Chapter 10

• Basilar membrane (mechanical process)


  When sound waves enter the cochlea through the oval window, they cause vibrations in the perilymph, a fluid within the cochlea. These vibrations travel through the vestibular canal, causing the basilar membrane to move in response to different frequencies.
• Basilar membrane (frequency-based displacement)


  The basilar membrane is tuned such that it vibrates differently along its length in response to different frequencies. It is thicker, narrower, and stiffer at the base (near the oval window) and becomes thinner, wider, and more flexible towards the apex. This means that high-frequency sounds cause maximal displacement at the base, while low-frequency sounds cause maximal displacement towards the apex.
• Basilar membrane (Location-specific Stimulation)


  As the basilar membrane vibrates, hair cells located on top of it bend in response to the movement. These hair cells are connected to auditory nerve fibers. The movement of hair cells triggers neural impulses in these fibers, with the location along the basilar membrane determining which nerve fibers are activated.
• Basilar Membrane (Place Coding)


  Place coding is fundamental to the perception of frequency. Essentially, the brain interprets signals from different regions of the basilar membrane as representing different frequencies. Thus, the specific location on the basilar membrane that vibrates the most corresponds to the frequency of the incoming sound.
• Basilar Membrane (Neural Transmission)


  The activated auditory nerve fibers transmit these signals to the brainstem and then to the auditory cortex, where they are further processed and interpreted as sound.
• Physiological Evidence for place codes for frequency (Frequency Tuning Curves)


  Physiological studies, such as those examining auditory nerve fiber profiles, demonstrate that different regions of the basilar membrane respond maximally to specific frequencies. This is evidenced by the characteristic frequency tuning curves of auditory nerve fibers, which show the frequency range over which each fiber is most responsive.
• Physiological Evidence for place codes for frequency (Tonotopic Organization)


  The tonotopic organization of the cochlea, where higher frequencies are processed near the base and lower frequencies near the apex, supports the idea of a place code. This organization is reflected in the spatial arrangement of auditory nerve fibers along the cochlea.
• Psychophysical Evidence for place codes for frequency (Psychophysical Tuning Curves)


  Masking experiments, where listeners are presented with a target tone and narrowband noise maskers at different frequencies, provide psychophysical evidence for place coding. These experiments demonstrate that the effectiveness of a masker in masking the target tone depends on its frequency relative to the target tone. When the masker frequency aligns with the region of the cochlea responding to the target tone, it is most effective in masking it.
• Psychophysical Evidence for place codes for frequency (Threshold Shifts)


  As the frequency of the masker deviates from the frequency of the target tone, the threshold for detecting the target tone increases. This suggests that the auditory system relies on specific regions of the cochlea to encode different frequencies, supporting the idea of place coding.
• Temporal coding (Phase locking)


  Auditory nerve fibers synchronize their firing to the peaks of the incoming sound wave. While individual fibers cannot fire at rates above a certain threshold (typically around 1000 spikes per second), they can synchronize their firing to coincide with the peaks of the sound wave.
• Temporal coding (Frequency Representation)


  For frequencies beyond the maximum firing rate of individual fibers, groups of fibers work together to represent the frequency. Each fiber contributes a spike at specific phases of the sound wave, resulting in a combined pattern of firing that corresponds to the frequency of the sound.
• Temporal coding (Volley Principle)


  Groups of auditory nerve fibers fire in synchrony to represent higher frequencies. While no single fiber can follow the entire waveform of a high-frequency sound, a population of fibers can collectively encode the frequency by firing in phase with different portions of the waveform.
• Evidence for volley principle (Physiological)


  Studies on auditory nerve responses have demonstrated phase locking and synchronization of firing patterns in response to sound waves. Physiological recordings have shown that groups of auditory nerve fibers fire in synchrony with specific phases of high-frequency sound waves.
• Evidence for volley principle (Behavioral)


  Experiments have shown that animals can discriminate between frequencies beyond the maximum firing rate of individual auditory nerve fibers, suggesting that temporal information is utilized in frequency perception.