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OBJECTIVEExperiential phenomena (EP), such as illusions and complex hallucinations, are vivid experiences created in one’s mind. They can occur spontaneously as epileptic auras or can be elicited by electrical brain stimulation (EBS) in patients undergoing presurgical evaluation for drug-resistant epilepsy. Previous work suggests that EP arise from activation of different nodes within interconnected neural networks mainly in the temporal lobes. Yet, the anatomical extent of these neural networks has not been described and the question of lateralization of EP has not been fully addressed. To this end, an extended number of brain regions in which electrical stimulation elicited EP were studied to test whether there is a lateralization propensity to EP phenomena.METHODSA total of 19 drug-resistant focal epilepsy patients who underwent EBS as part of invasive presurgical evaluation and who experienced EP during the stimulation were included. Spatial dispersion of visual and auditory illusions and complex hallucinations in each hemisphere was determined by calculation of Euclidean distances between electrodes and their centroid in common space, based on (x, y, z) Cartesian coordinates of electrode locations.RESULTSIn total, 5857 stimulation epochs were analyzed; 917 stimulations elicited responses, out of which 130 elicited EP. Complex visual hallucinations were found to be widely dispersed in the right hemisphere, while they were tightly clustered in the occipital lobe of the left hemisphere. Visual illusions were elicited mostly in the occipital lobes bilaterally. Auditory illusions and hallucinations were evoked symmetrically in the temporal lobes.CONCLUSIONSThese findings suggest that complex visual hallucinations arise from wider spread in the right compared to the left hemisphere, possibly mirroring the asymmetry in the white matter organization of the two hemispheres. These results offer some insights into lateralized differences in functional organization and connectivity that may be important for functional mapping and planning of surgical resections in patients with epilepsy.

Mating signals have evolved to attract target receivers, even to the point of exploiting receivers through perceptual manipulation. Signals, however, can also expose signalers to nontarget receivers, including predators and parasites, and thus have also evolved to decrease enemy attraction. Here we show that male tree frogs (Smilisca sila) reduce their attractiveness to eavesdropping enemies (bats and midges) by overlapping their calls at near-perfect synchrony with the calls of neighboring conspecifics. By producing calls that closely follow those of other males, synchronizing S. sila take advantage of an auditory illusion where enemies are more attracted to the leading call. Female S. sila, however, are less susceptible to this illusion. Thus, synchronization among signaling males can result in acoustic crypsis from predators without affecting female attraction. Given the widespread use of conspicuous mating signals and eavesdropping enemies, perceptual exploitation of eavesdroppers is likely a common driver of signal evolution.

Several studies have demonstrated the neural correlates of chronic tinnitus. However, we still do not understand what happens in the acute phase. Past studies have established Zwicker tone (ZT) illusions as a good human model for acute tinnitus. ZT illusions are perceived following the presentation of a notched noise stimulus, that is, broadband noise with a narrow band‐stop filter (notch). In the current study, we compared the neural correlates of the reliable perception of a ZT illusion to that which is not. We observed changes in evoked and total theta power in wide‐spread regions of the brain particularly in the temporal‐parietal junction, pregenual anterior cingulate cortex/ventromedial prefrontal cortex (pgACC/vmPFC), parahippocampus during perception of the ZT illusion. Furthermore, we observe that increased theta power significantly predicts a gradual positive change in the intensity of the ZT illusion. Such changes may suggest a malfunction of the sensory gating system that enables habituation to redundant stimuli and suppresses hyperactivity. It could also suggest a successful retrieval of the memory of the missing frequencies, resulting in their conscious perception indicating the role of higher‐order processing in the mechanism of action of ZT illusions. To establish a more concrete relationship between ZT illusion and chronic tinnitus, future longitudinal studies following up a much larger sample of participants who reliably perceive a ZT illusion to see if they develop tinnitus at a later stage is essential. This could inform us if the ZT illusion may be a precursor to chronic tinnitus.

It is pointed out that beyond reproducing the physically correct sound pressure at the eardrums, more effects play a significant role in the quality of the auditory illusion. In some cases, these can dominate perception and even overcome physical deviations. Perceptual effects like the room-divergence effect, additional visual influences, personalization, pose and position tracking as well as adaptation processes are discussed. These effects are described individually, and the interconnections between them are highlighted. With the results from experiments performed by the authors, the perceptual effects can be quantified. Furthermore, concepts are proposed to optimize reproduction systems with regard to those effects. One example could be a system that adapts to varying listening situations as well as individual listening habits, experience and preference.

Perception of sound fields reproduced by loudspeaker arrays which are driven by spatial-audio technologies, such as Wave-Field Synthesis or Higher-Order Ambisonics, is examined in the light of “Spatial Auditory Illusions”. The spatial-audio technologies are based on illusions as to which real sound-sources vanish perceptually in favor of virtual sources. Furthermore, spatial-audio technologies are able to synthesize sound fields which are very similar to sound fields as created by real sources. In this chapter, these illusions are first explored as a function of the acoustic (physical) properties of the synthesized sound fields. Then, the perceptual dimensions are reviewed of what is actually heard when being exposed to these synthesized sound fields.

Visual Abstract, Binaural beating is a perceptual auditory illusion occurring when presenting two neighboring frequencies to each ear separately. Several controversial claims have been attributed to binaural beats regarding their ability to entrain human brain activity and mood, in both the scientific literature and the marketing realm. Here, we sought to address those questions in a robust fashion using a single-blind, active-controlled protocol. To do so, we compared the effects of binaural beats with a control beat stimulation (monaural beats, known to entrain brain activity but not mood) across four distinct levels in the human auditory pathway: subcortical and cortical entrainment, scalp-level functional connectivity and self-reports. Both stimuli elicited standard subcortical responses at the pure tone frequencies of the stimulus [i.e., frequency following response (FFR)], and entrained the cortex at the beat frequency [i.e., auditory steady state response (ASSR)]. Furthermore, functional connectivity patterns were modulated differentially by both kinds of stimuli, with binaural beats being the only one eliciting cross-frequency activity. Despite this, we did not find any mood modulation related to our experimental manipulation. Our results provide evidence that binaural beats elicit cross frequency connectivity patterns, but weakly entrain the cortex when compared with monaural beat stimuli. Whether binaural beats have an impact on cognitive performance or other mood measurements remains to be seen and can be further investigated within the proposed methodological framework.

