The Meaning of Pupil Dilation | The Scientist Magazine®

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What do an orgasm, a multiplication problem and a photo of a dead body have in common? Each induces a slight, irrepressible expansion of the pupils in our eyes, giving careful observers a subtle but meaningful signal that thoughts and feelings are afoot.


For more than a century, scientists have known that our pupils respond to more than changes in light. They also betray mental and emotional commotion within. In fact, pupil dilation correlates with arousal so consistently that researchers use pupil size, or pupillometry, to investigate a wide range of psychological phenomena. And they do this without knowing exactly why our eyes behave this way. “Nobody really knows for sure what these changes do,” said Stuart Steinhauer, who directs the Biometrics Research Lab at the University of Pittsburgh School of Medicine.

While the visual cortex in the back of the brain assembles the images we see, a different, older part of our nervous system manages the continuous tuning of our pupil size, alongside other functions—like heart rate and perspiration—that operate mostly outside our conscious control. This autonomic nervous system dictates the movement of the iris, like the lens of a camera, to regulate the amount of light that enters the pupil.

The iris is made of two types of muscle: in a brightly lit environment, a ring of sphincter muscles that encircle and constrict the pupil down to as little as a couple of millimeters across; in the dark, a set of dilator muscles laid out like bicycle spokes, which can expand the pupil up to 8 millimeters—approximately the diameter of a chickpea.

Cognitive and emotional events can also dictate pupil constriction and expansion, though such events occur on a smaller scale than the light reflex, causing changes generally less than half a millimeter. But that’s enough. By recording subjects’ eyes with infrared cameras and controlling for other factors that might affect pupil size, like brightness, color, and distance, scientists can use pupil movements as a proxy for other processes, like mental strain.

Princeton psychologist Daniel Kahneman showed several decades ago that pupil size increases in proportion to the difficulty of the task at hand. Calculate 9 times 13, and you pupils will dilate slightly. Try 29 times 13, and they will widen further and remain dilated until you reach the answer or stop trying. As Kahneman says in his recent book, Thinking Fast and Slow, he could divine when someone gave up on a multiplication problem simply by watching for pupil contraction during the experiment.

Written By: Joss Fong
continue to source article at the-scientist.com

18 COMMENTS

  1.   Princeton psychologist Daniel Kahneman showed several
    decades ago that pupil size increases in proportion to the difficulty
    of the task at hand. Calculate 9 times 13, and you pupils will dilate
    slightly. Try 29 times 13, and they will widen further and remain
    dilated until you reach the answer or stop trying.

    Moving slightly off topic, this illustrates the limitations of testing methods, as it presumes significant levels of difficulty!
    These are only marginal differences for those familiar with arithmetical gymnastics.

    My first thought was, “Why does he think these questions are difficult?”

    9 x 13 = (10 x13) – 13 = 130 – 13 = 117

    29 x 13 = (30 x 13) -13 = (10 x13 x 3) – 13 = 390 -13 = 377

  2. A4D Yes, what you have shown seems pretty simplistic. Unfortunately, I recently learned (through firsthand market testing) that the average consumer cannot relate to square footage. If they know something is 18 sq ft, they have no idea of how much that really is and what they can do with a product this size. I was shocked at how the average person cannot relate to simple math. 

    9 x 13 = (9×10) + (9×3)= 117 By breaking it down, you can figure it out in your head.Lots of people cannot do this.

  3. If a math problem is processed as a challenge, could the unnamed “older part of our nervous system” be equating that challenge to a threat? A threat would trigger the fight or flight mechanism which might dilate the pupil to allow for the greatest opportunity to take in information (light information). The harder the problem the greater the threat.

    I know I feel threatened by difficult problems because they call into question my authority on a subject and I run the risk of losing credibility which can have a negative affect on my overall health.

    “Subsequent research found that the pupils of intelligent people (as defined by their SAT scores) dilated less in response to cognitive tasks compared to those of less intelligent participants, possibly indicating a more efficient use of brainpower.”

    Perhaps, the less intelligent participant is suffering a great threat level than the intelligent participant and therefore suffers a higher degree of fight or flight influence.

    Just a guess.     

  4. Alan4,
    I am currently teaching chemistry instead of my usual biology.  High school biology is descriptive while high school chemistry relies on math.  The average person understands math at a very very rudimentary level.  Anything outside the 12 times tables is “difficult”.

