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Medical JC #2: The common neural basis of seeing and feeling disgust

  1. Jan 29, 2006 #1

    hypnagogue

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    Wicker B, Keysers C, Plailly J, Royet JP, Gallese V, Rizzolatti G. (2003) Both of us disgusted in my insula: the common neural basis of seeing and feeling disgust. Neuron 40, 655–664

    http://www.sciencedirect.com/scienc...serid=10&md5=a58d6c91ce2a4cf27529d85a812f34ee

    A relatively recent and important discovery in the neurosciences is that of the mirror neuron system. Briefly, mirror neurons are activated both when a subject performs a particular kind of action and when the subject observes another person performing that same action. It has been argued that mirror neurons facilitate our ability to understand the actions of others by means of a sort of empathy-- observing others' actions automatically causes us to "simulate" those actions in our own minds as if we were doing them ourselves. In essence, the idea is that mirror neurons spontaneously put us in the other guy's shoes.

    The article I'll be presenting (reference above) discusses a similar kind of neural mirroring system, but one that is sensitive to emotion rather than action. In particular, the authors use fMRI imaging to analyze how the brain is activated during the perception of disgusting, pleasant, and neutral olfactory stimuli in self and others. The core finding is that some brain areas (left anterior insula and right anterior cingulate cortex) are active both when one perceives disgusting stimuli and when one observes the facial expressions of others who have been exposed to disgusting stimuli. The authors propose that this mirror neuron system for disgust underpins our ability to understand the same emotion in others, again essentially by means of a kind of automatically generated empathy.

    A brief review article on mirror neurons can be found here: A unifying view of the basis of social cognition. The authors of this paper discuss mirror neurons in both the motor and emotive modalities-- drawing extensively from the Wicker et al. paper presented here for the latter topic-- and put forth the case for mirror neurons as grounding empathic understanding of others. A more laymanesque discussion of mirror neurons is available at PBS's Nova site.
     
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  3. Jan 29, 2006 #2

    Moonbear

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    Sounds really interesting. I just read the abstract and all sorts of horrid odors are coming to mind as possible candidates for the test subjects here (something tells me this is one of those studies where you'd really want to be in the control group). :rofl:
     
  4. Feb 3, 2006 #3

    Moonbear

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    Discussion on this article should begin tomorrow (Saturday), whenever hypnagogue is ready to kick it off.
     
  5. Feb 4, 2006 #4
    That looks really cool but I wonder if the results would be slightly different had they used a different age group or females as test subjects.
     
  6. Feb 4, 2006 #5

    hypnagogue

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    OK, on to the discussion. First I'll give some basic background on some of the more technical aspects of the paper (I should have done that earlier-- my apologies); then I'll briefly discuss the hypothesis being tested, the experimental methods, and the results; and finally, I'll give my evaluation of how the results come to bear on the paper's hypothesis.

    Some background information
    "fMRI" is an acronym for "functional magnetic resonance imaging." fMRI is distinct from MRI insofar as MRI is used to obtain static, structural images of the body, whereas functional MRI is used to assess changes in the levels of neural activity in the brain as it performs some kind of task of interest. However, fMRI does not directly measure neural activity. When the activity of a cluster of neurons increases, they have increased metabolic demands, and to meet these demands an increased amount of blood is directed to the neurons. fMRI measures this blood flow by monitoring the differing magnetic properties of oxygenated and deoxygenated hemoglobin in order to obtain an indirect measure of neural activity. Thus in this paper you will see the term "BOLD signal," where BOLD is an acronym for "blood oxygen level-dependent." An increased BOLD signal in a given region of the brain implies an increased level of neural activity in that region.

    The experiment described in this paper uses fMRI imaging to assess brain activity that occurs during the tasks of interest (inhaling pleasant and disgusting odorants and observing others inahling pleasant, disgusting, and neutral odorants). In order to do this, they use what is called the "subtraction method."