The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link The solitaire illusion is an illusion of numerosity proposed by Frith and Frith. In the original version, an apparent number of elements was determined by the spatial arrangement of two kinds of elements (black and white marbles). In our study, an auditory version of the solitaire illusion was demonstrated. Participants were asked to judge if they perceived more drum or piano sounds. When half of the piano tones were perceived as lower in pitch than a drum sound and the other half higher, piano tones appeared to be arranged in small units, leading to numerosity underestimation. Conversely, when all piano tones were perceived to be higher in pitch than the drum sounds, they appeared to be arranged in a single large unit, leading to numerosity overestimation. Comparable to the visual version of the solitaire illusion, the clustering seems to be determined by Gestalt principles. In our auditory version, a clear reversal of the illusion (numerosity overestimation or underestimation) was observed when piano tones appeared to be arranged in a single large cluster or in several small clusters, respectively.

The speech-to-song illusion is a type of auditory illusion that the repetition of a part of a sentence would change people’s perception tendency from speech-like to song-like. The study aims to examine how pace, emotion, and language tonality affect people’s experience of the speech-to-song illusion. It uses a between-subject (Pace: fast, normal, vs. slow) and within-subject (Emotion: positive, negative, vs. neutral; language tonality: tonal language vs. non-tonal language) design. Sixty Hong Kong college students were randomly assigned to one of the three conditions characterized by pace. They listened to 12 audio stimuli, each with repetitions of a short excerpt, and rated their subjective perception of the presented phrase, whether it sounded like a speech or a song, on a five-point Likert-scale. Paired-sample t-tests and repeated measures ANOVAs were used to analyze the data. The findings reveal that a faster speech pace could strengthen the tendency of the speech-to-song illusion. Neither emotion nor language tonality show a statistically significant influence on the speech-to-song illusion. This study suggests that the perception of sound should be in a continuum and facilitates the understanding of song production in which speech can turn into music by having repetitive phrases and to be played in a relatively fast pace.

International audience; An auditory illusion caught the world’s attention recently. For the same noisy speech utterance, different people reported hearing either ‘Laurel’ or ‘Yanny’. The dichotomy highlights how perceptions are inferences from inherently ambiguous sensory information, even though ambiguity is often unnoticed.

In arbitrating between representational and relational theories of perception, perceptual illusions—cases in which a subject’s perceptual experience diverges from the way the world really is—constitute an important battleground. The debate has, however, been dominated by discussions of visual perception. In attempting to extend the debate to audition, it is appropriate to start by considering what is thought to be a key case of auditory illusion. I consider the phenomenon of the ‘missing fundamental’, as well as examining a notion that is often deployed by representationalists to explain it—namely, perceptual inference. Though it is frequently deployed as an explanatory concept by inferentialists, the notion of perceptual inference is somewhat opaque. Here, I formulate a ‘job description’ for perceptual inference, involving rule-following. I then identify two sets of cases that commonly prompt the invocation of perceptual inference: namely, cases of perceptual illusion, and cases of veridical perception where the perceptual content outstrips the information present in the stimulus. I then argue that an appeal to perceptual inference is unnecessary, in the case of the missing fundamental, for two reasons. Firstly, the missing fundamental is not, after all, a clear candidate for the ascription of inferential capacity to the auditory system: it is neither an illusion, nor is it the case that the stimulus is crucially impoverished. That is, it submits to a ‘direct’ explanation. Secondly, given the adequacy of a simpler explanation, and the difficulty between distinguishing between real rule-following and the mere appearance of it, I argue that we should avoid ascribing inferential capacity to the auditory system. I close by considering objections, and offering replies.

Early in the twentieth century, the Gestalt psychologists outlined principles governing the ability of the human visual system to construct integrated percepts of objects in visual scenes. By the close of the twentieth century, ample evidence suggested that the human auditory system follows similar principles of perceptual organization. Several Gestalt principles of grouping—proximity, similarity, common fate, good continuation, and familiarity, govern our ability to decompose complex mixtures of sounds into percepts of auditory objects in acoustic scenes. Auditory objects are perceptual groupings of sounds generated by the same source that are present at different times and in different parts of the frequency spectrum. The ability to form auditory objects likely plays an important role in allowing animals to navigate human-altered soundscapes. This chapter reviews studies of insects, fish, frogs, birds, and nonhuman mammals in which experimenters manipulated potential grouping cues and measured performance on behavioral tasks designed to reveal the animal’s perception of auditory objects. These studies employed techniques ranging from measuring natural behaviors in response to communication signals to operant conditioning of responses to artificial sounds such as pure tones. The totality of the studies reviewed here unequivocally reveals that nonhuman animals not only form auditory objects but that they also follow the Gestalt principles of grouping. These principles and their underlying mechanisms allow animals to perceptually organize the often noisy and complex acoustic environments in which they live.

Human listeners place greater weight on the beginning of a sound compared to the middle or end when determining sound location, creating an auditory illusion known as the Franssen effect. Here, we exploited that effect to test whether human auditory cortex (AC) represents the physical vs. perceived spatial features of a sound. We used functional magnetic resonance imaging (fMRI) to measure AC responses to sounds that varied in perceived location due to interaural level differences (ILD) applied to sound onsets or to the full sound duration. Analysis of hemodynamic responses in AC revealed sensitivity to ILD in both full-cue (veridical) and onset-only (illusory) lateralized stimuli. Classification analysis revealed regional differences in the sensitivity to onset-only ILDs, where better classification was observed in posterior compared to primary AC. That is, restricting the ILD to sound onset-which alters the physical but not the perceptual nature of the spatial cue-did not eliminate cortical sensitivity to that cue. These results suggest that perceptual representations of auditory space emerge or are refined in higher-order AC regions, supporting the stable perception of auditory space in noisy or reverberant environments and forming the basis of illusions such as the Franssen effect.

The challenges of daily communication require listeners to integrate both independent and complementary auditory information to form holistic auditory scenes. As part of this process listeners are thought to “fill in” missing information to create continuous perceptual streams, even when parts of messages are masked or obscured. One example of this filling-in process – the auditory continuity illusion – has been studied primarily using stimuli presented in isolation, leaving it unclear whether the illusion occurs in more complex situations with higher perceptual and attentional demands. In this study, young normal-hearing participants listened for long target tones, either real or illusory, in “clouds” of shorter masking tone and noise bursts with pseudo-random spectro-temporal locations. Patterns of detection suggest that illusory targets are salient within mixtures, although they do not produce the same level of performance as the real targets. The results suggest that the continuity illusion occurs in the presence of competing sounds, and can be used to aid in the detection of partially obscured objects within complex auditory scenes.

