Iterated Rippled Noise Stimuli Research Articles

The Maturation of the Acoustic Change Complex in Response to Iterated Ripple Noise in 'Normal'-Hearing Infants, Toddlers, and Adults.

Infants and toddlers are still being evaluated for their hearing sensitivity but not their auditory-processing skills. Iterated rippled noise (IRN) stimuli require the auditory system to utilize the temporal periodicity and autocorrelate the iterations to perceive pitch. This study investigated the acoustic change complex (ACC) elicited by IRN in "normal"-hearing infants, toddlers, and adults to determine the maturation of cortical processing of IRN stimuli. Cortical responses to filtered white noise (onset) concatenated with IRN stimuli (d = 10 milliseconds, gain = 0.7 dB: 4-32 iterations) were recorded in quiet, alert participants. Participants included 25 infants (2.5-15 months), 27 toddlers (22-59 months), and 8 adults (19-25 years) with "normal" hearing sensitivity. Cortical auditory-evoked responses were recorded for each participant, including the onset response to the noise and an ACC to the transition from noise to IRN. Group differences were assessed using repeated-measures analyses of variance. Most infants had a replicable onset (P) response, while only about half had a measurable ACC (PACC) response to the high-saliency IRN condition. Most toddlers had onset responses and showed a P-NACC response to the IRN16 and IRN32 conditions. Most of the toddler group had responses present to the onset and showed a P-NACC response to all IRN conditions. Toddlers and adults showed similar P-NACC amplitudes; however, adults showed an increase in N1ACC amplitude with increase in IRN iterations (i.e., increased salience). While cortical responses to the percept of sound as determined by the onset response (P) to a stimulus are present in most infants, ACC responses to IRN stimuli are not mature in infancy. Most toddlers as young as 22 months, however, exhibited ACC responses to the IRN stimuli even when the pitch saliency was low (e.g., IRN4). The findings of the current study have implications for future research when investigating maturational effects on ACC and the optimal choice of stimuli.

High-Field Functional Imaging of Pitch Processing in Auditory Cortex of the Cat.

The perception of pitch is a widely studied and hotly debated topic in human hearing. Many of these studies combine functional imaging techniques with stimuli designed to disambiguate the percept of pitch from frequency information present in the stimulus. While useful in identifying potential “pitch centres” in cortex, the existence of truly pitch-responsive neurons requires single neuron-level measures that can only be undertaken in animal models. While a number of animals have been shown to be sensitive to pitch, few studies have addressed the location of cortical generators of pitch percepts in non-human models. The current study uses high-field functional magnetic resonance imaging (fMRI) of the feline brain in an attempt to identify regions of cortex that show increased activity in response to pitch-evoking stimuli. Cats were presented with iterated rippled noise (IRN) stimuli, narrowband noise stimuli with the same spectral profile but no perceivable pitch, and a processed IRN stimulus in which phase components were randomized to preserve slowly changing modulations in the absence of pitch (IRNo). Pitch-related activity was not observed to occur in either primary auditory cortex (A1) or the anterior auditory field (AAF) which comprise the core auditory cortex in cats. Rather, cortical areas surrounding the posterior ectosylvian sulcus responded preferentially to the IRN stimulus when compared to narrowband noise, with group analyses revealing bilateral activity centred in the posterior auditory field (PAF). This study demonstrates that fMRI is useful for identifying pitch-related processing in cat cortex, and identifies cortical areas that warrant further investigation. Moreover, we have taken the first steps in identifying a useful animal model for the study of pitch perception.

Linear and log frequency rippled spectra

Roy Patterson’s Ph.D. dissertation investigated the pitch-shift of the residue and pitch perception has been a significant topic of interest to Roy ever since. I had the good fortune of collaborating with Roy on several studies involving pitch perception, especially on what we called regular interval stimuli (RIS), most notably iterated rippled noise (IRN). In addition to IRN stimuli which are characterized as stimuli with regularly spaced spectral peaks and valleys on a linear frequency axis, many studies have investigated stimuli that have regularly spaced spectral peaks and valleys on a logarithmic axis. Both set of stimuli produce a timbre/pitch-like sound quality. In some cases the sound quality of the two types of stimuli are difficult to perceptually separate. While RIS models of pitch processing (e.g., autocorrelation-based models) can account for many of the IRN pitch data, it is not clear what mechanisms produce the timbre/pitch-like qualities of log-frequency, rippled-spectra stimuli. The current paper involves three experiments designed to better understand auditory processing of rippled-spectra stimuli in order to determine if there may be some common perceptual elements that underlie the perception of such stimuli. [Research supported by the AFOSR.]

