Efficient Coding of Natural Sounds

Grace Wang HST 722 Topic Proposal

Background

Our natural environment consists of multiple sound sources and background noises with complex harmonic and temporal structuresStationary harmonic structure (vocalizations)Nonstationary structure (crunchy)

Have our brains developed to be optimized for processing natural sounds efficiently?

Efficient coding hypothesis

Shannon’s model for transferring data in communication systems (info theory)

Barlow applied info theory to model neural behavior Proposed spikes of neural populations was optimized to

efficiently represent naturally occurring images and sounds

Efficiency = reduce redundancy to maximize independence

Largely consistent with early stages of visual processing Is the same true for the auditory system?

Information in adjacent filters is highly redundant

Nearly identical statistics across filters and across sound types

Suggested natural sounds may be associated with bandwidth and translation invariance

from Attias and Schreiner (1997), “Temporal low-order statistics of natural sounds”

How do we remove redundancy to achieve statistically independent neural responses?

from Schwartz and Simoncelli (2000), “Natural sound statistics and divisive normalization in the auditory system”

Sound pressure waveform decomposition

Differs from spectrograms by retaining the phase information

from Smith and Lewicki (2005), “Efficient coding of time-relative structure using spikes”

1 1

( ) ( ) ( )mnM

m mi m i

m i

x t s t t

Time-frequency distribution of optimal filter

shapes Fourier transform

may be sufficient for animal vocalizations

Need wavelet analysis for environmental sounds and speech

from Lewicki (2002), “Efficient coding of natural sounds”

Decompose into modulation spectra

from Singh and Theunissen (2003), “Modulation spectra of natural sounds and ethological theories of auditory processing”

from Singh and Theunissen (2003), “Modulation spectra of natural sounds and ethological theories of auditory processing”

Discarding a lot of energy in white noise

from Singh and Theunissen (2003), “Modulation spectra of natural sounds and ethological theories of auditory processing”

from Singh and Theunissen (2003), “Modulation spectra of natural sounds and ethological theories of auditory processing”

Neural responses

Increasing selectivity along ascending pathway for songs in zebra finches

Optimal stimulus set in grasshoppers does not coincide completely with their natural soundsSuggested neurons are optimized for

behaviorally relevant sounds (Machens et al 2005)

Papers Background

Smith and Lewicki 2005: Efficient coding

Discussion Attias and Schreiner 1997: Demonstrate redundant

representation in peripheral auditory system Lewicki 2002: Fourier vs wavelet representation Singh and Theunissen 2003: Modulation spectra

Further reading Machens et al 2005: grasshoppers, behaviorally

relevant sounds Hsu et al 2004: zebra finches, increasing selectivity