According to the rhythm rule in metrical phonology, a word's stress alternates from the second syllable to the first when followed by a word with first syllable stress, a situation also known as a stress clash. For example, the second-syllable stressed word sixTEEN will be produced as SIXteen in the phrase SIXteen CANdles. Using pseudo-words in different rhythmic contexts, we demonstrate that stress clash resolution has a strong perceptual component and can be an auditory illusion. In Experiment 1, participants were more likely to misattribute stress in a clash condition (sixTEEN heard as SIXteen in sixTEEN CANdles) than in isolation (sixTEEN) or in a non-clash condition (sixTEEN caNALS). In Experiment 2, we failed to find support for the hypothesis that nuclear stress drove these ratings. We discuss these findings in light of various theories and mechanisms for parsing spontaneous speech.

Can what we imagine hearing change what we see? Whether imagined sensory stimuli are integrated with external sensory stimuli to shape our perception of the world has only recently begun to come under scrutiny. Here, we made use of the cross-bounce illusion in which an auditory stimulus presented at the moment two passing objects meet promotes the perception that the objects bounce off rather than cross by one another to examine whether the content of imagined sound changes visual motion perception in a manner that is consistent with multisensory integration. The results from this study revealed that auditory imagery of a sound with acoustic properties typical of a collision (i.e., damped sound) promoted the bounce-percept, but auditory imagery of the same sound played backwards (i.e., ramped sound) did not. Moreover, the vividness of the participants’ auditory imagery predicted the strength of this imagery-induced illusion. In a separate experiment, we ruled out the possibility that changes in attention (i.e., sensitivity index d′) or response bias (response bias index c) were sufficient to explain this effect. Together, these findings suggest that this imagery-induced multisensory illusion reflects the successful integration of real and imagined cross-modal sensory stimuli, and more generally, that what we imagine hearing can change what we see.

Human brain contains of approximately 100 billion neurons. Each neuron communicates with few ten thousands of other neurons in order to carry messages in the brain. Significant electrical activity is produced in the brain over synaptic joints of such neurons sending signals at very low frequencies below (50 Hz), thereby forming the brainwave pattern. The brainwaves are categorized as delta, theta, alpha and beta, as per different frequency ranges. In this paper, the effect of binaural beats on human mind is presented. Alpha binaural beats of 10 Hz are produced by creating the auditory illusion of 10 Hz in the brain by playing the binaural beats of 370 Hz and 380 Hz for left and right ear respectively. Binaural beats are effective only when heard through an earphone. In order to examine the effects of binaural beats on human brain, 10 people are subjected to these beats for 3 minutes. Using Matlab, the attention level and meditation levels are measured by alpha brainwaves, and the comparison graphs are plotted. Relative comparison is carried out for each persons state while listening to the binaural beats. This study provides scientific evidences for the common perception that alpha binaural beats and thus music can help a person in achieving a relaxed state of mind i.e., meditative state in a better way.

In a complex acoustic environment, acoustic cues and attention interact in the formation of streams within the auditory scene. In this study, a variant of the “octave illusion” [Deutsch (1974). Nature 251, 307–309] was used to investigate the neural correlates of auditory streaming, and to elucidate the effects of attention on the interaction between sequential and concurrent sound segregation in humans. By directing subjects' attention to different frequencies and ears, it was possible to elicit several different illusory percepts with the identical stimulus. The first experiment tested the hypothesis that the illusion depends on the ability of listeners to perceptually stream the target tones from within the alternating sound sequences. In the second experiment, concurrent psychophysical measures and electroencephalography recordings provided neural correlates of the various percepts elicited by the multistable stimulus. The results show that the perception and neural correlates of the auditory illusion can be manipulated robustly by attentional focus and that the illusion is constrained in much the same way as auditory stream segregation, suggesting common underlying mechanisms.

Background: There is evidence that experimentally elicited auditory illusions in the general population index risk for psychotic symptoms. As little is known about underlying cortical mechanisms of auditory illusions, an experiment was conducted to analyze processing of auditory illusions in a general population sample. In a follow-up design with two measurement moments (baseline and 6 months), participants (n = 83) underwent the White Noise task under simultaneous recording with a 14-lead EEG. An auditory illusion was defined as hearing any speech in a sound fragment containing white noise. Results: A total number of 256 speech illusions (SI) were observed over the two measurements, with a high degree of stability of SI over time. There were 7 main effects of speech illusion on the EEG alpha band-the most significant indicating a decrease in activity at T3 (t = -4.05). Other EEG frequency bands (slow beta, fast beta, gamma, delta, theta) showed no significant associations with SI. Conclusion: SIs are characterized by reduced alpha activity in non-clinical populations. Given the association of SIs with psychosis, follow-up research is required to examine the possibility of reduced alpha activity mediating SIs in high risk and symptomatic populations.

This study investigates the neural correlates and processes underlying the ambiguous percept produced by a stimulus similar to Deutsch's ‘octave illusion’, in which each ear is presented with a sequence of alternating pure tones of low and high frequencies. The same sequence is presented to each ear, but in opposite phase, such that the left and right ears receive a high–low–high … and a low–high–low … pattern, respectively. Listeners generally report hearing the illusion of an alternating pattern of low and high tones, with all the low tones lateralized to one side and all the high tones lateralized to the other side. The current explanation of the illusion is that it reflects an illusory feature conjunction of pitch and perceived location. Using psychophysics and electroencephalogram measures, we test this and an alternative hypothesis involving synchronous and sequential stream segregation, and investigate potential neural correlates of the illusion. We find that the illusion of alternating tones arises from the synchronous tone pairs across ears rather than sequential tones in one ear, suggesting that the illusion involves a misattribution of time across perceptual streams, rather than a misattribution of location within a stream. The results provide new insights into the mechanisms of binaural streaming and synchronous sound segregation. This article is part of the themed issue ‘Auditory and visual scene analysis’.

