Repetition enhancement for frequency-modulated but not unmodulated sounds: a human MEG study

Background

Decoding of frequency-modulated (FM) sounds is essential for phoneme identification. This study investigates selectivity to FM direction in the human auditory system.

Methodology/Principal Findings

Magnetoencephalography was recorded in 10 adults during a two-tone adaptation paradigm with a 200-ms interstimulus-interval. Stimuli were pairs of either same or different frequency modulation direction. To control that FM repetition effects cannot be accounted for by their on- and offset properties, we additionally assessed responses to pairs of unmodulated tones with either same or different frequency composition. For the FM sweeps, N1m event-related magnetic field components were found at 103 and 130 ms after onset of the first (S1) and second stimulus (S2), respectively. This was followed by a sustained component starting at about 200 ms after S2. The sustained response was significantly stronger for stimulation with the same compared to different FM direction. This effect was not observed for the non-modulated control stimuli.

Conclusions/Significance

Low-level processing of FM sounds was characterized by repetition enhancement to stimulus pairs with same versus different FM directions. This effect was FM-specific; it did not occur for unmodulated tones. The present findings may reflect specific interactions between frequency separation and temporal distance in the processing of consecutive FM sweeps.     

The generation of direction selectivity in the auditory system

Both human speech and animal vocal signals contain frequency-modulated (FM) sounds. Although central auditory neurons that selectively respond to the direction of frequency modulation are known, the synaptic mechanisms underlying the generation of direction selectivity (DS) remain elusive. Here we show the emergence of DS neurons in the inferior colliculus by mapping the three major subcortical auditory nuclei. Cell-attached recordings reveal a highly reliable and precise firing of DS neurons to FM sweeps in a preferred direction. By using in vivo whole-cell current-clamp and voltage-clamp recordings, we found that the synaptic inputs to DS neurons are not direction selective, but temporally reversed excitatory and inhibitory synaptic inputs are evoked in response to opposing directions of FM sweeps. The construction of such temporal asymmetry, resulting DS, and its topography can be attributed to the spectral disparity of the excitatory and the inhibitory synaptic tonal receptive fields.     

Auditory Cortical Areas Activated by Slow Frequency-Modulated Sounds in Mice

Species-specific vocalizations in mice have frequency-modulated (FM) components slower than the lower limit of FM direction selectivity in the core region of the mouse auditory cortex. To identify cortical areas selective to slow frequency modulation, we investigated tonal responses in the mouse auditory cortex using transcranial flavoprotein fluorescence imaging. For differentiating responses to frequency modulation from those to stimuli at constant frequencies, we focused on transient fluorescence changes after direction reversal of temporally repeated and superimposed FM sweeps. We found that the ultrasonic field (UF) in the belt cortical region selectively responded to the direction reversal. The dorsoposterior field (DP) also responded weakly to the reversal. Regarding the responses in UF, no apparent tonotopic map was found, and the right UF responses were significantly larger in amplitude than the left UF responses. The half-max latency in responses to FM sweeps was shorter in UF compared with that in the primary auditory cortex (A1) or anterior auditory field (AAF). Tracer injection experiments in the functionally identified UF and DP confirmed that these two areas receive afferent inputs from the dorsal part of the medial geniculate nucleus (MG). Calcium imaging of UF neurons stained with fura-2 were performed using a two-photon microscope, and the presence of UF neurons that were selective to both direction and direction reversal of slow frequency modulation was demonstrated. These results strongly suggest a role for UF, and possibly DP, as cortical areas specialized for processing slow frequency modulation in mice.     

Frequency modulated sound pattern analysis in the lesser bulldog bat: the role of interactions between adjacent frequency elements of complex sounds

A stereotyped approach phase vocalization response of Noctilio albiventris to artificial echoes simulating a virtual approaching object was used to assess the ability of the bat to analyze and extract distance information from the artificial echoes. The performance of the bats depended on the temporal pattern of frequency change of the continuously sweeping frequency modulated (FM) component of the signals. When the bats were presented with a CF/FM signal containing a time-reversed upward FM sweep, they responded with approach phase behavior at a performance level that was significantly below that seen with a CF/FM signal containing a naturally structured downward FM sweep. When the FM sweep was divided into a series of brief pure tone steps, the extent to which the bats showed a difference in their capability to process upward versus downward FM sweeps depended on the difference in frequency between the pure tone steps. The bats effectively processed downward but not upward FM sweeps when the difference in frequency between pure tone frequency elements of the FM sweeps was from about 100–200 Hz, but they effectually processed both downward and upward FM sweeps when the tonal elements composing the FM sweeps were separated by more than about 200 Hz. This suggests that the ability of the bats to effectively process downward but not upward FM sweeps is based on local interactions between adjacent frequency elements of the complex sounds.Abbreviations CF constant frequency - FM frequency modulated     

