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The general understanding is that when I touch an object, I perceive its (relative) temperature.

A metal object at room temperature, however, will often feel cool to the touch.

This leads me to believe that the skin senses heat flux in addition to, or instead of temperature. (The metal conducts heat well, and therefore creates a greater flux when touched than a more insulative material.)

I am unfamiliar with the chemical process by which temperature or flux is converted into meaningful signals.

Do humans perceive temperature or heat-flux? (or both?)

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  • $\begingroup$ related to: biology.stackexchange.com/questions/27535/… $\endgroup$
    – One Face
    Feb 12, 2015 at 1:53
  • $\begingroup$ check my answer now - I have written in greater detail $\endgroup$
    – One Face
    Feb 12, 2015 at 16:04
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    $\begingroup$ I am not really convinced by the arguments in support of flux perception; I'll also think about it but here is a point that you (and @CRags) can also think about: If our body could really perceive flux then it can anticipate the final temperature before it actually reaches. In differential control mechanisms, this is used to regulate the levels of any feature in advance (See PID controllers). I haven't heard of differential control of thermoregulation or any biological process, yet. Though I agree that one can perceive flux with conscious effort. $\endgroup$
    – WYSIWYG
    Feb 12, 2015 at 16:53
  • $\begingroup$ @WYSIWYG But the final temperature is already fixed (the set point) which the body is trying to maintain? $\endgroup$
    – One Face
    Feb 12, 2015 at 16:57
  • $\begingroup$ @CRags It is better if we discuss this on chat $\endgroup$
    – WYSIWYG
    Feb 12, 2015 at 16:59

3 Answers 3

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Short answer

Humans sense temperature differences.

Background
(Including edits based upon comments)

Because the question is "Do humans perceive temperature or heat-flux?", I will answer the answer from a psychophysical perspective, i.e., by dealing with sensory awareness.

Just as with many other sensory systems, temperature sensors in the human body adapt to ambient stimuli (i.e., constant absolute stimulus levels). In other words, after being exposed for a while to a certain temperature, one looses the perception of temperature per se (assuming temperatures are within comfortable limits of course, otherwise general discomfort will arise such as shivering or sweating etc.).

Therefore, temperature sensing in humans is not absolute, but differential, and occurs when differences occur relative to the baseline.

Holmes and Wood (1968) mention in their Introduction the following, and I quote:

...three variables of thermal stimulation [...] affect the sensitivity of human subjects to warm and cool stimuli. These are (1) the temperature to which the skin has been adapted, (2) the area of skin over which the thermal energy is applied, and (3) the rate at which the temperature of the skin is changed.

Variables (1) and (3) clearly emphasize the differential nature of temperature perception in humans. Variable (2) is a general psychophysical phenomenon, and basically shows that when a stimulus is sufficiently large, temperature differences will be more easily felt, while when the skin surface being stimulated is excessively small (say a needle with a small temperature difference is pressed against the skin) certain temperature differences may not be felt any longer.

With regard to your question on metal and whether heat-flux can be sensed: a metal surface will feel cool only when it is lower in temperature than your body. Suppose now you touch a metal object at room temperature, i.e. lower than your body temperature. In that case, as you already indicate, it will draw away heat quickly from your skin by conduction, creating a 'heat-flux' if you like. The extraction of heat thereby elicits a powerful stimulus, because a large temperature difference creeps into your skin as heat is drawn away from your skin and underlying tissues. As such, and contrary to other answers given elsewhere, I explicitly say yes, 'heat-flux' can be sensed as it per definition generates a temperature difference. This in contrast to touching insulators like wood or plastic, which will quickly adopt the temperature of the skin very locally where you touch it. However, as has been pointed out by others - this does not mean that temperature is measured by heat-flux sensors per se. In contrast, temperature is measured by temperature sensors that detect temperature differences (differential stimuli) between skin and external media. Because a temperature change is also a differential stimulus, it will also be detected, and hence 'heat-flux' will therefore be detected as well.

