Smell Illusions

[ashennell]

Quote:

Originally Posted by Glider

There really isn't such a thing as an olfactory illusion. The olfactory sense is our oldest sense and of all our senses it is the only one that does not project to the thalamus. It sends projections to the older parts of our brains though' date=' including limbic areas associated with emotion and memory, which is why smell is so evocative...............

[/quote']

 

 

I was thinkin of posting a reply, but when i read ur post i realised taht there was nothing more i cud add. Great response Gilder

 

??? But it's a poor argument!!

 

The description of the olfactory pathways is very interesting, but to assume that olfactory illusions are impossible just because of this seems very naive. There is no law that says that illusions require the thalamus or that primitive senses cannot be fooled. This is just assuption. It is certainly no basis for dismissing the possibility of illusions.

 

Try and find a working definition of 'illusion' that includes all your excepted illusions and that also rules out ethers as olfactory illusions. I think it would be quite difficult. I have also presented some examples of other illusions that are possible borderline olfactory illusions.

[Glider]

Aw...I see you're no novice at flattery either are ya, you smooth talker you!

 

However, I can't take full credit for all that with which you credit me. I don't remember saying olfactory illusions were impossible, and I doubt I would have assumed it. Nope, that doesn't sound like me at all. I don't remember implying that thalamic involvement was necessary for illusions to occur either.

 

As I recall, my 2 main points were 1) that whereas optical illusions most often work through 'top down' processes, smell, depending less on 'top down' processes for its function and interpretation is less likely to be fooled. 2) That as the basic olfactory sense depending on chemoreceptors is less likely to be fooled. For example, there are plants that generate a smell like rotting meat (Amorphophallus titanum).

 

It could be argued that this is an olfactory illusion because here we have a plant and not a rotting carcass. However, chemical analysis of the scent released by the plant shows the basic compounds are the same as those released by rotting meat. So, is that an illusion?

[m4rc]

I believe that I have thought of a possible olfactory illusion. Consider the following situation:

In a circular room, you set up some fans to create a strong wind going around the room. You then place a source of odor to the left of you (downwind). The wind will carry the smell around the room then back to you. If you smell while moving, trying to locate the source of the smell, it will seem that it is to your right when it is in fact to your left.

[ashennell]

I don't remember saying olfactory illusions were impossible

 

There really isn't such a thing as an olfactory illusion.

 

Sorry, you didn't say they were impossible - just that they don't exist.

 

Anyway, I'm not here just to start an arguement. I'm not entirely sure myself if olfactory illusions exist but I think it's worth debating. I think I was mainly annoyed because I had alrady replied your orignial post in a more dignified manner, in this post, but it didn't seem to merit any response.

 

I will try and tackle your two main points again briefky-

 

1) that whereas optical illusions most often work through 'top down' processes, smell, depending less on 'top down' processes for its function and interpretation is less likely to be fooled.

 

I think that hallicinaitons require interaction between top-down and bottom-up processes moreso than illusions. Yet you have already provided an example of an olfactory hallucination. So why not an olfactory illusion? I agree that this property of the olfactory system allows less scope for both illusions and hallucinations though.

 

2) That as the basic olfactory sense depending on chemoreceptors is less likely to be fooled.

 

Our sense of touch arguably allows even less room for confusion than our olfactory system. All that is required is a mapping from levels of pressure, vibration and muscular feedback. This is compared with the olfactory systems task of representing the presense of almost any molecule. But there are haptic illusions. One of my favourites is the 'Cutaneous rabbit' experiment.

 

 

It could be argued that this is an olfactory illusion because here we have a plant and not a rotting carcass. However, chemical analysis of the scent released by the plant shows the basic compounds are the same as those released by rotting meat. So, is that an illusion?

 

This is a tough question. At the moment I would say yes, because I can't find a way to define an illlusion that captures all illusions that we would easily except from those that don't seem right.

