to the nature of the task of recognizing sentences in quiet. Audiometric thresholds were the only significant predictor of vocoded sentence recognition. Although performance on the working memory task declined with advancing age, working memory scores did not predict sentence recognition. Younger listeners outperformed older listeners for recognizing noise-vocoded sentences in quiet. The negative impact of age was reduced when ample spectral information was available. Age-related limitations for recognizing vocoded sentences were not affected by the temporal envelope modulation rate of the signal, but instead, appear to be related to a generalized task limitation or to reduced audibility of the signal.

Neural Modulation Transmission Is a Marker for Speech Perception in Noise in Cochlear Implant Users.

Cochlear implants (CIs) restore functional hearing in persons with a severe hearing impairment. Despite being one of the most successful bionic prosthesis, performance with CI (in particular speech understanding in noise) varies considerably across its users. The ability of the auditory pathway to encode temporal envelope modulations (TEMs) and the effect of degenerative processes associated with hearing loss on TEM encoding is assumed to be one of the reasons underlying the large intersubject differences in CI performance. The objective of the present study was to investigate how TEM encoding of the stimulated neural ensembles of human CI recipients is related to speech perception in noise (SPIN). We used electroencephalography as a noninvasive electrophysiological measure to assess TEM encoding in the auditory pathway of CI users by means of the 40-Hz electrically evoked auditory steady state response (EASSR). Nine CI users with a wide range of SPIN outcome were included in the present study. TEM encoding was assessed for each stimulation electrode of each subject and new metrics; the CI neural modulation transmission difference (CIMTD) and the CI neural modulation transmission index (CIMTI) were developed to quantify the amount of variability in TEM encoding across the stimulated neural ensembles of the CI electrode array. EASSR patterns varied across the CI electrode array and subjects. We found a strong correlation (r = 0.89, p = 0.001) between the SPIN outcomes and the variability in EASSR amplitudes across the array as assessed with CIMTD/CIMTI. The results of the present study show that the 40-Hz EASSR can be used to objectively assess the neural encoding of TEMs in human CI recipients. Overall reduced or largely variable TEM encoding of the neural ensembles across the electrode array, as quantified with the CIMTD/CIMTI, is highly correlated with speech perception in noise outcome with a CI.

Effects of noise and reverberation on speech recognition with variants of a multichannel adaptive dynamic range compression scheme

Objective: Adaptive compression methods in hearing aids have been developed to maximise audibility while preserving temporal envelope modulations. Increasing the number of channels may improve listening comfort for loud sounds. However, the effects of this on speech recognition in different environmental conditions are unknown. This study evaluated the effects of different channel architectures and adaptive compression properties on speech recognition in noise and reverberation.Design: Sentences were mixed with steady or modulated noise at three signal-to-noise ratios (SNRs). These were processed with and without reverberation and amplified with four proprietary adaptive compression methods or linear amplification.Study sample: 36 listeners with mild to moderately-severe hearing loss.Results: Adaptive compression improved speech recognition over linear amplification to a small extent, with no significant differences among methods using 4 or 24 channels or a combination thereof. These effects remained across the different background noise and reverberation conditions.Conclusions: Increasing the number of channels does not negatively affect speech recognition in noise and reverberation when adaptive compression is used. If future research shows that increasing the number of channels improves listening comfort for loud sounds, these results indicate that adaptive compression methods with as many as 24 channels are viable options for hearing aids.

Processing of fast amplitude modulations in bat auditory cortex matches communication call-specific sound features.

