Psychophysical Distinctions: Investigating “Just Noticeable Differences”

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Updated on Dec 12, 2013

Grade Level: Middle School; Type: Life Science


  • Learn about the field of psychophysics.
  • Learn about "Just Noticeable Differences"

Research Questions:

How well can we detect small changes in weight?

A "Just Noticeable Difference," or JND, is the smallest detectable difference between a starting and secondary level of a particular sensory stimulus--in other words, the difference between a one dollar bill in your hand and two dollar bills in your hand. In this experiment, you will determine the JND between the weights of bags of pennies. The results graphed in comparison to the JND for weight calculated by Ernst Weber, the psychophysicist who first tested and described the JNDs of weights.


  • Sandwich bags
  • 280 pennies
  • Masking tape
  • Pen and paper
  • Blindfold or eye mask

Experimental Procedure

  1. Put different sets of pennies in each of six different baggies in the following amounts: 37, 38, 39, 40, 41, 42 and 43. The bag with 40 pennies is the standard against which others will be compared.
  2. Twist the baggies at the top, tape each with masking tap and mark the tape, using the pen, with the number of pennies in the bag.
  3. Blindfold your subject.
  4. Hand the subject the bag with 40 pennies in it, and ask them to feel the weight in either hand.
  5. Take it from them and hand them another bag and ask them to feel the weight with the same hand. Ask, “Are the bags the same weight or is one heavier? If one is heavier, which one?” The subject may re-check the weights of the bags until s/he is ready to come to a decision.
  6. Continue the same procedure using the remainder of the bags, always comparing them to 40-penny standard bag.
  7. Record on the data sheet the number of pennies that the subject FAILS to detect as weighing a different amount compared to the standard. So, if a subject can tell the difference between the standard and all other bags except the bag with 39 or 41 pennies, record 39 and 41 for that subject.
  8. Try to run as many subjects as possible; 30 is best.
  9. Assess the number of pennies difference that alerted each subject to a difference in weight. So, if the subject could not tell the difference between the standard and the bags with 39 and 41 pennies in it, that subject's penny difference is ‘1’ and the JND is 1/40 or .025.
  10. Calculate the average JND for all of the subjects.
  11. Create a bar graph comparing the JND you calculated for your subjects and the one that Ernst Weber calculated for his subjects who were also making weight assessments.
  12. Discuss your results.

Terms/Concepts: Psychophysics; Detection threshold; Just Noticeable Difference; Ernst Weber; Gustav Fechner; Weber-Fechner Law


Psychophysics: Introduction to Its Perceptual, Neural and Social Prospects, by SS Stevens, S. S. (Transaction Books, 1986)

Dr. LaCerra is an evolutionary neuroscientist, author of "The Origin of Minds" (with co-author, Roger Bingham, Harmony, 2002) and a columnist and contributing editor at "Spirituality & Health" Magazine.

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