Sensation & Perception

16 – The Auditory System

Administrative stuff

The final Friday, December 10 th, am pm Comprehensive, with an emphasis on things that haven’t been covered in previous exams. Like the midterms. Interpreting graphs, drawing diagrams, multiple choice and short written answers.

How to do well in class?

1.: Attend class! Class attendance correlates with GPA typically around 0.3 (Schuman et al., 1985) but can reach correlations of up to 0.4! (Larrieu, 2004)

2.: Study! (a lot) The effect of studying seems to be highly nonlinear. It only makes a significant difference if one studies more than 5 hours per day! (Schuman et al., 1985).

3. Don’t drink! Effects of alcohol consumption on GPA have been consistently found to be negative and significant (Finnell & Jones, 1975). The magnitude of the correlations is in the ballpark of -0.2 (Larrieu, 2004).

4. Don’t do drugs! Effects of Marihuana consumption on GPA have been found to be significantly negative. The magnitude of the correlation is in the ballpark of (Larrieu, 2004).

Summary Attend as many classes as possible Study at least 5 hours a day (outside of class) Don’t drink Don’t do drugs  Be a complete nerd (or act like one). Caveat: The effects outlined above might be non-independent and might not be causal.

The Auditory System

If a tree falls but there is no one there to listen, does it make a sound? Profound question?Silly question?

Why focusing on the visual system for most lectures? This approach has a good rationale: Rather studying one system very thoroughly than all systems superficially. Primary Goal: Achieving understanding of one system rather than amassing easily forgotten factoids about many system. Secondary Goal: Extend the concepts introduced in the visual system to other perceptual systems.

Why focusing on the visual system? More than 50 % of the macaque brain is visually responsive. It is certainly the most studied perceptual system Possibly also the best understood.

Now we will use this approach to Outline similarities and differences between the auditory and visual system. Essentially understanding the auditory system in terms of the concepts we introduced in the context of the visual system.

Functional classification of external perceptual systems Long range systems Vision (Seeing) Audition (Hearing) Short range systems Gustation (Taste) Olfaction (Smell) Somatosensation (Touch, Pain, etc.) Long range systems Vision (Seeing) Audition (Hearing) Vision and Audition are both long range, external perceptual systems.

The physical stimulus Vision: Electromagnetic waves (in humans with a wavelength of around nm) Audition: Pressure waves in air (in humans with a frequency of around Hz)

Refresher on wave-physics 1 Hz = 1 cycle per second (Unit of frequency) Conversion: Frequency (f) = Speed (c)/Wavelength(λ). Example 1: Speed of light in vacuum/air ≈ 3*10 8 m/s f = c/λ  3*10 8 / 400 * = 7.5 * = 750 TeraHz f = c/λ  3*10 8 / 700 * = 4.29 * = 429 TeraHz We see in a range of nm = THz Example 2: Speed of sound in air (at 20º C) ≈ 343 m/s λ = c/f  343 / 20 = 17.2 m λ = c/f  343 / = 1.72 cm We hear air waves with a wavelength of 1.72 cm to 17.2 m.

Stimulus comparison The physical stimulus of both the auditory system and the visual system can be described as a wave-phenomenon with a certain wavelength and frequency. However, the physical quality of the waves (electromagnetic vs. air-pressure) as well as the frequency-ranges (or wavelengths) are quite different.

Psychological qualities of sound Amplitude  Loudness Wavelength = Cyclelength. Frequency = 1/Wavelength  Pitch

Some examples 300 Hz, low. Big amplitude, loud 300 Hz, low. Small amplitude, not as loud 3000 Hz, high. Small amplitude, not as loud 3000 Hz, high. Small amplitude, loud

Intensity/Loudness The amplitude corresponds to Sound Pressure Sound Pressure levels is measured in decibels (dB SPL) Just like with luminances in vision, the auditory system is capable of hearing sounds over many orders of magnitude of sound pressure. Hence, the decibel scale is logarithmic. Multiplying the sound pressure by 10 adds 20 dB. Loudness estimate are largely a linear function of sound intensity as measured in dB.

Stimulus Correspondences Visual System Frequency = Hue, Color. Amplitude = Luminance Auditory System Frequency = Pitch Amplitude = Loudness Example: Light that contains all frequencies equally appears white: Sound that contains all frequencies equally is called “white noise”

Anatomy of the auditory system Contains primary receptors (transducers) Coarse amplification Equalizer system

The inner ear Filled with fluid Contains the Organ of Corti, which contains the transducers that transduce mechanical pressure waves into action potentials. The cochlea is tonotopically organized. Frequencies map orderly along the length of the cochlea. Hair cells near the Apex respond best to low frequencies Hair cells near the Base respond best to high frequencies Base = oval window, connection to Stapes

The inner ear (cochlea) The inner ear uses a place code to decode fundamental frequencies

The organ of corti Corresponds to the retina in it’s function as an array of primary transducers. Contains two membranes (Basilar membrane and tectorial membrane) and Hair cells. The hair cells sit on the basilar membrane. Their cilia move against the tectorial membrane. They bend. This causes action potentials by the hair cells.

Hair cells The primary transducing receptors of the auditory system They have cilia that bend Bending of the cilia of the inner hair cells generates action potentials. They are the transducers. The outer hair cells have modulatory functions, effectively narrowing the frequency tuning of the basilar membrane.

Hair cells

The auditory pathway Visual: Retina Chiasma SC/LGN V1 V2, etc. Auditory Cochlea SON IC MGN A1 A2, etc.

Big difference between the systems: The temporal fidelity in the auditory system is much higher than that in the visual system. This temporal fidelity is for example needed in sound localization, where interaural time differences are utilized (on the order of microseconds)

Temporal order judgements 12 Visual fusion threshold 40 ms Auditory fusion threshold 2 ms (!) This is not wrong, it’s british english

Visual and Auditory illusions There are illusions in both systems. Most of the auditory ones require a very controlled stimulus administration that is not possible here (e.g. headphones) Hence, the demonstration of Shepard’s Tones, an ever-ascending scale of tones should be sufficient (as a proof of concept) Psychophysics of the effect is well established, underlying cortical mechanisms are not.

Shepard’s Tones Locally, each sound will appear higher than the previous one. This is due to the relative spacing of tone components (1 octave apart) and appropriate scaling (amplitude). The brain makes a (wrong) perceptual decision, based on in-built assumptions

Interactions between the systems Neat separation of visual vs. auditory system is a textbook phenomena. Many real life phenomena have corresponding sights and sounds.  There is crosstalk between the systems, the brain uses multimodal information to disambiguate the perceptual world. Primary examples: The McGurk effect And Motion Disambiguation

The McGurk effect Auditory: Ba Visual: Ga Fused percept: Da Discovered by McGurk in 1976 by accident Neural mechanisms remain unknown

Motion disambiguation by sound

Back to the tree When it falls, it creates a pressure wave in the air. If there is no organism with an auditory system present that interprets these oscillations in the air as a sound, it will make no noise. Even if organisms with auditory systems are present, it will sound different to different organisms (if they have different auditory systems).