    The subtraction method involves taking brain images during both experimental conditions (e.g., observing pleased faces) and control conditions (e.g., observing neutral faces). Thus, in this paper there are terms like "pleasure – neutral condition," which refers to the activity in the brain that is significantly higher during the observation of pleased faces than it is during the observation of neutral faces. Once the relevant imaging data are acquired, statistical methods are used to assess what brain regions are significantly more active during the experimental condition, as compared to the control condition. (These regions of significantly higher neural activity during a given task as compared to a control condition correspond to the colored pixels on those well-known fMRI images that show which parts of the brain have "lit up" during a particular task.) The idea is that this methodology will isolate just those brain regions that are particularly relevant to performing the perceptual/cognitive/affective tasks in the experimental condition, while filtering out the activity of all those regions that are not selectively activated above baseline levels when performing the tasks. The subtraction method is widely used in fMRI studies, though it is not without its critiques. I will not discuss the merits and critiques of the subtraction method here, as that could occupy an entire discussion thread in and of itself.

    Another technical term used in this paper that might be unfamiliar is "voxel," which can be thought of as a 3-dimensional chunk of space ("voxel" is a marriage of the words "volume" and "pixel"). Voxels are the fundamental units of analysis into which a brain is conceptually divided up in fMRI imaging. The size of a voxel in a given analysis corresponds to the spatial resolution of the fMRI imaging used in that analysis.
     
    Last edited: Feb 4, 2006
  7. Feb 4, 2006 #6

    hypnagogue

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    Hypothesis
    The article begins with a simple observation: when we observe someone being disgusted after ingesting unsavory food, we know that we ourselves should avoid that food. This raises the question of exactly how it is that we can understand another's behavioral cues in this way.

    (By the way, it's worth noting that the authors of this paper are examining core disgust, which is the visceral reaction we have upon inhaling or tasting disgusting smells or foods. There are other forms of disgust for humans, such as being disgusted with another person's opinions. However, it has been argued that core disgust is the evolutionarily most ancient and basic form of disgust, and the more abstract disgust reactions we have are learned by accretion to core disgust, essentially as a kind of exaptation.1 If this is right, then more abstract forms of human disgust might have the same fundamental neural basis as core disgust, in which case the results in this paper could arguably generalize to all forms of disgust.)

    The authors suggest two kinds of hypotheses that could account for our ability to recognize and understand disgust in others. One kind is the "cold" hypotheses, which would account for disgust recognition by reference to perceptual or cognitive mechanisms that do not involve emotion. The other kind is the "hot" hypothesis, which holds that we understand the disgust of others because observing their disgusted behaviors automatically generates the emotion of disgust in ourselves.

    If the hot hypothesis is true, we should expect that observing disgust in another activates the same brain regions that become active when one experiences disgust onself. In fact, studies conducted prior to this one had shown the insular cortex (insula) to be active both during exposure to disgusting stimulants and during observation of disgusted facial expressions. However, none of these studies directly examined whether or not the same regions of the insula were active in the same subjects for both conditions. The purpose of this experiment was to directly demonstrate such a common neural substrate.

    Experimental methods
    In order to test the hot hypothesis, the experimenters set up a fairly straightforward experimental design. While in the fMRI scanner, subjects were asked to perform two kinds of tasks, visual runs and olfactory runs. In the visual runs, they passively observed videos of trained actors inhaling from a glass containing either a pleasant, disgusting, or neutral liquid, after which the actors performed a suitable facial reaction to indicate their emotional response. The neutral reactions served as control conditions for analyzing brain activity in response to pleased and disgusted reactions. In the olfactory runs, subjects were exposed to a series of pleasant and disgusting odorants by means of an anesthesia mask, separated by periods of rest. The periods of rest were used as control conditions for the disgusting and pleasant odorant conditions.