The endless scale illusion, obtained by cyclically repeating a chromatic scale made up of Shepard tones, has been used in a variety of musical works. Music psychology and neuroscience has been interested in this particular psychoacoustic phenomenon mainly for studying the cognitive processes of pitch perception involved. In the present study, we investigated the emotional states induced by the Shepard-Risset glissando, a variant of the Shepard scale. For this purpose we chose three musical stimuli: a Matlab-generated Shepard Risset glissando, Jean-Claude Risset's Computer Suite from Little Boy, which presents a Shepard-Risset glissando integrated in the aesthetic context of a composition, and an ordinary orchestral glissando taken from the opening of Iannis Xenakis's Metastasis. Seventy-three volunteers completed a listening experiment during which they rated their emotional response to these stimuli on a seven-point Likert scale and indicated whether they had experienced a disruption of equilibrium. Personality was also measured with the Five-Factor Model of personality traits. The results show that negative emotions were most strongly evoked during listening to each of the stimuli. We also found that the Shepard-Risset glissando illusion, both within the aesthetic context of a musical composition and on its own, was capable of evoking disruption of equilibrium, frequently leading to the associated feeling of falling. Moreover, generally for the Shepard-Risset glissando illusion, higher negative emotional ratings were given by individuals who had experienced a feeling of disturbance of equilibrium relative to those who had not had this experience. Finally, we found a complex pattern of relationships between personality and the subjective experience of the glissando. Openness to experience correlated positively with positive emotion ratings for the Computer Suite, while agreeableness correlated negatively with positive emotion ratings for the Matlab stimulus. Moreover, results indicated higher (Bonferroni-uncorrected) neuroticism for those who experienced an equilibrium disturbance relative to subjects who did not have this experience during listening to the Computer Suite. These findings suggest that musical paradoxes may be of interest not only for the insights they provide on our perceptual system, but also for the richness of the emotional experience elicited during listening.

We used a dynamic auditory spatial illusion to investigate the role of self-motion and acoustics in shaping our spatial percept of the environment. Using motion capture, we smoothly moved a sound source around listeners as a function of their own head movements. A lowpass filtered sound behind a listener that moved in the direction it would have moved if it had been located in the front was perceived as statically located in front. The contrariwise effect occurred if the sound was in front but moved as if it were behind. The illusion was strongest for sounds lowpass filtered at 500 Hz and weakened as a function of increasing lowpass cutoff frequency. The signals with the most high frequency energy were often associated with an unstable location percept that flickered from front to back as self-motion cues and spectral cues for location came into conflict with one another.

The octave or Deutsch illusion occurs when two tones, separated by about one octave, are presented simultaneously but alternating between ears, such that when the low tone is presented to the left ear the high tone is presented to the right ear and vice versa. Most subjects hear a single tone that alternates both between ears and in pitch; i.e., they hear a low pitched tone in one ear alternating with a high pitched tone in the other ear. The present study examined whether the illusion can be elicited by aperiodic signals consisting of low-frequency band-pass filtered noises with overlapping spectra. The amount of spectral overlap was held constant, but the high- and low-frequency content of the signals was systematically varied. The majority of subjects perceived an auditory illusion in terms of a dominant ear for pitch and lateralization by frequency, as proposed by Deutsch [(1975a) Sci. Am. 233, 92-104]. Furthermore, the salience of the illusion increased as the high frequency of the content in the signal increased. Since no harmonics were present in the stimuli, it is highly unlikely that this illusion is perceived on the basis of binaural diplacusis or harmonic binaural fusion.

An illusion is explored in which a spoken phrase is perceptually transformed to sound like song rather than speech, simply by repeating it several times over. In experiment I, subjects listened to ten presentations of the phrase and judged how it sounded on a five-point scale with endpoints marked "exactly like speech" and "exactly like singing." The initial and final presentations of the phrase were identical. When the intervening presentations were also identical, judgments moved solidly from speech to song. However, this did not occur when the intervening phrases were transposed slightly or when the syllables were presented in jumbled orderings. In experiment II, the phrase was presented either once or ten times, and subjects repeated it back as they finally heard it. Following one presentation, the subjects repeated the phrase back as speech; however, following ten presentations they repeated it back as song. The pitch values of the subjects' renditions following ten presentations were closer to those of the original spoken phrase than were the pitch values following a single presentation. Furthermore, the renditions following ten presentations were even closer to a hypothesized representation in terms of a simple tonal melody than they were to the original spoken phrase.

Audition is accepted as more reliable (thus dominant) than vision when temporal discrimination is required by the task. However, it is not known whether the characteristics of the visual stimulus, for example its familiarity to the perceiver, affect auditory dominance. In this study we manipulated familiarity of the visual stimulus in a well-established multisensory phenomenon, i.e., the sound-induced flash illusion. This illusion occurs when, for example, one brief visual stimulus (e.g., a flash) is presented in close temporal proximity with two brief sounds; participants perceive two flashes instead of one. We found that when the visual stimuli (faces or buildings) were familiar, participants were less susceptible to the illusion than when they were unfamiliar. As the illusion has been ascribed to early cross-sensory interactions between vision and audition, the present work offers behavioural evidence that high level processing of objects' characteristics such as familiarity, affects early temporal multisensory integration. Possible mechanisms underlying the effect of familiarity are discussed.

Leboe and Mondor (2008) demonstrated that participants will apply a change heuristic when making duration judgments. In this study we investigated whether participants would apply this same change heuristic when making judgments about the perceived intensity of a sound. In two experiments, participants were presented with two consecutive sounds on each of a series of trials and their task was to judge whether the second sound was louder or quieter than the first. In Experiment 1, participants were more likely to judge sounds that increased in frequency as louder in intensity than sounds that maintained a constant frequency. In Experiment 2, participants were more likely to judge sounds that either increased or decreased in frequency as louder in intensity than sounds that maintained a constant frequency. We interpret these results as evidence that reliance on a change heuristic leads to the illusion of increased intensity.

In the Speech-to-Song Illusion, repetition of a spoken phrase results in it being perceived as if it were sung. Although a number of previous studies have examined which characteristics of the stimulus will produce the illusion, there is, until now, no description of the cognitive mechanism that underlies the illusion. We suggest that the processes found in Node Structure Theory that are used to explain normal language processing as well as other auditory illusions might also account for the Speech-to-Song Illusion. In six experiments we tested whether the satiation of lexical nodes, but continued priming of syllable nodes may lead to the Speech-to-Song Illusion. The results of these experiments provide evidence for the role of priming, activation, and satiation as described in Node Structure Theory as an explanation of the Speech-to-Song Illusion.

