Sensation & Perception II: Audition, Equilibrioception, Somatosensation, Gustation, & Olfaction

Lecture 6: Sensation & Perception II

Introduction

This lecture covers advanced topics in sensation and perception, focusing on the psychological and physiological mechanisms underlying hearing (audition), balance (equilibrioception), touch (somatosensation), taste (gustation), and smell (olfaction). These sensory systems are essential for interpreting the environment and guiding behavior.

Audition & Equilibrioception

Audition (Hearing)

Audition is the process by which sound waves are detected and interpreted by the auditory system. It involves the transformation of mechanical vibrations into neural signals that are processed by the brain.

• Sound Waves: Vibrations in the air (measured in Hz for pitch and dB for loudness) strike the eardrum, causing it to move.

• Middle Ear: The eardrum's movement is transmitted via ossicles (malleus, incus, stapes) to the oval window, which moves fluid within the cochlea.

• Cochlea: Fluid movement in the cochlea causes the basilar membrane to vibrate. Hair cells on the membrane transduce these vibrations into electrical signals.

• Pitch Perception:

  • Place Theory: Pitch depends on the location of hair cells activated along the basilar membrane. High-frequency sounds stimulate the base; low-frequency sounds stimulate the apex.

  • Frequency Theory: Pitch depends on the frequency of basilar membrane vibration. Neurons fire in synchrony with the sound wave frequency.

• Auditory Pathway: Auditory nerve → inferior colliculus (midbrain) → medial geniculate nucleus (MGN; thalamus) → auditory cortex (A1; temporal lobe).

Example: Exposure to loud sounds (e.g., jet engines, rock concerts) can damage hair cells, leading to hearing loss.

Noise Levels and Effects

Sound	Noise Level (dB)	Effect
Jet engines (near)	140	Immediate pain, hearing damage
Rock concerts (varies)	110-140	Risk of hearing loss
Thunderclap (near)	120	Pain threshold
Power saw (chainsaw)	110	Potential hearing damage
Garbage truck/Cement mixer	100	Uncomfortable
Motorcycle (25 ft)	88	Annoying
Lawn mower	85-90	Annoying
Average city traffic	80	Annoying
Vacuum cleaner	70	Annoying
Normal conversation	50-65	Comfortable
Whisper	30	Comfortable
Rustling leaves	20	Comfortable

Equilibrioception (Balance)

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

• Vestibular System: Composed of the semicircular canals (detect rotational movement) and otolith organs (utricle and saccule; detect linear acceleration and gravity).

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

• Pathway: Vestibular ganglion → vestibular nuclei (brainstem) → integration with cerebellum and other brain regions.

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

Example: Riding in a car while reading can cause motion sickness due to conflicting sensory information.

Somatosensation, Gustation, & Olfaction

Somatosensation (Touch)

Somatosensation encompasses the perception of touch, pressure, pain, and temperature through specialized receptors in the skin and body.

• Receptors: Mechanoreceptors (e.g., Meissner's corpuscles, Pacinian corpuscles) detect different types of stimuli.

• Pathway: Sensory nerves → spinal cord → brainstem → primary somatosensory cortex (S1; parietal lobe).

• Sensory Homunculus: A map of the body in the brain, representing the relative sensitivity of different regions.

• Fiber Types: Myelinated fibers transmit signals rapidly; unmyelinated fibers transmit signals slowly.

Example: The fingertips have a high density of mechanoreceptors, allowing for fine tactile discrimination.

Gustation (Taste)

Gustation is the sense of taste, mediated by taste cells located in taste buds on the tongue.

• Taste Modalities: Salty (NaCl), sweet, sour, bitter, umami.

• Pathway: Taste cells → cranial nerves → brainstem → thalamus → gustatory cortex (insula; temporal lobe).

Example: Umami is the taste of glutamate, commonly found in foods like tomatoes and cheese.

Olfaction (Smell)

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

• Olfactory Receptors: Bipolar neurons located in the olfactory epithelium.

• Pathway: Olfactory bulb → olfactory nerve → primary olfactory cortex.

• Unique Feature: Olfactory signals bypass the thalamus and project directly to the cortex.

Example: The smell of freshly baked bread can trigger vivid memories due to direct connections between olfactory and limbic regions.

Additional info:

• Neuroimaging studies (e.g., Herz et al., 2004) show that olfactory cues can evoke emotional and autobiographical memories.