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Capacity of our senses | Notes from Planet Earth

The tremendous capacity of our senses to discriminate what’s going on around us is not often something we take the time to think about.

We’ve all heard about the extraordinary resolution of the eagle’s eye which is able to see small prey animals from great distances, and about the sonar capabilities of the bat which enables it to generate a changing three-dimensional view of all the objects and insects in its flight path, in spite of the absence of light. We’ve also known for some time about both the bear’s keen ability to detect faint odors and the direction from which they arrive at its nose, and the elephant’s ability to hear sounds having frequencies well below anything we can hear.

Work over the past couple of decades has shown that humans, whose eyes can see colors with wavelengths between 390 nanometers and 700 nanometers (a nanometer is a millionth of a millimeter), can distinguish something between 2.3 and 7.5 million colors within that range of wavelengths. I certainly have difficulty believing this but this number range has not been challenged to date as far as I know.

Similarly our ability to detect different musical tones within the frequency of human hearing range of 20 to 20,000 cycles per second has been investigated and found to be about 340,000 tones, again an almost unimaginable number. Clearly, however, this number of distinguishable tones often diminishes with advanced age.

This past March four scientists published their efforts to determine how many olfactory stimuli, or different odors, humans could distinguish. The project was made more difficult because there was no known “range” of odors “beneath” or “above” which we cannot smell. Single odor molecules do have to be volatile. That is the molecule has to exist as a gas in the vapor phase so that it can be transported by air to the olfactory nerve endings high up in our nose. Thus there is a restricted range of temperatures that will allow us to smell some molecule. If it were too cold, the molecule would not move through the air but instead be frozen in some ice-like state. While if it were too warm our olfactory nerve endings would be destroyed and made insensible. Research done in the early part of the 20th Century suggested that humans seemed able discriminate about 10,000 different odors. In most cases what we sense as an odor is a mixture of many different molecules. For example, it has been determined that the scent of a rose is composed of a mixture of some 275 different components. Therefore, what we can distinguish as a different odor is usually just a mixture containing different amounts (from zero to 100%) of each the various vapors we are able to detect with our nose. These scientists controlled their tests so as to eliminate those differences based solely on the quantity of a particular odorant mixture in the sample being tested. The 26 human subjects they used were given three vials to smell. Two contained the same mixture but in different concentrations in the odorless solvent used to dissolve the odorants, while the third vial contained a different mixture. In these double-blind tests, the subjects had to pick out the one containing the different mixture.

The astounding results obtained indicated that humans are able to discriminate more than one trillion different odors! Clearly, most of us rarely make significant use of this phenomenal capacity.

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