Localization

sound localization- two plans Azimuth (horizontal) and elevation (vertical)

  • binaural (need two ears)

    • inter aural time different (ITD)

      • waves hit nearer ear first, we are sensitive to these small time differences

    • inter aural intensity difference (IID or ILD)

      • our head creates a sound shadow, so higher intensity in nearer ear

both azimuth and elevation are relative to head position - more precisely ear level

takes about 0.6 milliseconds for sound to travel width of head - this creates cone of confusion

ITD only works for sound below about 1500-1600 hz (depending on size of head) - phase info gets ambiguous if more than one cycle occurs within the time it takes to travel between two ears

Jeffrey’s Neural Coincidence Model: how neuron’s might code for ITD

  • sound directly ahead - hits ears at the same time

  • from closer to left ear -different time

Azimuth - IID - good for high frequencies

low frequencies can wrap around the head

high frequencies will hit the head and create sound shadow

sound localization experiment:

  • two blocks, each w 7 diff frequencies

  • interaural time difference block

    • tone onset slightly earlier in one ear than the other

  • interaural intensity difference block

    • tone slightly louder in one ear than the other

  • ITD- basically fit ideal pattern - good localization for low frequencies but not for high

  • good localization for all frequencies for IID

    • in the real world IID is caused by sound shadow from head

    • sound shadow only exists for higher frequencies (low wrap around head)

    • here we used headphones and IID was programmed into stimulus regardless of frequency, so it always existed and head couldn’t shadow

sound localization in elevation: acoustic cues to sound elevation

  • pinna shape alters which frequencies get transferred effectively depending on how they hit the folds

  • funnels duplicate front back symmetry of pinna

  • spectral cues - the spectrum of the frequencies get modulated by the folds

  • head related transfer function (HRTS) - pinna and head effect the intensities of different frequencies as a function of elevation

sound localization - distance:

  • monaural - need one ear - changes in stimulus quality

cues to distance:

  • loudness - if familiar sound

  • frequency - high frequencies fade faster as sound travels through air

  • movement parallax

  • reflection - higher % of reflected sound for far sounds

auditory pathway:

  • auditory nerves terminate on cochlear nucleus (in brain stem)

  • cochlear nucleus

    • dorsal cochlear nucleus - frequency (what)

    • ventral cochlear nucleus - sound localization (where)

    • dorsal/ventral is opposite of vision

  • from cochlear nucleus → superior olive

  • superior olive

    • binaural inputs - sound localization (mostly IIT with some ITD)

  • from superior olive → inferior colliculus

  • inferior colliculus

    • sound localization (ITD)

    • startle response

    • then medial geniculate of thalamus → primary auditory cortex

  • primary auditory cortex (A1)

    • has tonotopic organization

    • cortical magnification around speech frequencies

  • secondary auditory cortex

    • important role in localization and analysis of complex sounds (speech, object recognition)

tinnitus - phantom perception of ringing in the ears

  • development : usually begins w hearing loss caused by hair cell damage, cortical input reduced for those frequencies

  • brain compensates for lack of input by increasing sensitivity

  • if compensation overshoots, can produce spontaneous activity that is interpreted as sound

  • activity often occurs in thalamus and primary auditory cortex

  • created long term changes/strengthening in pathways

  • auditory thalamus to A1 connections strengthen and have spontaneous finding

  • strengthens connections between auditory cortex and limbic system (including amygdala)

types of tinnitus

  • subjective tinnitus - majority of cases - brain firing in absence of sound (steady tone)

  • rarely - objective tinnitus - if you place a mic in persons ear can hear the sound

    • vascular issues - turbulent blood flow near the ear causes sound

      • rhythmic synchronized w heart beat

    • muscular - involuntary contractions of middle ear muscles

      • clicking noises

treatment for tinnitus

  • CBT - doesn’t reduce noise but helps make it less distressing

  • hearing aids - particularly those w hearing loss and restores audio input, so cuts down on gain due to lack of input

important role of hearing in language - use to communicate w others, communicate w selves

hearing language:

  • speech frequencies vary from 200 Hz to 8,000 Hz

  • consonants convey most of the information

  • most of the energy in spoken language is vowels

  • consonants alone are just pops and ticks, only make sense when paired with a vowel

  • conclusions:

    • poor signal to noise ratio

    • perceptual units of speech are larger than single letter-phonemes

evidence for feature/phoneme detectors in language

  • create an ambiguous stimulus

    • heard as “time” or “dime”

  • adapt a person to “di” sound - they will hear “time”