“The observation that the aversiveness of pain escalates super-linearly with [pain] intensity [...]”
It seems to me that the data mostly don’t support this claim, or at least that there are plausible alternative explanations for the data. The main issues I’d like to raise are:
1. Whether aversiveness varies linearly or super-linearly with pain intensity can only be determined if the pain intensity is represented on a ratio scale (e.g. with the same intensity difference between 0-1 and 9-10). In several studies, however, the ratio scale consists of values arbitrarily assigned to verbal descriptions of pain intensity (e.g. source [22]: “2: just noticeable, 3: weak, 4: mild, 5: moderate, 6: strong, 7: severe, 8: excruciating”). If the participants react mainly to the verbal descriptions (and less to the assigned numbers), which seems plausible to me, then we can’t draw any conclusions from the mathematical relationship between the (arbitrary) numbers and the aversiveness. Another interpretation could be that the assumed difference in pain intensity between “just noticeable” and “weak” is simply much smaller than the assumed difference in pain intensity between “severe” and “excruciating”, even though the assigned values differ by 1 in both cases. The same concern applies to [19], [20], but not to [21].
2. Even when pain intensity is represented only by numbers (and not by verbal descriptions), it’s not obvious how participants interpolate between the highest and lowest numbers. Take [21]: In the appendix it says, “pain intensity is rated using a pain scale 0–10, with 0=no pain and 10=worst pain imaginable”. Do participants treat these numbers as if they were on a ratio scale (e.g. with the same intensity difference between 0-1 and 9-10)? It’s not unreasonable to think so, but we just don’t know because they’re not asked to do so. It may well be that they are interpolating on a log scale, where intensities are compressed the higher they go (or something else entirely). For example, if pain intensities are interpreted logarithmically, the identical plot for aversiveness would imply that aversiveness varies *linearly* with intensity, not super-linearly.
3. “Research in this area indicates that as sensations, including pain, increase in intensity, their perceived aversiveness grows exponentially [24]”. The source [24], however, only seems to show that as *physical stimuli* (such as loudness or visual flash rate) increase in intensity, their perceived aversiveness grows super-linearly. This doesn’t say anything about the relationship between *pain intensity* (or other sensations) and perceived aversiveness. An alternative explanation might be that pain intensity varies super-linearly with physical stimuli, and perceived aversiveness varies *linearly* with pain intensity. Also, according to [24], perceived aversiveness doesn’t vary “exponentially”, as you claim, but according to a “power function” which means e.g. y=x^2, not y=2^x. I haven’t checked the other psychophysics sources cited, but I wouldn’t be surprised if they suffered from the same limited explanatory power.
4. It seems hard even in theory for participants to distinguish between pain intensity and pain aversiveness. I’m somewhat convinced by behavioral studies (such as [21]) that express aversiveness in terms of trade-offs (a measurement distinct from pain intensity), but less so by the other study designs.
5. Some of the sources are very old ([24] from 1957, [25] from 1959), which in and of itself doesn’t invalidate the claims, but still surprised me a little. Do they still represent the current state of science?
I’d be interested to hear your reaction to these points. I’m sure there are things I’m missing or that I’m getting wrong.
Thank you for the detailed comments, it is really nice to see you were so thorough with the text and the studies we cite, these are good points!
If we understand you well, with a few exceptions (as in the case of the time trade-off study, [21]), what you mention is that the superlinearity observed could be a by-product of participants of the studies interpreting the designed scales of intensity (even the numerical ones) as not equidistant, hence a non-equidistant plot of aversiveness could, for example, be observed if the relationship between intensity and aversiveness was in fact linear.
My impression, however, is that we are referring to different things when we say pain intensity (and this is our fault for not being clearer on the text, we will make the distinction clearer). My understanding is that ‘pain intensity’ is often used simply as a synonym for aversiveness or unpleasantness, as opposed to physical intensity of stimuli/pain signals.
As I see it, assessments of pain intensity (unpleasantness) are typically made on what can only be understood as an ordinal scale. Whether evidence on pain unpleasantness is collected with verbal descriptors or numerical descriptors, these unpleasantness scales (i.e. the intensity scales used) can only be interpreted as ordinal exactly because we do not know what type of understanding of the scale study participants have. Even though some authors use the data collected (e.g., pain intensity/unpleasantness ratings in scales from 1-10 or 1-100) and calculate things such as areas under the curve for plots of ‘intensity’ (meant as unpleasantness) ratings x duration, the data is still ordinal for practical purposes and unsuitable for these operations.
