Lecture 6: Sensory Systems: Audition, Equilibrioception, Somatosensation, Gustation, and Olfaction

Lecture 6: Sensation & Perception II

Introduction

This study guide covers the major sensory systems discussed in introductory psychology, focusing on audition (hearing), equilibrioception (balance), somatosensation (touch), gustation (taste), and olfaction (smell). Each system is explored in terms of its anatomy, physiological mechanisms, and pathways to the brain.

Audition & Equilibrioception

Audition (Hearing)

Audition is the process by which sound waves are detected and interpreted by the auditory system. The ear converts mechanical vibrations into neural signals that are processed by the brain.

• Sound Waves: Characterized by frequency (pitch) and amplitude (loudness). Frequency is measured in Hertz (Hz) and determines pitch. Amplitude is measured in decibels (dB) and determines loudness.

• Pathway of Sound:

  1. Sound waves vibrate the eardrum (tympanic membrane).

  2. Vibrations are transmitted via ossicles (malleus, incus, stapes) to the oval window of the cochlea.

  3. Movement of fluid in the cochlea activates hair cells on the basilar membrane.

  4. Neural signals travel via the auditory nerve to the inferior colliculus (midbrain), then to the medial geniculate nucleus (MGN) of the thalamus, and finally to the primary auditory cortex (A1) in the temporal lobe.

• Basilar Membrane:

  • Different regions respond to different frequencies (tonotopic organization).

  • The base is stiff and responds to high frequencies; the apex is flexible and responds to low frequencies.

• Theories of Pitch Perception:

  • Place Theory: Pitch depends on the location of hair cells activated along the basilar membrane.

  • Frequency Theory: Pitch depends on the rate of vibration of the basilar membrane.

  • Volley Theory: Neurons fire in alternating patterns to encode higher frequencies.

Example: Noise Levels

Everyday sounds vary in intensity. Exposure to high decibel levels can damage hearing.

Sound	Noise Level (dB)	Example
Jet engines (near)	140	Workplace hazard
Rock concerts	110-140	Hearing risk
Thunderclap (near)	120	Brief exposure
Power saw	110	Construction
Garbage truck	100	Street noise
Motorcycle (25 ft)	88	Traffic
Lawn mower	85-90	Yard work
Average city traffic	80	Urban environment
Vacuum cleaner	70	Home appliance
Normal conversation	50-65	Social interaction
Whisper	30	Quiet communication
Rustling leaves	20	Nature

Equilibrioception (Balance)

Equilibrioception is the sense of balance and spatial orientation, primarily mediated by the vestibular system in the inner ear.

• Anatomy:

  • Semicircular canals: Detect rotational movements.

  • Otolith organs (utricle and saccule): Detect linear acceleration and gravity.

• Mechanism:

  • Movement of fluid in the semicircular canals bends hair cells, generating neural signals.

  • Signals travel via the vestibular nerve to the brainstem and cerebellum.

• Applications:

  • Balance, posture, and coordination.

  • Motion sickness results from conflicting signals between vestibular and visual systems.

Somatosensation, Gustation, & Olfaction

Somatosensation (Touch)

Somatosensation encompasses the senses of touch, pressure, pain, temperature, and proprioception.

• Receptors:

  • Mechanoreceptors (touch, pressure)

  • Thermoreceptors (temperature)

  • Nociceptors (pain)

  • Proprioceptors (body position)

• Pathway:

  • Signals travel via sensory nerves to the primary somatosensory cortex (S1) in the parietal lobe.

• Example:

  • Light touch is detected by Meissner's corpuscles; pain by free nerve endings.

Gustation (Taste)

Gustation is the sense of taste, mediated by taste buds located on the tongue and oral cavity.

• Five Basic Tastes:

  • Sweet

  • Sour

  • Salty

  • Bitter

  • Umami (savory)

• Pathway:

  • Taste signals travel via cranial nerves to the gustatory cortex (insula, temporal lobe).

• Example:

  • Sweet taste is detected by receptors sensitive to sugars.

Olfaction (Smell)

Olfaction is the sense of smell, involving detection of airborne chemicals by olfactory receptors in the nasal cavity.

• Pathway:

  • Odor molecules bind to receptors in the olfactory epithelium.

  • Signals travel via the olfactory nerve to the primary olfactory cortex (temporal lobe).

• Example:

  • Detection of floral scents by specific olfactory receptors.

References

• Boland S & Souza T (2004). Neuroimaging evidence of odor-evoked memory. Neuropsychologia, 42(3), 371-374.

• Herz et al. (2003). Neuropsychologia, 2003.08.009.

Additional info:

• Equations for sound: $f = \frac{1}{T}$ where $f$ is frequency and $T$ is the period of the wave.

• Tonotopic organization: The spatial arrangement of where sounds of different frequency are processed in the brain.