Brief pitch-priming facilitates infants’ discrimination of pitch-evoking noise: Evidence from event-related potentials

Pitch is derived by the auditory system through complex spectrotemporal processing. Pitch extraction is thought to depend on both spectral cues arising from lower harmonics that are resolved by cochlear filters in the inner ear, and on temporal cues arising from the pattern of action potentials contained in the cochlear output. Adults are capable of extracting pitch in the absence of robust spectral cues, taking advantage of the temporal cues that remain. However, recent behavioral evidence suggests that infants have difficulty discriminating between stimuli with different pitches when resolvable spectral cues are absent. In the current experiments, we used the mismatch negativity (MMN) component of the event related potential derived from electroencephalographic (EEG) recordings to examine a cortical representation of pitch discrimination for iterated rippled noise (IRN) stimuli in 4- and 8-month-old infants. IRN stimuli are pitch-evoking sounds generated by repeatedly adding a segment of white noise to itself at a constant delay. We created IRN stimuli (delays of 5 and 6ms creating pitch percepts of 200 and 167Hz) and high-pass filtered them to remove all resolvable spectral pitch cues. In experiment 1, we did not find EEG evidence that infants could detect the change in the pitch of these IRN stimuli. However, in Experiment 2, after a brief period of pitch-priming during which we added a sine wave component to the IRN stimulus at its perceived pitch, infants did show significant MMN in response to pitch changes in the IRN stimuli with sine waves removed. This suggests that (1) infants can use temporal cues to process pitch, although such processing is not mature and (2) that a short amount of pitch-priming experience can alter pitch representations in auditory cortex during infancy.

Development of pitch processing: Infants' discrimination of iterated rippled noise stimuli with unresolved spectral content

Sound frequency is extracted at the level of the cochlea, and is represented by two neural codes: a spectral (place) code that is maintained by tonotopic maps extending into primary auditory cortex, and a temporal code based on the periodicity of action potentials in auditory nerve fibers. To date, little work has examined infants' ability to perceive pitch when spectral content cannot be resolved by cochlear filters; the present experiments do so using high-pass filtered iterated rippled noise (IRN) stimuli. Using a conditioned head-turn paradigm, most 8-month-old infants showed above-chance discrimination of a change from 167 to 200 Hz in the fundamental frequency (F0) of such high-passed filtered IRN stimuli, but only when first exposed to a training target stimulus that emphasized pitch through the addition of a sine wave tone to the IRN stimulus at the F0. However, even after this period of pitch priming, performance was quite poor relative to that found in previous studies using stimuli with resolved spectral content. These results support the idea that 8-month-olds can perceive pitch when only unresolved spectral content is present in the stimulus, but that such processing is not yet robust.

Perception of Pure Tones and Iterated Rippled Noise for Normal Hearing and Cochlear Implant Users

Cochlear Implant (CI) users typically perform poorly on musical tasks, especially those based on pitch ranking and melody recognition. It was hypothesized that CI users would demonstrate deterioration in performance for a pitch ranking and a melody recognition task presented with iterated rippled noise (IRN) in comparison to pure tones (PT). In Addition, it was hypothesized that normal hearing (NH) listeners would show fewer differences in performance between IRN and PT for these two tasks. In this study, the ability of CI users and NH subjects to rank pitches and to identify melodies created with IRN and PT was assessed in free field in a sound-isolated room. CI subjects scored significantly above chance level with PT stimuli in both tasks. With IRN stimuli their performance was around chance level. NH subjects scored significantly above chance level in both tasks and with all stimuli. NH subjects performed significantly better than CI subjects in both tasks. These results illustrate the difficulties of CI subjects to rank pitches and to identify melodies.

