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responds。 Beyond a certain level； further increase in physical intensity no longer produces
a subjective change in intensity； because the system is saturated。
6。 Response Latency。 Every system is a transducer； in that it converts energy from one medium
to another so that it can be processed。 This transduction process takes about 20—30
milliseconds； and about 200—500 milliseconds following the stimulus； you bee aware
of the sensation。 Thus； we live 200 milliseconds in the past。
The Sensory System
In learning about sensation； it is important for your class to be aware that we have three different
types of sensory systems； each of which performs different functions。
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PSYCHOLOGY AND LIFE
1。 Exteroceptors。 These sensory receptors take data from the external world。 Types of
exteroceptors include distal and proximal receptors。 Distal receptors include those
associated with vision。 Objects rarely make direct contact with the eye； rather they are
discerned at a distance； with no need for contact in order to experience the sensation。
Proximal receptors are associated with touch； taste； and possibly olfaction。 Thermal
radiation does not always require proximity； you can tell that the sun is warm via your
distal receptors–you do not have to touch it。 In most instances； proximal systems require
direct contact with the stimulus。
2。 Interoceptors。 These are internal system monitors； they work to keep you aware of the
internal working of your body； such as letting you know when you are hungry； thirsty； in
pain； nauseated； fatigued； and so on。
3。 Proprioceptors。 These receptors monitor the position of the body or limbs relative to some
reference point。 They let you know where you are physically located in space。
Proprioceptors are found in the vestibular system； where they permit maintenance of your
physical position； in the pressure receptors of the skin； in the muscle stretch receptors of
your muscles； and in the joint movement receptors of your limbs。
Auditory Localization
We use our ears to point our eyes in the direction of sound…producing events。 For this to happen；
the auditory system must be able to perceive the direction from which a sound is originating； and
the system’s perception of space must be integrated with the visual system’s perception of space。
Unlike the eye； the ear has no direct coding of spatial direction。 Information about the sound’s
direction is perceived by paring the stimulation in one ear with that in the other。 In this respect；
sound localization is much like the visual…depth cue of binocular disparity。
There are two basic sources of information about sound ing from the left or right； the sound
entering one ear differs from that entering the other in both intensity and time。 When a sound es
from directly in front of your head； its intensity is equal at your two ears。 In the case of high…
frequency sounds ing from the side； your head creates a sound shadow； making the sound less
intense at the ear farthest away from the sound than at the ear closest to the sound。 It is only for
high frequencies that there is information about how far to one side or another a sound is located。
The other major source of information about the horizontal direction of a sound is the time at which
it arrives at your two ears。 When a sound es from directly in front of your head； the arrival times
are the same because your two ears are the same distance away from the sound。 However； when the
sound es from the side； the sound wave must travel farther to reach the ear on the far side。 Even
though this extra distance takes only a little extra time—less than one…thousandth of a second—it is
enough to tell us which side sound is ing from。
The direction of sounds from left to right； or right to left； is probably the most important part of
spatial hearing； but it is not the only part。 You can also tell whether a sound is ing from above
or below—the sound of a jet streaking overhead or of an object dropped at your feet。 You are not
able to perceive vertical direction from simple arrival times or intensities； however。 It is the shape of
the external ear that allows you to perceive the vertical dimension of space。 Notice that your ear is
asymmetrical。 There are many plex； sound…reflecting folds in the pinna above the ear canal； and
few below it。 These differences in the shape of the external ear make subtle changes in the sound
wave that enters your ears； depending on the vertical direction of the sound source。 Somewhere in
the auditory centers of the brain； these differences are detected and decoded； allowing you to
perceive upward and downward directions of environmental sounds。
We are left with the problem of perceiving the third dimension of depth—how far away the source
of a sound is from us。 A sound that is near is louder than one that is far away； so you might think
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CHAPTER 4： SENSATION
that intensity would provide all the information you need about the distance （or depth） of the
source of a sound。 Unfortunately； it is not that easy。 A low…intensity sound at the ear might have
e from either a loud sound far away or a soft one nearby。 This situation is analogous to the
relations among retinal size； object distance； and object size in visual perception。 If the sound is one
whose usual intensity you know； such as someone speaking in a normal voice or the sound of an
average car engine； you can perceive its approximate distance by sound using intensity
information。 If the sound is one whose usual intensity you do not know； you cannot tell how far
away it is by hearing it； you have to look。 Because you can locate the direction that the sound is
ing from using your ears； you can use them to point your eyes in the correct direction； which
can then do the job of judging distance。
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PSYCHOLOGY AND LIFE
BIOGRAPHICAL PROFILES
Hermann von Helmholtz （1821–1894）
Hermann Von Helmholtz obtained his M。D。 in Berlin and served subsequently as an Army surgeon
for seven years。 Following his military service； he studied math and physics and held academic
appointments over the next 30 years at Bonn； Heidelberg； and Berlin； initially as a physiologist；
later as a physicist。 Helmholtz； whose versatility and intellectual brilliance manifested itself in
various disciplines； is considered one of the true giants in the history of science。
Helmholtz’s prominence in physiology came chiefly from his discovery of the rate of neural
conduction； a finding that surprised many of his contemporaries who had assumed that nerve
impulses must travel at or near the speed of light。 In addition； he invented the opthalmoscope while
researching vision； and was involved in the development of theories of color vision and pitch
perception that remain influential today。 His published works include the three…volume series
Physiological Optics （1856—1866）。
Ernst Heinrich Weber （1795—1879）
Ernst Weber taught anatomy and physiology at the University of Leipzig； Germany； from 1820 until
the end of his career。 He is remembered in psychology for his studies of psychophysical relations；
especially for the sensations of temperature and touch。 Weber was the first to investigate the two…
point threshold for touch； observing that sensitivity to touch varied across different parts of the
body and demonstrating that regions of the body are differentially sensitive to tactile stimulation。
Weber’s analysis of difference thresholds led to the finding that the size of the difference threshold
remains a constant fraction of the stimulus intensity； an orderly relationship referred to as Weber’s
Law。
Ronald Melzack （b。 1929）
Ronald Melzack was raised and educated in Montreal； Canada； obtaining his Ph。D。 from McGill
University in 1958。 He conducted research in pain sensation at the University of Oregon Medical
School from 1954 to 1957。 Following this； he was a visiting lecturer at University College； London；
and spent a year conducting physiological research in Italy at the University of Pisa。 He was
appointed to the faculty of the Massachusetts Instit