In music that is perceived as metrically structured, events coinciding with the main beat are called metrically accented. Are these accents purely cognitive, or do they perhaps represent illusory increases in perceived loudness or duration, caused by heightened attention to main beats? In four separate tasks, musicians tried to detect a small actual increase or decrease in the loudness or duration of a single note in melodies comprising 12 notes. Musical notation prescribed a meter (6/8) implying a main beat coinciding with every third note. Effects of metrical accentuation on detection performance were found in all four tasks. However, they reflected primarily an increase in sensitivity to physical changes in main beat positions, likely to be due to enhanced attention. There was no evidence of biases indicating illusory phenomenal accents in those positions. By contrast, and independent of metrical structure, pitch accents due to pitch contour pivots were often mistaken for increases in loudness.

The human auditory system perceptually restores short deleted segments of speech and other sounds (e.g. tones) when the resulting silent gaps are filled by a potential masking noise. When this phenomenon, known as ‘auditory induction’, occurs, listeners experience the illusion of hearing an ongoing sound continuing through the interrupting noise even though the perceived sound is not physically present. Such illusions suggest that a key function of the auditory system is to allow listeners to perceive complete auditory objects with incomplete acoustic information, as may often be the case in multisource acoustic environments. At present, however, we know little about the possible functions of auditory induction in the sound-mediated behaviours of animals. The present study used two-choice phonotaxis experiments to test the hypothesis that female grey treefrogs, Hyla chrysoscelis, experience the illusory perceptual restoration of discrete pulses in the male advertisement call when pulses are deleted and replaced by a potential masking noise. While added noise restored some attractiveness to calls with missing pulses, there was little evidence to suggest that the frogs actually experienced the illusion of perceiving the missing pulses. Instead, the added noise appeared to function as an acoustic appendage that made some calls more attractive than others as a result of sensory biases, the expression of which depended on the temporal order and acoustic structure of the added appendages.

It is argued that speech perception, just like visual perception, relies on a good match between memorized experience and current sensation: when sensation meshes with expectations, listeners believe they perceive ‘real’ linguistic objects in spite of possibly severe variation and degradation in the acoustic signal. Reviews of the acoustic and perceptual correlates of the features [nasal] and [voice] in various speech styles illustrate how multiple perceptual cues to a simple phonological distinction may be dispersed across syllables, and how absence of one or several such cues may be compensated by the presence of others, or by recovery processes that rely on listeners’ knowledge and expectations. Visual illusions are discussed which have apparent parallels with auditory illusions and with well-known aspects of speech perception. These include particular types of physical structure (e.g. abrupt changes, edges), enhancement of properties of a given object by juxtaposed information which either changes the percept of those properties, or else provides a context that changes the percept of what the object is, and influences of familiarity and probability which can be profound enough to fly in the face of contrary sensory evidence. These data are used to support the hypothesis that perceived linguistic units, including distinctive features, are ephemeral (and illusory) ‘auditory objects’, which are created by the listening brain using domain-general processes that underpin meaningful behaviour.

Objective: To define brain activity corresponding to an auditory illusion of 3 and 6 Hz binaural beats in 250 Hz or 1000 Hz base frequencies, and compare it to the sound onset response. Methods: Event-Related Potentials (ERPs) were recorded in response to unmodulated tones of 250 or 1000 Hz to one ear and 3 or 6 Hz higher to the other, creating an illusion of amplitude modulations (beats) of 3 Hz and 6 Hz, in base frequencies of 250 Hz and 1000 Hz. Tones were 2000 ms in duration and presented with approximately 1 s intervals. Latency, amplitude and source current density estimates of ERP components to tone onset and subsequent beats-evoked oscillations were determined and compared across beat frequencies with both base frequencies. Results: All stimuli evoked tone-onset P50, N100 and P200 components followed by oscillations corresponding to the beat frequency, and a subsequent tone-offset complex. Beats-evoked oscillations were higher in amplitude with the low base frequency and to the low beat frequency. Sources of the beats-evoked oscillations across all stimulus conditions located mostly to left lateral and inferior temporal lobe areas in all stimulus conditions. Onset-evoked components were not different across stimulus conditions; P50 had significantly different sources than the beats-evoked oscillations; and N100 and P200 sources located to the same temporal lobe regions as beats-evoked oscillations, but were bilateral and also included frontal and parietal contributions. Conclusions: Neural activity with slightly different volley frequencies from left and right ear converges and interacts in the central auditory brainstem pathways to generate beats of neural activity to modulate activities in the left temporal lobe, giving rise to the illusion of binaural beats. Cortical potentials recorded to binaural beats are distinct from onset responses. Significance: Brain activity corresponding to an auditory illusion of low frequency beats can be recorded from the scalp. © 2009 International Federation of Clinical Neurophysiology.

The human auditory system is efficient at restoring sounds of interest. In noisy environments, for example, an interrupted target sound may be illusorily heard as continuing smoothly when a loud noise masks the interruptions. In quiet environments, however, sudden interruptions might signal important events. In that case, restoration of the target sound would be disadvantageous. Achieving useful perceptual stability may require the restoration mechanism to adapt its output to current perceptual demands, a hypothesis which has not yet been fully evaluated. In this study, we investigated whether auditory restoration depends on preceding auditory scenes, and we report evidence that restoration adapts to the perceived continuity of target sounds and to the loudness of interrupting sounds. In the first experiment, listeners adapted to illusory and non-illusory tone sweeps (targets) and interrupting noise, and we observed that the perceived continuity of the target and the loudness of the interrupting noise influenced the extent of subsequent restorations. A second experiment revealed that these adaptation effects were unrelated to the adapted spectra, indicating that non-sensory representations of the perceived auditory scene were involved. We argue that auditory restoration is a dynamic illusory phenomenon which recalibrates continuity hearing to different acoustic environments.