    There are several things to like about the careful manner in which the experimental design was set up. The odors they administered to the subjects in the olfactory runs had been evaluated for hedonicity and intensity ratings given by test subjects in a previous study, providing an empirical basis on which to justify the labelling of each odorant as either pleasant or disgusting. The administration of the odors was carefully monitored in order to synchronize with subjects' breathing patterns, ensuring that the odors were indeed smelled by the subjects. The stimuli provided in the visual runs were videos of actors rather than static pictures of facial expressions, making for a more ecologically and empirically valid design. (In particular I found interesting the reference to the 2003 Kilts et al. study suggesting that dynamic and static facial expressions are analyzed with different neural structures; if true, that really casts many previous experiments utilizing static facial expressions in a different light.) The choice of exclusively right-handed subjects is a standard one in brain imaging studies, in order to rule out variations in regional brain function that might correlate with different lateralization across subjects. I don't think the inclusion of only male subjects was a particularly significant experimental flaw, though it would have been nice if the authors had given some rationale for this decision. All in all, the experimental design seems very tight and well controlled. I do have one fairly considerable complaint about the design, but I will save that for my discussion of the paper's conclusion.

    I should say at this point that I am no expert on the more technical aspects of fMRI data acquisition and analysis. As a result, I cannot offer any substantial endorsements or critiques of the specific data acquisition techniques and statistical analyses detailed by the authors, and so I'll just move on to their reported findings.

    Results
    The authors found significant activations for various experimental conditions in the insula, anterior cingulate cortex, and amygdala, all of them largely in accordance with previous findings. I'll focus here on the insula, as that is where they found their most significant and well-supported results. (As an aside, I've noticed that in Figure 2 of the html version of this article, it's somewhat difficult to differentiate the splotches of brain activity colored in green from the blue-ish background of the brain. It's much easier to look at this figure in the PDF version of the article, where the brain is colored in a light gray.)

    In the olfactory runs, inhalation of disgusting odorants was found to correlate with heightened bilateral activity in the anterior insula, whereas pleasant odorants activated a more posterior site on the right insula, with no regions of overlap between the two conditions. In the visual runs, the insula was activated by observation of disgusted facial expressions but not observation of pleased facial expressions. Most importantly, a region of overlap between the disgust – neutral condition and the disgusting odorant – rest condition was found in the left anterior insula, and this region was selective for disgust. Thus, direct evidence was found that experiencing and recognizing disgust share a common neural basis.

    The authors go on to provide an impressive and convincing set of converging evidence from independent studies to support their finding. This converging evidence really runs the gamut, approaching the issue from all sides. It includes results from analysis of the cytoarchitecture and neuronal connectivity of the insula, single-cell recordings from the insula, brain imaging of subjects during the experience of disgust and observation of disgust in others, electrical brain stimulation (ESB) of the insula with corresponding verbal reports, and lesions studies of subjects with damaged insulae. As the authors explain, all of this data is consistent with the experimental findings that the anterior insula is responsive to both the experience and observation of disgust.

    The ESB and lesion studies seem particularly powerful here. They are more than just consistency checks for the authors' findings insofar as they actually may provide a basis for stronger conclusions about the insula's role in mediating disgust than could be drawn from the imaging and anatomical evidence alone. The ESB findings indicate that some level of activation of the insula is sufficient for producing the subjective experience of nausea and sickness (both intimately related to core disgust), while the lesion studies suggest that the insula may be necessary for both the experience and recognition of disgust.

    1. Rozin P, Haidt J, McCauley CR. Disgust. In: Lewis M, Haviland JM, eds. Handbook of emotions. New York: Guilford Press, 1993: 575–94.
     
    Last edited: Feb 5, 2006
  8. Feb 4, 2006 #7

    hypnagogue

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    Discussion
    I find this paper to be a convincing demonstration that the firsthand experience of disgust and the recognition of disgust in the facial expressions of others shares a common neural basis in the left anterior insula. If the authors had left it at that, I would be hardpressed to find any significant flaws in their experimental design, results, and conclusions.

    However, in the introduction the authors explicitly stake out some territory that they never fully explore. They begin by discussing the "cold" and "hot" hypotheses of disgust recognition; the hot hypothesis conjectures that individuals recognize disgust in others by experiencing disgust themselves upon seeing a disgusted facial expression. In their own words, the hot hypothesis states that "in order to understand the facial expression of disgust displayed by others, a feeling of disgust must occur also in the observer" (emphasis added).