Bats use a large repertoire of calls for social communication. In the bat Phyllostomus discolor, social communication calls are often characterized by sinusoidal amplitude and frequency modulations with modulation frequencies in the range of 100-130 Hz. However, peaks in mammalian auditory cortical modulation transfer functions are typically limited to modulation frequencies below 100 Hz. We investigated the coding of sinusoidally amplitude modulated sounds in auditory cortical neurons in P. discolor by constructing rate and temporal modulation transfer functions. Neuronal responses to playbacks of various communication calls were additionally recorded and compared with the neurons' responses to sinusoidally amplitude-modulated sounds. Cortical neurons in the posterior dorsal field of the auditory cortex were tuned to unusually high modulation frequencies: rate modulation transfer functions often peaked around 130 Hz (median: 87 Hz), and the median of the highest modulation frequency that evoked significant phase-locking was also 130 Hz. Both values are much higher than reported from the auditory cortex of other mammals, with more than 51% of the units preferring modulation frequencies exceeding 100 Hz. Conspicuously, the fast modulations preferred by the neurons match the fast amplitude and frequency modulations of prosocial, and mostly of aggressive, communication calls in P. discolor. We suggest that the preference for fast amplitude modulations in the P. discolor dorsal auditory cortex serves to reliably encode the fast modulations seen in their communication calls. NEW & NOTEWORTHY Neural processing of temporal sound features is crucial for the analysis of communication calls. In bats, these calls are often characterized by fast temporal envelope modulations. Because auditory cortex neurons typically encode only low modulation frequencies, it is unclear how species-specific vocalizations are cortically processed. We show that auditory cortex neurons in the bat Phyllostomus discolor encode fast temporal envelope modulations. This property improves response specificity to communication calls and thus might support species-specific communication.

Indexical properties influence time-varying amplitude and fundamental frequency contributions of vowels to sentence intelligibility

The present study investigated how non-linguistic, indexical information about talker identity interacts with contributions to sentence intelligibility by the time-varying amplitude (temporal envelope) and fundamental frequency (F0). Young normal-hearing adults listened to sentences that preserved the original consonants but replaced the vowels with a single vowel production. This replacement vowel selectively preserved amplitude or F0 cues of the original vowel, but replaced cues to phonetic identity. Original vowel duration was always preserved. Three experiments investigated indexical contributions by replacing vowels with productions from the same or different talker, or by acoustically morphing the original vowel. These stimulus conditions investigated how vowel suprasegmental and indexical properties interact and contribute to intelligibility independently from phonetic information. Results demonstrated that indexical properties influence the relative contribution of suprasegmental properties to sentence intelligibility. F0 variations are particularly important in the presence of conflicting indexical information. Temporal envelope modulations significantly improve sentence intelligibility, but are enhanced when either indexical or F0 cues are available. These findings suggest that F0 and other indexical cues may facilitate perceptually grouping suprasegmental properties of vowels with the remainder of the sentence. Temporal envelope modulations of vowels may contribute to intelligibility once they are successfully integrated with the preserved signal.

Exploring the role of feedback-based auditory reflexes in forward masking by schroeder-phase complexes.

Several studies have postulated that psychoacoustic measures of auditory perception are influenced by efferent-induced changes in cochlear responses, but these postulations have generally remained untested. This study measured the effect of stimulus phase curvature and temporal envelope modulation on the medial olivocochlear reflex (MOCR) and on the middle-ear muscle reflex (MEMR). The role of the MOCR was tested by measuring changes in the ear-canal pressure at 6 kHz in the presence and absence of a band-limited harmonic complex tone with various phase curvatures, centered either at (on-frequency) or well below (off-frequency) the 6-kHz probe frequency. The influence of possible MEMR effects was examined by measuring phase-gradient functions for the elicitor effects and by measuring changes in the ear-canal pressure with a continuous suppressor of the 6-kHz probe. Both on- and off-frequency complex tone elicitors produced significant changes in ear canal sound pressure. However, the pattern of results was not consistent with the earlier hypotheses postulating that efferent effects produce the psychoacoustic dependence of forward-masked thresholds on masker phase curvature. The results also reveal unexpectedly long time constants associated with some efferent effects, the source of which remains unknown.

Mismatch negativity (MMN) objectively reflects timbre discrimination thresholds in normal-hearing listeners and cochlear implant users

