1 29 2 3 Chapter 9 Audition Coding Tonotopic Organization fourier analysis breaking complex tone down into simpler components Coding Frequency the cochlea place theory place code certain frequencies activate hair cells at different parts of higher frequency sounds activate regions of cochlea closer to base lower frequency sounds activate regions of cochlea closer to apex more accurate for high frequencies frequency matching hair cells initiate action potentials at the same rate as the frequency does ex 100 Hz tone 100 action potentials second ways to frequency match 1 phase locking each auditory nerve fiber fires at a particular point in the sound wave works for low frequency sounds up to about 250 300 Hz fiber can only fire so fast bc of refractory period so can t phase lock to higher frequency sounds 2 volley principle multiple neurons divide and conquer high frequency sounds by individually locking onto a particular part of the sound wave cross fiber patterning receive information for the total response of all neurons combined rather than individual neurons 1 1 29 2 3 louder sound more hair cells activated because louder sounds displace more of Coding Amplitude firing rate rate law increases with amplitude used for high frequency sounds basilar membrane used for low frequency sounds Auditory Nerve hair cells are found along the cochlea and synapse on AN fibers different parts of cochlea respond to specific frequencies response of individual AN fibers should be related to where they are i e what hair cells they synapse with along the cochlea characteristic frequency frequency to which particular AN fibers is most sensitive labeled line coding coding based solely on which axon is being activated not going to work well because sounds are hardly ever pure lowest dB 2 tone suppression decrease in the firing rate of 1 AN fiber stimulated by a particular tone when a second tone is presented at the same time perceived as sounding a bit quieter because of mechanical changes to basilar membrane complex sounds are by definition frequency combinations those different frequencies don t just add together as multiple responses of AN fibers they actually suppress one another a little 2 1 29 2 3 lower frequencies have greater effect than high brain can t determine the frequency of a sound based on what fiber carries the signal because loud sounds cause larger bulge in vestibular canal more stereocilia bent more hair cells activated wider range of frequencies more AN fibers with different CFs are activated all firing at once not useful info for the brain in terms of perceiving a specific frequency rate saturation loud sounds increase in firing rate AN fibers not as selective for their CFs at levels well above threshold Figure 9 17 each line is from the same AN fiber iointensity curves cross fiber patterning rate intensity functions for 6 AN fibers all synapsing on the same hair cell all have same characteristic frequency Figure 9 18 low spontaneous fibers won t fire until the sound gets louder saturate more slowly more frequency selective as dB increases high spontaneous fibers very sensitive to low levels of sound but saturate quickly fire 40 100 times sec even without noise at only 40 dB firing almost as fast as they ever will Brain Pathway dendrites synapse at base of hair cells 3 1 29 2 3 somas of all synapsing nerve fibers live in spiral ganglion all of the axons make up the auditory portion of the 8th cranial nerve axons synapse in ipsilateral cochlear nuclei all structure of the auditory pathway from BM to cortex have tonotopic organization pathway is bilateral almost all structures get info directly or indirectly from both ears damage to pathway brainstem medulla pons midbrain thalamus 1 cochlear nuclei medulla ipsilateral info from 8th CN left side gets info from left right from right different types of specialized neurons that respond preferentially to different aspects of a sound onset of a sound at a given frequency some fire APs with same rate as AN fibers that synapse upon them aka coding that began in cochlea continues into brainstem 2 superior olive in pons first place in brain to get info form both ears via cochlear nuclea important in sound localization medial superior olive interaural time difference lateral superior olive interaural intensity level difference 3 inferior colliculus midbrain map of auditory space superior colliculi map of visual space integrated 4 important for orienting toward a sound tectospinal pathway goes back down to cervical SC to help direct neck 1 29 2 3 muscles to turn head 4 medial geniculate nucleus MGN thalamus relay filter for auditory info heading to cortex projects to primary auditory cortex receives projections back from cortex 5 primary auditory cortex superior part of temporal lobe A1 primary auditory cortex a lot of processing has already occurred prior to cortex processing becomes more sophisticated as it moves through cortex any sound elicits activity tonotopic organization A2 belt zone secondary auditory cortex auditory association cortex parabelt zone even more complex sounds elicit activity processes other sensory info as well more complex sounds elicit activity doesn t care for pure tones Speech Processing 5 1 29 2 3 A1 deep in lateral sulcus belt zone parabelt zone too when listening to speech or music but at this level the activation is balanced between the hemispheres as sounds become more complex they are processed more anteriorly and ventrally to A1 language is usually in the left hemisphere lateralized see lateralization as well as anterior ventral shift as the sounds become more clearly a part of a language parts differ in activation pattern one part when speech forms intelligible sentences middle part when categorizing sounds bilaterally activated when listening to complex sounds and speech Broca s area important in the motor production of speech to a much lesser extent in Wernike s area important in language comprehension right hemisphere prosody rhythmic musical aspects of speech question statement comprehension intonation 6 1 How are amplitude and frequency coded in the cochlea Study Questions Frequency Place code Frequency matching phase locking volley High frequency sound Low frequency sound 1 29 2 3 Amplitude Firing rate increases w amplitude of hair cells activated louder more cells 2 Here are some terms to know place code temporal code frequency matching phase locking volley principle 2 tone suppression rate
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