    I don't feel as if the authors have adequately followed through on this claim. For instance, it could have been that viewing actors in the disgusting odorant condition did not cause subjects to actually feel disgusted, despite the observed insula activation. (For instance, perhaps some necessary threshold of activation for the experience of disgust to occur was not achieved.) The experimental design gives us no indication of the subjective state of the subjects upon viewing disgust reactions, even though the hot hypothesis is defined by its prediction of what the nature of that subjective state should be. The authors seem to try to hide this at times by equivocating between the idea that recognition of disgust entails experience of disgust on the one hand, and the idea that recognition of disgust and experience of disgust share a common neural substrate (which is a weaker claim).

    I really would have liked to see some measure of the subjects' subjective report during both olfactory and visual runs. It could have been something as simple as asking whether they felt positive or negative after each olfactory or visual event, and rating the intensity of that emotion on a seven point scale. If subjects tended to rate their experience more negatively following "disgust" conditions than "neutral" conditions, this would have been some nice, direct empirical support for the hot hypothesis. Alas, we are left in the dark.

    Certainly subjective report has well-known flaws, but at this stage of mind and brain science it's really our only direct way of getting at what a subject is actually feeling. Had subjective reports been gathered, the hot hypothesis could have been more fully and fairly evaluated. I can think of a number of interesting side-analyses that could have been done with such data as well: for instance, it would have been interesting to compare the degree to which the average subjective rating of one's emotional state compares to the average level of insula activity for the "disgust" and "disgusting odorant" conditions, respectively. Do they correlate to the same degree in both conditions, or is there some kind of interaction such that the same level of insula activity is predictive of different subjective ratings depending on whether the subject observes another being disgusted or is exposed to a disgusting odorant himself? This would have provided valuable insight into the hot hypothesis in particular and the neural correlates of consciousness for disgust in general.

    I find this ommission particularly unfortunate since the authors recognized the importance of the ESB findings to establishing the idea that anterior insula activity causes feelings of disgust, and is not just a mechanism used for processing disgust information in general.

    I should say in closing that even if successful recognition of disgust does not always entail the experience of disgust, this would not spell doom for the hot hypothesis if it were to be slightly reworded. The hot hypothesis seems to refer specifically to consciously felt emotion where perhaps it need only refer to affective information processing in general. It has been demonstrated that there exist kinds of information processing in the brain that are distinctly "affective" in terms of the nature of the perceptual input and its effect on resulting behavior, but nonetheless do not register in consciousness (e.g. see here). It seems to me the hot hypothesis would retain much of its explanatory power, and perhaps would even be more plausible, if it allowed some role for unconscious emotion processing as well as consciously felt emotion.
     
    Last edited: Feb 4, 2006
  9. Feb 4, 2006 #8

    hypnagogue

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    Sorry for the late edits. I only did some minor rewording-- no any substantive changes-- in case anyone started reading already.
     
  10. Feb 5, 2006 #9

    Monique

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    Nice paper and presentation of it. I read through the whole thing except the methods and tables part (not had the time yet).

    The first comment I want to make is: why males. Isn't it well accepted that females are more sensitive to reading and feeling the emotions of others? Did the authors think that this would activate irrelevant centers and that males would thus be a better subject? I would have liked to see both a male and a female group.

    They draw that conclusion based on experiments by others where 1) the electrical stimulation of the anterior ventral insula causes unpleasant sensations in the throat, mouth and stomach and 2) trauma to the anterior insula ablates the ability to feel disgust (and to recognize it in others). It would have been nice to know whether the participants claimed to feel any emotions or not as you suggested.