Responses of neurons in the rat auditory and associative cortices to tonal stimuli

Activity of single neurons and mass evoked potentials (EP) were recorded from the auditory (area 41) and associative (area 39) cortices in acute experiments on rats anesthetized with urethane, nembutal, or chloralose; pure tones were used as acoustic stimuli. The EP appearing in response to a wide range of sound tones on the surface of the auditory and associative cortices were dissimilar in their latency and shape. For neurons exhibiting stable responses, the frequency-threshold curves (FTC) were plotted.Weak and variable responses of neurons were observed under slight urethane anesthesia. Nembutal anesthesia increased the responsiveness of neurons and the probability of appearing of late components in the responses. Chloralose anesthesia was characterized by extension of frequency range perceived by a neuron, while its sharpness of tuning remained unchanged. Under all types of anesthesia employed, the responses recorded from the associative cortex neurons had longer latencies than those recorded from the auditory cortex neurons. Neurons exhibiting the frequency selectivity were much less numerous in the associative cortex than in the auditory cortex. The former neurons were often characterized by intermittent FTC and they responded to a more extended frequency range. No clear tonotopic organization was found in the associative cortex.Neirofiziologiya/Neurophysiology, Vol. 25, No. 5, pp. 343–349, September–October, 1993.     

Responses of midbrain auditory neurons in rats to FM and AM tones presented simultaneously

Speech and other communication signals contain components of frequency and amplitude modulations (FM, AM) that often occur together. Auditory midbrain (or inferior colliculus, IC) is an important center for coding time-varying features of sounds. It remains unclear how IC neurons respond when FM and AM stimuli are both presented. Here we studied IC neurons in the urethane-anesthetized rats when animals were simultaneously stimulated with FM and AM tones. Of 122 units that were sensitive to the dual stimuli, the responses could be grossly divided into two types: one that resembled the respective responses to FM or AM stimuli presented separately ("simple" sensitivity, 45% of units), and another that appeared markedly different from their respective responses to FM or AM tones ("complex" sensitivity, 55%). These types of combinational sensitivities were further correlated with individual cell's frequency tuning pattern (response area) and with their common response pattern to FM and AM sounds. Results suggested that such combinational sensitivity could reflect local synaptic interactions on IC neurons and that the neural mechanisms could underlie more developed sensitivities to acoustic combinations found at the auditory cortex.     

Multiple mechanisms shape selectivity for FM sweep rate and direction in the pallid bat inferior colliculus and auditory cortex

The inferior colliculus and auditory cortex of the pallid bat contain a large percentage of neurons that are highly selective for the direction and rate of the downward frequency modulated (FM) sweep of the bat’s echolocation pulse. Approximately 25% of neurons tuned to the echolocation pulse respond exclusively to downward FM sweeps. This review focuses on the finding that this selectivity is generated by multiple mechanisms that may act alone or in concert. In the inferior colliculus, selectivity for sweep rate is shaped by at least three mechanisms: shortpass or bandpass tuning for signal duration, delayed high-frequency inhibition that prevents responses to slow sweep rates, and asymmetrical facilitation that occurs only when two tones are presented at appropriate delays. When acting alone, the three mechanisms can produce essentially identical rate selectivity. Direction selectivity can be produced by two mechanisms: an early low-frequency inhibition that prevents responses to upward sweeps, and the same asymmetrical two-tone inhibition that shapes rate tuning. All mechanisms except duration tuning are also present in the auditory cortex. Discussion centers on whether these mechanisms are redundant or complementary.     

Similarities of FM and AM receptive space of single units at the auditory midbrain

Complex sounds, including human speech, contain time-varying signals like frequency modulation (FM) and amplitude modulation (AM) components. In spite of various attempts to characterize their neuronal coding in the mammalian auditory systems, a unified view of their responses has not been reached. We compared FM and AM coding in terms of receptive space with reference to the input-output relationship of the underlying neural circuits. Using extracellular recording, single unit responses to a novel stimulus (i.e. random AM or FM tone) were obtained at the auditory midbrain of the anesthetized rat. Responses could be classified into three general types, corresponding to selective sensitivity to one of the following aspects of the modulation: (a) steady state, (b) dynamic state, or (c) steady-and-dynamic states. Such response typing was basically similar between FM and AM stimuli. Furthermore, the receptive space of each unit could be characterized in a three-dimensional Cartesian co-ordinate system formed by three modulation parameters: velocity, range and intensity. This representation applies to both FM and AM responses. We concluded that the FM and AM codings are very similar at the auditory midbrain and may likely involve similar neural mechanisms.     