So the effort in this work was to try and see if it would be possible to make such a conversion of pain unpleasantness (from ordinal to ratio scale), and determine the distance among the categories of unpleasantness on a ratio scale. Thus, in the case of the psychophysics studies, I do not see three parameters: intensity of physical stimuli, pain intensity and pain aversiveness, but only two: intensity of physical stimuli and aversiveness (we will correct the text where we mention that the relationship is exponential).
On the availability of studies, unfortunately, all of them were riddled with limitations. Although designs such as the time trade-off approach (used in study [21]) are much better, to our surprise the literature is extremely scarce. So no good studies, and no recent studies either, as far as we are aware. This is why we currently prefer a disaggregated approach, as we do not see how, with the evidence at hand, it is possible to estimate the equivalence weights with any precision .
“If we understand you well […] the relationship between intensity and aversiveness was in fact linear.” → Yes, that entire section is well put and a succinct summary of what I was trying to get at.
“My understanding is that ‘pain intensity’ is often used simply as a synonym for aversiveness or unpleasantness, as opposed to physical intensity of stimuli/pain signals.” -> I see, that wasn’t clear to me when I read your post. I think it’s good to make those distinctions clearer (as you suggested). E.g. some scholars argue that pain without aversiveness (or painfulness) is possible (mainly in pain asymbolia – maybe you’re aware of this: https://direct.mit.edu/books/book/2549/Feeling-Pain-and-Being-in-Pain ). It might be a good idea to distinguish pain and aversiveness conceptually or else explicitly say that you treat them as synonymous (in which case you should modify the claim “Pain’s aversiveness escalates disproportionally with its intensity” and similar claims, because it becomes contradictory)
“So the effort in this work was to try and see if it would be possible to make such a conversion of pain unpleasantness (from ordinal to ratio scale), and determine the distance among the categories of unpleasantness on a ratio scale.” -> if you put it that way it seems clearer (at least to me). I’m still not sure if that’s a substantial point, since adding or subtracting categories would change the relationship between aversiveness and the ordinal scale, but I see what you mean. I guess there is *some* value in saying that stronger verbal descriptions tend to comprise more levels of aversiveness, but to me it’s a bit of a stretch to talk about non-linearity if the x-axis in the plot is actually an ordinal axis.
“So no good studies, and no recent studies either, as far as we are aware. This is why we currently prefer a disaggregated approach, as we do not see how, with the evidence at hand, it is possible to estimate the equivalence weights with any precision.” -> yeah, that makes sense.
“The observation that the aversiveness of pain escalates super-linearly with [pain] intensity [...]”
It seems to me that the data mostly don’t support this claim, or at least that there are plausible alternative explanations for the data. The main issues I’d like to raise are:
1. Whether aversiveness varies linearly or super-linearly with pain intensity can only be determined if the pain intensity is represented on a ratio scale (e.g. with the same intensity difference between 0-1 and 9-10). In several studies, however, the ratio scale consists of values arbitrarily assigned to verbal descriptions of pain intensity (e.g. source [22]: “2: just noticeable, 3: weak, 4: mild, 5: moderate, 6: strong, 7: severe, 8: excruciating”). If the participants react mainly to the verbal descriptions (and less to the assigned numbers), which seems plausible to me, then we can’t draw any conclusions from the mathematical relationship between the (arbitrary) numbers and the aversiveness. Another interpretation could be that the assumed difference in pain intensity between “just noticeable” and “weak” is simply much smaller than the assumed difference in pain intensity between “severe” and “excruciating”, even though the assigned values differ by 1 in both cases. The same concern applies to [19], [20], but not to [21].
2. Even when pain intensity is represented only by numbers (and not by verbal descriptions), it’s not obvious how participants interpolate between the highest and lowest numbers. Take [21]: In the appendix it says, “pain intensity is rated using a pain scale 0–10, with 0=no pain and 10=worst pain imaginable”. Do participants treat these numbers as if they were on a ratio scale (e.g. with the same intensity difference between 0-1 and 9-10)? It’s not unreasonable to think so, but we just don’t know because they’re not asked to do so. It may well be that they are interpolating on a log scale, where intensities are compressed the higher they go (or something else entirely). For example, if pain intensities are interpreted logarithmically, the identical plot for aversiveness would imply that aversiveness varies *linearly* with intensity, not super-linearly.
3. “Research in this area indicates that as sensations, including pain, increase in intensity, their perceived aversiveness grows exponentially [24]”. The source [24], however, only seems to show that as *physical stimuli* (such as loudness or visual flash rate) increase in intensity, their perceived aversiveness grows super-linearly. This doesn’t say anything about the relationship between *pain intensity* (or other sensations) and perceived aversiveness. An alternative explanation might be that pain intensity varies super-linearly with physical stimuli, and perceived aversiveness varies *linearly* with pain intensity. Also, according to [24], perceived aversiveness doesn’t vary “exponentially”, as you claim, but according to a “power function” which means e.g. y=x^2, not y=2^x. I haven’t checked the other psychophysics sources cited, but I wouldn’t be surprised if they suffered from the same limited explanatory power.
