Auditory Computation Overview: I. Physiological Foundations II. Elements of Auditory Computation

I. Physiological Foundations Auditory perception – subjective experience: “What you hear is not what is out there in the outside world, but instead what has been created all inside of your head.” What is the sound of falling tree in no-man’s land?

What is sound? Sound is the psychological sensation that is internally generated in the brain as a response to an external physical stimulus called the ‘sound’ wave. Two necessary ingredients of sound: (1) Source (2) Listener

(1) Source (Sound Wave)

A sound wave is the pressure cycle (compression & rarefaction) of air molecules.

Intensity of sound wave is measured in decibel.

(2) Listener (Ear)

Steps of Auditory Computation 1: Outer ear (eardrum) (sound detection) 2: Middle ear (sound amplification) 3: Inner ear (neural pulse generation) 4: Superior olive (relay station - interaural mixing ) 5: Auditory cortex (conscious perception)

Auditory Projection System

Middle Ear – sound amplification

Inner Ear -- Cochlea (coiled tube) - neural pulse generation

Cross-sectional View of Cochlea

Close-up View of Cochlea

Traveling wave inside Cochlea

Steps of Auditory Computation 1: Outer ear (eardrum) (sound detection) 2: Middle ear (sound amplification) 3: Inner ear (neural pulse generation) 4: Superior olive (relay station - interaural mixing ) Q: What computational advantages ?? 5: Auditory cortex (conscious perception )

Auditory Projection System

II. Elements of Auditory Computation 1. Frequency-to-location coding 2. Frequency sensitive cells 3. Frequency coding in cortex 4. Sound localization

1. Frequency-to-location Coding - Each group of cells has its own fundamental frequency, to which it maximally responds. Note the nonlinearity of the map. Why nonlinear??

2. Frequency sensitive cells

3. Frequency Coding in Auditory Cortex

Topographic mapping of auditory information: Similar frequencies are mapped to nearby points in the cortex Q: What computational advantages of such topo map??

topographic map in sensation of touch

4. Sound Localization by interaural time difference(ITD)

Neural Circuit for Computing ITD via Time-to-Position Coding: Coincidence detector for each frequency channel -- physiologically confirmed

Phase Ambiguity in Sound Localization

Ambiguity Resolution by Across-frequency Comparison -- physiologically confirmed

Neural Circuit for Across-frequency Comparison

Audition is a computation.