Sequencing the Cortical Processing of Pitch-Evoking Stimuli using EEG Analysis and Source Estimation

Cues to pitch include spectral cues that arise from tonotopic organization and temporal cues that arise from firing patterns of auditory neurons. fMRI studies suggest a common pitch center is located just beyond primary auditory cortex along the lateral aspect of Heschl’s gyrus, but little work has examined the stages of processing for the integration of pitch cues. Using electroencephalography, we recorded cortical responses to high-pass filtered iterated rippled noise (IRN) and high-pass filtered complex harmonic stimuli, which differ in temporal and spectral content. The two stimulus types were matched for pitch saliency, and a mismatch negativity (MMN) response was elicited by infrequent pitch changes. The P1 and N1 components of event-related potentials (ERPs) are thought to arise from primary and secondary auditory areas, respectively, and to result from simple feature extraction. MMN is generated in secondary auditory cortex and is thought to act on feature-integrated auditory objects. We found that peak latencies of both P1 and N1 occur later in response to IRN stimuli than to complex harmonic stimuli, but found no latency differences between stimulus types for MMN. The location of each ERP component was estimated based on iterative fitting of regional sources in the auditory cortices. The sources of both the P1 and N1 components elicited by IRN stimuli were located dorsal to those elicited by complex harmonic stimuli, whereas no differences were observed for MMN sources across stimuli. Furthermore, the MMN component was located between the P1 and N1 components, consistent with fMRI studies indicating a common pitch region in lateral Heschl’s gyrus. These results suggest that while the spectral and temporal processing of different pitch-evoking stimuli involves different cortical areas during early processing, by the time the object-related MMN response is formed, these cues have been integrated into a common representation of pitch.

Brainstem pitch representation in native speakers of Mandarin is less susceptible to degradation of stimulus temporal regularity

It has been demonstrated that neural encoding of pitch in the auditory brainstem is shaped by long-term experience with language. To date, however, all stimuli have exhibited a high degree of pitch saliency. The experimental design herein permits us to determine whether experience-dependent pitch representation in the brainstem is less susceptible to progressive degradation of the temporal regularity of iterated rippled noise (IRN). Brainstem responses were recorded from Chinese and English participants in response to IRN homologues of Mandarin Tone 2 (T2 IRN). Six different iterations steps were utilized to systematically vary the degree of temporal regularity in the fine structure of the IRN stimuli to produce a pitch salience continuum ranging from low to high. Pitch-tracking accuracy and pitch strength were computed from the brainstem responses using autocorrelation algorithms. Analysis of variance of brainstem responses to T2 IRN revealed that pitch-tracking accuracy is higher in the native tone language group (Chinese) relative to the non-tone language group (English) except for the three lowest steps along the continuum, and moreover, that pitch strength is greater in the Chinese group even in severely degraded stimuli for two of the six 40-ms sections of T2 IRN that exhibit rapid changes in pitch. For these same two sections, exponential time constants for the stimulus continuum revealed that pitch strength emerges 2–3 times faster in the tone language than in the non-tone language group as a function of increasing pitch salience. These findings altogether suggest that experience-dependent brainstem mechanisms for pitch are especially sensitive to those dimensions of tonal contours that provide cues of high perceptual saliency in degraded as well as normal listening conditions.

Iterated rippled noise discrimination at long durations

Iterated rippled noise (IRN) was used to study discrimination of IRN stimuli with a lower number of iterations from IRN stimuli with a higher number of iterations as a function of stimulus duration (100-2000 ms). Such IRN stimuli differ in the strength of the repetition pitch. In some cases, the gain used to generate IRN stimuli was adjusted so that both IRN stimuli in the discrimination task had the same height of the first peak in the autocorrelation function or autocorrelogram. In previous work involving short-duration IRN stimuli (<500 ms), listeners were not able to discriminate between IRN stimuli that had different numbers of iterations but the same height of the first peak in the autocorrelation function. In the current study, IRN discrimination performance improved with increases in duration, even in cases when the height of the first peak in the autocorrelation was the same for the two IRN stimuli. Thus, future studies involving discrimination of IRN stimuli may need to use longer durations (1 s or greater) than those that have been used in the past.

The perceptual tone‐noise ratio of iterated rippled noise in listeners with hearing loss.

Previous work has demonstrated that iterated rippled noise (IRN) produces a weaker pitch in hearing‐impaired (HI) listeners than in normal‐hearing (NH) subjects (e.g., Leek and Summers, 2001). In this study, the hypothesis was tested that the weaker pitch is a result of an increased perceptual noisiness of IRN for HI subjects. The tone‐noise ratio (T/N) was developed by Patterson et al. (1996) to describe the relationship between the strength of the tone component and the strength of the noise component for a given IRN. The T/N was measured using an adaptive procedure to find the tone‐plus‐noise level that matched each IRN stimulus. Four NH and 4 HI subjects took part in the study. The stimuli were IRNs with delays ranging from 4 to 16 ms and with one to eight iterations. There was a significant effect of delay and of iteration number, with longer delays and more iterations producing greater T/N. There were no significant differences between subject groups. These findings suggest that the weaker complex pitch observed in listeners with hearing loss is not due to an abnormally noisy IRN percept but must be attributed to other impaired processing mechanisms. [Work supported by NIH.]