The development of music psychology in Britain over the last few decades is outlined from the personal viewpoint of the author, who was actively involved in research and teaching in the field. The important ideas and influential researchers of the period are described, together with their influence on the author's own research interests. More recent developments are also reviewed showing the great broadening of research activity in the whole subject area. I came into Psychology from a scientific background at a time of rising interest in the questions of perception and cognition. Although a keen amateur musician, I decided to specialize in visual perception, inspired by Richard Gregory (no relation), Fergus Campbell, and Horace Barlow at the University of Cambridge. At that time, in the 1960s, psychological studies on vision still had a valuable role to play in advancing knowledge. However, gradually advances in physiological research seemed to be explaining more and more of the phenomena of visual perception, and there remained much less scope for purely psychological studies. I became interested in another developing field at that time, the idea of cerebral dominance. For normal right-handed people the left cerebral hemisphere seemed to control most speech functions, whilst the right hemisphere seemed more associated with spatial processing. Kimura (1964), using the technique of dichotic listening, had demonstrated that the perception of melody was probably also a right hemisphere function. The idea of this dichotomy, language in the left hemisphere, music in the right, first aroused my interest in music psychology. What about the perception of rhythm? This occurs in both music and in language but is probably more fundamental to music. With a graduate student, Lorna Roberts, we asked listeners to adjust the precise timing of brief auditory stimuli presented alternately to the two ears, and concluded that it was the right hemisphere that was involved in rhythm perception (Gregory, Harriman, & Roberts, 1972). A further study using the technique of delayed auditory feedback on rhythmic tapping again suggested right hemisphere involvement (Roberts & Gregory, 1973). At this time the techniques for presenting precisely controlled auditory stimuli required considerable ingenuity. The experiment requiring regular brief pulses of sound used an audiometer controlled by external logic circuits, and delayed auditory feedback was produced by the delay between the record and playback heads of a tape-recorder. My developing interest in this field led to a study of Deutsch 's (1974) auditory illusion. She reported that alternating 2-note chords, presented with one note to each ear, were usually perceived by righthanded listeners as a sequence of high tones to the right ear and low tones to the left ear. With a colleague, Ian Christensen, we studied this with harmonic and dissonant chords, and also varying the relative intensity of the notes (Christensen & Gregory, 1977). For harmonic chords only a single tone was heard, but where it was heard depended critically on which ear received the louder note. For dissonant chords the two individual notes were more often heard. This supported an alternative study by Efron and Yund (1974), which demonstrated that individuals differ in their ear dominance for pitch perception. I spent a sabbatical year in California working with Robert Efron, William Yund, and also Pierre Divenyi, a talented musician working on language. This improved my knowledge of both the theoretical bases of auditory and musical perception, and of the use of computers to control auditory and musical stimuli. My research during this year also led to studies showing that there was no clear relationship between ear dominance and handedness (Gregory, 1982), and that ear dominance was not simply explained by either attentional bias or by hemispheric specialization in the processing of tonal stimuli (Gregory, Efron, Divenyi, & Yund, 1983). …

The "Deutsch's illusion" occurs in most people when a dichotic pair of tones spaced an octave apart is presented repeatedly in alternation, so that when the right ear receives the high tone, the left ear receives the low tone, and vice versa. The illusory percept consists typically in a single low tone heard at one ear alternating with a single high tone heard at the other ear. Here, we investigate whether the frequency interval between the tones and their duration play a role in the perception of the illusion. By testing 74 subjects we demonstrate that the illusion is not confined to tones spaced an octave apart but it is perceivable also with tones separated by a major seventh, a minor ninth, a major ninth, and a minor tenth. Regarding duration, the present results show that the illusion is stronger with tones lasting 500 than 200 ms. The present results suggest that the perceptual mechanisms at the basis of the illusion are not strictly linked to the frequency relationships between the dichotic tones.

Consciousness takes two forms, transitive and intransitive. Transitive consciousness is a matter of being conscious of something or other whereas intransitive consciousness has no object, as being conscious or awake. Of the different forms of transitive consciousness, perceptual, somatic, kinaesthetic and so on, cognitive neuroscience has concentrated on determining the neural concomitants of perceptual consciousness. To be conscious of a percept is to be aware of it and this requires attending to it. This work sets out a hypothesis as to what brain areas are involved in a schizophrenia subject attending and becoming aware of hallucinations. First, the different areas of cortex that support different visual and auditory illusions of percepts are considered. Next it is argued that endogenous activity in these areas of cortex give rise to hallucinations of percepts that are similar to the percepts that these same areas support during illusions. The basis of such endogenous activity, it is suggested, is to be found in the paucity of afferent synapses to these cortical areas. This may occur as a consequence of loss and regression of synapses due to a degenerative disease or because of abnormal synapse formation and regression during childhood and adolescence, as is likely to be the case in schizophrenia. Finally the neural basis of attention and awareness of these hallucinations are considered for subjects suffering from schizophrenia, and a set of important questions posed that await elucidation through future experimental studies.

In the introduction the importance of illusions is illustrated as being a peephole through which the processes of sensation and perception can be watched. Three illusions will exemplify this, one for the visual modality (the goose orientation illusion), one for the tactile modality (Aristotle's illusion), and one for the auditory modality (streaming illusion). Subsequently, three “classic” time illusions are discussed: 1 - illusion of a divided time interval, 2 - subjective rhythmization, and 3 - the kappa effect. These latter three illusions are dealt with in view of more recent studies.

Auditory scene analysis is essential in daily life to extract necessary information from complex acoustic environment and also from intricate development of music compositions. Auditory illusions and ambiguity are important factors in auditory scene analysis and have been studied extensively. We here report a novel form of ambiguity involving two illusory melodies implied by a very simple stimulus consisting of two sustained tones of different frequencies and an intermittently repeated tone of a frequency between the sustained tones. The measured time elapsed before a first perception change showed that illusion, ambiguity and disambiguation actually took place. We anticipate that the proposed illusion and ambiguity will provide a well-controlled approach for behavioral and neurophysiological studies of music recognition because of the simplicity of stimulus.

The dream of perfect re-creation of sound has always consisted of the reproduction of sounds such that they seem to be exact copies of an original signal and the plausible re-creation of complex sound environments, the possibility to be immersed in sound. The goal seems to be challenging, especially if reproduction via headphones is considered. In spite of an adequate technical realization the creation of a perfect auditory illusion is not predictable in a reliable way. Contemporary investigations show clearly that in addition to the static, source and listener dependent modifications to headphone sound, attention must be paid to cognitive effects: The perceived presence of a room rendering changes depending on the expectations of a listener. Prominent effects are acoustic divergence between the listening room and the synthesized scene, visibility of the room, and prior knowledge triggered by the location where the listener has been before. Cognitive effects are mostly time variant which includes anticipation and assimilation processes caused by training and adaptation. The contribution presents experiments showing some of these contextual effects. Quality features are shifted by adaptation and training. Furthermore, this contribution presents some proposals on how to get to the next level of fidelity in headphone listening.

