Notes

How what we have seen so far translates into encoding of information? Neighboring cells in the cortex tend to prefer the same/neighboring frequencies.

• Place principle of f determination: the frequency (f) of a sound that activates a particular hair cells depends on the location of the hair cell along the basilar membrane. This spatial organization is maintained all the way to the cerebral cortex. The auditory cortex shows that specific brain neurons are activated by specific sound frequencies (tonotopic organization). The brain knows which frequencies compose the sound by the movements of hair cells in the cochlear.
• Volley principle of f determination: low frequencies are discriminated by firing of the auditory nerve fibers at the same frequency as the sound wave. The frequency of firing of the cell corresponds to the frequency of the sound. If the frequency of the sounds is too high, i.e., the neuron cannot fire so many APs in a second, 2 cells can coordinate and fire alternatively.
• Loudness. As the amplitude of vibration increases, a larger proportion of the basilar membrane vibrates, causing more and more of the hair cells to move. This leads to spatial summation of impulses and transmission through a greater number of nerve fibers.

Signals from both ears are transmitted to both sides of the brain, with preponderance to contralateral pathway. Many collateral fibers to RAS of brain stem (loud sound). Tonotopic organization is maintained from cochlea to auditory cortex. Where high frequency sounds excite neurons at one end, whereas low frequency sounds excite neurons at the opposite end. The first auditory cortex is excited by the MGN, whereas the auditory association areas are excited secondarily by impulses from the first auditory cortex.

The circuits are quite complex because we use these circuits for sound localization.

Tonotopic Organization

Different frequencies are organized in a regular fashion, which represents the tonotopical organization.

Discrimination of "Sound Patterns" by the First and Second Auditory Cortex Destruction of both (but not one) first auditory cortices will reduce greatly one's sensitivity to hearing. Interpretation of the meaning and sequence of sound tones in the auditory signals - second auditory cortex.

There is a close association between the areas involved in the processing of auditory information (Auditory Cortex) and those involved in processing language (Wernicke's Area).

Physiology and Psychophysics There is a complicated relation between the physics of the signals and what we perceive.

• Cochlea performs mechanical spectral analysis of sound signal.
• Pure tone induces traveling wave in basilar membrane, maximum mechanical displacement along membrane is function of frequency (place coding).
• Displacement of basilar membrane changes with compression and rarefaction (frequency coding).
• The pitch stay to frequency as loudness stays to intensity.

Perception of Pitch

• Along the basilar membrane, hair cell response is tuned to frequency
  • Each neuron in the auditory nerve responds to acoustic energy near its preferred frequency.
  • Preferred frequency is place coded along the cochlea. Frequency coding believed to have a role at lower frequencies.
• Higher auditory centers maintain frequency selectivity and are "tonotopically mapped".
• Pitch is related to frequency for pure tones.
• For periodic or quasi-periodic sounds the pitch typically corresponds to inverse of period.
• Some have no perceptible pitch (e.g., clicks, noise).
• Sounds can have same pitch but different spectral content, temporal envelope ... timbre. The timbre is what allows us to hear the difference between the same frequency played by different instruments.

Perception of Loudness

• Intensity is measured on a logarithmic scale in decibels.
• Range from threshold to pain is about 120 dB-SPL (Sound Pressure Level).
• Loudness is related to intensity but also depends on many other factors (attention, frequency, harmonics, ...). Attention because you can isolate particular frequencies and/or locations (e.g., trumpet in a band).