 

For example, a mirage is a visual illusion caused by the refraction of light as it enters a layer of warm air near the ground. The illusion is actually an image of the sky or other distant objects being refracted back. This looks like water because water does exactly the same thing. So we could say that our visual system is not being tricked but providing an accurate description of the visual scene. It is our interpretation of the result that it faulty - our assuption that this effect is only produced by water. Is this not the same as assuming that a certain smell is only produced by rotting meat?

 

Edit: bad english

[Glider]

Sorry, you didn't say they were impossible - just that they don't exist.

I should make my position clearer. As far as I know, olfactory illusions per se don't exist.

 

Anyway, I'm not here just to start an arguement. I'm not entirely sure myself if olfactory illusions exist but I think it's worth debating. I think I was mainly annoyed because I had alrady replied your orignial post in a more dignified manner, in this post, but it didn't seem to merit any response.

 

I will try and tackle your two main points again briefky-

 

 

 

I think that hallicinaitons require interaction between top-down and bottom-up processes moreso than illusions. Yet you have already provided an example of an olfactory hallucination. So why not an olfactory illusion? I agree that this property of the olfactory system allows less scope for both illusions and hallucinations though.

Hallucination don't require bottom up processes at all. They don't require the processing of incoming stimuli because they are generated at 'the top'.

 

Illusions require the interaction between bottom up and top down processes, because in order to work they require that top down systems interfere with the objective interpretation of bottom up stimuli.

 

 

Our sense of touch arguably allows even less room for confusion than our olfactory system. All that is required is a mapping from levels of pressure, vibration and muscular feedback. This is compared with the olfactory systems task of representing the presense of almost any molecule. But there are haptic illusions. One of my favourites is the 'Cutaneous rabbit' experiment.

Well, there are a number of haptic illusions, including the thermal grill illusion which induces the experience of (painful) noxious heat on exposure to stimuli of innocuous intensity. This is attributed to the interplay between two types of spinothalamic tract projection neurons (see for example http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8023144&dopt=Abstract). So the evidence would suggest that our sense of touch does in fact allow considerable room for confusion, particularly as afferent signals entering via spinal nerves undergo a level of processing at the level of the dorsal horn.

 

The olfactory sense on the other hand undergoes no primary processing as it relies on cranial nerves which project directly to the brain.

 

 

This is a tough question. At the moment I would say yes, because I can't find a way to define an illlusion that captures all illusions that we would easily except from those that don't seem right.

I would have to disagree with you there. In my opinion, the fact that the compounds that smell like rotting meat were interpreted as smelling like rotting meat denies the claim that it's an illusion. I think the fact that the compounds were generated by a plant and not rotting meat is not relevant in this case. The compounds are the same, so there was no misinterpretation by the sense.

 

For example, a mirage is a visual illusion caused by the refraction of light as it enters a layer of warm air near the ground. The illusion is actually an image of the sky or other distant objects being refracted back. This looks like water because water does exactly the same thing. So we could say that our visual system is not being tricked but providing an accurate description of the visual scene. It is our interpretation of the result that it faulty - our assuption that this effect is only produced by water. Is this not the same as assuming that a certain smell is only produced by rotting meat?

 

Edit: bad english

I see your point and in this instance it is a good argument. However, there are many more visual illusions than mirages: Muller lyer, Kanitson triangle, Necker cube, Pinna-Brelstaff, any of the figure-ground illusions and any number of other visual phenomena that demonstrate the interferance of top down processes with the accurate perception of bottom up stimuli. As far as I know, there is nothing like this to demonstrate the same effect in the olfactory sense.

[ashennell]

Hallucination don't require bottom up processes at all. They don't require the processing of incoming stimuli because they are generated at 'the top'.

 

Illusions require the interaction between bottom up and top down processes' date=' because in order to work they require that top down systems interfere with the objective interpretation of bottom up stimuli.[/quote']

 

Sorry, I didn't make my point very well. Hallucinations are generated in disregad to any bottom-up information, you are correct about this but this was not really central to the point I was trying to make.