Reference
Holmes & Wood, Perception & Psychophysics 1968; 3:81-4

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  • $\begingroup$ "I say yes, heat-flux is definitely sensed as it per definition generates a temperature difference." I disagree with this statement. The temperature difference causes heat flux and not vice versa. Heat flux is dependent on the nature of the material and this may differ for skin types. But ultimately thermal equilibrium is reached. $\endgroup$
    – WYSIWYG
    Feb 12, 2015 at 12:51
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    $\begingroup$ I believe this answer is most correct of the three submitted. The disagreement over the phrase, "heat-flux is definitely sensed as it per definition generates a temperature difference" seems to be largely a case of semantics. Temperature difference will cause heat flux, but flux will always be accompanied by a temperature difference. The key is that the temperature of the skin will decrease over time, not that the skin and surface were different temperatures to start with. $\endgroup$
    – Ruthalas
    Feb 12, 2015 at 14:55
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    $\begingroup$ SYSIWYG's answer added valuable information on the chemical sensing, which seems to be absolute in some regards, but this is mitigated perceptually by the effects of habituation. (C Rags makes good points about the body's drive to achieve homeostasis, but I feel this only impacts conscious perception minimally in normal operating conditions.) Thanks everyone! $\endgroup$
    – Ruthalas
    Feb 12, 2015 at 14:57
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    $\begingroup$ @ChrisStronks Check my answer below $\endgroup$
    – One Face
    Feb 12, 2015 at 16:03
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    $\begingroup$ @th3ophilos - This is most definitely a heated and fluxed discussion here :) The best questions generate discussions. Hence - my compliments to your excellent question! Thanks for your careful analysis of the answers. Most definitely WYSIWYG has added important points about the definition of heat flux (which was indeed a source of confusion). He excels in bringing terminology down to its core. However, his answer was at the physics and protein level (bottom-up), while I have chosen a top-down answer, which is a choice one has to make given the question and/or based on ones expertise. $\endgroup$
    – AliceD ♦
    Feb 12, 2015 at 23:34
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This is the modified answer in response to the discussion:

Facts:

  1. There are warmth and cold receptors in the body at two places: The Peripheral receptors and the Central Receptors

  2. The peripheral receptors are present in skin and the central receptors in the body core at multiple sites the notable site being the hypothalamus

  3. The Temperature receptors have two patterns of firing - The tonic response (sustained firing rate due to steady rise and maintainance in temperature over long time) and the phasic response (also called dynamic response - response to short changes in the temperature)

  4. The Warmth receptors fire maximally at 44 - 46 degrees celcius. The firing stops almost completely at about 48-49 degrees celcius beyond which there is no effective thermoregulation

  5. The Cold receptors fire maximally ar 20 - 35 degrees celcius.

enter image description here

What is measured?

  1. The peripheral receptors sense the temperature of skin.

  2. The central receptors measure the core body temperature.

Only the temperature is measured as WYSIWYG mentions.

However the frequency of stimulus (How frequently the nerves fire - this depends on the temperature range in which the respective receptors fire) and the two types of receptors present on the skin allows the brain to interpret the flux. So Chris Stronks is correct in telling the heat flux is perceived.

Mechanism of feeling cold:

When you touch a cold object the heat is lost through conduction and radiation through the portion of skin that is in contact with the object. The cold will cause vasoconstriction of that part of skin - This means that the temperature of that part of skin rapidly equilibriates with that of the object. As there is rapid cooling (assuming that the object is cold) the rate of firing of the warmth receptors decrease and the rate of firing of the cold receptors increase. This allows the brain to interpret that the skin is losing heat (the flux).

The radiation part of heat loss is more appreciated when you pass a uncurtained window during the winter - the temperature difference is so great that very high loss of heat occurs through radiation that you shiver. You have to note that during these instances there is no decrease in the core body temperature. The actual decrease in core body heat needs a long exposure to such conditions. In the scenarios detailed above the heat loss is primarily from skin which reflects the ambient conditions (very quickly). Thus the peripheral receptors effect Anticipatory Control which is effectively measuring the flux of heat through skin.

The opposite occurs when you suddenly stand beneath the sun coming from a cold room - the radiant temperature of the sun being so great will cause the skin temperature to rapidly rise causing the body to feel very hot (though the core body temperature has not changed much).

Interesting details that may increase your understanding (though not directly related to the question):

The heat control effected through sensing the change in the skin temperature is called Anticipatory Control. In this the brain "anticipates" what is going to happen - for example a sudden cold draught - and will respond to it - reponds by increasing the heat production (shivering).

The heat control effected through sensing the core body temperature is Negative Feedback Control. In this the brain responds to increase in the core body temperature and takes steps to remedy this (the response will bring the temperature back to the set point).

For more see:Boron & Boulpaep Textbook of Medical Physiology, 2nd Edition, Chapter 59.

