Sensation:
1.0 Introduction:
 We interact with the world around us by means of a number of
sensory systems
 Sensation is the process of obtaining information about the
world through these sensory systems
 Sensation is distinct from the process of perception
 Aristotle identified 5 senses: sight, hearing, taste, smell, skin
senses
 Today we add to two more senses to Aristotle’s list:
 the kinesthetic sense (proprioception), which is involved
in sensing body position
 the vestibular sense, which is involved in sensing linear
and angular movement
 Sense organs transduce the energy of external stimuli into forms
of energy our nervous systems can interpret. There are 3 types
of transduction:
 Chemical
 Mechanical
 Thermal
 We encode quality (type) and intensity (strength) of stimuli
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 Note: different organisms have different sensory systems in
terms of both ranges of sensitivity and types of sensory
capability
 Organisms also differ in terms of which senses are dominant
 The different sensory capabilities of different organisms are the
result of evolution and the different environmental pressures
experienced by various species
 Different organisms can live in very different realities as a result
of having different sensory systems. The subsequent reality that
organisms live in is called an Umweld (Ethological psychology
term).
 We live in a uniquely human Umweld
2.0 Psychophysics
Absolute thresholds:
 The value of a stimulus for which the stimulus is detected 50%
of the time
Just noticeable differences (JND’s)
 The amount of change in intensity of a stimulus required to
detect a difference between the intensity of the two stimuli 50%
of the time (always in reference to a single sensory modality)
 Webber (1934) – the more intense a stimulus is to begin with,
the larger the change needed for it to be noticed. Webber
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discovered that that this ratio has a particular constant for each
sensory modality.
 Fechner (1860) – our sensitivity to stimuli of various intensities
is logarithmic – perceived magnitude increases rapidly at first,
then more slowly
Signal detection:
 The probability of detection depends upon both intensity of the
stimulus and response bias
Probability of detection = expectations + cost / benefit of
making correct Vs incorrect response
 This is a more sophisticated method for identifying thresholds
than using absolute thresholds
3.0 Vision:
 The eyes are sensitive to white light and its’ components
which represent only a small portion of the
electromagnetic spectrum
 The eye has two systems:
 One for forming images on the retina (optics)
 One for transducing the image into neural signals
(neurological)
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 Visual transduction involves:
 Number of neurons firing
 Which neurons fire
 How frequently neurons fire
Forming images on the retina:
 Normal eye
 Myopia
 Hyperopia
Parts of the eye:
Paths of each retina to the visual cortex:
Cones:
 Photopic (day vision – require high intensity light)
 High resolution
 Colour
Rods:
 Scotopic (night vision – washed out by daylight)
 Low resolution
 Monochromatic
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 Three types of cones:
 Blue – 450-500 nm (short)
 Green- 500-570 nm (medium)
 Red - 620-700 nm (long)
Colour blindness:
 Trichromats:
 Dichromats:
 Monochromats:
have all 3 types of cones
have only 2 of the 3 types of cones
have only 1 of the 3 types of cones
 Colour blindness is genetic and affects 2% of males and 0.035%
of females
Night adaptation:
 Time profile (rod / cone break)
 Use of red light
4.0 Other Sensory Systems (see text)
5.0 Sensory Integration (in-class discussion)
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