244 Neuroanatomy: Draw It to Know It
Auditory Physiology ( Advanced )
Here, we will learn the physiolog y of tonotopy (the top-
ographic organization of sound) and also the physiolog y
of binaural and monaural sound localization. First, we
will show the tonotopy of select regions of the auditory
pathway. Indicate that the base of the cochlea encodes
high-frequency sounds and that the apex encodes low-
frequency sounds. Th en, within the ventral and dorsal
cochlear nuclei, show that low-frequency sounds lie
anterior whereas high-frequency sounds lie posterior.
Finally, indicate that within the transverse temporal g yri
(Heschl’s g yri) (the primary auditory cortex), low-fre-
quency sounds localize laterally whereas high-frequency
sounds localize medially. Th us, there is preservation of
tonotopy throughout the central auditory system.^7 , 8
Each cerebral hemisphere receives similar input from
each set of cochlear nuclei, so when there is a unilateral
lesion to any of the steps in the central auditory pathway,
hearing is preserved from the duplication of auditory
information in the contralateral cerebral hemisphere.
Th e auditory system has this central duplication of sen-
sory information whereas other sensory systems, such as
the visual and somatosensory systems, do not because
the peripheral receptors of the visual and somatosensory
systems are topographically arranged to encode for stim-
ulus localization, whereas the cochlea is tonotopically
arranged, instead.
Tonotopy informs the brain about sound frequency
but not location; therefore, sound localization must
occur through other means. Sound reaches each ear (the
near ear and far ear) at slightly diff erent times and inten-
sities. Th ese are the two main interaural (between ear)
diff erences: the interaural time diff erence and the inter-
aural level diff erence (where “time” refers to the time the
sound reaches each ear and “level” refers to the sound
intensity level at each ear). Sound reaches the near ear
before it reaches the far ear to create the interaural time
diff erence, and the head, itself, attenuates the sound
before it reaches the far ear to create the interaural level
diff erence. In accordance with the duplex theory of
sound localization, time diff erences are best detected for
low-frequency sounds whereas level diff erences are best
detected for high-frequency sounds.
Now, let’s incorporate these interaural diff erences
into the core pathways for binaural sound localization.
First, in coronal view, on both sides of the brainstem,
draw the following superior olivary complex nuclei:
the medial nucleus of the trapezoid body, medial supe-
rior olivary nucleus, and lateral superior olivary nucleus.
Low-frequency sounds are encoded within the spheri-
cal bushy cells of the ventral cochlear nucleus. Indicate
that they project bilaterally to the medial superior
olivary nuclei, which are sensitive to interaural time
diff erences.
High-frequency sounds are encoded within the ven-
tral cochlear nucleus in both the spherical bushy cells
and globular bushy cells. Both cell types project to the
lateral superior olivary nucleus, which is sensitive to
interaural level diff erences. Show that the spherical bushy
cells send exclusively ipsilateral, excitatory projections to
the lateral superior olivary nucleus. On the contrary,
using the ventral cochlear nucleus on the opposite side,
show that the globular bushy cells send excitatory pro-
jections to the contralateral medial nucleus of the trape-
zoid body, which in turn sends inhibitory projections
to the ipsilateral lateral superior olivary nucleus (thus
the globular bushy cells inhibit the contralateral lateral
superior olivary nucleus via the medial nucleus of the
trapezoid body).^9 – 16
Th ese pathways establish the physiolog y of binaural
sound localization, but plug one of your ears, close your
eyes, and snap your fi ngers at diff erent points in space.
Even with only one ear, you can still localize sound stim-
uli fairly well because of your monaural sound localiza-
tion skills. Monaural localization of sound relies on the
head-related transfer function, which shapes sound
waves as they propagate toward the eardrum. Th rough
monaural sound localization mechanisms, the head,
torso, and pinna distort sound waves into spectral pat-
terns that the central auditory system uses in the local-
ization of sound.^15