 

My point, expressed differently, is that if a perceptual system is capable of creating completely ficticious percepts then there is always the possibility that it my misrepresent incoming stimuli. If we have the freedom to create then surely we must have the freedom to modify. I believe this point is still valid.

 

While the definition of a hallucination is creation of a percept without any sensory cause I'm not sure if it's meaningful outside of it's clinical origin. There are many perceptual effects that would seem to be halfway between an illusion and an hallucination. Metamorphosias and Chromatopsias are both groups of visual illusions that are the result of, what would seem like, domination by top-down information in the perceptual pathway. They are also subjective effects. I Think it makes sense to treat illusions and hallucinations as the two ends of a spectrum rather than as discrete categories.

 

So the evidence would suggest that our sense of touch does in fact allow considerable room for confusion, particularly as afferent signals entering via spinal nerves undergo a level of processing at the level of the dorsal horn.

 

I think your way of looking at this makes a lot more sense. I kinda felt my argument here was poor when I was typing it. However, I'm not sure that the amount of precortical processing is important. We both agree that illusions require interaction between top-down and bottom-up info. While passing through the thalamus provides one extra point where this interaction may occur I assume that you would agree that most visual illusions result from computations at levels higher than the LGN, somewhere in the cortex. If olfactory illusions to exist then they would most probably result from processing in the olfactory cortex.

 

I would have to disagree with you there. In my opinion, the fact that the compounds that smell like rotting meat were interpreted as smelling like rotting meat denies the claim that it's an illusion.

 

I agree that this issue is contentious. I only (edit)accept this as an illusion because of its similarity to the mirage example. I think there are two ways of reasoning about this situation. One way would be as you have outlined above. Another way would be to say the the chemical in question is the smell of both rotting meat and of Amorphophallus titanum, i.e. associating the smell with the chemical and not the causes. There is no basis for the olfactory system to determine which is the correct cause , the assumption of one over another is a mistake at some level. Although this assuption is a valid (and useful) one in most cases. The beetles that head towards Amorphophallus titanum in expectation of rotten meat probably feel tricked.

 

Perhaps I should find an example of two completely unrelated compounds that have exactly the same smell. Would this be acceptible as an olfactory illusion?

 

However, there are many more visual illusions than mirages: Muller lyer, Kanitson triangle, Necker cube, Pinna-Brelstaff, any of the figure-ground illusions and any number of other visual phenomena that demonstrate the interferance of top down processes with the accurate perception of bottom up stimuli. As far as I know, there is nothing like this to demonstrate the same effect in the olfactory sense.

 

there are indeed lots of types of illusion. One type that you mention are based on ambiguity - where there are 2 or more valid interpretations of a stimulus. Perhaps there are compounds that smell differently in different contexts.

 

Rather than debating what constitutes an illusion and what properties of a perceptual system are required for an illusion I will try and find an example that we both agree is an olfactory illusion.

 

I will start here:

 

Olfactory illusion. In some cases, a perfumer can identify by smell alone all the components of a fragrance, even if some of these components are themselves complex mixtures, such as essential oils. However, in other cases, blending of ingredients can create the illusion that a certain material or class of material is present when it is not. In other words, it is possible to create an olfactory illusion. In the first case, the sense of smell appears to provide us with an extremely effective analytical device, yet the second case shows that it can be deceived.

 

from this site

 

Now don't laugh. I also can't belive that I have been reduced to drawing quotes from a summary of a Perfumer & Flavorist artical. Hey, it's the first thing I found. I would get the whole article but I'm guessing my Uni dosn't stock Perfumer & Flavorist.

 

I'll do a more in depth search later.

[Glider]

there are indeed lots of types of illusion. One type that you mention are based on ambiguity - where there are 2 or more valid interpretations of a stimulus. Perhaps there are compounds that smell differently in different contexts.

As far as the rest of it goes' date=' I think we're approaching concensus, but I think this is the crux of the matter.