Applications of Static and Dynamic Iterated Rippled Noise to Evaluate Pitch Encoding in the Human Auditory Brainstem

This paper presents a new application of the dynamic iterated rippled noise (IRN) algorithm by generating dynamic pitch contours representative of those that occur in natural speech in the context of EEG and the frequency following response (FFR). Besides IRN steady state and linear rising stimuli, curvilinear rising stimuli were modeled after pitch contours of natural productions of Mandarin Tone 2. Electrophysiological data on pitch representation at the level of the brainstem, as reflected in FFR, were evaluated for all stimuli, static or dynamic. Autocorrelation peaks were observed corresponding to the fundamental period (tau) as well as spectral bands at the fundamental and its harmonics for both a low and a high iteration step. At the higher iteration step, both spectral and temporal FFR representations were more robust, indicating that both acoustic properties may be utilized for pitch extraction at the level of the brainstem. By applying curvilinear IRN stimuli to elicit FFRs, we can evaluate the effects of temporal degradation on 1) the neural representation of linguistically-relevant pitch features in a target population (e.g., cochlear implant) and 2) the efficacy of signal processing schemes in conventional hearing aids and cochlear implants to recover these features.

Spectral resolvability of iterated rippled noise

A forward-masking experiment was used to estimate the spectral ripple of iterated rippled noise (IRN) that is possibly resolved by the auditory system. Tonal signals were placed at spectral peaks and valleys of IRN maskers for a wide variety of IRN conditions that included different delays, number of iterations, and stimulus durations. The differences in the forward-masked thresholds of tones at spectral peaks and valleys were used to estimate spectral resolvability, and these results were compared to estimates obtained from a gamma-tone filter bank. The IRN spectrum has spectral peaks that are harmonics of the reciprocal of the delay used to generate IRN stimuli. As the number of iterations in the generation of IRN stimuli increases so does the difference in the spectral peak-to-valley ratio. For high number of iterations, long delays, and long durations evidence for spectral resolvability existed up to the 6th harmonic. For all other conditions spectral resolvability appeared to disappear at harmonics lower than the 6th, or was not measurable at all. These data will be discussed in terms of the role spectral resolvability might play in processing the pitch, pitch strength, and timbre of IRN stimuli. [Work supported by a grant from NIDCD.]

Estimates of pitch strength for musicians and nonmusicians

To measure the strength of the pitch of iterated rippled noise (IRN), 19 adults were tested in an operant conditioning procedure. Seven adults had music training and currently played an instrument; 12 adults had no training and did not currently play an instrument. To generate IRN, a 500-ms Gaussian noise stimulus was delayed by 5 or 6 ms (pitches of 200 or 166 Hz) and added to the original for 16 iterations. IRN stimuli having one delay were presented repeatedly. On signal trials the delay changed for 6 s. Stimulus level roved from 63-67 dBA (background of 28 dBA). Adults learned to press a button when the stimulus changed. Testing started with IRN stimuli having 0-dB attenuation (i.e., maximal pitch strength). Stimuli having weaker pitches (i.e., progressively greater attenuation applied to the delayed noise) followed. Strength of pitch was quantified as the maximum attenuation for which pitch was discerned. For each subject, threshold attenuation for pitch strength was extrapolated as the 71% point on a psychometric function depicting percent correct performance as a function of attenuation. Mean thresholds revealed that the pitch percept was similar for both nonmusically trained (18.70 dB) and musically trained adults (18.73 dB).

Estimates of the strength of repetition pitch in infants

To measure the strength of the pitch of iterated rippled noise (IRN), 24 7- to 8-month-olds and 24 adults were tested in an operant conditioning procedure. To generate IRN, a 500-ms Gaussian noise was delayed by 5 or 6 ms (pitches of 200 and 166 Hz) and added to the original noise for 16 iterations. IRN stimuli having one delay were presented repeatedly, and on signal trials the delay changed for 6 s. Overall stimulus level roved from 63–67 dBA. Infants learned to turn their heads toward the sound, and adults learned to press a button when the delay of the stimulus changed. Testing started with IRN stimuli having 0 dB attenuation (i.e., maximal pitch strength). Then, stimuli having weaker pitches (i.e., progressively greater attenuation applied to the delayed noise) were presented. Strength of pitch can be quantified as the maximum attenuation for which pitch can be discerned. For each subject, threshold attenuation for pitch strength was extrapolated as the 71% point on a psychometric function depicting percent-correct performance as a function of attenuation. Mean thresholds revealed that the pitch percept was significantly weaker for infants (6.9 dB) than for adults (19.1 dB).