Bats and frogs and animals in between: evidence for a common central timing mechanism to extract periodicity pitch

Widely divergent vertebrates share a common central temporal mechanism for representing periodicities of acoustic waveform events. In the auditory nerve, periodicities corresponding to frequencies or rates from about 10 Hz to over 1,000 Hz are extracted from pure tones, from low-frequency complex sounds (e.g., 1st harmonic in bullfrog calls), from mid-frequency sounds with low-frequency modulations (e.g., amplitude modulation rates in cat vocalizations), and from time intervals between high-frequency transients (e.g., pulse-echo delay in bat sonar). Time locking of neuronal responses to periodicities from about 50 ms down to 4 ms or less (about 20–300 Hz) is preserved in the auditory midbrain, where responses are dispersed across many neurons with different onset latencies from 4–5 to 20–50 ms. Midbrain latency distributions are wide enough to encompass two or more repetitions of successive acoustic events, so that responses to multiple, successive periods are ongoing simultaneously in different midbrain neurons. These latencies have a previously unnoticed periodic temporal pattern that determines the specific times for the dispersed on-responses.     

Spectral and Temporal Acoustic Features Modulate Response Irregularities within Primary Auditory Cortex Columns

Assemblies of vertically connected neurons in the cerebral cortex form information processing units (columns) that participate in the distribution and segregation of sensory signals. Despite well-accepted models of columnar architecture, functional mechanisms of inter-laminar communication remain poorly understood. Hence, the purpose of the present investigation was to examine the effects of sensory information features on columnar response properties. Using acute recording techniques, extracellular response activity was collected from the right hemisphere of eight mature cats (felis catus). Recordings were conducted with multichannel electrodes that permitted the simultaneous acquisition of neuronal activity within primary auditory cortex columns. Neuronal responses to simple (pure tones), complex (noise burst and frequency modulated sweeps), and ecologically relevant (con-specific vocalizations) acoustic signals were measured. Collectively, the present investigation demonstrates that despite consistencies in neuronal tuning (characteristic frequency), irregularities in discharge activity between neurons of individual A1 columns increase as a function of spectral (signal complexity) and temporal (duration) acoustic variations.     

Processing of frequency-modulated stimuli in the chick auditory cortex analogue: evidence for topographic representations and possible mechanisms of rate and directional sensitivity

Summary Responses of units in the auditory forebrain (field L/hyperstriatum ventrale-complex) of awake domestic chicks were studied to frequency-modulated (FM) signals and isointensity tone bursts, presented to the ear contralateral to the recording sites. FM signals, linear frequency sweeps in the range of 50 Hz to 10.25 kHz, differed in the rate of change of frequency (RCF) and in the direction of modulation. The majority of RCF response functions obtained could be classified as predominantly ascending and bell shaped. Best rates of change of frequency (BRCFs), assigned to these functions, covered a range of nearly 3 orders of magnitude. BRCFs of the same units for upward (positive BRCFs) and for downward modulations (negative BRCFs) were correlated. The lowest BRCF encountered among all units for a given isointensity ON-response bandwidth (F _on ) increased as a function of F _on . F _on was derived from the responses to tone bursts of various frequencies at 70 dB SPL. As F_ON tended to increase with the best frequency (BF) of units the lowest BRCF encountered among all units for a given BF also increased as a function of BF. Positive and negative BRCFs of a unit were also correlated with the slopes of onset latency-frequency relationships below and above BF, respectively. FM responses were optimal, when the frequency-specific latency differences at a given unit were compensated by the direction and rate of frequency change in the signal. FM-directional sensitivity varied with BF. Most units with BFs below about 2 kHz preferred upward modulations, while those with BFs above 2 kHz preferred downward modulations. Directional preference and sensitivity correlated with asymmetric distributions of inhibitory sidebands around BF, as derived from the analysis of OFF-responses. Maximum directional sensitivity for a given BRCF increased with BRCF. BRCF and FM-directional sensitivity were topographically organized on neuronal planes harboring units with similar BFs (isofrequency planes). Highest BRCFs were observed in the input-layer L₂ of field L. BRCF declined along a rostrocaudal isofrequency axis in all 4 subdivisions of the auditory forebrain. Similarly, response strength shifted from rostral to caudal as a function of RCF. FM-directional sensitivity was organized in a subdivision-specific fashion. Units in the input-layer of field L (L₂), and even more so in the hyperstriatum ventrale, were fairly insensitive to the direction of modulation, whereas units in the postsynaptic layers of field L (L₁ and L₃) exhibited higher degrees of directional sensitivity. Directional sensitivity also declined along the rostrocaudal isofrequency axis of field L. Two simple models of connectivity in the chick auditory forebrain are presented, which could be sufficient to explain these results. One is based on a tonotopic arrangement of afferent synapses on dendrites and somata of units in L₂, the other on local lateral inhibition in the postsynaptic layers of field L.Abbreviations BF best frequency (kHz) - BRCF best rate of change of frequency (kHz/s) - DS index of FM-directional sensitivity - F _on ON-response bandwidth (kHz) - F _off OFF-response bandwidth (kHz) - FM frequency modulation - RCF rate of change of frequency (kHz/s)     

