Size-weight Illusion

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Overview

Students experiment with the size-weight illusion and then compare the magnitude of the effect to when procedures are introduced to attenuate this effect.

Learning goals

  • To understand how previous experience can influence perception.
  • To understand what factors, contribute to mental shortcuts in making predictions for action.
  • To practice how to experimentally isolate variables that contribute to perceptual effects.

Materials

  • Whiteboards
  • Cans – A big and a small, per table.
  • Cloth bags with drawstrings – two per table.
  • Optional / nice-to-have:
    • Kitchen scale
    • Mini Whiteboards
    • Dry Erase Markers and Erasers

Setup (before class)

  • Instructor prep:
    • Fill cans with any material you may have available, ensuring that they weight the exact same.
  • Room/tech setup:
    • Distribute materials across tables, so that each group has a pair of cans, and cloth bags.

Procedure

  1. Give students a standardized way of comparing the weight of the cans across several trials.
    • Example: 5-point scale ranging from small can weights much more than the big can to big can weights much more than the small can.
  2. Students will take turns lifting both cans at the same time, and rating their perception of their weight.
    • They should organize to tally up the ratings across their group, and ultimately compute an average.
  3. Discuss the results obtained by each group with the whole class. What was the trend in ratings? Which can was generally perceived as heavier?
  4. Repeat the previous experiment, now with the difference of having the cans placed inside the provided cloth bags.
    • Students should close their eyes and have someone else in the group place the bags on their hands by the drawstrings, so that they have no idea, either by vision or touch, about which can is in which bag.
    • Have them rate the perceived weight on their left and right hand, with someone in the group translating that into the big/small can scale.
  5. Discuss the results with the class. And finally allow students to measure the weight of the cans to see what was ” reality”.

Debrief & discussion questions

  • Was there a trend in how the weight of the cans was perceived?
  • Did this change when the cans were held by the drawstring instead of on your hands?
  • What could be the reason for this illusion to appear?
  • Is there any benefit for why our brains may work this way?

Expected outcomes (what you’ll likely observe)

  • The majority of the students should report the small can as being heavier than the large.
  • This outcome tends to be quite reliable. It can be difficult to resist the illusion even if you aware explicitly aware of the “gimmick” of the activity.
  • When introducing the cloth bag variation of the experiment, the perception of different weights across both cans should completely disappear.
  • Ideally, we would like to guide the discussion as to the “why” this happens towards a broad range of factors, such as:
    • Conflict of expectations between a small object weighting more than you would expect, and a large one, less.
      • This is coming from both our sense of vision as we look at the cans, as well as the sense of touch, for contact with the cans, or even proprioception, for the degree of aperture of our hands to hold the cans.
    • Different weight distribution of large can vs small can on the hand.
      • This one is not the main point of this activity, but it is an important “confound” to discuss.
    • The cloth bag, and specifically also holding the cans by the drawstrings, serves to counteract both all of these points, and isolate the weight of the cans from these “distractors”.
  • We also want students to discuss that this is not a “bug” of our brains, but instead, a feature with its own tradeoffs. This effect, much like many other illusions, is a result of the use of pre-existing expectations in creating mental shortcuts in faster perception of the environment.

Variations

  • If you have time, allow students to come up with their own methodology for the experiment.
    • This will likely work better if the whole class agrees on one approach, so as to make the groups more comparable, but this is also a point worth discussing.
  • Different variations of the experiment are also possible. For example, the cloth bag and drawstring version can be the last one, with increasing steps towards isolating each factor contributing to the illusion one by one, such as first just having students close their eyes, change the way in which they hold the cans, etc.

Tips and Troubleshooting

  • Make sure to tell students not to handle the cans outside of the “trials”. The more they manipulate them, the weaker the effect will be.
  • You may want to have students organize a dedicated “note taker”. to record the trials.

References

  • I was first introduced to this activity by my supervisor Ingrid Johnsrude who brought it to Western University after having used it during her time as a professor at Queens University.
  • That version was adapted by Krista MacPherson to be used with students at Western at introductory psychology courses.
  • Later, it was further adapted and adjusted by me during my implementation in these same courses.

Personal Notes

  • This activity has been very consistent with many different groups of students, and is very flexible with regards to time.
  • It can be quite reliably be introduced in any sensation and perception module.

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