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Although there is much that we do sense, there is even more that we do not. Dogs, bats, whales,
and some rodents all have much better hearing than we do, and many animals have a far richer
sense of smell. Birds are able to see the ultraviolet light that we cannot (see Figure 4.3
"Ultraviolet Light and Bird Vision") and can also sense the pull of the earth’s magnetic field.
Cats have an extremely sensitive and sophisticated sense of touch, and they are able to navigate
in complete darkness using their whiskers. The fact that different organisms have different
sensations is part of their evolutionary adaptation. Each species is adapted to sensing the things
that are most important to them, while being blissfully unaware of the things that don’t matter.
Measuring Sensation
Psychophysics is the branch of psychology that studies the effects of physical stimuli on sensory
perceptions and mental states. The field of psychophysics was founded by the German
psychologist Gustav Fechner (1801–1887), who was the first to study the relationship between
the strength of a stimulus and a person’s ability to detect the stimulus.
The measurement techniques developed by Fechner and his colleagues are designed in part to
help determine the limits of human sensation. One important criterion is the ability to detect very
faint stimuli. The absolute threshold of a sensation is defined as the intensity of a stimulus that
allows an organism to just barely detect it.
In a typical psychophysics experiment, an individual is presented with a series of trials in which
a signal is sometimes presented and sometimes not, or in which two stimuli are presented that are
either the same or different. Imagine, for instance, that you were asked to take a hearing test. On
each of the trials your task is to indicate either “yes” if you heard a sound or “no” if you did not.
The signals are purposefully made to be very faint, making accurate judgments difficult.
The problem for you is that the very faint signals create uncertainty. Because our ears are
constantly sending background information to the brain, you will sometimes think that you heard
a sound when none was there, and you will sometimes fail to detect a sound that is there. Your
task is to determine whether the neural activity that you are experiencing is due to the
background noise alone or is a result of a signal within the noise.