Developmental psychology.pdf

126 Modes of Awareness
TRADITIONAL SENSES
With this background in awareness of stimulation, we turn to the five traditional
senses, beginning with vision. Visual experience is one of the "higher senses" in human
beings, although it is limited and specialized in the frog.
By inserting microelectrodes in different parts of the frog's optic nerve, five
different kinds of fibers have been identified. These include on-fibers, which respond
to the onset of illumination; off-fibers, activated by the cessation of illumination; and
on-off fibers, which operate both ways. In addition, edge receptors respond to a distinct
line or edge in the visual field, regardless of whether this edge is stationary or in motion.
They appear to be a highly useful survival mechanism for the frog, for flying predators,
as they circle their prey, cast a shadow across the land.
Perhaps the most intriguing mechanisms are the receptors known as bug detectors,
for they only respond to small, moving objects, such as bugs, flies, spiders, and
other insects, which comprise the frog's favorite meal. These receptors are not activated
by large objects or even by small stationary ones; hence the frog is not responsive
to dead flies. They also are not activated by changes in the amount of light, unlike the
on-off cells. In short, the frog's visual system contains several highly specialized nerve
cells (Manturana, 1959; Manturana, Lettvin, McCulloch, & Pitts, 1960).
This research illustrates a typical model for the investigation of sensory processes
in any species. It begins with some physical energy, such as the light waves
emitted by the flying bug; then there is a physiological condition, when this energy has
been converted into neural impulses; and finally, there is the psychological experience,
as these impulses are combined with other neural activities in the brain, producing an
awareness of the moving insect. Sensation involves a physical-physiologicalpsychological
sequence of events.
Visual Ability
Light, the stimulus for visual experience, is considered to be waves of radiant energy
emanating from a source. These waves occur in sequence, but the human eye is attuned
only to a narrow range of light wavelengths, from approximately 400 to 700 nanometers.
Within this range, the shorter wavelengths are perceived as violet, the intermediate
ones as blue, green, and yellow, and the longer ones as red. The experience of
hue, which refers to what we commonly call color, is largely a function of wavelength
(Plate 4).
Visual phenomena also can be described in terms of intensity or brightness,
which means that the experience may vary from dark to light. Even a yellowish hue
may be dark or light. This experience is correlated chiefly with wave amplitude, which
is the size of the wavelength (Figure 5.3).
Finally, colors vary in richness or saturation. Highly saturated colors are the
purest colors—the reddest reds, the yellowest yellows, and so forth—as determined by
several physical properties of light, including the mixture of different wavelengths.
When we speak of saturation, we are referring to differences in purity among hues of
the same level of intensity. In other words, we are referring to the degree to which any
given hue differs from gray of the same brightness (Plate 5).
Figure 5.3
Wave Characteristics. Wavelength
is the distance between consecutive
crests. Amplitude is the maximum
displacement of a wave from the zero
point.
Wave i N.
Amplitude т _Д
Wavelength
A A