Dissociable Neural Response Signatures for Slow Amplitude and Frequency Modulation in Human Auditory Cortex

Natural auditory stimuli are characterized by slow fluctuations in amplitude and frequency. However, the degree to which the neural responses to slow amplitude modulation (AM) and frequency modulation (FM) are capable of conveying independent time-varying information, particularly with respect to speech communication, is unclear. In the current electroencephalography (EEG) study, participants listened to amplitude- and frequency-modulated narrow-band noises with a 3-Hz modulation rate, and the resulting neural responses were compared. Spectral analyses revealed similar spectral amplitude peaks for AM and FM at the stimulation frequency (3 Hz), but amplitude at the second harmonic frequency (6 Hz) was much higher for FM than for AM. Moreover, the phase delay of neural responses with respect to the full-band stimulus envelope was shorter for FM than for AM. Finally, the critical analysis involved classification of single trials as being in response to either AM or FM based on either phase or amplitude information. Time-varying phase, but not amplitude, was sufficient to accurately classify AM and FM stimuli based on single-trial neural responses. Taken together, the current results support the dissociable nature of cortical signatures of slow AM and FM. These cortical signatures potentially provide an efficient means to dissect simultaneously communicated slow temporal and spectral information in acoustic communication signals.     

Frequency tuning, latencies, and responses to frequency-modulated sweeps in the inferior colliculus of the echolocating bat, Eptesicus fuscus

Neurons in the inferior colliculus (IC) of the awake big brown bat, Eptesicus fuscus, were examined for joint frequency and latency response properties which could register the timing of the bat's frequency-modulated (FM) biosonar echoes. Best frequencies (BFs) range from 10 kHz to 100 kHz with 50% tuning widths mostly from 1 kHz to 8 kHz. Neurons respond with one discharge per 2-ms tone burst or FM stimulus at a characteristic latency in the range of 3–45 ms, with latency variability (SD) of 50 μs to 4–6 ms or more. BF distribution is related to biosonar signal structure. As observed previously, on a linear frequency scale BFs appear biased to lower frequencies, with 20–40 kHz overrepresented. However, on a hyperbolic frequency (linear period) scale BFs appear more uniformly distributed, with little overrepresentation. The cumulative proportion of BFs in FM₁ and FM₂ bands reconstructs a scaled version of the spectrogram of FM broadcasts. Correcting FM latencies for absolute BF latencies and BF time-in-sweep reveals a subset of IC cells which respond dynamically to the timing of their BFs in FM sweeps. Behaviorally, Eptesicus perceives echo delay and phase with microsecond or even submicrosecond accuracy and resolution, but even with use of phase-locked FM and tone-burst stimuli the cell-by-cell precision of IC time-frequency registration seems inadequate by itself to account for the temporal acuity exhibited by the bat. Accepted: 21 June 1997     

Learning-induced dynamic receptive field changes in primary auditory cortex of the unanaesthetized Mongolian gerbil

