is now used to predict the accuracy
of judgments by systematically taking into account two kinds
of errors made by observers--reporting the presence of stimuli
when there is none and reporting the absence of a stimulus when
one is actually present. Difference thresholds relate to the
smallest detectable difference between two stimuli, known as
a just noticeable difference. According to Weber’s law,
a just noticeable difference is a constant proportion of the
intensity of an initial stimulus.
Sensory adaptation occurs
when we become accustomed to a constant stimulus and change
our evaluation of it. Repeated exposure to a stimulus results
in an apparent decline in sensitivity to it.
What are the major senses,
and what are the basic mechanisms that underlie their operation?
Vision depends on sensitivity
to light, electromagnetic waves that are reflected off objects
outside the body. The eye shapes the light into an image that
is transformed into nerve impulses and interpreted by the brain.
When light first enters
the eye, it travels through the cornea and transverses the pupil,
a dark hole in the center of the iris. The size of the pupil
opening adjusts according to the amount of light entering the
eye. Light then enters the lens, which, by a process called
accommodation, acts to focus the light rays onto the rear of
the eye. On the rear of the eye is the retina, which is composed
of light sensitive nerve cells called rods and cones. The rods
and the cones are unevenly spaced over the retina, with the
greatest concentration of cones occurring in an area called
the fovea.
The visual information gathered
by the rods and cones are transferred via bipolar and ganglion
cells through the optic nerve, which leads to the optic chiasm--
the point where the optic nerve splits. Because the image on
the retina is reversed and upside down, images from the right
half of the retina are actually from the field of vision to
the left of the person, and vice-versa. Moreover, because of
the phenomenon of adaptation, it takes time to adjust to situations
that are either measurably lighter or measurably darker that
the previous environment.
Color vision seems to be based
on two processes described by the trichromatic theory and opponent
process theory. The trichromatic theory suggests that there
are three kinds of cones in the retina, each of which is responsive
to a certain range of colors. The opponent-process theory presumes
pairs of different types of cells in the eye. These cells work
in opposition to each other .
Sound, motion, and balance
are centered in the ear. Sounds, in the form of vibrating air
waves, enter through the outer ear and travel through the canal
until they reach the eardrum. The vibrations of the eardrum
are transmitted into the middle ear, which consists of three
bones the hammer, the anvil and the stirrup. These bones transmit
vibrations to the oval window, a thin membrane leading to the
inner ear. In the inner ear, vibrations move into the cochlea,
which encloses the basilar membrane. Hair cells on the basilar
membrane change the mechanical energy of sound waves into nerve
impulses which are transmitted to the brain. In addition to
processing sound, the ear is involved in the sense of balance
and motion through the semicircular canals and otholiths. The
place theory of hearing and the frequency theory explain the
processes by which we distinguish sounds of varying frequency
and intensity.
How do perceptual constancies and organization
influence what we see?
Perception is the process by which we sort
out, interpret, analyze, and integrate stimuli to which our
senses are exposed. Perceptual processes can be generally divided
into three namely : recognition, spatial localization and perceptual
constancies.
Localization involves segregation, perceiving
distance and perceiving motion. In segregation, the Gestalt
school has also noted that we divide what we see into a figure
(an object) and ground (the background). Perception also follows
the gestalt laws of organization. These laws provide a series
of principles by which we organize bits and pieces of information
into meaningful wholes, known as gestalts. Among the most important
laws are those of closure, proximity, similarity and simplicity.
We actively impose organization on what we see according to
these various rules. Others have proposed two new laws: the
law of enclosure and the law of connectedness.
In perceiving distance, we make use of the
difference in view of the world from each eye. This difference
is called binocular disparity. Monocular cues operate even when
only one eye is used. The most important monocular clues include
relative size, relative height, clearness, linear perspective
and texture gradient. Two types of depth information are obtained
through motion: motion parallax ( the apparent pattern of object
motion that is seen when an observer travels past those objects)
and kinetic depth effect (the apparent depth that depends on
object motion).
Recognition can be either be
a bottom up process or a top down process. Processing of perceptual
stimuli occurs in both a top down and bottom up fashion. In
top down processing, perception is guided by higher level knowledge,
experience, expectations and motivations. In bottom up processing,
perception involves recognition and processing of information
about the individual components of stimuli.
Perceptual constancies
are perceptions that remain constant or stable when visual information
changes. For instance, apparent size remains constant when we
move further from an object even though movement shrinks the
retinal image of the object. Unconscious inference theory and
ecological theory offer two opposing explanations for perceptual
constancies.
How do visual illusions manifest themselves
?
Visual illusions are physical stimuli that
consistently produce errors in perception, causing judgment
that do not accurately reflect the physical reality of the stimulus.
Among the best-known illusions is the Muller-Lyer illusion.
Most evidence suggests that visual illusions are a result of
errors in the brain’s interpretation of visual stimuli.
