SUBCORTICAL AUDITION SEPT 14, 2015 – DAY 9 Brain & Language LING NSCI Fall 2015

Course organization Fun with I am still working on grading. 9/14/15Brain & Language - Harry Howard - Tulane University 2

THE QUIZ WAS THE REVIEW, BUT … review 9/14/15Brain & Language - Harry Howard - Tulane University 3

Changes in book The discussion of the philosophy of perception was moved to the beginning of this chapter, "Subcortical audition" and expanded to include ecological and evolutionary psychology. The discussion of phonation was moved to the beginning of the next chapter, "Auditory cortex". 9/14/15Brain & Language - Harry Howard - Tulane University 4

SUBCORTICAL AUDITION 9/14/15Brain & Language - Harry Howard - Tulane University 5

The central auditory pathway Anatomical layout 9/14/15Brain & Language - Harry Howard - Tulane University 6

The central auditory pathway Functional layout 9/14/15Brain & Language - Harry Howard - Tulane University 7

The central auditory pathway It preserves tonotopy 9/14/15Brain & Language - Harry Howard - Tulane University 8

JAN SCHNUPP Coding in the Auditory System (2015) 9/14/15Brain & Language - Harry Howard - Tulane University 9

What is there to encode? The cochlea produces a huge amount of information. The basilar membrane's sensitivity to frequency creates creates a place code or frequency-to-place code (tonotopy) [which is maintained throughout the ascending auditory pathway > labelled-line code] "Increased sound levels lead to larger amplitude vibrations of the basilar membrane, causing stronger mechanical stimulation and hence larger depolarizing currents in the hair cells, which leads to greater neurotransmitter release onto the spiral ganglion cells and hence higher firing rates in the auditory nerve. The relationship between sound intensity and firing rate is monotonic, giving rise to a relatively simple rate code." 9/14/15Brain & Language - Harry Howard - Tulane University 10

but … auditory nerve fibers differ in their spontaneous rates of firing, and the fibers with the highest spontaneous rates are also the ones with the lowest thresholds, that is, those that show significant increases in their firing rates at the lowest sound intensities. Indeed, it appears that, in order to facilitate the encoding of a potentially very wide range of sound intensities, the auditory nerve implements a “division of labor,” in which ca 75% of the so-called high spontaneous rate fibers specialize in encoding weak sound levels, and it falls to the remaining 25% or so of medium and low spontaneous rate fibers to differentiate the levels of more intense sounds. 9/14/15Brain & Language - Harry Howard - Tulane University 11

and The mechanical tuning of the cochlear filters is not particularly sharp, and becomes less sharp at higher sound intensities. This means that the frequency resolution that is achievable with place coding within the tonotopic array must be rather limited. … higher harmonics are therefore increasingly closely spaced, and resolving individual higher harmonics quickly becomes impossible. Conclusion: the auditory system cannot rely mainly on place coding 9/14/15Brain & Language - Harry Howard - Tulane University 12

Temporal coding Cyclical changes in sound amplitude are mirrored in the activation, and hence depolarization, of cochlear hair cells. This, in turn, is reflected in periodic increases and decreases in the probability of neurotransmitter release, and consequently, auditory nerve fibers are more likely to fire near the “crests of the sound wave,” than at its troughs. Downstream neurons can phase lock to such cyclical changes. 9/14/15Brain & Language - Harry Howard - Tulane University 13

NEXT TIME On to auditory cortex; there will be a reading 9/14/15Brain & Language - Harry Howard - Tulane University 14