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How many senses do humans have?

[There's a New Scientist article, 29 Jan 2005 by Bruce Durie, on how many senses we have.]

The only wrong answer is Aristotle's answer of 5: vision, hearing, touch, taste, smell.

Defensible answers are:

• 3: the number of physical types of stimulus: light (photons), chemicals (smell, taste, and internal sensors), mechanical (touch and hearing).
• 9: vision, hearing, smell, taste, touch, pain, mechanoreception (balance etc.), temperature, interoreceptors (e.g. blood pressure, bladder stretch).
• 21
• 33

Of course the real answer is that this is the wrong way to look at it. Sensing doesn't cause perception: real perception is all about integrating information across senses, across time, across space if you are (as is normal) moving around partly in order to perceive better.

External chemical sensing; Senses of smell; Olfaction

1. Taste.
   1. The taste buds on the tongue detect 5 different flavours.
   2. Most perceived taste comes from Olfaction on exhaled air from the oral cavity.
2. Olfaction by the Olfactory bulb and nerve, analysing airborne molecules inhaled by the nose.
3. Trigeminal: airborne molecules are often also detected by other sensors in the whole nose and oral cavity, transmitted by the trigeminal nerve, perceived as hot/cold, but combined as part of an odour percept.
4. Vomeronasal: there is some but insufficient evidence, both behavioural, anatomical, and from brain scans, that humans have a further set of detectors which in animals respond to pheromones, whose sensing we are unconscious of but which do affect us. (We are largely unconscious of some other things, such as a shortage of oxygen in the air, which undoubtedly have huge effects on us.)

The theory of how olfaction works is still undecided, but it seems clear enough that it is like colour perception in that: a) There are a number of different receptor types b) the same stimulus (odour molecule) reacts with several receptor types at once; so that c) it is the ratio (relative strength) of responses that tells a person which odour it, rather than having one receptor type per detectable smell.

Dogs (bloodhounds) vs. humans: sensitivity to odours 10 million to one.
Human sensitivity to a strong odour can be 9 parts per trillion.
A silkworm moth can detect a single molecule of pheromone.
There are some cases of significant differences in what humans smell: like "colour blindness".

Leffingwell,J.C. (2005) "Olfaction: Update no.5" Leffingwell Reports vol.2 no.1