Contains fulltext : 159633.pdf (Publisher’s version ) (Open Access) Visual information can alter auditory perception. This is clearly illustrated by the well-known McGurk illusion, where an auditory/aba/ and a visual /aga/ are merged to the percept of 'ada'. It is less clear however whether such a change in perception may recalibrate subsequent perception. Here we asked whether the altered auditory perception due to the McGurk illusion affects subsequent auditory perception, i.e. whether this process of fusion may cause a recalibration of the auditory boundaries between phonemes. Participants categorized auditory and audiovisual speech stimuli as /aba/, /ada/ or /aga/ while activity patterns in their auditory cortices were recorded using fMRI. Interestingly, following a McGurk illusion, an auditory /aba/ was more often misperceived as 'ada'. Furthermore, we observed a neural counterpart of this recalibration in the early auditory cortex. When the auditory input /aba/ was perceived as 'ada', activity patterns bore stronger resemblance to activity patterns elicited by /ada/ sounds than when they were correctly perceived as /aba/. Our results suggest that upon experiencing the McGurk illusion, the brain shifts the neural representation of an /aba/ sound towards /ada/, culminating in a recalibration in perception of subsequent auditory input. 7 p.

Shepard's tones are a typical example for auditory illusion. They consist in a series of computer generated tones, which prohibit relative pitch discrimination. As a result, when repetitively played in sequence, the illusion of an ever-ascending scale is evoked. In order to investigate this aural phenomenon, fMRI time series were acquired during presentation of a conventional block-designed paradigm as well as during continuous presentation of Shepard's tones. With respect to the different setups of the two experiments, two fundamentally different methods were applied in order to conduct data analysis. Common Statistical Parameter Mapping served to evaluate the time series obtained with the block-designed paradigm. For the continuous experiment, a novel wavelet-based multifractal analysis was used, recently proposed as a classification tool for fMRI time series. This approach applies the wavelet transform to extract multifractal spectra from time-signals. For reasons of quantification, we introduced an ameliorated method for visual inspection of the multifractal properties. The results proved existence of characteristic neural responses to continuously presented Shepard's tones. Interestingly, the same was not restricted to the auditory cortex, but also involved areas of the visual cortex. Related impact on the imaged cognitive areas, primary motor cortex, and primary sensory cortex could not be observed. We further provide evidence that pitch misjudgment does not occur in temporal concurrence with the repetition of the whole scale, but according to whether the main perceived frequency is located in the sensitive range of auditory perception or not. We remark that this is the first time, continuously stimulated brain areas could be detected by means of fMRI.

When a long glide with a short temporal gap in its middle crosses with a continuous short glide at the temporal midpoint of both glides, the gap is perceived in the short glide instead of in the long glide. In the present article, we tested possible explanations for this “gap transfer illusion” by obtaining points of subjective equality of the pitches and durations of the two short tones that are subjectively divided by the gap. The results of two experiments showed that neither an explanation in terms of envelope patterns nor explanations in terms of combination tones or acoustic beats could account for the perception of the short tones in the gap transfer illusion. Rather, the results were compatible with the idea that the illusory tones were formed by the perceptual integration of onsets and offsets of acoustically different sounds. Implications for the perceptual construction of auditory events are discussed.

This article reports the first study of the glissando illusion, which was created and published as a sound demonstration by Deutsch [Deutsch, D. (1995). Musical illusions and paradoxes. La Jolla: Philomel Records (compact disc)]. To experience the illusion, each subject was seated in front of two stereophonically separated loudspeakers, with one to his left and the other to his right. A sound pattern was presented that consisted of a synthesized oboe tone of constant pitch, together with a sine wave whose pitch repeatedly glided up and down (the glissando). These two components alternated continuously between the loudspeakers such that when the oboe tone emanated from the loudspeaker on the left, the glissando emanated from the loudspeaker on the right; and vice versa. The oboe tone was perceived correctly as switching between loudspeakers; however, the segments of the glissando appeared to be joined together seamlessly, such that a single, continuous tone was heard, which appeared to be moving slowly around in space in accordance with its pitch motion. Right-handers (n = 22) tended strongly to hear the glissando move between left and right, and also between low and high in space, as its pitch moved between low and high. More specifically, it was frequently heard as tracing an elliptical path aligned diagonally between a position low and to the left when its pitch was lowest, and high and to the right when its pitch was highest. Non-right-handers (n = 42) perceived the illusion in statistically different ways. The handedness correlates and other implications of the glissando illusion are discussed. © 2007 Elsevier Ltd. All rights reserved.

Many natural stimuli have perceptual ambiguities that can be cognitively resolved by the surrounding context. In audition, preceding context can bias the perception of speech and non-speech stimuli. Here, we develop a neuronal network model that can account for how context affects the perception of pitch change between a pair of successive complex tones. We focus especially on an ambiguous comparison-listeners experience opposite percepts (either ascending or descending) for an ambiguous tone pair depending on the spectral location of preceding context tones. We developed a recurrent, firing-rate network model, which detects frequency-change-direction of successively played stimuli and successfully accounts for the context-dependent perception demonstrated in behavioral experiments. The model consists of two tonotopically organized, excitatory populations, E up and E down, that respond preferentially to ascending or descending stimuli in pitch, respectively. These preferences are generated by an inhibitory population that provides inhibition asymmetric in frequency to the two populations; context dependence arises from slow facilitation of inhibition. We show that contextual influence depends on the spectral distribution of preceding tones and the tuning width of inhibitory neurons. Further, we demonstrate, using phase-space analysis, how the facilitated inhibition from previous stimuli and the waning inhibition from the just-preceding tone shape the competition between the E up and E down populations. In sum, our model accounts for contextual influences on the pitch change perception of an ambiguous tone pair by introducing a novel decoding strategy based on direction-selective units. The model's network architecture and slow facilitating inhibition emerge as predictions of neuronal mechanisms for these perceptual dynamics. Since the model structure does not depend on the specific stimuli, we show that it generalizes to other contextual effects and stimulus types.

