INTRODUCTION TO HEARING
WHAT IS SOUND?
amplitude Intensity measured in decibels.
SAME PITCH DIFFERENT TIMBRE
* * * All points on this curve have the same perceived loudness as the standard (*) EQUAL LOUDNESS CURVES
SUMMARY Sound is variation in pressure Frequency, pitch Intensity (SPL, “sound pressure level”), loudness, decibels Timbre, harmonics Equal loudness Hearing thresholds
THE EAR
Amplification: Bones 1.3x Area 17 x Total = 22.1x Density: Air: 1.2 kg/m 3 Water: 1000 kg/m 3 Ratio:
LESS STIFF STIFF HIGH FREQUENCIES LOW FREQUENCIES
SUMMARY Outer, middle, inner ear Oval window, round window, Basilar membrane, tectorial membrane, hair cells
THEORIES OF HEARING
PLACE THEORY (which fibres, labelled lines) Von Békésy (Nobel prize 1961) 1 - Travelling wave; stiffness varies 2 - one place most active for a given frequency 3 - Tonotopic code; coded as place PERIODICITY THEORY (how they are firing, temporal code) 1 - sound coded as pattern
MODEL OF THE BASILAR MEMBRANE Varies in stiffness… RESONANCE Traveling wave:
WHERE THE WAVE HAS ITS HIGHEST AMPLITUDE DEPENDS ON ITS FREQUENCY
Evidence against place -- Missing fundamental -- which can be masked -- some animals have no basilar membrane Evidence against periodicity -- cells can’t fire fast enough -- diplacusis Evidence for place -- physiology (basilar membrane) (cells tuned for frequencies) -- masking Evidence for periodicity -- multiple cells could do it -- phase locking of cells
MASKING
Evidence against place -- Missing fundamental -- which can be masked -- some animals have no basilar membrane Evidence against periodicity -- cells can’t fire fast enough -- diplacusis Evidence for place -- physiology (basilar membrane) (cells tuned for frequencies) -- masking Evidence for periodicity -- multiple cells could do it -- phase locking of cells
PHASE LOCKING
Temporal coding up to about 4,000 hz… but each spike takes about 2ms… therefore only up to around 500 hz…..
But could share it out over several cells One cell might only be able to follow every 4 th cycle….. … but others can share the task
Evidence against place -- Missing fundamental -- which can be masked -- some animals have no basilar membrane Evidence against periodicity -- cells can’t fire fast enough -- diplacusis Evidence for place -- physiology (basilar membrane) (cells tuned for frequencies) -- masking Evidence for periodicity -- multiple cells could do it -- phase locking of cells
THE CASE OF THE MISSING FUNDAMENTAL Pitch determined by fundamental….
So what happens if we remove the fundamental? What does it sound like?
Play it
Training a goldfish... CAN AN ANIMAL WITH NO BASILAR MEMBRANE DISTINGUISH FREQUENCIES?
Evidence against place -- Missing fundamental -- which can be masked -- some animals have no basilar membrane Evidence against periodicity -- cells can’t fire fast enough -- diplacusis Evidence for place -- physiology (basilar membrane) (cells tuned for frequencies) -- masking Evidence for periodicity -- multiple cells could do it -- phase locking of cells
Place theory sound coded as place Periodicity theory sound coded as pattern Duplicity below 1-4 kHz, coded by periodicity above 1-4 kHz, coded by place
AUDITORY SYSTEM
Auditory cortex Auditory thalamus Superior colliculus Inferior colliculus cochlea Cochlear nucleus Superior olive The Auditory System
The Auditory System (cortical route) Cochlear nucleus Inferior colliculus thalamus cortex
The Auditory System (sub-cortical route) Cochlear nucleus Superior olive Inferior colliculus Superior colliculus
The Auditory System
Cells in primary auditory cortex: -- tonotopically arranged -- respond to more complex features (eg howler monkey calls) primary auditory cortex
The Superior Colliculus
AUDITORY LOCALIZATION
Auditory localization 1 inter-aural time of arrival differences -- circle of confusion 2 inter-aural intensity differences 3 pinnae (up/down front/back etc..) 4 head movements
1 inter-aural time of arrival differences
2 inter-aural intensity differences
3 pinnae (up/down front/back etc..)
4 head movements
AUDITORY SCENE ANALYSIS