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Copyright © 2009 Allyn & Bacon

How You Know the World

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Chapter 7: Mechanisms ofPerception – Hearing, Touch,Smell, Taste, and Attention


The Case of the Man Who Could See Only One Thing at a Time

This chapter focuses on the four exteroreceptive sensory systems besides vision that interpret external stimuli

Why would a man be unable to see two objects simultaneously when he can see each individually?

What could cause this deficit?


Principles of Sensory System Organization

Primary – input mainly from thalamic relay nuclei For example, striate cortex receives input from

the lateral geniculate nucleus Secondary – input mainly from primary and

secondary cortex within the sensory system Association – input from more than one sensory

system, usually from secondary sensory cortex


Principles of Sensory System Organization (continued)

Hierarchical organization Specificity and complexity increases with each

level Sensation – detecting a stimulus Perception – understanding the stimulus

Functional segregation – distinct functional areas within a level

Parallel processing – simultaneous analysis of signals along different pathways


Two models of sensory system organization


Auditory System: Sound The relation between the physical and perceptual dimensions of sound


The Ear Sound waves enter the auditory canal

of the ear and then cause the tympanic membrane (the eardrum) to vibrate

This sets in motion the bones of the middle ear, the ossicles, which trigger vibrations of the oval window


The Ear (continued)

Anatomy of the ear

AuditoryNerve

Cochlea

Semicircular canals

(unwound) Roundwindow

Oval window Tympanic membrane

Tectorialmembrane

Haircells

Basilarmembrane

Organof Corti

Auditorynerve


The Ear (continued)

Sound wave > eardrum > ossicles (hammer, anvil, stirrup) > oval window

Vibration of the oval window sets in motion the fluid of the cochlea

The cochlea’s internal membrane, the organ of Corti, is the auditory receptor organ


The Ear (continued)

Organ of Corti Composed of two membranes

Basilar membrane – auditory receptors, hair cells, are mounted here

Tectorial membrane – rests on the hair cells

Stimulation of hair cells triggers action potentials in the auditory nerve


The Ear (continued)

Cochlear Coding Different frequencies produce maximal

stimulation of hair cells at different points along the basilar membrane

Tonotopic (frequency) organization of the basilar membrane and most other auditory system components


Some pathways ofthe auditory system

from one ear tothe cortex

The Ear(continued)


Subcortical Mechanisms of Sound Localization The lateral and medial superior olives

react to differences in what is heard by the two ears Medial – arrival time differences Lateral – amplitude differences

Both project to the superior colliculus The deep layers of the superior colliculus is laid

out according to auditory space, allowing location of sound sources in the world; the shallow layers are laid out retinotopically


Primary and Secondary Auditory Cortex

Two or three areas of primary auditory cortex About seven areas of secondary auditory

cortex Functional columns (cells of a column

respond to the same frequency) Tonotopic organization Secondary areas do not respond well to pure

tones and have not been well-researched


Effects of Damage to the Auditory System

Lesions of auditory cortex in rats results in few permanent hearing deficits

Lesions in monkeys and humans hinder sound localization and pitch discrimination

Deafness in humans Total deafness is rare, due to multiple pathways Two kinds: conductive deafness (damage to

ossicles) and nerve deafness (damage to cochlea) Partial cochlear damage results in loss of hearing at

particular frequencies

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