Timbre perception is poor in cochlear implant (CI) users. Most behavioral studies assess timbre discrimination with real instrumental sounds differing in more than one timbre dimension. This study focuses on objective measures of individual discrimination of single timbre dimensions. A psychoacoustic and electrophysiological case-control study of timbre discrimination was conducted in 15 CI users and 15 normal-hearing (NH) listeners. Just noticeable differences (JNDs) for temporal envelope (TE) modulation differences and spectral distribution (S) differences of complex tones were measured in an adaptive three-alternatives forced-choice procedure. From individual JNDs, individual four tone pairs were computed: temporal envelope modulation/spectral distribution timbre discrimination, and above/below JND. With these tone pairs, four randomly arranged oddball paradigms were presented to derive mismatch negativity (MMN), which reflects the ability to automatically detect acoustic changes. JNDs could be derived for all NH listeners. Discrimination of spectral distribution differences and particularly temporal envelope modulation differences were more difficult for CI users. MMN was present in all NH listeners and CI users when the timbre difference was above the individual JND, but not when below JND. Thus, the timbre difference between ‘deviant’ and ‘standard’ tones reflected the individual threshold for automatic detection of timbre changes. We conclude that the timbre perception skills of NH listeners and CI users in particular can be assessed timbre dimension specifically with psychoacoustic measurements and MMN recordings. MMN occurrence reflects the individual JND for temporal envelope modulation and spectral distribution differences and can be used as a clinical tool to monitor auditory-verbal therapy after CI surgery.

Importance of envelope modulations during consonants and vowels in segmentally interrupted sentences

The present study investigated the importance of overall segment amplitude and intrinsic segment amplitude modulation of consonants and vowels to sentence intelligibility. Sentences were processed according to three conditions that replaced consonant or vowel segments with noise matched to the long-term average speech spectrum. Segments were replaced with (1) low-level noise that distorted the overall sentence envelope, (2) segment-level noise that restored the overall syllabic amplitude modulation of the sentence, and (3) segment-modulated noise that further restored faster temporal envelope modulations during the vowel. Results from the first experiment demonstrated an incremental benefit with increasing resolution of the vowel temporal envelope. However, amplitude modulations of replaced consonant segments had a comparatively minimal effect on overall sentence intelligibility scores. A second experiment selectively noise-masked preserved vowel segments in order to equate overall performance of consonant-replaced sentences to that of the vowel-replaced sentences. Results demonstrated no significant effect of restoring consonant modulations during the interrupting noise when existing vowel cues were degraded. A third experiment demonstrated greater perceived sentence continuity with the preservation or addition of vowel envelope modulations. Overall, results support previous investigations demonstrating the importance of vowel envelope modulations to the intelligibility of interrupted sentences.

Predicting the intelligibility of reverberant speech for cochlear implant listeners with a non-intrusive intelligibility measure

Reverberation is known to reduce the temporal envelope modulations present in the signal and affect the shape of the modulation spectrum. A non-intrusive intelligibility measure for reverberant speech is proposed motivated by the fact that the area of the modulation spectrum decreases with increasing reverberation. The proposed measure is based on the average modulation area computed across four acoustic frequency bands spanning the signal bandwidth. High correlations (r=0.98) were observed with sentence intelligibility scores obtained by cochlear implant listeners. Proposed measure outperformed other measures including an intrusive speech-transmission index based measure.

Importance of sentence-level and phoneme-level envelope modulations during vowels in interrupted speech

Speech interrupted by noise has been used as a simplified case for listening to speech in the presence of a fluctuating masker. The present study investigated the importance of overall vowel amplitude and intrinsic vowel amplitude modulation to sentence intelligibility. Eight young normal-hearing listeners participated in the experiment. Sentences were processed according to three conditions that replaced vowel segments with noise matched to the long-term average speech spectrum. Vowels were replaced with (1) low-level noise that distorted the overall sentence envelope, (2) segment-level noise that restored the overall syllabic amplitude modulation of the sentence, and (3) segment-modulated noise that further restored faster temporal envelope modulations during the vowel. Results demonstrated incremental benefit with increasing resolution of the temporal envelope. An additional seven listeners participated in a separate experiment that replaced vowels with a vowel masker instead of noise. The vowel masker was modified to have a flattened fundamental frequency at the mean of the replaced segment. The vowel masker either had a standard temporal envelope or was modulated by the envelope of the replaced vowel. Listeners who heard vowel maskers performed more poorly than those who heard noise maskers. No benefit of segment modulation was observed with the vowel masker.

