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

Lecture 6: Sensation & Perception II

Overview

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 brain. The ear is the primary organ responsible for hearing, and it consists of several key structures that convert sound waves into neural signals.

• Sound Waves: Vibrations in the air that are characterized by frequency (pitch) and amplitude (loudness).

• Frequency: Number of cycles per second (measured in Hertz, Hz); determines pitch. $f = \frac{1}{T}$ where $T$ is the period of the wave.

• Amplitude: Height of the wave; determines loudness.

• Outer Ear: Collects sound waves and funnels them to the eardrum (tympanic membrane).

• Middle Ear: Contains ossicles (malleus, incus, stapes) that amplify vibrations and transmit them to the oval window of the cochlea.

• Inner Ear (Cochlea): Fluid-filled structure where sound waves cause movement of the basilar membrane, activating hair cells (sensory receptors).

• Basilar Membrane: Varies in stiffness along its length; high-frequency sounds activate hair cells at the base, low-frequency sounds at the apex.

• Auditory Pathway: Cochlea → Auditory nerve → Inferior colliculus (midbrain) → Medial geniculate nucleus (MGN; thalamus) → Primary auditory cortex (A1; temporal lobe)

Pitch Perception Theories

• Place Theory: Pitch is determined by the location of hair cells activated along the basilar membrane.

• Frequency Theory: Pitch is determined by the rate of neural firing in response to sound frequency.

• Volley Principle: Groups of neurons fire in alternating patterns to encode higher frequencies.

Noise Levels and Hearing Safety

Exposure to loud sounds can damage hair cells and lead to hearing loss. The following table summarizes common sound sources and their noise levels:

Sound	Noise Level (dB)	Example
Jet engines (near)	140	Very loud, risk of hearing damage
Rock concerts	110-140	Hearing protection recommended
Thunderclap (near)	120	Brief exposure, risk of damage
Power saw (chainsaw)	110	Occupational hazard
Normal conversation	50-65	Safe
Whisper	30	Very quiet
Rustling leaves	20	Minimal

Equilibrioception (Balance)

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

• Vestibular Organs:

  • Semicircular Canals: Detect rotational movements of the head.

  • Otolith Organs (Utricle and Saccule): Detect linear acceleration and gravity.

• Mechanism: Movement of fluid in the semicircular canals and otolith organs bends hair cells, generating neural signals.

• Vestibular Pathway: Vestibular ganglion → Vestibular nerve → Brainstem → Cerebellum → Various cortical areas

• Motion Sickness: Occurs when there is a mismatch between visual and vestibular inputs.

Somatosensation, Gustation, & Olfaction

Somatosensation (Touch)

Somatosensation refers to the perception of bodily sensations, including touch, pressure, pain, temperature, and proprioception.

• Primary Somatosensory Cortex (S1): Located in the parietal lobe; processes sensory input from the body.

• Types of Sensory Receptors:

  • Mechanoreceptors: Detect touch and pressure.

  • Nociceptors: Detect pain.

  • Thermoreceptors: Detect temperature changes.

• Pathway: Sensory neurons → Spinal cord → Thalamus → S1

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

• Taste Pathway: Taste receptor cells → Cranial nerves → Gustatory cortex (insula; temporal lobe)

• Function: Helps identify nutrients and avoid toxins.

Olfaction (Smell)

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

• Olfactory Pathway: Olfactory receptor cells → Olfactory nerve → Primary olfactory cortex (temporal lobe)

• Role in Memory: Olfactory cues are strongly linked to emotional and episodic memory.

Example: Olfactory Memory

Neuroimaging studies (e.g., Boland & Souza, 2004) show that odor-evoked memories activate specific brain regions associated with emotional processing.

Summary Table: Sensory Systems

System	Receptors	Primary Cortex	Main Function
Audition	Hair cells (cochlea)	Auditory cortex (A1)	Hearing
Equilibrioception	Hair cells (vestibular organs)	Parietal/temporal cortex	Balance
Somatosensation	Mechanoreceptors, nociceptors, thermoreceptors	Somatosensory cortex (S1)	Touch, pain, temperature
Gustation	Taste buds	Gustatory cortex (insula)	Taste
Olfaction	Olfactory receptors	Olfactory cortex	Smell

Additional info: Some details about neural pathways and receptor types were expanded for clarity and completeness.