In this work we report an illusion of proximity of a sound source created by a sonic crystal placed between the source and a listener. This effect seems, at first, paradoxical to naïve listeners since the sonic crystal is an obstacle formed by almost densely packed cylindrical scatterers. Even when the singular acoustical properties of these periodic composite materials have been studied extensively (including band gaps, deaf bands, negative refraction, and birrefringence), the possible perceptual effects remain unexplored. The illusion reported here is studied through acoustical measurements and a psychophysical experiment. The results of the acoustical measurements showed that, for a certain frequency range and region in space where the focusing phenomenon takes place, the sonic crystal induces substantial increases in binaural intensity, direct-to-reverberant energy ratio and interaural cross-correlation values, all cues involved in the auditory perception of distance. Consistently, the results of the psychophysical experiment revealed that the presence of the sonic crystal between the sound source and the listener produces a significant reduction of the perceived relative distance to the sound source. Fil: Spiousas, Ignacio. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnología. Laboratorio de Acústica y Percepción Sonora; Argentina Fil: Etchemendy, Pablo Esteban. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnología. Laboratorio de Acústica y Percepción Sonora; Argentina Fil: Vergara, Ramiro Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnología. Laboratorio de Acústica y Percepción Sonora; Argentina Fil: Calcagno, Esteban. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnología. Laboratorio de Acústica y Percepción Sonora; Argentina Fil: Eguia, Manuel Camilo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Quilmes. Departamento de Ciencia y Tecnología. Laboratorio de Acústica y Percepción Sonora; Argentina

The presentation of two sinusoidal tones, one to each ear, with a slight frequency mismatch yields an auditory illusion of a beating frequency equal to the frequency difference between the two tones; this is known as binaural beat (BB). The effect of brief BB stimulation on scalp EEG is not conclusively demonstrated. Further, no studies have examined the impact of musical training associated with BB stimulation, yet musicians' brains are often associated with enhanced auditory processing. In this study, we analysed EEG brain responses from two groups, musicians and non-musicians, when stimulated by short presentation (1 min) of binaural beats with beat frequency varying from 1 Hz to 48 Hz. We focused our analysis on alpha and gamma band EEG signals, and they were analysed in terms of spectral power, and functional connectivity as measured by two phase synchrony based measures, phase locking value and phase lag index. Finally, these measures were used to characterize the degree of centrality, segregation and integration of the functional brain network. We found that beat frequencies belonging to alpha band produced the most significant steady-state responses across groups. Further, processing of low frequency (delta, theta, alpha) binaural beats had significant impact on cortical network patterns in the alpha band oscillations. Altogether these results provide a neurophysiological account of cortical responses to BB stimulation at varying frequencies, and demonstrate a modulation of cortico-cortical connectivity in musicians' brains, and further suggest a kind of neuronal entrainment of a linear and nonlinear relationship to the beating frequencies.

“Out of the body” tactile illusion refers to the phenomenon in which one can perceive tactility as if emanating from a location external to the body without any stimulator present there. Taking advantage of such a tactile illusion is one way to provide and realize richer interaction feedback without employing and placing actuators directly at all stimulation target points. However, to further explore its potential, it is important to better understand the underlying physiological and neural mechanism. As such, we measured the brain wave patterns during such tactile illusion and mapped out the corresponding brain activation areas. Participants were given stimulations at different levels with the intention to create veridical (i.e., non-illusory) and phantom sensations at different locations along an external hand-held virtual ruler. The experimental data and analysis indicate that both veridical and illusory sensations involve, among others, the parietal lobe, one of the most important components in the tactile information pathway. In addition, we found that as for the illusory sensation, there is an additional processing resulting in the delay for the ERP (event-related potential) and involvement by the limbic lobe. These point to regarding illusion as a memory and recognition task as a possible explanation. The present study demonstrated some basic understanding, how humans process “virtual” objects and the way associated tactile illusion is generated will be valuable for HCI (Human-Computer Interaction).

Speech and song are readily differentiated from each other in everyday communication, yet sometimes listeners who have formal music training will hear a spoken utterance transform from speech to song when it is repeated (Deutsch, Henthorn, & Lapidis, 2011). It remains unclear whether music training is required to perceive this illusory transformation or whether implicit knowledge of musical structure is sufficient. The current study replicates Deutsch et al.'s findings with musicians and demonstrates the generalizability of this auditory illusion to casual music listeners with no formal training. We confirm that the illusory transformation is disrupted when the pitch height of each repetition of the utterance is transposed, and we find that raising the pitch height has a different effect on listeners' ratings than does lowering it. Auditory illusions such as this may offer unique opportunities to compare domain-specific and domain-general processing in the brain while holding acoustic characteristics constant.

Although vision is considered the dominant modality, recent studies demonstrate the influence of other modalities on visual perception. For example, in the sound-induced flash illusion, two auditory stimuli cause one visual flash to be perceived as two. We report an extension of the sound-induced flash illusion to the tactile-visual domain, yielding the touch-induced flash illusion. Observers reported seeing two flashes on the majority of trials when a single flash was presented concurrently with two task-irrelevant brief tactile stimuli. Somatosensory stimulation changed the sensitivity (d') of detecting visual stimuli, which suggests that the observed effect is at least partly due to perceptual interactions. Together with other recent findings, these results challenge the notion that the processing of visual information is independent of activity in other modalities.

In four experiments, we investigated the influence of timbre on perceived interval size. In Experiment 1, musically untrained participants heard two successive tones and rated the pitch distance between them. Tones were separated by six or seven semitones and varied in timbre. Pitch changes were accompanied by a congruent timbre change (e.g., ascending interval involving a shift from a dull to a bright timbre), an incongruent timbre change (e.g., ascending interval involving a shift from a bright to a dull timbre), or no timbre change. Ratings of interval size were strongly influenced by timbre. The six-semitone interval with a congruent timbre change was perceived to be larger than the seven-semitone interval with an incongruent timbre change (interval illusion). Experiment 2 revealed similar effects for musically trained participants. In Experiment 3, participants compared the size of two intervals presented one after the other. Effects of timbre were again observed, including evidence of an interval illusion. Experiment 4 confirmed that timbre manipulations did not distort the perceived pitch of tones. Changes in timbre can expand or contract the perceived size of intervals without distorting individual pitches. We discuss processes underlying interval size perception and their relation to pitch perception mechanisms.