Learning-induced changes of the spectro-temporal characteristics of primary auditory cortex (AI) units were studied by response plane analysis of recordings from the AI in unanaesthetized Mongolian gerbils. Using response planes obtained prior to and after auditory discrimination training bins of significant change were identified and their spectro-temporal distribution was studied. Bins of significant changes were generally found to be distributed over the entire spectro-temporal receptive field but occurred most frequently within the first 100 ms of response in the spectral neighbourhood (1.5 octaves) of the frequency of the reinforced conditioned stimulus. Training-induced response decreases occurred early after 10 ms for reinforced conditioned tones and tones in the frequency neighbourhood. Response increases occurred so early only for non-reinforced tones in the neighbourhood of the reinforced frequency and occurred later (after 40 ms) for the reinforced tones. The results are discussed in the light of dynamic disinhibition. Accepted: 13 August 1997     

Responses to pure tones and linear FM components of the CF — FM biosonar signal by single units in the inferior colliculus of the mustached bat

The responses of 682 single-units in the inferior colliculus (IC) of 13 mustached bats (Pteronotus parnellii parnellii) were measured using pure tones (CF), frequency modulations (FM) and pairs of CF-FM signals mimicking the species' biosonar signal, which are stimuli known to be essential to the responses of CF/CF and FM-FM facilitation neurons in auditory cortex. Units were arbitrarily classified into 'reference frequency' (RF), 'FM2' and 'Non-echolocation' (NE) categories according to the relationship of their best frequencies (BF) to the biosonar signal frequencies. RF units have high Q10dB values and are tuned to the reference frequency of each bat, which ranged between 60.73 and 62.73 kHz. FM2 units had BF's between 50 and 60 kHz, while NE units had BF's outside the ranges of the RF and FM2 classes. PST histograms of the responses revealed discharge patterns such as 'onset', 'onset-bursting' (most common), 'on-off', 'tonic-on','pauser', and 'chopper'. Changes in discharge patterns usually resulted from changes in the frequency and/or intensity of the stimuli, most often involving a change from onset-bursting to on-off. Different patterns were also elicited by CF and FM stimuli. Frequency characteristics and thresholds to CF and FM stimuli were measured. RF neurons were very sharply tuned with Q10dB's ranging from 50-360. Most (92%) also responded to FM2 stimuli, but 78% were significantly more sensitive (greater than 5 dB) to CF stimuli, and only 3% had significantly lower thresholds to FM2. The best initial frequency for FM2 sweeps in RF units was 65.35 +/- 2.138 kHz (n = 118), well above the natural frequency of the 2nd harmonic. FM2 and NE units were indistinguishable from each other, but were quite different from RF units: 41% of these two classes had lower thresholds to CF, 49% were about equally sensitive, and 10% had lower thresholds to FM. For FM2 units, mean best initial frequency for FM was 60.94 kHz +/- 3.162 kHz (n = 114), which is closely matched to the 2nd harmonic in the biosonar signal. Very few units (5) responded only to FM signals, i.e., were FM-specialized. The characteristics of spike-count functions were determined in 587 units. The vast majority (79%) of RF units (n = 228) were nonmonotonic, and about 22% had upper-thresholds.(ABSTRACT TRUNCATED AT 400 WORDS)     

Response properties of FM-FM combination-sensitive neurons in the auditory cortex of the mustached bat

Summary For echolocation, the mustached bat,Pteronotus parnellii rubiginosus, emits orientation sounds (pulses) and listens to echoes. Each pulse is made up of 8 components, of which 4 are constant frequencies (CF_1–4) and 4 are frequency-modulated (FM_1–4). Target-range information, conveyed by the time delay of the echo FM from the pulse FM, is processed in this species by specialized neurons in a part of the auditory cortex known as the FM-FM area. These cortical neurons are responsive to pulse-echo pairs at specific echo delays (Fig. 1). The essential components in the sound pair include the pulse FM₁ followed by an echo FM_n (n=2, 3 or 4). Downward sweeping FM₁-FM_n sounds that are similar to those the animal naturally hears during echolocation are the most effective in evoking facilitative responses. Most FM-FM neurons, however, still exhibit facilitative responses to stimulus pairs consisting of upward sweeping FM sounds and/or pure tones at frequencies found in FM sweeps (Figs. 2 and 3). The magnitude of facilitation is altered by changes in echo rather than pulse amplitude (Figs. 5 and 6). Neurons characterized by shorter best delays (or echoes from closer targets) do not require larger best echo amplitudes for facilitation.Abbreviations CF constant frequency - FM frequency modulation - H _n CF — FM harmonics of the mustached bat biosonar signal - CF _n CF components of the harmonics - FM _n FM components of the harmonics - PCF _n pulse CF_n - ECF _n echo CF_n - PFM _n pulse FM_n - EFM _n echo FM_n - PH _n pulse H_n - EH _n echo H_n - BA best amplitude for facilitation - BD best delay for facilitation - PST peri-stimulus-time - PSTC peri-stimulus-time-cumulative - dB SPL dB re 20 Pa     

Neuromagnetic evidence for early auditory restoration of fundamental pitch

Background

Understanding the time course of how listeners reconstruct a missing fundamental component in an auditory stimulus remains elusive. We report MEG evidence that the missing fundamental component of a complex auditory stimulus is recovered in auditory cortex within 100 ms post stimulus onset.

