7.5 More revision questions
Describe how phase locking transmits information about the frequency of a sound (include the volley principle).
Phase locking is the consistent firing of an auditory neuron at the same phase of each cycle of a sound wave. At low frequencies, the neuron will fire action potentials at some constant location on the wave (peak or trough, for example) so that the frequency of the sound can be determined from the frequency of the neuron's action potentials. For higher-frequency sounds, neurons may not fire on every cycle even though they fire at the same point on the cycle. A group of such neurons can encode the frequency of the sound wave if their activity is pooled.
The place theory of frequency coding is regarded as being of minimal use for very low frequencies. Why is this?
Recall that low frequencies create a rather broad or flat pattern of vibratory activity on the basilar membrane – nearly the entire membrane moves although the peak in the wave is towards the base. High frequencies on the other hand create a wave that is very localised – only a small part of the membrane moves. This means that for low frequencies, the displacement pattern on the membrane is much less specific and localisable than the peak displacement at high frequencies.
Describe how the movement of the basilar membrane provides the brain with information about a signal's (a) frequency and (b) intensity.
(a) The travelling wave on the basilar membrane has a peak amplitude at a location determined by the frequency of the incoming sound wave. For low-frequency sounds the peak is located at the apical end of the membrane while for high-frequency sounds the peak is located at the basal end. Hair cells are located along the basilar membrane. Cells that are located at the position where the wave peaks are stimulated, resulting in the generation of action potentials in the auditory nerve fibres contacting those hair cells. Fibres on the outside of the auditory nerve innervate the basal hair cells and therefore fire in response to high-frequency sounds whereas fibres on the inside of the auditory nerve innervate apical hair cells and therefore fire in response to low-frequency sounds. So when a sound of a certain frequency stimulates the basilar membrane, the brain receives information about the frequency of the sound, as a consequence of which the fibres in the auditory nerve fire action potentials at the highest relative rate.
(b) The higher the intensity of the sound impinging on the ear, the greater the amplitude of the wave produced on the basilar membrane. The higher amplitude wave causes nerve fibres to fire at a greater rate (the number of action potentials per second is higher for a high-amplitude wave compared to a low-amplitude wave) or causes more neurons to fire (auditory fibres have different thresholds and the greater the displacement of the membrane the greater the number of neurons that reach threshold).
Use diagrams similar to Figure 24 to illustrate how the firing pattern of auditory neurons connected to two different (but close together) positions on the basilar membrane could encode information about the frequency and intensity of the following signals: (a) a low-intensity low-frequency tone; (b) a low-intensity high-frequency (but < 1 kHz) tone; (c) a high-intensity low-frequency tone; and (d) a high-intensity, high-frequency (but < 1 kHz) tone.
Frequency is determined by the place code (which neurons are firing) and by a temporal code (neurons fire in bursts that phase lock to the stimulus frequency). Intensity is determined by the firing rate (more spikes per burst for louder sounds) and the number of neurons (at high intensity, more spikes are produced from both positions). (see Figure 31).
Describe the method used to determine the characteristic frequency of a single auditory nerve fibre?
To determine the characteristic frequency of an auditory nerve fibre you would construct a tuning curve. Tuning curves indicate the sound pressure level at the eardrum that is just sufficient to elicit a detectable increase in the firing rate of an auditory nerve fibre, as a function of the frequency of a pure tone stimulus. For each fibre the lowest intensity of a pure tone that will produce a detectable response across a range of pure tones is determined. The frequency of the tone for which the threshold of a given fibre is lowest is called the critical or characteristic frequency.
Where in the higher auditory centres does binaural processing of information begin? What is the nature of the information used that enables us to localize sounds? How does the operation of excitatory and inhibitory inputs enable the auditory system to use this information?
Binaural processing of information begins in the superior olivary complex, using interaural time delays and interaural intensity differences. In the MSO, neurons increase their firing rate in response to sounds from both ears, and will increase their discharge rate even further when sounds reach both ears with a certain delay (interaural time delays, EE units). In the LSO, neurons increase their firing rate in response to sounds in the ipsilateral ear and are inhibited from firing by sounds in the contralateral ear. So stimulation from both ears may decrease the firing rate of neurons compared to stimulation by one ear (EI units).
Activity 1 The Senses and Hearing animation
This is a good time to break from reading and look at two interactive activities. The first involves exploring the structure and function of the ear, the middle and inner ear, the auditory pathways to the brain and the auditory cortex. There is also an explanation of sound in this activity. The second activity is to look at an animation of the transmission of sound from the outer to the inner ear.