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How many senses do humans have?

[There's a New Scientist article, 29 Jan 2005 by Bruce Durie, "Senses special: Doors of perception" on how many senses we have. If you are at Glasgow University then the best way to get the link to work may be to FIRST login to your library account an some window; THEN click the link above.]

But in any case, Aristotle's answer of 5 is definitely wrong: 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, touch, taste, smell, pain, mechanoreception (balance etc.), temperature, interoreceptors (e.g. blood pressure, bladder stretch).
• 21 (see table below)
• 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.
      But also at least two types of receptor elsewhere in the mouth for chilli-hot, and its opposite creamy-cool-soothing.
   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.
   But there are some women with excellent olfaction but no olfactory bulb whatever.
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 is, 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 amongst people in what a given stimulus smells of: like "colour blindness". Thus you cannot trust a trained expert (a perfumer on scent, or oenologist on wine) to know what you will like, nor even what you will experience.

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