Need help preparing for the Psych/Soc section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Weber's Law for sensation. Watch this video to get all the MCAT study tips you need to do well on the psychology section of the exam!

If the light in a room is extremely dim, we’ll probably notice if it gets a little brighter or dimmer. However, if a light is extremely bright, we probably won’t notice if the light becomes slightly brighter or dimmer. Weber's Law states that our sensitivity to a change in a stimulus decreases as the magnitude of the stimulus increases. A term that's very important for this is what is called the just noticeable difference (JND). The JND is the minimum amount that the magnitude of a stimulus has to be changed to be noticeable. Returning to our initial example, if the light is dim (low magnitude), its JND is low. If the light is bright (high magnitude), its JND is high.

JND can be quantitatively understood as the fraction of change in the original stimulus magnitude.

The ratio of the JND to the magnitude of the original stimulus is the Weber fraction, which is a constant that describes the percent change necessary to detect a change in stimulus at all magnitudes of the stimulus. For example, if you have a 10 lb backpack, and you require a 0.5 lb weight to detect a change in weight, there is a 5% change in stimulus magnitude required for that change to be detected. In other words, the Weber fraction is 0.5/10, or 5%. Therefore, if we had a backpack of any weight, it would need to increase or decrease in weight by 5% before we noticed the change. For example, we would predict a 30 lb backpack to require a 1.5 lb increase or decrease in weight to notice a change.

So let's say you have a person that's carrying a 10lb backpack, and you start adding weights to the backpack to find out how much weight you have to add in order for the person to be able to detect a change in the weight of the backpack. And let's say for instance, that a .5lb weight is required to be added for the person to detect the change. This would mean that the JND is 0.5lbs, and the Weber faction is 0.5lbs divided by 10 or 5%. This implies that a 5% change in the stimulus magnitude is required for that change to be detected.

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