 

In a nutshell I think it's the quality of the stimulus [i']medium[/i] that makes the difference (and is the 'bone' of our contention) rather than the stimuli themselves. For example, light is always light. When light (the stimulus medium) reflects from an image (the stimulus), we detect exactly and only the pattern of light reflected from the image. So, when discussing illusions, and particularly those based upon ambiguity (e.g. the Necker cube and the figure-ground illusions), we know the ambiguity is imposed by top down processes and is not a function of the stimulus medium (light).

 

However, the same cannot be said of smell because this relies on chemicals as a stimulus medium, and many of them (i.e. different types). So, in this case, the stimulus medium can be messed with.

 

Therefore to create a mix of aromatic chemicals that smell like a rose, even if these chemicals are not related to those produced by a rose, would be the same as taking a figure ground image (say vase/faces) and manipulating the light in such a way that it evoked mainly the image of faces rather than a vase (i.e. to suppress the vase schema). In other words, to knock out the ambiguity. Without that ambiguity, could it still be called an illusion?

 

I suppose in one sense it could. To evoke the sense of a rose through a scent relies on top down processes insofar as the subject would need prior experience of the object and the stimulus medium would have to trigger the association between the stimulus medium (scent) and the schema of the stimulus object (rose).

 

However, could it be said to have 'fooled' the sense? I don't think so. Relying (as it does) on mainly bottom up processes, the 'artificial' scent would still have to smell like a rose to evoke the schema of a rose. This is the same as saying that a drawing (or even a 3-D hologram) of a vase is fooling the sense because it evokes the schema of a vase.

 

Thus it cannot be said that the rose schema evoked by the 'artificial' scent of a rose was imposed on an ambiguous stimulus through top down processes because there was no ambiguity. Artificial or not, the scent smells of rose. If the scent was ambiguous, it would not evoke the schema of a rose.

 

 

Rather than debating what constitutes an illusion and what properties of a perceptual system are required for an illusion I will try and find an example that we both agree is an olfactory illusion.

 

I will start here:

 

 

 

from this site

 

Now don't laugh. I also can't belive that I have been reduced to drawing quotes from a summary of a Perfumer & Flavorist artical. Hey, it's the first thing I found. I would get the whole article but I'm guessing my Uni dosn't stock Perfumer & Flavorist.

 

I'll do a more in depth search later.

I'm not laughing. It's an interesting read. As you said above, smells can be influenced by context and psychological state (e.g. when hungry, food smells are detected much more quickly and are rated as much stronger than when not hungry). Expectancies also influence smell, but overall this is to a much lesser degree than with vision due to the large difference in direction of processing. Smell is mainly bottom up (although there must still be top down processes in order to make sense of the input), whilst vision is clearly hugely influenced by top down processes and so can be made to impose percepts that do not exist.

[Igor Suman]

"Do any of you out there know if there is such a thing called smell illusions?"

 

There are false sensations (paresthesia) of smell, e.g. some people experience a false smell of burning when they have a stroke or TIA.

[YT2095]

well I get that "false" (it isn`t false) burning smell quite regularly, and I`ve had no stroke nor do I have a brain problem! (some may disagree ).

 

I put it down to Sinus problems, so that`s not the ONLY 2 causes

[ashennell]

Some more potential smell illusions :

 

The first are links are regarding a study of the self-perception of bad breath and indicate that in some cases this may be exaggerated in individuals with low-self esteem. Question : Is this a hallucination (complete fabrication) or illusion (exaggeration of already existing smell)? I don't know.

 

http://www.findarticles.com/p/articles/mi_m1175/is_6_34/ai_82261836

http://members.rediff.com/drkhosla/News/news69.html

 

This is an interesting little snipped from a Criminal Psychology website:

 

http://www.nimblewisdom.com/Criminal_Psychology/Section_103_6_The_Illusions_of_the_Olfactory_Sense

 

 

There are two abstracts of studies that looked at context dependant peception of odours - one of which relys on non-olfactory information.