    BTW if you want to see a cool math demo check out the TED talk given by Arthur Benjamin.  It is truly jaw dropping!!!

    And, anecdotally, I had some students deriving Avagadro’s number in a lab the other day.  The answer was 6.02 x 10 to the 23rd power.  This student called me over and showed me her answer.  It was 6.02 x 10 to the negative 47th power. (sorry, i do not know how to type exponents)…  Her question was 
    “Am I close????”

    She was 70 orders of magnitude off!!!  Am I close????

  5. I remember seeing in a book by Desmond Morris two portraits of a woman that seemed identical. You had to choose on which portrait the woman looked more sympathetically. Most people choose the same portrait, although they could not say why. On that portrait her pupils were slightly wider. We measure pupils even without knowing it. 

  6. What, 50 yrs ago was referred to as ‘Mental Arithmetic’ as I recall, taught at about age 10 after learning the multiplication tables. Later we learned approximation in order to gauge the order of magnitude expected in calculation. A recent amusing example- when buying the single remaining peach on a tray, I carried both to the till. Young lady scans the barcode and announces “$48 please”. Her supervisor was unable to convince her of the error…

  7. Even more difficult- Americans trying to evaluate area, volume and weight in SI (‘metric’) units!! Were you aware that the U.S. survey ‘foot’ is different to the European/Imperial ‘foot’? As for bushel, cord, quart, std bucket, etc. the US system is a nightmare!
    Sorry, wandered off topic big time…

  8. @rdfrs-2fb5b47de18fa6e64715b36d59775ac6:disqus 
    I was at a store recently and the bill was $4.87.  I handed the clerk $5.12.  She looked from her hand (with the money in it) to my face and back to her hand 8 times.  I told her to enter it into the register.  When it told her to give me a quarter in change she smiled a huge smile and handed me my change.

  9. She was 70 orders of magnitude off!!!  Am I close????

    Probably about as close as an AGW denier who says the climatologists’ calculations are wrong!
    ……  Then there are those YEC calculations!!!!!! ???

  10. Nodhimmi
    Even more difficult- Americans trying to evaluate area, volume and weight in SI (‘metric’) units!!

    There have been some VERY expensive mistakes because of this!

    The Mars Climate Orbiter (formerly the Mars Surveyor ’98 Orbiter) was a 338 kilogram (750 lb) robotic space probe launched by NASA on December 11, 1998 to study the Martian climate, atmosphere, surface changes and to act as the communications relay in the Mars Surveyor ’98 program, for Mars Polar Lander.

    ▬▬▬▬▬▬▬▬▬▬
    However, on September 23, 1999, communication with the spacecraft was lost as the spacecraft went into orbital insertion, due to ground based computer software which produced output in Imperial units of pound-seconds (lbf×s) instead of the metric units of newton-seconds (N×s) specified in the contract between NASA and Lockheed.
    ▬▬▬▬▬▬▬▬▬▬
    The spacecraft encountered Mars at an improperly low altitude, causing it to incorrectly enter the upper atmosphere and disintegrate.
    ▬▬▬▬▬▬▬▬▬▬
     The cost of the mission was $327.6 million total for the orbiter, $193.1 million for spacecraft development, $91.7 million for launching it, and $42.8 million for mission operations.
    ▬▬▬▬▬▬▬▬▬▬
     - http://en.wikipedia.org/wiki/M

    There were probably some very wide-eyed people looking on!

    http://upload.wikimedia.org/wi

  11. Kahneman has some recent pop science books that are well worth reading. Plus Arthur Benjamin has an excellent video lecture series available with ‘The Great Courses’ fka TTC.

    From what I’ve read what makes mental arithmetic calculation harder or easier is the various mental buffer sizes of the individual. They are like on-chip cache in CPU designs. Very rapid store and access, short term, but very limited in capacity and a highly volatile form of memory. Different people have different capacities, probably genetically set.
    It might be important to establish whether a child has a relatively low buffer capacity. Because it’s probably easily compensated for by technique and is the kind of thing that would vary randomly. It also probably isn’t correlated with other aspects of intelligence, that also vary randomly. Ignoring this potential impediment might be dangerous for the fate of humanity: You might end up with a smart kid who becomes maths averse. Creating a serious risk of encumbering the world with more lawyers and politicians.

