1. THE AUDITORY SYSTEM: HEARING
Like light, sound travels in waves and like vision, in order to hear, we first need to receive the sensory input of the waves and then convert them into neural impulses sent to, and processed by, the brain

2. THE STIMULUS INPUT: SOUND
Sound waves are vibrating molecules traveling through the air at the speed of…sound (761.2 mph at sea level at 59° F)

3. THE STIMULUS INPUT: SOUND
→ sound has amplitude which affects loudness, and… → frequency (length) which affects pitch; long waves/low frequency/low pitch

4. THE STIMULUS INPUT: SOUND
Sound wave frequencies are measured in hertz (Hz) and amplitude in decibels (dB) → the range of human hearing for Hz is between 20 and 20,000 → 0 dB represents an absolute threshold for hearing*, 60 dB is normal conversation and prolonged exposure to more than 85 dB is risky

5. THE STIMULUS INPUT: SOUND
* Absolute thresholds for sound vary based on frequency, with the lowest being at frequencies near 2000 Hz, thus loudness is somewhat influenced by frequency

6. THE EAR AND AUDITORY PROCESSING
The process of converting sound waves to neural impulses begins in the outer/external ear, proceeds through the middle ear, and then to the inner ear → in the outer ear sound is conducted by vibrating air, in the middle ear it is done by vibrating bones, and in the inner ear by waves in a fluid

7. THE STIMULUS INPUT: SOUND
The External Ear: sound is collected by the visible outer ear and channeled through the auditory canal → sound waves hit the eardrum, a tight membrane, causing it to vibrate

8. THE STIMULUS INPUT: SOUND
The Middle Ear: the eardrums’ vibrations are transmitted via the ossicles (three tiny bones) that concentrate the vibrations on to the cochlea → the ossicle bones are the hammer, the anvil, and the stirrup, which lever sound inward

9. THE STIMULUS INPUT: SOUND
The Inner Ear: the ossicles cause the oval window to vibrate, thus causing waves in the fluid that fills the cochlea, which then vibrates the basilar membrane → the receptor cells on the basilar membrane called hair cells bend in response, triggering nerve cells to fire

10. THE STIMULUS INPUT: SOUND
→ axons of the nerve cells form the auditory nerve which sends messages to the auditory cortex in the temporal lobe via the thalamus

11. THEORIES OF AUDITORY PERCEPTION
How does the brain turn sound waves into the perception of hearing (the theories below focus specifically on pitch)? → von Helmholtz’s Place Theory says sound waves trigger activity at different places along the basilar membrane, which are detected by the brain and pitch is perceived

12. THEORIES OF AUDITORY PERCEPTION
→ Frequency Theory suggests instead that the brain reads pitch by monitoring the frequency/rate of the neural impulse going up the auditory nerve (3,000 Hz = 3,000 vibrations of the basilar membrane per second)

13. THEORIES OF AUDITORY PERCEPTION
* After doing battle for many long years, like trichromatic and opponent-process theories for color vision, these two have called a truce and given each other credit: place theory explains high frequencies and frequency theory low