The temporal representation of the delay of iterated rippled noise in the ventral cochlear nucleus of the guinea-pig.

1. We have examined the temporal discharge patterns of single units from the ventral cochlear nucleus (VCN) of anaesthetized guinea-pigs in response to iterated rippled noise (IRN). The pitch range evoked by the stimuli was from 32 to 1000 Hz. 2. Single units were classified into four groups using existing classification schemes: primary-like (PL), onset (O), sustained chopper (CS) and transient chopper (CT). For all unit types the delay of the IRN stimuli was well represented in the all-order interspike interval histograms (ISIHs). 3. A subset of the onset units (onset-chopper, OC) showed a clear preference for some delays of the IRN in their first-order interval statistics. We describe this delay preference as 'periodicity tuning'. The delay at which the pitch estimate was at its maximum was designated its best periodicity. The range of best periodicities for OC units was 3.75-13 ms (between 77 and 267 Hz). 4. The other unit types also showed enhancement of the first-order interval statistics at the delay of the IRN. The range of best periodicities was 1.4-8.8 ms (113-714 Hz) for the CT group, 2.25-10.8 ms (93-444 Hz) for the CS group and 0.5-4.6 ms (217-2000 Hz) for the PL group. 5. The correlation between the maximum interval enhancement observed in response to the IRN stimuli and the peak in the first-order ISIH in response to white noise was 0.81 for OC units, 0.72 for CS units, 0.44 for CT units and -0.15 for PL units. 6. These results demonstrate that all unit types in the VCN can enhance the representation of the delay of IRN using first-order interspike intervals (ISIs) over a range of periodicities. CS and OC units show the greatest range of best periodicities and they are well-suited to encode the delay of IRN in their first-order ISIs for a wide range of pitches.

Temporal representation of iterated rippled noise as a function of delay and sound level in the ventral cochlear nucleus.

The discharge patterns of single units in the ventral cochlear nucleus (VCN) of anesthetized guinea pigs were examined in response to iterated rippled noise (IRN) as a function of the IRN delay (which determines the IRN pitch) and the IRN sound level. Delays were varied over five octaves in half-octave steps, and sound levels were varied over a 30- or 50-dB range in steps of 5 dB. Neural responses were analyzed in terms of first-order and all-order inter-spike intervals (ISIs). The IRN quasi-periodicity was preserved in the all-order ISIs for most units independent of unit type or best frequency (BF). A deterioration of the temporal all-order code was found, however, when the neural response was influenced by inhibition. The IRN quasi-periodicity was also preserved in first-order ISIs for a limited range of IRN delays and levels. Sustained Chopper units (CS) in the VCN responded with very regular ISIs when the IRN delay corresponded to the unit's chopping period; i.e., the unit showed an increased proportion of intervals corresponding to the IRN delay (interval enhancement) relative to an equal-level, white-noise stimulation. This interval enhancement has a band-pass characteristic with a peak corresponding to the chopping period. Moreover, for CS units in rate saturation, the chopping period, and thus the interval enhancement to the IRN, did not vary with level. Units classified as onset-chopper also show a band-pass interval enhancement to the IRN stimuli; however, they show more level-dependent changes than CS units. Primary-like (PL) units also show level-dependent changes in their ability to code the IRN pitch in first-order intervals. The range of delays where PL units showed interval enhancement was broader and extended to shorter delays. Based on these findings, it is suggested that CS units may play an important role in pitch processing in that they transform a higher-order interval code into a first-order interval place code. Their limited dynamic range together with the preservation of the temporal stimulus features in saturation may serve as a physiological basis for the perceived level independence of pitch.

The role of the envelope in processing iterated rippled noise.

