Sensation Perception 16 The Auditory System Administrative stuff

Ψ Sensation & Perception

Ψ 16 – The Auditory System

Administrative stuff

The final • Friday, December 10 th, 10. 30 am -12. 30 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 -0. 25 (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. ) Vision and Audition are both long range, external perceptual systems.

The physical stimulus • Vision: • Electromagnetic waves (in humans with a wavelength of around 400 -700 nm) • Audition: • Pressure waves in air (in humans with a frequency of around 20 -20000 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*108 m/s f = c/λ 3*108 / 400 * 10 -9 = 7. 5 * 1014 = 750 Tera. Hz f = c/λ 3*108 / 700 * 10 -9 = 4. 29 * 1014 = 429 Tera. Hz We see in a range of 400 -700 nm = 429 -750 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 / 20000 = 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. 3000 Hz, high. Big amplitude, loud Small amplitude, loud 300 Hz, low. 3000 Hz, high. Small amplitude, not as loud

Intensity/Loudness • The amplitude corresponds to Sound Pressure • Sound Pressure levels is measured in decibels (d. B 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 d. B. • Loudness estimate are largely a linear function of sound intensity as measured in d. B.

Stimulus Correspondences • Visual System • Auditory System • Frequency = Hue, Color. • Amplitude = Luminance • 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 Equalizer system Coarse amplification Contains primary receptors (transducers)

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

Visual: The auditory pathway Auditory Retina Cochlea Chiasma SON SC/LGN IC V 1 MGN V 2, etc. A 1 A 2, 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 1 2 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 Mc. Gurk effect • And Motion Disambiguation

The Mc. Gurk effect • • • Auditory: Ba Visual: Ga Fused percept: Da Discovered by Mc. Gurk 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).