    Another note I have is that they mention the Carr et all (2003) study on page 658 last paragraph. The study showed activation of the anterior insula/inferior frontal gyrus during both the observation and imitation of facial expressions. Indicating that it IS a cold reaction. Then they conclude that since this contradicts the current study, it could have been a hot reactions where the subjects immitating emotions were actually feeling it. I don't follow that logic and it seems flawed to me. Why didn't they ask participants to imitate the emotion of disgust and analyze those results?

    Then Table 1, in the upper right paragraph on page 656 they say: "among the activated structures, two are of particular interest for the present study: the amygdala and the insula". Did they draw this conclusion a priori? Why didn't they look at the other activated structures in detail?
     
  11. Feb 5, 2006 #10

    hypnagogue

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    Right, and that does constitute some indirect evidence for the hot hypothesis, but this experiment still leaves ample room for skepticism. It's kind of odd-- the authors note that there has been indirect evidence that the anterior insula mediates both experience and recognition of disgust, but that it was never shown directly until this study. And yet, they seem content with the indirect evidence that disgust is experienced while one is recognizing disgust in others, without attempting to test this directly as well. Worse, the thing they fail to directly analyze is really what constitutes the heart of their hot hypothesis.

    It may be that the authors felt the evidence they presented was sufficient, but if they feel that way, they must be operating on the assumption that any kind of significantly elevated activity in the anterior insula gives rise to subjective feelings of disgust. But given the evidence in this experiment and in the other referenced studies, I think the most we could really say with a high degree of confidence is the following:

    1) If one is experiencing disgust, it follows that one's anterior insula is activated significantly more than normal (from the imaging and lesion studies);
    2) If the anterior insula are activated in some particular way, it follows that one experiences disgust (from the ESB studies).

    Even if we take it for granted that (1) and (2) are true, it is still at least logically possible for something like the following statement to be true as well:

    3) There exists some level of significantly elevated neural activity in the anterior insula such that this activity will register in an fMRI scan, but will not correlate with a first person experience of disgust.

    I can think of a couple prima facie plausible scenarios in which (3) might be true. One, there might be some threshold of overall neural activity that needs to be achieved in the anterior insula in order for a person to experience disgust; if such a threshold exists, it might be higher than the level of activity that would show up in an fMRI scan, but lower than the level of activity elicited in ESB. Two, it could be that different patterns of firing on the level of individual neurons show up in an fMRI scan as identical levels of gross activation on the level of clusters of neurons in the anterior insula; further, it could be that one of these patterns somehow correlates with feelings of disgust while the other does not.

    So really, it seems like the author's case for the hot hypothesis is anything but an open and shut case. They easily could have addressed this by adding the appropriate subjective measures to their study.

    The Carr study isn't evidence for a cold reaction, I think. It does raise some questions as you point out, but to me it seems neutral on whether or not there is some emotional component involved. To say that it definitely demonstrates a cold reaction, you would need to have (again) subjective data from the subjects in that study, indicating whether and to what degree they felt emotional changes in the course of imitating various expressions.

    Also, I'm not aware of any specific studies on this at the moment, but it does seem plausible to me that imitating facial expressions might elicit some kinds of corresponding emotions. It has been shown for instance that merely inducing subjects to nod up and down (like a "yes") or shake their head side to side (like a "no") unconsciously influences their evaluations of positively and negatively valenced products (ref). So that's some evidence (I'm sure there's more) that mere body movements can indeed exert influence on cognitive/affective processes (at least on an unconscious level), even though we usually think of the flow of causation going exclusively from the latter to the former. If this effect can exist on a subconscious level, it's at least plausible that it can occur on a conscious level as well, pending further research.

    I suppose the reason they singled out the amygdala and insula was because those are two regions that had previously been shown to be activated during disgust. If they found other regions of significant activity that had not been reported previously, they might have chalked it up to random chance rather than a truly significant result, or perhaps they just didn't think it was worth it to spend the extra time, space, and effort on previously unsupported results.

    Actually, if you look at the rightmost column in Table 1, you'll see that the sizes of the activated regions are largest for the amygdala and anterior insula (though some areas of the frontal gyrus also have high values). So, I'll hazard a guess that they may also have found these areas more significant because larger volumes of neural tissue within them were activated. I can't say for sure though.
     
