Acoustic Illusions & Sound Segregation Reading Assignments for Quizette 3 Chapters 10 & 11

TLA 5: Hearing in Space HIS DUE Nov. 8 Ingredients  Large area  Pen & paper Activity  Close your eyes and listen for 1 minute  Try to identify the sound sources Write-up  How many sounds did you hear?  What made those sounds distinctive?

Timbre Illusion: Shepard’s Scale The musical equivalent of M. C. Escher When does this scale reach the highest note?  PLAY DEMO What is Sheppard’s scale?  Notes are actually chords Highest/lowest notes in chord are replaced for successive frequency changes Illusion of timbre  Misattribution of fundamental frequency

Timbre Illusion: Interval Size Two intervals, which interval is larger?  PLAY DEMO Note brightness of timbre for each interval  PLAY DEMO again Bright-to-dark sounds larger  Works for musicians & untrained listeners  Timbre affects pitch perception

Deutsch Melody Melody played across two ears  Within each ear large intervals (1 octave)  Ascending/Descending melody can only be heard if ears are combined Listener hears 2 coherent scales  Melody is constructed by suppressing irrelevant tones Suggests ‘What’ or ‘Where’ theory of brain

What or Where? Is auditory perception duplicitous?  What: Objects & sequences Process of organization Melody Object recognition  Where: location, location, location Interaural processing Spectral cues for spatial position Motion Neural evidence (e.g., Berlin & Zatorre, 2000; Read et al., 2002; Romanski et al., 2000)  Dorsal pathway - spatial  Ventral pathway - identification

What causes sounds to group into streams? Gestalt grouping (see Bregman, 1990 for review)  Perceptual laws to combine cues Bottom-up processes/Automatic Hierarchical organization  Highly applicable to auditory perception DEMO – Galloping Sounds  Large Step then Small Step  Bach Chorale 1 & 2  Traffic Laws of proximity (close in time), similarity (same timbre/pitch)  Law of Pragnänz: simplest solution

Neural Correlates of Stream Segregation Summation of cortical response hypothesis  (DRAW) Will a stimulated area summate with another stimulated area on the tonotopic map? Tetanic + rapid stimulation increases probability of summation (Fishman et al., 2001) ALTERNATIVELY? Lesions of temporal cortex (in and around primary auditory cortex) (Peretz and colleagues, 1999; 2001)  Poster temporal gyrus affects melodic grouping Intervals, melodic contours  Anterior temporal gyrus affects meter Rhythmic grouping, temporal combinations