eyes, containing photoreceptor cells responding to visible light. The
associated perceptual experience is that of visually seeing the world.
Similarly, the auditory system has the ears as its organs of recep-
tion, containing hair cells responding to mechanical vibration. The
associated perceptual experience is that of hearing sounds. In like
manner, there is the tongue and taste, the nose and smell, and the skin
and tactile experience. These five channels of sensory perception are
sometimes referred to as the five primary senses.
In addition, there is the vestibular sense, for which the organs of re-
ception are the semicircular canals of the inner ear, the receptor cells
are hair cells, and the physical stimuli are gravity and acceleration.
The associated perceptual experience is that of balance. There is also
proprioception, in which stretch receptors in the muscles and joints
gather information related to muscle tension and joint movement.
The resulting information is used to tune body alignment and coordi-
nated movement. Vestibular and proprioceptive pathways, although
crucial to executing balanced movements, are often not considered
among the “primary” senses because the collection and use of infor-
mation by these channels are mostly out of awareness and not used
in the same way to inform our conscious perception of the world. We
mostly become aware of these senses when something is wrong and
balance and coordination are impaired, as when we spin around until
dizzy.
Human sensory perception is sophisticated, elaborate, and quite
sensitive. And like all forms of sensory perception, it is limited—that
is, not capable of detecting vast ranges of physical phenomena that
are known, by other means, to exist. This can be illustrated by taking
a look at vision. The visual pathway in humans responds to a limited
range of energies within the electromagnetic energy spectrum (Fig.