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Chapter 7

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PSYC 212
Evan Balaban

PSYC 212 | Chapter 7 – The Auditory System: Music and Speech Perception A. Music Perception Musical instruments do produce complex tones made up of a fundamental frequency and a series of harmonics. - Instruments have a number of different preferred states of vibration (resonance). When played, all of the resonant vibrational modes are activated at once, with each producing its own characteristic frequency  this is what produces a complex tone Musical pitch: the fact that the pitch of a pure tone of a certain frequency would be perceived to be the same as a complex tone containing the same fundamental frequency and harmonics (under normal conditions only) A1. Musical Pitch and Timbre If complex tones did not produce a single pitch, we would not have music perception Pitch sensations from complex tones Each harmonic component associated with middle C produces a travelling wave in the basilar membrane  its peak occurs where its frequency corresponds to the tonotopic frequency The harmonic components together make an orderly pattern of weak wave activity on the basilar membrane. Fundamental frequency is towards apex, while the harmonics (2f, 3f, …) is towards base Since the harmonics produce peak undulations according to tonotopic map  there is regulation in spatial arrangement  each harmonic representation has a fixed distance relationship to each other on the basilar membrane. The brain registers the spatial relationship - Neural activity triggered at each of the regions forms a coherent signal that is transformed into a pitch sensation o Spatial pattern of peak wave activity on basilar membrane forms the basis in producing a single pitch sensation by a complex musical tone o A different tone would produce another regular pattern on the basilar membrane, but the locations of wave activity are different depending on the fundamental frequency and harmonics of the tone. Musical scales – general concepts Octave: fundamental interval of musical pitch; 2:1 frequency ratio - To move one octave higher in pitch, fundamental frequency needs to be doubled - To move one octave lower, fundamental frequency needs to be halved - Effective range of musical pitch spans ~7 octaves o But human hearing frequency range spans 10 o Since tones with fundamental frequency >4500Hz are not for musical sounds (people have trouble assigning pitch values to them) Musical scales – chromatic intervals and tone height Musical tones are represented by alphabetic notational system: Repetition: musical tones one octave apart sound similar, but one is higher than the other Musical pitch within an octave forms a discrete transition - Any octave is divided into smaller intervals in a precise and dependant manner - Tone chroma: relative position of tones within an octave; represented circularly - Equally tempered scale: each octave is broken down into 12 divisions (semitones) which can be divided into 100 cents; solmization  do, re mi, … - Tone height: which octave pitch is in; depicted as a spiraling series of octaves Musical scales – staff notation Western staff notation: based on graphical signs that represent different musical pitch sensation, each being comprise of a certain fundamental frequency Perfect Pitch Relative pitch: ability to accurately identify tone intervals; relies on implicit use of standard pitch Absolute/perfect pitch: fast ability to name an isolated tone w/out implicit use of any reference Since we are more concerned with pitch transitions in music, actual pitch values are not processed when it reaches the brain. - We have trouble naming the pitch because the information is not directly relevant to the cognitive processes underlying music perception Debate if absolute pitch is genetic; studies show that is it more common in identical twins Tone quality – musical tone can be characterized by 3 attributes: pitch, loudness, and timbre Timbre: difference in tone quality that arises from subtle differences in neural activation each point of the basilar membrane due to different intensity distributions of harmonics - Different musical instrument = different intensity distribution among the harmonics for the same note o Each instrument favors certain harmonics over others 3 categories for musical instruments: chordophones, aerophones, percussions Mersenne’s laws: relationship between physical characteristics of a string & the tone produced 1/2  f = (1/2L)(T/p) Piano range of fundamentals: 27 to over 4000 Hz. A2. Perceptual Aspects of Music Musical perception relies on two qualities: - Tonal superpositions: different musical tones are sounded together in unison to produce a compound sound - Tonal Sequences: different (sometimes same) musical tones are sounded in rapid succession Tonal superpositions – monophonic versus polyphonic Polyphonic tones: superpositions of multiple tones; used in most Western music Auditory system can define the constituents of a chord or triad, despite its complex sound signal We can identify both tones in a doublet, but we “should” only sense 1 tone w/different timbre. When same note played on 2 different instruments: we correctly perceive two notes of the same pitch but different timbres - Due to timing of the two tones - Initiation/attack of two cones played by 2 different instruments is rarely synchronized - The very small timing asynchrony might help the brain to bind harmonic components of one musical tone to distinguish it from the other Tonal superpositions – consonance and dissonance – frequency content of music tones Consonance: when 2 or more notes are played simultaneously & generate a pleasant sensation - Associated with calm and stability - Occur when two or more tones contain harmonic frequencies that coincide and reinforce each other. - But overlapping harmonics can be consonant: if the harmonics of two tones are well separated and produce distinct activation patterns of the basilar membrane o Distinct and separable frequency pattern present among 2 or more tones played simultaneously produce consonance Dissonance: when 2 or more notes played simultaneously generate an unpleasant sensation - Associated