    Last edited: Feb 5, 2006
  12. Feb 6, 2006 #11

    Monique

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    If they found other regions, it would have been a highly significant finding. You don't do experiments to repeat the work that has already been done (unless you want to validate the work).

    They should have used arguments to support that the other regions are not significant, I still wonder whether the other areas show significant overlap or not in feeling and recognizing disgust.
     
  13. Feb 7, 2006 #12
    i have a couple of questions and comments

    1. Why wouldn't they run the experiment in reverse; ie first smell and then see the visual. It seems like this would show that the feeling of disgust and the viewing of disgust activate the same response no matter what is experienced first. It may seem trivial but if it works in one direction it should in the other if this is the main mechanism working in this situation.
    Maybe its not important but I would have liked to see that it was the same just as a control.

    2. I also agree that there is no evidence here that the subjects are experiencing disgust even though similar areas of the brain are activated. It would seem easy enough to have gotten the subjects view of their experiences, in both the visual and olfactory tests.

    3. Why is disgust so significantly associated with this phenomenon and pleasure not? This implies to me that this is specific to disgust and not a general mechanism of how we understand how "others" feel. Could it be that the same part of the brain is activated for all unpleasant things but the actual distinct experiences of disgust are different for smelling something nasty and seeing someone with a disgusted face? Looking at the picture of the guy with the disgusted facial expression is pretty unpleasant in itself.
     
  14. Feb 7, 2006 #13

    Monique

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    I thought about the same thing, the way I interpreted the set-up is that when you let them watch the video first and then experience disgust makes a stronger case than first experiencing disgust and then being referenced back to that personal experience event. I'd think that it could potentially elicit a strong reaction that would be correlated to the memory of smelling and not the recognition of disgust. Does that make sense?

    Because recognizing disgust can give you an evolutionary advantage: don't eat something that is violently rejected by someone else. Recognizing pleasure probably does not have a core-function and as mentioned in the paper could be spread throughout the brain.

    I wonder where recognition of sorrow lies in the brain, which is a very powerful mechanism (I even think more powerful than disgust).
     
  15. Feb 7, 2006 #14
    I totally agree with that.
     
  16. Feb 8, 2006 #15

    hypnagogue

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    Actually, I'd expect that the results of the visual runs would differ if you did the olfactory runs first. Coming into contact with the pleasing and disgusting odorants first would probably prime a person to react more strongly to seeing others being pleased and disgusted, as the mental representations of those experiences would still be fresh in the mind (as Monique argued above). This would hurt the authors' case, because a skeptic could argue that the anterior insula activation upon viewing disgusted faces was caused by one's immediate memory of just having smelled some disgusting odors, rather than being a general result that always occurs when one sees disgusted facial expressions.

    The authors' brief explanation of this is as follows:

    "The lack of overlap between the observation of pleased facial expressions and the olfaction of pleasant odorants is probably due to the fact that, in contrast to the emotion of disgust, which is tightly linked to bad odorants/tastes, the emotion of pleasure can be triggered by many stimuli, only few of which are olfactory or gustatory. There is therefore a strong link between bad tastes/smells, the emotion of disgust, and the facial expression of disgust, while there is a much weaker link between pleasant odors/smells, the emotion of pleasure, and pleased facial expressions."

    So basically they seem to be arguing that there are common neural substrates for the observation and experience of disgust, but not for pleasure, because disgust is highly specific to the taste and smell sensory modalities whereas pleasure can be elicited by a wider range of sensory modalities.

    My take on this explains the results not in terms of sensory modalities, but rather biological/evolutionary context. We experience disgust from tasting or smelling certain chemicals because it is highly likely that ingestion of these chemicals is biologically harmful; the disgust reaction includes both a negative emotional valence and spontaneous bodily reactions (retching or tightness of the threat, crinkling of the nose, etc.) designed to prevent ingestion of the offending substances and to encourage avoidance of them in the future. The biological harmfulness of disgusting substances for a given species is largely constant and context independent; if a substance is disgusting (and hence, likely harmful) to a conspecific, it is almost certainly going to be harmful for you as well. Therefore, it is to one's advantage to have the same behavioral reaction to a substance just from observing disgust in another as one would get from experiencing disgust oneself.

