Mechanisms of Sensory Perception: Vestibular and Auditory Systems

Mechanisms of the Vestibular System

Vestibular System Sensitivity to Acceleration: Mechanism

The vestibular system is sensitive to acceleration (linear or rotational) through the following mechanism:

• Semicircular Canals: There are three semi-circular canals, rotated 90 degrees relative to each other.
• Each canal is sensitive to acceleration in a different plane.
• The canals are filled with fluid (endolymph), which lags behind head movement, stimulating sensory hair cells.

Detecting Head Orientation (Tilt) via the Vestibular System

If the vestibular system is sensitive to acceleration, it can also detect the orientation (tilt) of the head because of its sensitivity to gravity. The otolith organs (utricle and saccule) contain dense structures (otoconia) that shift under gravitational force, signaling static head position.

Adaptation of the Vestibulo-Ocular Reflex (VOR)

Yes, the vestibulo-ocular reflex (VOR) can be adapted.

Example: Adaptation occurs when visual input conflicts with vestibular input (e.g., as demonstrated in example slide 16 regarding the vestibulo-ocular system).

Location of Adaptation: This adaptation occurs primarily in the cerebellum.

Sensory Input Changes in Posture Maintenance

Posture is maintained through the combination of visual, vestibular, and somatosensory input. The relative contribution of each of these inputs does change depending on environmental conditions.

We must be able to adapt these inputs depending on the conditions we are in:

• Example: When you are in a dark room, you do not have much visual information, requiring adaptation.
• Small Differences/Changes: Somatosensory input tends to dominate.
• Bigger Differences/Changes: There is a greater relative input from the vestibular system.

Auditory System: Sound Processing and Localization

External Ear Features for Sound Localization

The features of the external ear (pinna) are relevant for sound localization, particularly in the vertical plane (elevation). The mechanism works as follows:

• Sound reaches the eardrum via multiple pathways (some longer, some shorter) due to reflections off the pinna's complex shape.
• Differences in the time of arrival cause some signals to add together (constructive interference) and some to subtract from each other (destructive interference).
• This results in different frequencies being emphasized or attenuated, which affects the gain.

Defining Gain and Notch Frequency

Gain: For a certain input, gain is the size of the output signal you receive.

Notch Frequency: This is the frequency where gain is negative (i.e., frequencies are significantly out of phase and cancel each other out).

Understanding the Head-Related Transfer Function (HRTF)

The Head-Related Transfer Function (HRTF) is a measurement that describes how the outer ear, head, and torso transform sound waves before they reach the inner ear. It is essentially a comparison between the sound received at one ear and the other.

The HRTF depends on the torso, head position, and the shadowing of the body. How our own body and its orientation changes the sound allows us to detect where the sound originates.

Features of the HRTF allow localization in three dimensions:

• Elevation (Vertical Plane): Determined by the notch filter effect created by the pinna.
• Distance:
  • Power of the signal (decrease in power as sound distance increases).
  • Removal of high frequencies for distant sounds (high frequencies do not travel well).
• Azimuth (Horizontal Direction): Determined by the time difference between the two ears (Interaural Time Difference, ITD), which is most effective for low frequencies.

The Essential Role of the Middle Ear in Hearing

The middle ear plays a crucial role in hearing by transmitting sound from motion in air (outer ear) to motion in fluid (inner ear). This transmission is necessary because fluid is much harder to move than air, requiring an increase in gain to overcome the impedance mismatch.

The middle ear increases gain in two ways:

1. Mechanical Lever: The ossicles act as a mechanical lever system.
2. Difference in Area: The area of the oval window is approximately 21 times smaller than the area of the eardrum (tympanic membrane), concentrating the force.

The middle ear also protects the inner ear from loud noise through the contraction of muscles within the middle ear (acoustic reflex).