Determination of the Direction
from Which Sound Comes
A person determines the horizontal direction from which sound comes
by two principal means: (1) the time lag between the entry of sound into one
ear and its entry into the opposite ear, and (2) the difference between the
intensities of the sounds in the two ears.
The first mechanism functions best at frequencies below 3000 cycles
per second, and the second mecha-nism operates best at higher frequencies
because the head is a greater sound barrier at these frequencies. The time lag
mechanism discriminates direction much more exactly than the intensity
mechanism because it does not depend on extraneous factors but only on the
exact interval of time between two acoustical signals. If a person is looking
straight toward the source of the sound, the sound reaches both ears at exactly
the same instant, whereas if the right ear is closer to the sound than the left
ear is, the sound signals from the right ear enter the brain ahead of those
from the left ear.
The two aforementioned mechanisms cannot tell whether the sound is
emanating from in front of or behind the person or from above or below. This
dis-crimination is achieved mainly by the pinnae
of the two ears. The shape of the pinna changes the quality of the sound entering the ear, depending on the direc-tion
from which the sound comes. It does this by emphasizing specific sound
frequencies from the dif-ferent directions.
Neural
Mechanisms for Detecting Sound Direction. Destruc-tion of the auditory cortex on both
sides of the brain, whether in human beings or in lower mammals, causes loss of
almost all ability to detect the direction from which sound comes. Yet the
neural analyses for this detection process begin in the superior olivary nuclei in the brain stem, even though the neural
pathways all the way from these nuclei to the cortex are required for
interpretation of the signals. The mechanism is believed to be the following.
The superior olivary nucleus is divided into two sec-tions: (1) the
medial superior olivary nucleus and
(2) the lateral superior olivary nucleus.
The lateral nucleus is concerned with detecting the direction from which the
sound is coming, presumably by simply comparing the difference in intensities of the sound reaching the two ears and
sending an appropriate signal to the audi-tory cortex to estimate the
direction.
The medial superior olivary
nucleus, however, has a specific mechanism for detecting the time lag betweenacoustical signals entering the two ears.
This nucleuscontains large numbers of neurons that have two major
dendrites, one projecting to the right and the other to the left. The
acoustical signal from the right ear impinges on the right dendrite, and the
signal from the left ear impinges on the left dendrite. The intensity of
excitation of each neuron is highly sensitive to a specific time lag between
the two acoustical signals from the two ears. The neurons near one border of
the nucleus respond maximally to a short time lag, while those near the
opposite border respond to a long time lag; those in between respond to
intermediate time lags. Thus, a spatial pattern of neuronal stimulation
develops in the medial superior olivary nucleus, with sound from directly in
front of the head stimulating one set of olivary neurons maximally and sounds
from different side angles stimulating other sets of neurons on opposite sides.
This spatial orientation of signals is then transmitted to the auditory cortex,
where sound direction is determined by the locus of the maximally stimulated
neurons. It is believed that all these signals for determining sound direction
are transmitted through a different pathway and excite a different locus in the
cerebral cortex from the transmission pathway and termination locus for tonal
patterns of sound.
This mechanism for detection of sound direction indicates again how
specific information in sensory signals is dissected out as the signals pass
through different levels of neuronal activity. In this case, the “quality” of
sound direction is separated from the “quality” of sound tones at the level of
the superior olivary nuclei.
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2024 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.