Hard times for the pitch of complex sounds in reverberant environments

Reverberation is present in most everyday listening environments, with sound reaching our ears directly from the source(s), and indirectly after reflecting from nearby surfaces. Reverberation smears spectral transitions through time and disrupts temporal envelope modulation in harmonic complex sounds such as voiced portions of speech. Envelope modulation can provide a cue to the pitch of complex sounds and is relied upon by hearing impaired listeners. We have examined the effects of reverberation on the temporal representation of the dynamic pitch of frequency‐modulated harmonic complex sounds in the responses of single units from the ventral cochlear nucleus. Without reverberation most units provide a strong representation of the modulated pitch in their short‐term interspike interval distributions. Relatively mild reverberation degrades the representation based on envelope modulation, whilst more severe reverberation removes it. The representation of pitch based on fine structure is more robust to the e...

Dominance of missing fundamental versus spectrally cued pitch: individual differences for complex tones with unresolved harmonics.

In a two-alternative, forced-choice experiment, subjects had to compare the pitches of two sounds, A and B. Each sound was composed of four successive harmonics of a fundamental frequency between 100 to 250 Hz, added in cosine or Schröder phase. The harmonic frequencies of A were lower than those of B; the missing fundamental frequency of A was higher than that of B. The dominance of the missing fundamental versus the spectrally cued pitch--a pitch percept corresponding to spectral components--was measured as a function of nA, the lowest harmonic in A. The pitch percept is dominated by the missing fundamental if the harmonics are resolved (nA<7). If the harmonics become unresolved and are added in Schröder phase, the dominance shifts to a spectrally cued pitch (7<nA<13; 75% of the subjects). In the cosine phase condition, many subjects could detect the fundamental pitch well into the unresolved harmonic range (nA>20). For others, the transition was in the realm of partly resolved harmonics. This shows that the temporal envelope modulation of stimuli with only four unresolved harmonics can give a relatively clear fundamental pitch percept. However, this percept varies considerably among subjects.

Human language identification with reduced segmental information.

We conducted human language identification experiments using signals with reduced segmental information with Japanese and bilingual subjects. American English and Japanese excerpts from the OGI Multi-Language Telephone Speech Corpus were processed by spectral-envelope removal (SER), vowel extraction from SER (VES) and temporal-envelope modulation (TEM). The processed excerpts of speech were provided as stimuli for perceptual experiments. We calculated D indices from the subjects' responses, ranging from � 2 to þ2 where positive/negative values indicate correct/incorrect responses, respectively. With the SER signal, where the spectral-envelope is eliminated, humans could still identify the languages fairly successfully. The overall D index of Japanese subjects for this signal was þ1:17. With the VES signal, which retains only vowel sections of the SER signal, the D index was lower (þ0:35). With the TEM signal, composed of white-noise-driven intensity envelopes from several frequency bands, the D index rose from þ0:29 to þ1:69 corresponding to the increasing number of bands from 1 to 4. Results varied depending on the stimulus language. Japanese and bilingual subjects scored differently from each other. These results indicate that humans can identify languages using signals with drastically reduced segmental information. The results also suggest variation due to the phonetic typologies of languages and subjects' knowledge.

Effects of temporal envelope modulation on acoustic signal recognition in a vocal fish, the plainfin midshipman.

Amplitude modulation is an important parameter defining vertebrate acoustic communication signals. Nesting male plainfin midshipman fish, Porichthys notatus, emit simple, long duration hums in which modulation is strikingly absent. Envelope modulation is, however, introduced when the hums of adjacent males overlap to produce acoustic beats. Hums attract gravid females and can be mimicked with continuous tones at the fundamental frequency. While individual hums have flat envelopes, other midshipman signals are amplitude modulated. This study used one-choice playback tests with gravid females to examine the role of envelope modulation in hum recognition. Various pulse train and two-tone beat stimuli resembling natural communication signals were presented individually, and the responses compared to those for continuous pure tones. The effectiveness of pulse trains was graded and depended upon both pulse duration and the ratio of pulse to gap length. Midshipman were sensitive to beat modulations from 0.5 to 10 Hz, with fewer fish approaching the beat than the pure tone. Reducing the degree of modulation increased the effectiveness of beat stimuli. Hence, the lack of modulation in the midshipman's advertisement call corresponds to the importance of envelope modulation for the categorization of communication signals even in this relatively simple system.