Wilderness Underfoot: Seeing, hearing, touching, smelling, tasting

StoryImage( ‘/Images/Story//Auto-img-111765465514118.jpg’, ”, ‘Five senses and counting: Seeing – is made of at least two senses, light and color. If you divide colors into red, green and blue, the number goes up to four. Hearing – so far it appears to be one sense. Smelling – is one sense, unless you divide out the 2,000 different odor receptors. Tasting – includes four senses that detect sweet, salt, sour, and bitter. Another is “Umani,” allowing us to taste meat’s flavor. Touching – is two senses, divided into light touch and pressure. Pain – not the same as touch, it may be considered three distinct senses, including cutaneous (skin) somatic (outer body) and visceral (inner body.) Temperature – might be one overall sense, or two if we count hot and cold. And more – blood pressure, blood oxygen, cerebrospinal pH, thirst, hunger, body balance, joint position, acceleration, muscle stretch, bladder stretch… !’);

It was Aristotle who first described the list of five senses, 2,300 years ago. School children have been learning his list ever since: seeing, hearing, touching, smelling and tasting. But it turns out that Aristotle’s view was an oversimplification. We now know that the actual number of distinct senses humans possess is at least twice that, and probably more.

Pinning down the exact number is difficult because the lines are often blurred when it comes to where one sense ends and another begins. For example, is sight a single sense, or should it be broken down into the perception of light and the primary colors of light – red, green, and blue? Different kinds of nerves detect each of these qualities. And what about touch? Do we lump together tactile stimulation with temperature, surface pressure, limb and joint position, balance, hunger, thirst, and the feeling that it is time to race to the restroom? Modern research suggests these are all distinct senses.

If the senses are divided up based on how nerves are triggered to send signals to our brains, the total count skyrockets. Just to name a few, we have nerves that sense blood pressure, pH and chemical levels, blood glucose, muscle stretch, and pain occurring in the skin, the outer body, and the organs. But a more important question, and one asked by Aristotle back in the beginning, is of perception. How do we perceive these signals?

The brain’s handling of the senses turns out to be extremely complex. Clues to how it all works come from learning more about those people for whom it isn’t exactly working, as in the case of synesthesia. This is a strange condition in which senses are mixed or joined. A person with synesthesia might perceive numbers as having colors, or she might associate tastes or smells with letters of the alphabet. We all have a small bit of sense-mixing. Think of how color affects our perception of food, or how we describe colors as hot, cold, cheerful or depressing. This suggests that our senses are all intricately networked, and why they may be difficult to distinguish.

From the June 1-7, 2005, issue

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