Iterated rippled noise (IRN) is generated by a cascade of delay and add (the gain after the delay is 1.0) or delay and subtract (the gain is -1.0) operations. The delay and add/subtract operations impart a spectral ripple and a temporal regularity to the noise. The waveform fine structure is different in these two conditions, but the envelope can be extremely similar. Four experiments were used to determine conditions in which the processing of IRN stimuli might be mediated by the waveform fine structure or by the envelope. In experiments 1 and 3 listeners discriminated among three stimuli in a single-interval task: IRN stimuli generated with the delay and add operations (g = 1.0), IRN stimuli generated using the delay and subtract operations (g = -1.0), and a flat-spectrum noise stimulus. In experiment 2 the listeners were presented two IRN stimuli that differed in delay (4 vs 6 ms) and a flat-spectrum noise stimulus that was not an IRN stimulus. In experiments 1 and 2 both the envelope and waveform fine structure contained the spectral ripple and temporal regularity. In experiment 3 only the envelope had this spectral and temporal structure. In all experiments discrimination was determined as a function of high-pass filtering the stimuli, and listeners could discriminate between the two IRN stimuli up to frequency regions as high as 4000-6000 Hz. Listeners could discriminate the IRN stimuli from the flat-spectrum noise stimulus at even higher frequencies (as high as 8000 Hz), but these discriminations did not appear to depend on the pitch of the IRN stimuli. A control experiment (fourth experiment) suggests that IRN discriminations in high-frequency regions are probably not due entirely to low-frequency nonlinear distortion products. The results of the paper imply that pitch processing of IRN stimuli is based on the waveform fine structure.

Pitch strength of iterated rippled noise when the pitch is ambiguous

Two versions of a cascade of delay, gain following delay, and add circuits were used to generate iterated rippled noise (IRN) stimuli. IRN stimuli produce a repetition pitch whose pitch strength relative to the noise percept can be varied by changing the type of circuit, the attenuation, or the number of iterations in the circuit. The repetition pitch of IRN is different when the delayed noise is subtracted (gain < 0) rather than added (gain > 0) to the undelayed noise. In the case of subtraction, IRN pitch is often ambiguous having two or more pitches. Listeners were asked to use pitch strength to discriminate between various pairs of IRN stimuli generated with different gains, different network circuits, and different number of iterations. For most conditions the gain was less than one. The data were described by a description based on an exponential function of the largest peak of the autocorrelation function of IRN stimuli [W. A. Yost, J. Acoust. Soc. Am. 100, 511-518 (1996)] processed such that the spectral dominance region is emphasized. These results suggest that the strength of the pitch of IRN stimuli can be described by temporal processing mechanisms as might be revealed by autocorrelation.

The relative strength of the tone and noise components in iterated rippled noise

Rippled noise is constructed by delaying a random noise and adding it back to the original. Iterated rippled noise (IRN) is constructed by repeating the delay-and-add process. IRN produces a two-component perception, i.e., a buzzy tone with a pitch equal to the reciprocal of the delay and a background noise that sounds like the original random noise. The perceived tone/noise ratio increases with the number of iterations. The effective tone/noise ratio in IRN sounds with 1–16 iterations was measured in a discrimination matching experiment; each IRN was paired with a range of standard sounds, having varying proportions of a broadband noise and a complex tone, to find the point where their perceived tone/noise ratios are the same. The experiment shows that the tone/noise ratio of the matching standard increases 3.8 dB per doubling of the number of iterations in the IRN stimulus. Spectral models of auditory perception explain the pitch of IRN in terms of peaks in the region of the first five to eight harmonics of the reciprocal of the delay. However, the matching data are unaffected when the sound is high-pass filtered at the twelfth harmonic of the delay—above the region of resolved harmonics. We show that a wide range of time-domain auditory models can explain the discrimination matching data by applying autocorrelation, either to the IRN waveform, or to the neural activity patterns produced by the cochlea in response to IRN waves.

Pitch strength of iterated rippled noise.

Two versions of a cascade add, attenuate, and delay circuit were used to generate iterated rippled noise (IRN) stimuli. IRN stimuli produce a repetition pitch whose pitch strength relative to the noise percept can be varied by changing the type of circuit, the attenuation, or the number of iterations in the circuit. Listeners were asked to use pitch strength to discriminate between various pairs of IRN stimuli which differed in the type of network used to generate the sounds, the number of iterations, and the attenuation in the network. A description based on an exponential function of the first peak of the autocorrelation function of IRN stimuli is consistent with the results. The discrimination data were well fit by a function based on the difference in the exponential functions of the first peak of the autocorrelation functions. A magnitude estimation experiment indicated that pitch strength was an exponential function of the height of the first peak in the autocorrelation function. These results suggest that the strength of the pitch of IRN stimuli is based on temporal processing as might be revealed by autocorrelation.