Painful memories, and effortful actions

How does the brain evaluate a painful episode?  When you look back at an unpleasant episode of your life, how does your impression of it now relate to the actual experience that you had during the episode?

Surprisingly, when we recall a painful experience we seem not to evaluate it based on its duration, or its temporal integral, or its mean pain.  That is, it does not matter very much if one experience was on average more painful than another, nor does it matter that one experience was longer than another.  Rather, we seem to evaluate the totality of a painful experience using two factors: magnitude of the peak of the pain, and the magnitude of the pain as the episode ended.  Here, I will describe the basic experiments that led to these ideas, and then suggest a new interpretation of rather puzzling results regarding how the brain evaluates effort in simple motor control tasks.

Cold water bath

In 1993, Kahneman and colleagues asked 32 volunteers at University of California Berkeley to put both their hands in a cold water bath for 5 seconds.  Next, one hand was chosen at random and placed in cold water for 60 seconds (or 90).  After a brief rest period, the other hand was placed in cold water for 90 seconds (or 60).  In these two episodes the temperature of the water was the same for the first 60 seconds (21 degrees Centigrade).  However, in the last 30 seconds of the 90 second episode, the temperature was increased by 1.1 deg.  So in the 90 second episode one hand always experienced a longer period of discomfort, but the episode for that hand ended with slightly warmer water. 

During the time that their hand was in water the subjects used their other hand to adjust a knob to continuously indicate their discomfort.  As you would expect, the discomfort increased immediately as the hand was placed in the cold water, reached a peak at around 60 seconds, and then declined for the next 30 seconds. 

After the two episodes were completed, the subjects were told that they would need to put their hand in cold water one more time but that they could choose which episode they wanted.  The main dependent variable was the subject’s choice for this third episode.  Logically, no one should pick the episode that lasted 90 seconds.  But remarkably, most subjects (22 of 32, 69%) preferred to repeat the longer episode.  Indeed, most subjects indicated that the longer episode had caused less overall discomfort!

This suggested that when people evaluate painful episodes, what matters is not the duration, but rather the magnitude of the pain as the episode ended.  However, a potential confound with the cold water experiment is that we know that memory fades with time, and so perhaps evaluating the pain of an episode relies more on the ending because the memory of the early parts have faded.  Perhaps if the subjects were asked to remember the episode a few days later, they would not recall it the same way as a few minutes after the end of the episode.  Was this temporal decay the reason for the seemingly illogical choice?  To test for this, Kahneman and colleagues performed a new experiment.

The perceived pain of a medical procedure

Redelmeier and Kahneman (1996) asked patients that were undergoing colonoscopy (n=154) or lithotripsy (a procedure to destroy hardened masses, n=133) to give assessment of their pain by pointing to a scale at one minute intervals.  The colonoscopy lasted from 4-67 minutes, and the lithotripsy lasted from 18-51 min.  One hour after the procedure the patients were asked to judge the total amount of pain experienced using the same scale. 

To check for reliability of the evaluations, some of the patients were asked to recall the experience 6 months (colonoscopy) or 1 year (lithotripsy) later and again evaluate the total pain.  The retrospective ratings at 6 months and 1 year were correlated at r=0.77 and r=0.54 for the two groups.  For the colonoscopy group the ratings at 6 months had the same mean as at 1 hour, for the lithotripsy group the average ratings at 1 year were 15% higher than at 1 hour. 

In the colonoscopy procedure the pain intensity was higher at start than at end, whereas in the lithotripsy procedure pain intensity was low in the first few minutes and ended higher. 

Having collected these data, the investigators asked what aspect of the painful experience was a predictor of the immediate ratings at 1 hour, or the follow-up ratings at 6 months or 1 year.  Duration of the procedure was not a predictor of the immediate or follow-up ratings.  Rather, peak pain was the most powerful predictor of both ratings (r=0.6 for each), and end pain was the second most powerful predictor (r=0.4 for each).  These correlations held for both of the procedures.  The combination of the two factors increased the correlations to about 0.67 and 0.65 for immediate and follow-up ratings.

So people’s impression of the relative pain they endured during an episode remained fairly consistent at 1 hour and at many months after the episode.  Their impressions were predicted by two aspects of their actual experience: magnitude of the peak of the pain, and magnitude of the end pain.  Duration of the episode played little or no role.  

When we remember a painful episode, the most salient aspects of that episode seem to be the peak of the pain, and how it ended.  To improve our perception of a difficult episode, it may be more beneficial to prolong it and gradually reduce the pain, rather than shorten it and abruptly end the pain. 

Perception of effort

This idea of peak-end perception of pain may help us understand a rather puzzling result in the field of motor control.  One of the fundamental questions in motor control is how the brain evaluates effort.  The variables of interest are force and time, and the question is with regard to our perception of effort as a function of these variables.

In 2004, +Konrad Kording, +Daniel Wolpert and colleagues performed an experiment in which volunteers held a robotic arm and experienced a sinusoidal-like force profile of peak F and duration T.   Next, they experienced another force pattern of peak F’ and duration T’.  They then asked their volunteers which force they would like to experience again.  They were told that they should choose the force that required the least effort.  In this way, the investigators estimated indifference curves, i.e., curves along which the subjects were indifferent to changes in peak force and duration.

The rather unexpected result was that as the duration of a force pattern increased (beyond about 200ms), the indifference curve also increased.  This means that given a choice between some peak force and short duration, vs. the same peak force and longer duration, the subjects picked the longer duration!  

How could a longer duration of an effortful task be preferable to a shorter duration?

A close look at how the force patterns were produced provides a possible answer.  The forces were sinusoidal with a period that depended on T.  So as the duration increased, the rate at which the force changed decreased.  This means that for a longer duration force, the forces gradually came to an end, whereas for a short duration force, the forces rapidly came to an end.  People preferred the gradually ending force, despite the fact that they would be producing the forces for a longer amount of time.

The peak-end hypothesis of pain perception may have relevance to how the brain measures effort.

Acknowledgements: I am grateful to +Alaa Ahmed of University of Colorado for discussions regarding these ideas.

References

Kahneman D, Fredrickson BL, Schreiber CA, and Redelmeier DA (1993) When more pain is preferred to less: adding a better end. Psychological Science 4:401-405.

Kording KP, Fukinaga I, Howard IS, Ingram JN, and Wolpert DM (2004) A neuroeconomic approach to inferring utility functions in sensorimotor control.  PLoS Biology 2:e330.

Redelmeier DA, and Kahneman D (1996) Patients’ memories of painful medical treatments: real-time and retrospective evaluations of two minimally invasive procedures.  Pain 66:3-8.