    Buffering capacity can be changed slightly by practise and stressing their limits. What happens more complex mental calculations is that people lose track of the intermediate steps because the buffers are only good for a few moments. When learning mental arithmetic it can be important to augment the buffers with written notes of the intermediate calculation products. As a blend of mental and paper arithmetic. Eventually people get less dependent on the paper notes after they’ve mastered the internal algorithms and are less distracted by directing the process strategy and more focussed on actually performing it. (People might be consuming buffer capacity just to comprehend the algorithms. A problem that goes away once the algorithms are learnt more permanently.)

    One of the reasons there’s an expectation that Asian students might be slightly better at maths is that they get a head start early on in life and therefore are less likely to incur an expectation of maths anxiety. Maths anxiety might be a consequence of the mind shedding intermediate data by clipping off staged calculations that exceed available buffer memory. (Combined with being ridiculed by the teacher or other students.) The head start for Asians comes from the reduced buffer capacity required to translate names for numerals in everyday maths problems – which in English language has several additional processing steps, consuming precious buffer capacity, compared to Asian languages.

    Examples being the names for higher order numbers: English for 23 is not ‘two three’ but ‘twenty three’. For some everyday mental arithmetic problems the ‘twenty’ phrase has first to be mentally translated into ‘two of ten’. Same for ‘one thousand one hundred and twenty three’. So there’s a translation processing burden even before running the calculation, plus the reverse re-encoding the output back into everyday language.

    In contrast many Asian languages have no linguistic equivalent of ‘one thousand one hundred and twenty three’. Everyday dialects cut straight through to the relevant numerals and would be expressed as the equivalent of ‘one one two three’ which is more amenable to direct translation into 1123 using less mental processing power and buffer capacity.

  12. This reminds me of when I was a little kid in grade school. The teacher was teaching us about money and asked which coins and bills we would give if the total came to say $4.87 (just for demonstration.) I replied (say for demonstration) a five dollar bill, a dime, and two pennies. “NO!” she exclaimed loudly. Then she “corrected me.” A boy then turned to me and said “I know what your doing. You want a quarter back” I never forgot this though it was over 40 years ago. I still give cash in a way to try to eliminate extra coins as change.

  13.  Yep Nodhimmi we Americans have different ways of measuring. Those metric lessons I learned in school back in the 70s didn’t seem to stick with the general population except for the liter (pop (soda) bottles.) Educating the youth for metric change fell like a TON of bricks. Considering much of the manufacturing is done in China, I expect that this will change in time.

  14. Pete H  In contrast many Asian languages have no linguistic equivalent of ‘one thousand one hundred and twenty three’. Everyday dialects cut straight through to the relevant numerals and would be expressed as the equivalent of ‘one one two three’ which is more amenable to direct translation into 1123 using less mental processing power and buffer capacity.

    It is quite amazing how the Romans coped with this handicap
    when counting on from 1123 !!

    MCXXIV

  15. QuestioningKat
    Yep Nodhimmi – we Americans have different ways of measuring. Those metric lessons I learned in school back in the 70s didn’t seem to stick with the general population except for the liter (pop (soda) bottles.)
    Educating the youth for metric change fell like a TON of bricks.

    In the UK we still use two systems, but science uses SI /metric and °C or K and 1000kg tonnes.
    France has used the metric system since Napoleon!

    The metric system is so simple compared to gallons, pounds, feet, inches etc.

    A 10x10x10 centimetre cube is a litre.  A litre of water weighs a kilogram.

  16. In the long run they didn’t. Buy it might have been this numerical handicap that slowed the pace of change and prolonged their Western empire, for a while at least.

    The Romans coped for OK with everyday numbers for financial transactions for centuries. But then things feel apart, literally, sometime after their plutocrats invented fiat currency and then inevitably fell for the superficial attractions of quantitative easing monetary inflation to fund the military and public officials. The resulting wealth transfer from the increasingly poor to the increasingly very wealthy changed the balance of power sufficiently to allow them to entirely shift the tax burden from the rich to the poor. The collapse didn’t take all that long when it started. (A side effect and attempt to mitigate social disruption was the conversion of the hierarchy of Roman government into the catholic church and the feudal system of the middle ages.)

    The only reason that present era plutocrats like Ben Bernanke and colleagues are taking slightly longer than the Romans to similarly destroy and feudalise the economies of the known world is that they’ve got zero on their side.

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