Developmental psychology.pdf

130 Modes of Awareness
*l knew a guy who was color
blind and didn't even know it until
he took a test for the army. He
passed the rest of the test easily
and had 20/20 vision, but they
made some kind of restriction on
what he could do as a Gl because
sometimes he couldn't tell red and
green apart very well.
The opponent-process theory also postulates three types of receptor mechanisms
in the retina, but a two-way reaction is assumed in each case. The receptors
respond to red-green, yellow-blue, and black-white stimulation, each functioning in
reciprocal fashion. When the yellow-blue structure is stimulated by yellow, it develops
increasing sensitivity to blue and decreasing sensitivity to, yellow.
The phenomenon of visual afterimages lends some support to this view. When
you look at a bright green light, the color remains for a moment after you cease looking
at it, an outcome called a positive afterimage. This carryover is soon replaced by an
image of the complementary color, known as the negative afterimage, which here will
have a reddish hue. In the red-green, yellow-blue, and black-white pairings of positive
and negative afterimages, we have the basic elements of the opponent-process theory
(Plate 7).
Furthermore, additive mixtures of red and green, obtained with a rotating
color wheel, do not yield reddish green, just as yellow and blue do not give yellowish
blue. The outcome is gray, whereas mixtures of noncomplementary colors show both
components, blue and red giving violet, yellow and red giving orange, and so forth. The
reason, according to opponent-process theory, is that each receptor can respond only
to one complementary color; excitation of one process automatically opposes the other.
Subtractive mixtures are another matter, for here yellow and blue colored pencils, for
example, give green, as you well know.
Opponent-process theory remains relevant despite discovery of the three types
of cones proposed in the trichromatic theory. The third receptor, through black-white
responsiveness, is presumed to account for the experience of brightness, and on these
bases the overall evidence appears to support a combination of both theories. The trichromatic
theory seems appropriate at the receptor level, inasmuch as there are three
types of cones, and the opponent-process appears relevant at a higher level, in terms
of neural activities closer to the brain, involving such phenomena as afterimages, color
mixture, and perhaps color blindness (Hurvich & Jameson, 1957).
The most common deficiency in color blindness, in which the individual is unable
to see colors, is the inability to discriminate reds and greens of the same brightness.
Yellow-blue color blindness is encountered less frequently. These defects always
involve both red and green or both yellow and blue. When one color is absent, the
opposing or complementary color is absent as well, apparently providing still further
evidence for the opponent-process view. To be adequate, a theory of color vision must
explain why so many people suffer red-green color blindness and why the periphery
of the human retina is not as sensitive to red and green as to blue and yellow
(Plate 8).*
Capacity for Hearing
The other "higher sense" in human beings is hearing, a most critical source of information
in our daily lives. Here sound waves, the physical stimuli for hearing, are produced
by vibrating bodies and transmitted through the air to the eardrum, where they
stimulate nerve fibers. When the resulting nerve impulses reach the brain, auditory
experiences occur.
The experience of pitch, which is the relative position of a tone on a scale, is
determined partly by the frequency of vibrations. Low frequencies produce low pitch;
high frequencies produce high pitch. The human ear is attuned to frequencies ranging
from 20 to 20,000 hertz, but it is most sensitive between 2,000 and 4,000 hertz.
The loudness or strength of a sound is primarily a function of the sound pressure
activating the eardrum. It depends upon the degree of displacement of the vibrating
body from the resting position. As an example, consider the difference in
displacement between a lightly plucked and vigorously struck guitar string.