4. It seems hard even in theory for participants to distinguish between pain intensity and pain aversiveness. I’m somewhat convinced by behavioral studies (such as [21]) that express aversiveness in terms of trade-offs (a measurement distinct from pain intensity), but less so by the other study designs.
5. Some of the sources are very old ([24] from 1957, [25] from 1959), which in and of itself doesn’t invalidate the claims, but still surprised me a little. Do they still represent the current state of science?
I’d be interested to hear your reaction to these points. I’m sure there are things I’m missing or that I’m getting wrong.
Thank you for the detailed comments, it is really nice to see you were so thorough with the text and the studies we cite, these are good points!
If we understand you well, with a few exceptions (as in the case of the time trade-off study, [21]), what you mention is that the superlinearity observed could be a by-product of participants of the studies interpreting the designed scales of intensity (even the numerical ones) as not equidistant, hence a non-equidistant plot of aversiveness could, for example, be observed if the relationship between intensity and aversiveness was in fact linear.
My impression, however, is that we are referring to different things when we say pain intensity (and this is our fault for not being clearer on the text, we will make the distinction clearer). My understanding is that ‘pain intensity’ is often used simply as a synonym for aversiveness or unpleasantness, as opposed to physical intensity of stimuli/pain signals.
As I see it, assessments of pain intensity (unpleasantness) are typically made on what can only be understood as an ordinal scale. Whether evidence on pain unpleasantness is collected with verbal descriptors or numerical descriptors, these unpleasantness scales (i.e. the intensity scales used) can only be interpreted as ordinal exactly because we do not know what type of understanding of the scale study participants have. Even though some authors use the data collected (e.g., pain intensity/unpleasantness ratings in scales from 1-10 or 1-100) and calculate things such as areas under the curve for plots of ‘intensity’ (meant as unpleasantness) ratings x duration, the data is still ordinal for practical purposes and unsuitable for these operations.
So the effort in this work was to try and see if it would be possible to make such a conversion of pain unpleasantness (from ordinal to ratio scale), and determine the distance among the categories of unpleasantness on a ratio scale. Thus, in the case of the psychophysics studies, I do not see three parameters: intensity of physical stimuli, pain intensity and pain aversiveness, but only two: intensity of physical stimuli and aversiveness (we will correct the text where we mention that the relationship is exponential).
On the availability of studies, unfortunately, all of them were riddled with limitations. Although designs such as the time trade-off approach (used in study [21]) are much better, to our surprise the literature is extremely scarce. So no good studies, and no recent studies either, as far as we are aware. This is why we currently prefer a disaggregated approach, as we do not see how, with the evidence at hand, it is possible to estimate the equivalence weights with any precision .
“If we understand you well […] the relationship between intensity and aversiveness was in fact linear.”
→ Yes, that entire section is well put and a succinct summary of what I was trying to get at.
“My understanding is that ‘pain intensity’ is often used simply as a synonym for aversiveness or unpleasantness, as opposed to physical intensity of stimuli/pain signals.”
-> I see, that wasn’t clear to me when I read your post. I think it’s good to make those distinctions clearer (as you suggested). E.g. some scholars argue that pain without aversiveness (or painfulness) is possible (mainly in pain asymbolia – maybe you’re aware of this: https://direct.mit.edu/books/book/2549/Feeling-Pain-and-Being-in-Pain ). It might be a good idea to distinguish pain and aversiveness conceptually or else explicitly say that you treat them as synonymous (in which case you should modify the claim “Pain’s aversiveness escalates disproportionally with its intensity” and similar claims, because it becomes contradictory)
“So the effort in this work was to try and see if it would be possible to make such a conversion of pain unpleasantness (from ordinal to ratio scale), and determine the distance among the categories of unpleasantness on a ratio scale.”
-> if you put it that way it seems clearer (at least to me). I’m still not sure if that’s a substantial point, since adding or subtracting categories would change the relationship between aversiveness and the ordinal scale, but I see what you mean. I guess there is *some* value in saying that stronger verbal descriptions tend to comprise more levels of aversiveness, but to me it’s a bit of a stretch to talk about non-linearity if the x-axis in the plot is actually an ordinal axis.
“So no good studies, and no recent studies either, as far as we are aware. This is why we currently prefer a disaggregated approach, as we do not see how, with the evidence at hand, it is possible to estimate the equivalence weights with any precision.”
-> yeah, that makes sense.
Thanks! I will revise the post to make sure it is clearer!
Great!