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.
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