    On the other hand, we experience basic sensory pleasures largely when it is the case that a given stimulus satisfies some kind of idiosyncratic, allostatic need of the organism in some given time and context. Unlike the fixed, context-independent harmfulness of disgusting substances, an organisms's allostatic needs are fluid and context dependent. For instance, a hot shower might be welcome after a long day in the cold, but might be quite uncomfortable in a tropical climate. Or to take a more immediately relevant example, if one's blood sugar level is low and one is hungry, it is likely that eating a sweet tasting candy bar will elicit some kind of gustatory pleasure. However, try eating the same candy bar after an exceedingly large and satisfying meal, and it will no longer be so pleasant-- it might even become unpleasant or disgusting.

    Because there is no fixed, context-independent relationship between the types of stimuli that elicit basic sensory pleasures and their biological utility, it makes biological and evolutionary sense that observation of pleasure in another should not automatically trigger strong pleasure-related cognition and affect in the observer. If I see you enjoying a candy bar, it's not always to my advantage to share your experience and desire a candy bar myself, because I may have just had a large meal myself. On the other hand, if I see you retching after drinking a cup of bad milk, it will always be to my advantage to share your experience and be aversive to the milk, because that milk will be harmful to me no matter what.

    I don't think there is a single brain region responsible for all unpleasant feelings. Certainly the insula wouldn't qualify. To the best of my knowledge, the most ubiquitous neural system involved with negative emotions might be the periaqueductal gray (PAG), which is involved to some extent with fear and anxiety, anger, pain, and panic/grief/loneliness. However, the PAG would probably best be thought of as a component shared by many neural systems involving unpleasant feelings rather than a general system for feeling unpleasant in itself, since its role in mediating these respective negatively valenced emotions is elaborated to a great extent by distinct neural systems downstream. In fact, the PAG is also involved with some positive forms of mentality, as it plays a role in pain modulation and animals choose to self-stimulate certain parts of it (but not others).
     
  17. Feb 8, 2006 #16
    [QUOTE/]
    Because there is no fixed, context-independent relationship between the types of stimuli that elicit basic sensory pleasures and their biological utility, it makes biological and evolutionary sense that observation of pleasure in another should not automatically trigger strong pleasure-related cognition and affect in the observer. If I see you enjoying a candy bar, it's not always to my advantage to share your experience and desire a candy bar myself, because I may have just had a large meal myself. On the other hand, if I see you retching after drinking a cup of bad milk, it will always be to my advantage to share your experience and be aversive to the milk, because that milk will be harmful to me no matter what.
    .[/QUOTE]

    Do you think then for certain stimuli that are not related to the taste-smell response but are pleasurable could illicit similar responses? For example, there are pleasurable experiences that one could argue have evolutionary benefits. One strong example is sexual pleasure. You need to have sex to propagate the species. Do you think that maybe that watching a sexual act would stimulate the same areas as during a sexual act?
     
  18. Feb 8, 2006 #17
    Thats an interesting question, I thought I read somewhere before that they did a study on something similar to that.
     
  19. Feb 8, 2006 #18

    hypnagogue

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    It's possible. I imagine you might have mirror neurons for motoric actions responding purely to the physical movement. In addition you might also have mirror neurons for emotion responding.

    My argument above might or might not predict that you'd have mirror neurons for sexual emotions being activated depending on how you elaborate it. On the one hand, it's probably true that the evolutionary utility to having sex is pretty universal and not influenced much by context (though that might vary between males and females, and depending on to what extent the species in question uses up time and resources to care for its young, etc.). On the other hand, watching others have sex might not be a terribly relevant trigger for inducing immediate action in the environment in most cases (as watching another being disgusted upon smelling bad milk would be), at least not for humans (but maybe it would be for another species where the females only go into heat during certain times and are more or less 'freely available' to interested males during those times, etc.).

