Page 381 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Central Processing of Acoustic the impulses from the averted ear reach the
Information CNS later (nucleus accessorius, ! D5). Effects
(1) and (2) are additive effects (! B). The exter-
Various qualities of sound must be coded for nal ear helps to decide whether the sound is
signal transmission in the acoustic pathway. coming from front or back, above or below.
These include the frequency, intensity and Binaural hearing also helps to distinguish a
direction of sound waves as well as the dis- certain voice against high background noise,
e.g., at a party. Visibility of the speaker’s mouth
tance of the sound source from the listener.
Central Nervous System and Senses conducted in separate fibers of the auditory mined because high frequencies are at-
of
Tones
Frequency
imaging.
various
also facilitates comprehension.
frequencies are “imaged” along the cochlea,
Distance to the sound source can be deter-
tenuated more strongly than low frequencies
pathway and centrally identified. Assuming
during sound wave conduction. The longer the
that a tone of 1000 Hz can just be distin-
sound wave travels, the lower the proportion
guished from one of 1003 Hz (resembling true
conditions), the frequency difference of 3 Hz
of high frequencies when it reaches the
corresponds to a relative frequency differential
listener. This helps, for instance, to determine
whether a thunderstorm is nearby or far away.
threshold of 0.003 (! p. 352). This fine differ-
ential capacity is mainly due to frequency im-
Auditory pathway (! D). The auditory
aging in the cochlea, amplification by its outer
ral ganglion of the cochlea project from the
along the auditory pathway (! p. 313 D). This
cochlea (! D1) to the anterolateral ( ! D2),
posteroventral and dorsal cochlear nuclei
fine tuning ensures that a certain frequency
12 hair cells (! p. 366), and neuronal contrast nerve fibers with somata positioned in the spi-
has a particularly low threshold at its “imag-
(! D3). Afferents in these three nuclei exhibit
ing” site. Adjacent fibers are not recruited until tonotopicity, i.e., they are arranged according
higher sound pressures are encountered. to tone frequency at different levels of com-
Intensity. Higher intensity levels result in plexity. In these areas, lateral inhibition
higher action potential frequencies in afferent (! p. 313 D) enhances contrast, i.e., suppresses
nerve fibers and recruitment of neighboring noise. Binaural comparison of intensity and
nerve fibers (! A). The relative intensity differ- transit time of sound waves (direction of
ential threshold is 0.1 (! p. 352), which is very sound) takes place at the next-higher station
crude compared to the frequency differential of the auditory pathway, i.e. in the superior
threshold. Hence, differences in loudness of olive (! D4) and accessory nucleus (! D5). The
sound are not perceived by the human ear next stations are in the nucleus of lateral lem-
until the intensity level changes by a factor of niscus (! D6) and, after most fibers cross over
over 1.1, that is, until the sound pressure to the opposite side, the inferior quadrigeminal
changes by a factor of over !1,1 = 1,05. bodies (! D7). They synapse with numerous
Direction. Binaural hearing is needed to afferents and serve as a reflex station (e.g.,
identify the direction of sound waves and is muscles of the middle ear; ! p. 366). Here,
based on the following two effects. (1) Sound sensory information from the cochlear nuclei
waves that strike the ear obliquely reach the is compared with spatial information from the
averted ear later than the other, resulting in a superior olive. Via connections to the superior
lag time. The change in direction that a normal quadrigeminal bodies (! D8), they also ensure
human subject can just barely detect (direction coordination of the auditory and visual space.
threshold) is roughly 3 degrees. This angle By way of the thalamus (medial geniculate
delays the arrival of the sound waves in the body, MGB; ! D9), the afferents ultimately
averted ear by about 3 · 10 s (! B, left). (2) reach the primary auditory cortex (! D10) and
-5
Sound reaching the averted ear is also per- the surrounding secondary auditory areas
ceived as being quieter; differences as small as (! p. 311 E, areas 41 and 22). Analysis of com-
1 dB can be distinguished. A lower sound pres- plex sounds, short-term memory for compari-
sure results in delayed firing of actions poten- son of tones, and tasks required for “eaves-
368 tials, i.e., in increased latency (! B, right). Thus, dropping” are some of their functions.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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