Audition
THE AUDITORY MECHANISMS OF DIFFERENT SPECIES
20-20,000 Hz :: human audible frequency range
basilar membrane :: contains hearing receptors. longer basliar membrane allows processing for a wider range of frequencies. low freq ends near apex of membrane. high freq ends near the base. house auditory receptors, hair cells, that are moved by vibrations in the fluid that are then converted to neural impulses that the brain can understand
low freq has long wavelength = one cycle will end near the apex of membrane. high freq has short wavelength = one cycle will end near the base
THE STIMULUS - SOUND WAVES
band of compressed air molecules :: causes eardrum to get pushed slightly inwards
band of less dense air molecules :: causes eardrum to move slightly outwards
loudness :: higher amplitude (or height of wave) = higher loudness. measured w decibels
decibels :: dB. Logarithmic scale. the perceived loudness of a sound doubles for every 10 decibel inc
pitch :: wavelength; from one peak to the next. measured in Hertz. longer wavelength = lower pitch = lower frequency
hertz :: Hz. rep the number of cycles per second
timbre :: like purity. perceived complexity of a sound.
THE EAR
external ear :: pinna, auditory canal, ear drum
pinna :: funky external ear part. collects sounds
auditory canal :: amplifies the incoming sound waves like a horn
ear drum :: back wall of the ear canal. vibrating at the frequency of the incoming sound waves
middle ear :: other side of the ear drum. the ossicles. smallest bones in the body. amplification of sound
inner ear :: oval window, cochlea
cochlea :: fluid filled tube coiled like a snail shell. transfer the changes in fluid to neural impulses
round window :: in the middle ear and helps equalize pressure differences in the cochlea
oval window :: separates the inner ear from the middle ear. transmits sound information from the ossicles to the inner ear
- then to basilar membrane
- the displacement of the oval window causes movement of both the basilar membrane and the round window
structures of the ear in order :: external ear, typanic membrane, middle ear, oval window, cochlea
AUDITORY PATHWAY: FROM RECEPTORS TO AUDITORY CORTEX
hair cells :: form synapses w bipolar cells, whose axons make up the cochlear nerve (branch of the main auditory nerve) outer hair cells to inner hair cells, 4:1
inner hair cells :: less numerous. more connections to the brain. send pitch information. myelinated. mainly contribute to the signal in the cochlear nerve
outer hair cells :: more numerous. fewer connections to brain. amplify sound. unmyelinated.
tonographic organization :: the hair cells connect to the cochlear nerve such that nighbouring regions of hair cells remain together. organized by frequency
sound with similar frequencies are processed beside each other in A1
AUDITORY LOCALIZATION
unlike in vision, in audition there is no direct spatial representation of the stimulus
interaural cues :: interaural time differences, interaural intensity differences, and pinna cues
interaural time difference :: the diff in time it takes for a sound to travel to one ear when compared to the other
interaural intensity difference :: the diff in intensity at one ear compated to another. sound will be louder at the closer ear. works better for high frequency sounds
with high frequncy sounds we can rely on the difference in intensity caused by the head which casts a “sound shadow” → when the intensity dimishes the intensity at the distal ear. cant see sound shadow with low frequency sounds bc if the wavelength of the shound is larger than the actual diameter of the listener’s head, the sound wave can actually diffract around the head, withough losing much of its intensity.
pinna cues :: sound direction produced by the characteristic folds + ridges of our pinnae. important for determining sound elevation
ECHOLATION IN BATS
echolocation :: indiv is able to form perceptual ‘image’ of the objects in the surrounding environment by emitting a sound and then analyzing the time and frequency information that is contained in the retruning echoes
co-evolution :: adaptation of traits of one species can direct affect the adaptation of traits in another species