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  • $\begingroup$ I disagree. There is no way to perceive heat flux. $\endgroup$
    – WYSIWYG
    Feb 12, 2015 at 9:05
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    $\begingroup$ I think this is the correct answer, but you should point out that receptors measure temperature rather than heat flux. However, the main process that causes body temperature to increase/decrease is heat flux. $\endgroup$
    – ChrKoenig
    Feb 12, 2015 at 11:09
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    $\begingroup$ @WYSIWYG I am saying that the flux is deduced by the brain. I clearly pointed out that flux is not measured and only temperature is measured. But coupling the fact that the skin temperature rapidly equilibrates with the ambient temperature and the rate of firing of the temperature receptors, the flux is perceived and the response is effected $\endgroup$
    – One Face
    Feb 12, 2015 at 16:45
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    $\begingroup$ @CRags - Nice answer !! I +1d already :) $\endgroup$
    – AliceD ♦
    Feb 12, 2015 at 23:19
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    $\begingroup$ So I'd say Humans sense changes in temperature (which would be absolute, but are modified by our body's adaptation to a specific environment), while our subconscious uses the same information to guess at flux and compensate. (Sorry for the multiple comments, I keep accidentally posting them with the return key and then timing out the editing period.) $\endgroup$
    – Ruthalas
    Feb 13, 2015 at 16:45
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Flux is defined as amount of heat transferred per unit area per unit time.

Our body does not perceive heat flux. It perceives temperature and tries to adjust heat exchange mechanisms until thermal homeostasis is achieved (in all warm blooded animals). This is a feedback controlled process.

If it were to measure heat flux then the body cannot sense if it is at 47⁰C and the environment is at 60⁰C or if it is at 37⁰C and the environment is at 50⁰C. The heat sensors in our body have evolved to function at 37⁰C. Their conformation changes with increase/decrease in temperature thereby opening or closing the ion channels. The conformation is a function of temperature. If you put that channel in 45⁰C and try to measure a temperature higher that that then it wont work in the same way.

Body function and metabolism are sensitive to absolute temperature. Also see this post.

So while temperature sensing is absolutely necessary even for cold-blooded animals, plants and other organisms, there is no mechanism to measure heat flux. You can just measure the effect of heat not the flow of heat. For e.g. you cannot say if a body is cooling more rapidly than the other just by perceiving it for a short duration. It is possible to consciously calculate heat flux but it is neither measured nor (afaik) is fed in as a parameter to any control systems that regulate body temperature.

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  • $\begingroup$ I've got two examples that I can't really bring in line with that: First, why then do we feel such a difference between, for example, 10°C cold water and air? Second, what about the density of the surrounding medium? I was once told that you would still freeze in the thermosphere (up to 2000°C) because of its low density. $\endgroup$
    – ChrKoenig
    Feb 12, 2015 at 9:49
  • $\begingroup$ @Hav0k heat perception is different from the density problem that you are talking about. It is because of the relationship between boiling/melting points and pressure. State transition is dependent on not just temperature but pressure too. Now heat essentially needs molecules for its effect. There cannot be a hot vacuum. Temperature is a measure of the heat (so it is not possible that something is 2000⁰C and not hot) $\endgroup$
    – WYSIWYG
    Feb 12, 2015 at 11:33
  • $\begingroup$ @Hav0k Regarding your first question. It is like asking why does polyester feel different from wool. The perception of a material is not solely because of its temperature. For perceiving temperature the heat signal has to be relayed to the thermoreceptor neurons. This will depend only on the thermal conductance of your skin; not of the material that you are touching. Moreover this just affects sensitivity. $\endgroup$
    – WYSIWYG
    Feb 12, 2015 at 11:37
  • $\begingroup$ @ChrisStronks Can you explain how. I cannot see the reason. Imagine two bodies are cooling by dissipating heat with different rates. You get to touch both of them at a point when they have the same temperature. How will you tell which one is cooling faster unless you make another measurement. $\endgroup$
    – WYSIWYG
    Feb 12, 2015 at 12:45
  • $\begingroup$ I disagree strongly with some of your statements. Most of all your last sentence "...you cannot say if a body is cooling more rapidly than the other just by perceiving it for a short duration" is nothing less than exactly the opposite of how sensing in general works. Moreover, your answer has a tendency to explain things teleologically, like 'why would the body need data on temperature differences?'. This may often work in evolution, but in physiogical mechanisms it can easily put you on an incorrect mindset $\endgroup$
    – AliceD ♦
    Feb 12, 2015 at 12:46

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