    On the whole, I would guess that watching sexual acts would indeed activate some kind of mirror neuron system for sexual emotion in humans, perhaps even more so for males than females.
     
  20. Feb 12, 2006 #19

    Q_Goest

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    Hey H'. Interesting presentation here, nice job! Sorry I've not been around for a while, I've been traveling almost constantly now for the past 4 weeks (business).

    Regarding fMRI, I wonder how one can discern which areas of the brain are working on some specific task and why there might be "mirror" neurons operating there. I assume there are differences in the level of activity that can be detected. Could someone comment on how it is various areas of the brain are found to correlate to the given task while an fMRI is being performed? How do we know which areas of the brain are most active and how do we know these are mirror neurons?

    I suspect it has something to do with the tables provided by the paper (esp. Table 1). Table 1 is broken up into left and right brain areas along with type of stimuli. It then has x,y,z coordinates of those locations (I think) and a t value (what's that?) and number of voxels that are "activated" during the test.

    *

    Why do you think this issue wasn't cause for rejection of the paper or cause for rewording of the evidence provided? If this paper is reviewed by people intimately involved with this type of research, how is it a paper can be published with missleading or highly speculative information?
     
  21. Feb 20, 2006 #20

    hypnagogue

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    I gave a brief overview of basic fMRI methodology in post #5 of this thread that seems to address some of your questions here. If I explained something inadequately could you be more specific in pinpointing what your question(s) might be?

    As for mirror neurons-- mirror neurons are just defined to be those neurons that are active both when one is 'doing something' (performing some motor task, or experiencing some emotion, etc.) and when one observes another doing that same thing. Accordingly, it's pretty straightforward to demonstrate empirically whether a neuron (or group of neurons) is a mirror neuron or not. For instance, in this paper it was found via fMRI imaging that the left anterior insula is activated both when one experiences disgust and when one observes others displaying disgust reactions. So the left anterior insula meets the criteria for containing mirror neurons for disgust.

    Table 1 just shows all the brain regions which were found to be significantly more active when inhaling disgusting and pleasant odorants, respectively, than when not inhaling any odorants. (A given region is determined to be significantly more active in one condition than in another by using statistical analysis on the imaging data.) The "Hem." column specifies whether a given activated region was located in the right or left brain hemisphere, since many brain structures (e.g. the amygdala) are mirrored on both sides of the brain. The x,y,z are 3D coordinates of these regions according to two coordinate systems.

    The t-value is a statistical measure used when comparing the means of two sets of data (for instance, the "disgusting odorant" imaging data set and the "rest" data set). Very basically speaking, the higher the t-value, the more statistically significant the result. (And the higher the statistical significance is, the lower odds we would have of observing this data if it were generated randomly, i.e. if there really was no difference between the mean activations in the two conditions.)

    A quick google search picked up a couple of potentially helpful links on t-values:
    http://helios.bto.ed.ac.uk/bto/statistics/tress4a.html#Student's t-test
    http://sahs.utmb.edu/pellinore/intro_to_research/wad/differences.htm

    I don't know if I would call their conclusions "highly speculative"; I'd prefer "vulnerable to skepticism." They have what I'd consider to be fairly decent indirect evidence for their hot hypothesis. They don't by any means make an open and shut case, but they aren't wildly speculating either.

    Still, if nothing else it would have been nice if they came out and presented it like this. As it stands they kind of dance around the issue, starting off talking very explicitly about feeling disgust while observing disgust in others, but at the end just showing a common neural basis for feeling and observing disgust in separate conditions and apparently equivocating the two. How they managed to get that past the referees, who knows. I wouldn't call it a fatal error-- the direct evidence of a common neural basis for feeling and observing disgust in itself is an important and interesting finding-- but it is somewhat sloppy and assumptive.
     
    Last edited: Feb 20, 2006
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