Methodology

Two outside tones of four-tone complex stimuli were held constant (1200 Hz and 2400 Hz), while two inside tones were systematically modulated (between 1300 Hz and 2300 Hz), such that the restored fundamental (also knows as “virtual pitch”) changed from 100 Hz to 600 Hz. Constructing the auditory stimuli in this manner controls for a number of spectral properties known to modulate the neuromagnetic signal. The tone complex stimuli only diverged on the value of the missing fundamental component.

Principal Findings

We compared the M100 latencies of these tone complexes to the M100 latencies elicited by their respective pure tone (spectral pitch) counterparts. The M100 latencies for the tone complexes matched their pure sinusoid counterparts, while also replicating the M100 temporal latency response curve found in previous studies.

Conclusions

Our findings suggest that listeners are reconstructing the inferred pitch by roughly 100 ms after stimulus onset and are consistent with previous electrophysiological research suggesting that the inferential pitch is perceived in early auditory cortex.     

Functional topography of cat primary auditory cortex: response latencies

Minimum onset latency (L_min) of single- and multiple-unit responses were mapped in the primary auditory cortex (AI) of barbiturate-anesthetized cats. Contralateral L_min for multiple units was non-homogeneously distributed along the dorso-ventral/isofrequency axis of the AI. Responses with shorter latencies were more often located in the central, more sharply tuned region while longer latencies were more frequently encountered in the dorsal and ventral portions of the AI. For single units, a large scatter of L_min values was found throughout the extent of the AI including cortical depth. The relationship between L_min and previously reported spectral, intensity and temporal parameters was analyzed and revealed statistically significant correlations between minimum onset latency and the following response properties in some but not all studied animals: sharpness of tuning of a frequency response area 10 dB above threshold, broadband transient response, strongest response level, monotonicity of rate/level functions, dynamic range, and preferred frequency modulation sweep direction. This analysis suggests that L_min is determined by several independent factors and that the prediction of L_min based on relationships with other spectral and temporal response properties is inherently weak. The spatial distribution and the functional relationship between these response parameters may provide an important aspect of the time-based cortical representation of specific features in the animal's natural environment. Accepted: 13 August 1997     

Stream Segregation in the Perception of Sinusoidally Amplitude-Modulated Tones

Amplitude modulation can serve as a cue for segregating streams of sounds from different sources. Here we evaluate stream segregation in humans using ABA- sequences of sinusoidally amplitude modulated (SAM) tones. A and B represent SAM tones with the same carrier frequency (1000, 4000 Hz) and modulation depth (30, 100%). The modulation frequency of the A signals (f_modA) was 30, 100 or 300 Hz, respectively. The modulation frequency of the B signals was up to four octaves higher (Δf_mod). Three different ABA- tone patterns varying in tone duration and stimulus onset asynchrony were presented to evaluate the effect of forward suppression. Subjects indicated their 1- or 2-stream percept on a touch screen at the end of each ABA- sequence (presentation time 5 or 15 s). Tone pattern, f_modA, Δf_mod, carrier frequency, modulation depth and presentation time significantly affected the percentage of a 2-stream percept. The human psychophysical results are compared to responses of avian forebrain neurons evoked by different ABA- SAM tone conditions [1] that were broadly overlapping those of the present study. The neurons also showed significant effects of tone pattern and Δf_mod that were comparable to effects observed in the present psychophysical study. Depending on the carrier frequency, modulation frequency, modulation depth and the width of the auditory filters, SAM tones may provide mainly temporal cues (sidebands fall within the range of the filter), spectral cues (sidebands fall outside the range of the filter) or possibly both. A computational model based on excitation pattern differences was used to predict the 50% threshold of 2-stream responses. In conditions for which the model predicts a considerably larger 50% threshold of 2-stream responses (i.e., larger Δf_mod at threshold) than was observed, it is unlikely that spectral cues can provide an explanation of stream segregation by SAM.