 

http://chemse.oxfordjournals.org/cgi/content/abstract/3/2/183

http://chemse.oxfordjournals.org/cgi/content/abstract/16/4/349

[Igor Suman]

"Do any of you out there know if there is such a thing called smell illusions?"

 

There are false sensations (paresthesia) of smell' date=' e.g. some people experience a false smell of burning when they have a stroke or TIA.[/quote']

 

When I said "false" I meant other people could not smell it, it did not exist in reality, it was a neurogenic fault. Paresthesias are "real" and can be extreme (painful parasthesia = dysesthesia), however they are false sensations.

 

PS

medications can cause parasthesias, e.g anti-convulsants (for epilepsy) and antihypertensives (for high blood pressure).

[ashennell]

When I said "false" I meant other people could not smell it, it did not exist in reality, it was a neurogenic fault. Paresthesias are "real" and can be extreme (painful parasthesia = dysesthesia), however they are false sensations.

 

These phenomena are classed as hallicinations as there is no sensory basis for the perception.

[Igor Suman]

These phenomena are classed as hallicinations as there is no sensory basis for the perception.

 

I have sciatica, this is a referred pain in my buttock&thigh due to compression of the sciatic nerve where it enters the spine.

 

According to your definition (above) sciatica is a hallucination, (it is actually dysethesia).

 

Surely only false sensations which are psychogenic can be described as hallucinations.

[ashennell]

Hi, sorry I meant to quote your previous post, i.e :-

 

There are false sensations (paresthesia) of smell, e.g. some people experience a false smell of burning when they have a stroke or TIA.

 

There are classed as halluccinations and the definition I gave of halluccinations is brief but essentially accurate.

 

http://en.wikipedia.org/wiki/Hallucination

 

Parasthesia is a specific name for a somatosensory condition. I don't think you can apply it to other senses such as the olfactory system. Parasthesia, in my opinion, is neither an illusion or a hallucination. We are quite aware that the sensation is not from some stimulus in the environment but from within our own sensory system. It is not a case of misinterpretation of ambiguous bottom-up signals.

 

So I agree with you - parasthesia is not what I would call a halluccination.

 

Surely only false sensations which are psychogenic can be described as hallucinations.

 

I think the most important criteria is that a some external ficticious cause is created. Psychogenic sounds like terminology that is impossible to pin down.

[Igor Suman]

Ashennell said "Parasthesia is a specific name for a somatosensory condition.

I don't think you can apply it to other senses such as the olfactory system".

 

Why not apply the term "paresthesia" to other all senses ?,

e.g. Tinnitus is "auditory paresthesia",

so why is the phrase "olfactory paresthesia" unapplicable ?.

[ashennell]

Why not apply the term "paresthesia" to other all senses ?

 

Because the definition includes reference to the somatosensory sense:

 

Paresthesia

: a sensation of pricking, tingling, or creeping on the skin having no objective cause and usually associated with injury or irritation of a sensory nerve or nerve root. (from http://www.dictionary.com)

 

There are lots of types of Tinnitus and many different causes. Some forms of tinnitus are casuses by similar type of nerve damage/dysfunction that are also sometimes the cause of paresthesia.

[Igor Suman]

[size="3

[squishyspong]

Yep its possible. It is possible for people to experience smells after seisures when the smell isnt really there. A lot of things can cause seisures.

[ashennell]

Yep its possible. It is possible for people to experience smells after seisures when the smell isnt really there. A lot of things can cause seisures.

 

These count as hallucinations and not as illusions.

[bwalkabout2003]

Well, I'm not sure if this is what it's called, but I have them. My morning coffee comes on before I wake up and I NEVER smell even when going to get a cup. However, some mornings I wake and the smell of coffee is so strong it could be brewing in my bed, BUT, when I go to get a cup, the coffee maker has not even kicked on yet. Can someone explain that to me?

 

Thanks