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Problem 8: Listen to This (Why are higher frequencies influenced by…

- - - - Tuned neurons/sine wave: neurons that respond to pure tones (Wolfe)
        
        Two-toned suppression
        
        Neural adaptation
- - - - A. Skin/outer ear
        B. Bone/Auricle (pinna):fleshy, visible frm outside part, channel sounds to canal. Aids localization of sound.
        C. Auditory Canal: ‘external auditory meatus’, amplifier inc amplitude of certain sound frequencies (2000-7000 Hz-speech related)
        D. Eardrums: ‘tympanum’, vibrates incoming sound waves, move faster for low-frequency sounds
    - - E. Middle ear: shapes way ear responds to sounds, amplifies strength of sound-induced vibrations, consists of the 3 tiny bones(each is an ossicle).
        F.Malleus/hammer: large, moveable flap of skin; receives vibration from tympanic membrane, attached to the incus
        G.Incus, anvil (Wolfe): Middle of 3 ossicles, connects the malleus and the stapes.
        H.Stapes/stirrup: connects to inner ear (by oval window, long tube w/ fluid- conduction deafness possible).
        
        Acoustic reflex: protects ear from intense sounds by inc pressure by 15x inc b/w eardrum and oval window or level action of malleus and incus.
        
        conductive hearing loss = middle ear issues
        
        Low to moderate sound pressures = stapes push directly on oval window. High pressure = muscles attached to malleus and stapes contract (so force on oval window is decreased).
    - - Acoustical Law (cochlea performs fourier analysis on complex sounds = responds optimally to diff frequencies.)
        
        I.Cochlea/Inner ear: Extension of labyrinth; membrane w/ sensory cell hairs; a shell form.
        J.Vestibular canal: one of 3 tubes tht is the cochlea. Oval window is at the base of this; connects to tympanic canal by helicotrema.
        K.Auditory nerve fibers: collection of neurons tht convey info from hair cells in the cochlea to (afferent) and from (efferent) the brain stem. If there’s an issue here it’s called: sensorineural hearing loss
        L. Eustachian tube: Keeps air in middle ear equal to its surrounding; allows air to flow into the two cavities in middle ear; route bacteria. E.g. head cold.
      - 3 canals: vestibular, eustachian, cochlear duct. Ohm’s
      - 3 membranes: Reissner, tectorial and basilar.
  - - - Place coding/place principle (Wolfe): tuning diff parts of cochlea to diff frequencies, info about particular frequency of incoming sound along partition has greatest mechanical displacement. Doesn’t explain missing fundamental.
    - - (Wolfe) Stereocilium (in inner hair cell): hairlike extensions on tip of hair cells in the cochlea. The hair has nerve fibers, neurons located in spiral ganglion (connected to base of hair in cochlea).
        
        Type 1 fibers/afferent fibers: 95% of spiral ganglion, makes single connection w/ single inner hair; sounds carry through to the brain.
        Others are type 2 fibers/efferent: each one connects to 10 outer hair; smaller = slower conduction speed; a feedback system (can modify smaller hair) w/ info coming from the brain.
      - Inner and Outer hair cell interaction: Two-tone suppression
- - - - Wolfe: goes to media geniculate nuclei (part of thalamus that relays auditory signals to temporal cortex and receives info from auditory) where we can tell where it’s coming from.
        
        After, it goes to primary auditory projection area (A1) in temporal cortex.
        
        Belt and parabelt area: underneath A1 responds to complex characteristics of sounds
    - - onset neurons (burst of responses immediately aft onset of tone). Wolfe: attack (beginning of complex sound)
      - pauser neurons (burst followed by pause the weaker response till tone is off)
      - chopper neurons (bursts of firing aft short pauses, then dec bursts)
      - primary-like neurons (initial burst, then lower level for duration of tone)
      - offset neurons (reduce response rate below spontaneous activity when its on, then burst when its off). Wolfe: decay (end of complex sound)
  - - - Harmonic spectra (frequency components higher than fundamental).
      - Fundamental frequency (ppl can hear this even if it’s removed-neurons are phase locked, patterns!).
        
        Frequency principle: pitch is encoded in terms of overall frequency of firing in the auditory nerve.
    - - Wavelength: distance from one peak of wave to the next (that s 1 cycle).
      - Bandwidth: range of frequencies from narrow to wider, used in complex sounds
      - Frequency: # of cycles the wave can complete in a second; expressed in Hertz.
      - Other dimensions: volume (sense of how it fills the space), density (feeling of compactness/hardness), consonance or dissonance (go together/clash)
  - - - Phase angle: a specific compression-rarefaction of a wave; 0-360 degrees w/ rarefaction peak at 270
        
        In phase
        
        Out of phase
        
        Relative phase
      - Active noise suppression: principle to reduce unwanted noises; subtract relative phase to mimic same pressure amplitude.