The five senses traditionally ascribed to humans are vision, hearing, taste, smell and touch. A sixth "sense" could be proprioception, the perception of body position, which is important for balance and agility in movement. It could also include perception of stimuli from within the body, such as pain, hunger, or thirst.
The Limits of Vision
Human vision is the eyes' ability to sense electromagnetic radiation within a limited range of 380 to 780 nanometers. Through an effect called "flicker fusion," the eyes normally can't detect a flicker above about 60 hz in a light source, according to research done by NASA. Thus, a motion picture image appears to move smoothly, despite being a series of still images. Sensitivity varies across the retina; it's concentrated in the macula, which is the center of view. That's why you can see your hand held straight out to the side, but you probably don't have enough acuity to count the fingers.
Human Hearing Is Tuned
The normal range of human hearing is from 20 Hz to 20,000 Hz. The ear funnels sound waves, actually the vibration of air molecules, to the eardrum. This then also vibrates, setting into motion a chain of small bones, called ossicles, which stimulate the cochlea, a fluid-filled organ, which then stimulates nerves. The outer ear, called the pinna, faces forward to favor gathering sound from ahead, above and below. It includes complex ridges that selectively funnel frequencies into the ear canal. This helps you detect the direction of incoming sound.
Taste and Smell Are Linked
Taste (gustation) and smell (olfaction) are related senses. Unlike vision or hearing, there's no set range of sensitivity. The tongue can sense flavors that are sweet, sour, salty, bitter and savory. Part of the perception of flavors comes from aromas reaching olfactory nerve cells in the nostrils. PubMed Health states that these senses are connected to the involuntary nerve system, so they can trigger bodily reactions from vomiting to salivation.
Touch is Electric
The sense of touch is part of the somatosensory system, which also includes senses of pain, tickling and itching, along with awareness of body position and movement, called proprioception. Touch sensations can be sorted into sub-categories, such as sharp pain, aching pain, and tactile stimulations such as pressure and vibration. The sensory receptors in the skin are called Merkel cells, and they reside at the base of the epidermis and around hair follicles. Researchers at Columbia University report that their function is similar to the nerve cells in the cochlea, turning sensations like vibrations or texture into electrical signals.
Other Ways of Sensing
The number of described senses beyond the traditional five varies by source. Harvard Medical School says the number varies even among researchers within their institution. The list can include temporal perception, the sense of the passage of time, and interoception, sensations coming from within organs. Equilibrioception is the sense of balance, and thermoception is the ability to feel hot and cold.
- NASA: The Windows of Visibility
- American Macular Degeneration Foundation: Anatomy of a Normal Human Eye
- MIT: Human Ear
- PubMed Health: How Does Our Sense of Taste Work?
- Nature: Epidermal Merkel Cells Are Mechanosensory Cells that Tune Mammalian Touch Receptors
- Harvard Medical School: Extra Sensory Perceptions