with tension and harshness - Occur if the harmonic representation are too close, especially for lower harmonics o Results in interference between their activations in basilar membrane  Clashing Tonal sequences – melody, tempo, and rhythm Melody: group of notes with distinct and separable frequency pattern played in time - Melodic sequences harmonize w/emotional centers of brain to produce pleasure - Characterized by a precise set of pitch (frequency) changes as a function of time Tempo: perceived speed of tonal presentations in a melodic sequence - Can produce grouping effect Rhythm: perceptual organization of a melody intro subunits based on the temporal character of the sequence; occurs naturally Contour: nature of pitch changes with time. We are able to identify a melody regardless of a transposition to a different pitch range, as long it is applied equally to all of the notes Tonal sequences – cognitive theories Gestalt Theory: principles largely deduced on the basis of intuitive reasoning and observations gathered from simple experiments on visual perception 3 Gestalt principles applied to group musical sequences: Proximity: tones appearing close together in time will be grouped as a perceptual unit Similarity: tones with a similar pitch or timbre are perceptually bound together into the same group Common fate: tonal sequences whose pitch or intensity pattern change in similar ways at the same time tends to form a common unit A3. Music and the Brain Musical stimulation: - cyclically changing flux of neural signals that propagates through the auditory centers and triggers neural circuits that may have a periodic/cyclical response pattern - Rhythmic sound pattern of music resonates with some internal rhythm in the brain that leads to the activation of higher emotional centers Planuum temporale: near the auditory cortex - Enlarged in left hemisphere of professional musicians/people with perfect pitch - Processes speech sounds - Might be responsible for analytical tasks common to language and music perception, even though music perception is dominant in the right hemisphere Using fMRI: Musical stimulation produces much greater brain activity in the temporal lobe B. Speech Perception B1. Speech Production Language: encompasses any form of communication (non-verbal, hand communication, …) Articulation: process that uses the vocal apparatus  speech Speech is the only sensory stimulus that is actually produced by humans to form active sensory signal The process of vocalization Speech is produced by the vocal apparatus (subglottal system, larynx, vocal tract): Subglottal system: composed of trachea (windpipe), lungs, & muscles that expand and contract chest (move air in and out). Air moves from nasal/oral cavities through: Pharynx, larynx, trachea Glottis (vocal folds): as air goes down larynx it passes through the opening in the glottis - Made of two strips of ligaments controlled through muscles to change its size - Inhalation: remains open  air can go into lungs - Exhalation: rapid open/closing of glottis, interrupts airflow, causes vibrations (buzzing) - Vibrations caused by fluctuation of air pressure. The pressure of air in the lungs separates the vocal folds, then as air passes through, the folds close due to the drop in pressure. The vocal cords closing causes air in the throat to vibrate that causes initial vocal sound. These sounds are then modified by the vocal tract to produce speech. - Rate at which the glottis opens/closes = fundamental frequency o Males have lower vibrational frequency thickening/lengthening of vocal folds Vocal tract (tongue, teeth, palate, lips, various cavities/structures in nasal/oral chambers): - Different speech sounds are made by contorting the structures or producing a constriction along the pathway to modulate the movement of air o Tongue and lips are the biggest contributors to this The nature of speech sounds Wordssyllablesphonemes Phonemes: smallest unit of speech - Basic speech sound that distinguishes one work element from another - Restricted to consonants and vowels, but can also be used to indicate alterations in pitch/rhythm Vowels: produced through changes in the shape of the mouth/lips  ~200 Consonants: require more articulatory movement  ~600 - Almost always require a constriction or impediment within the vocal tract, usually by tongue movement - Described by their place of articulation & the manner of articulation Acoustic properties of speech sounds – general principles Speech is a time-varying signal in which the twin properties of sounds (frequency and intensity) are both changing in complex ways as the speech signal unfolds Pressure waveform (pressure (intensity) change over time): - Advantage: can be stored & later used to create the exact sound by mechanical devices - Disadvantage: do not reveal details of the frequency content in the stimulus Possible solution: identify principal frequency components associated with various phonemes Vowels tend to be clustered more toward lower frequency range Consonants cover a wider range of frequencies with high variability  So cannot use this method or “solution” either Acoustic properties of speech sound – spectrographic representation Sound spectrograph: instrument that analyses audio signals and determines the distribution of sound frequencies contained within the signal. - Can perform this function repeatedly over times as speech signal is unfolding, so can provide a readout of how the frequencies change with ongoing speech - Graph (spectrogram): Plot sound frequency and time with respect to each other such that frequency is on the y-axis and time on the x-axis; sound intensity is shown by the darkness of the markings o Displays the changing nature of sound frequencies and associated intensity values through the duration of the speech signal - Formants: bands of resonant frequencies o On a spectrogram: dark (high intensity) band, arises due to a response in the vocal tract o Are numbered: lowest = F1, next = F2,… - Spectrogram of every individual is unique: voiceprint - Not useful in visual speech B2. Speech Comprehension What exactly is language? Language is based on complicated processes that require us to co
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