Sensation - Studying the Gateways of Experience

Here is a useful way to think about the character of conscious experience. Imagine three ascending steps. The first step is associated with sensation. Sensation refers to the raw data of experience. The second step is associated with perception. Perception refers to organized experience. The third step is associated with cognition. Cognition refers to knowing.

Vision: Seeing Is Believing

In the front of the eye is the cornea, a kind of window that allows light to enter the eye. Because the cornea has a convex shape, it also is somewhat responsible for bending light waves and making them converge on the lens.

The lens is used to focus light waves, and it produces an inverted, or upsidedown, image on the retina. The retina is a photosensitive neurological structure. Think of it as a target. The center of the target is called the fovea, and it plays a dominant role in visual acuity and color vision. The outer rim of the target, the periphery, plays an important part in signal detection and brightness vision. The neurons in the retina are called photoreceptors because they are light sensitive. The optic nerve conveys the retina’s activity pattern to the brain.

The two kinds of photoreceptors are the cones and the rods. They have been given these names because of the shapes of their cell bodies. The cones are located primarily in the fovea. The rods are located primarily in the periphery. As already indicated, color vision is associated with the fovea, suggesting that the cones have a lot to do with this particular quality of sensation.

Hearing: The Sound of Music

In order to experience the sensation of sound, it is necessary to have a functioning ear. These are the principal structures and functions of the ear. The eardrum, already mentioned, is also known as the tympanic membrane. Its vibrations induce a series of events. The motion of the tympanic membrane is conveyed to a structure called the oval window. The conveyance of the motion is made possible by the motion of three linked bones called the malleus (“hammer”), the incus (“anvil”), and the stapes (“stirrup”).

Vibrations of the oval window in turn set up vibrations within a fluid contained in the cochlea, a bony structure reminiscent of a snail shell. A nervous system structure within the cochlea called the basilar membrane plays a role in hearing similar to the role that the retina plays in vision. The auditory nerve conveys the basilar membrane’s activity pattern to the brain.

There are three basic sensations associated with hearing. First, pitch is the ability to hear sounds ranging from low to high. Second, loudness is associated with the magnitude of a sound. Third, timbre refers to the quality of a tone. In general, the quality of a note played on a piano has more timber, or “richness,” than a note of the same pitch played on a flute.

Taste: “This Is Too Salty”

The stimuli that control much of the sense of taste are various chemical compounds such as those associated with salt, sugar, or lemon juice. The units that make taste possible are clusters of neurons located on the tongue called taste buds. The taste buds respond in such a way that they produce four basic taste sensations. These sensations are quite familiar. They are known as sweet, salty, bitter, and sour.

Touch: Of Pain and Pressure

There are four skin senses:

1. The sensation of light touch can be induced by placing very little pressure on the surface of the skin or by slowly stroking the skin. Neurons located near the surface of the skin are the ones that give us the sensation of light touch.
2. Deep touch can be induced by placing substantial pressure on the surface of the skin. Deep touch is also known as the sensation of pressure. Neurons located well below the surface of the skin are the ones that give us the sensation of deep touch.
3. Temperature is induced by variations in the amount of heat being conducted to or away from the skin. When heat is being conducted toward the skin, we usually experience an increase in warmth. When heat is being conducted away from the skin, we usually experience an increase in cold.
4. Pain is a skin sense induced by tissue damage. A hard blow to the body or being cut by a knife will usually cause pain. Be clear that the kind of pain being described here is not the only type of pain. But the kind of pain associated with the skin is called cutaneous pain. Neurons in the skin that can detect tissue damage are the ones that give us this particular pain sensation.

Smell: The Nose Knows

The sense of smell allows us to detect the presence of some, but not all, airborne chemical substances. The sense of smell is also known as olfaction. The receptor organ that makes the sense of smell possible is called the olfactory epithelium, and it is located high in the nose. It is to smell what the retina is to vision. Several kinds of neurons differentially sensitive to chemicals in gaseous forms induce the various smell sensations.

Kinesthesis: Can You Touch the Tip of Your Nose with Your Eyes Closed?

Kinesthesis, also known as proprioception, is the capacity to know the position in space of various parts of your body. The receptor neurons for kinesthesis are located in the connective tissue surrounding the body’s joints as well as within the joints themselves.

The Sense of Balance: Walking in an Upright Position

The sense of balance informs you that you are walking in an upright position. What you are sensing is the relationship of your body, and in particular your head, to the Earth’s gravitational field. The sense of balance is made possible by receptor neurons located in the semicircular canals. Located in the inner ear, the canals are tubular bones filled with fluid. The movement of this fluid stimulates the firing of receptor neurons within the canals, and the information is transmitted to the brain.

The sense of balance is also known as the vestibular sense. A vestibule is a small antechamber or passageway. This is one way to describe the semicircular canals, important components of the apparatus involved in the sense of balance.