If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

# Weber's law and thresholds

Created by Ronald Sahyouni.

## Want to join the conversation?

• What does the Weber's law and threshold have to do with sensory perception? or the nervous system
• Sensation is proportional to the logarithm of the stimulus intensity.
• When he explains the equation at , isn't the explanation backwards? Shouldn't the Weber
fraction be the incremental threshold over the background intensity?
• He gave the correct equation in , he just made an error when reexplaining in .
• What are the exact threshold percentages in determining weight differences? Sound differences? Temperature difference?
• I think it is a relative threshold, if you are lifting 500 KG and you add 70 grams you might not notice it. And when you are lifting a wedding ring which weighs 20 grams, and you add 70 grams to it - you will notice it. The same hold true for light brightness and sound.

"Weber's Law states that the ratio of the increment threshold to the background intensity is a constant. So when you are in a noisy environment you must shout to be heard while a whisper works in a quiet room. And when you measure increment thresholds on various intensity backgrounds, the thresholds increase in proportion to the background." - The quote source : http://www.cis.rit.edu/people/faculty/montag/vandplite/pages/chap_3/ch3p1.html

I think the percentage is relative to where you measure it. And I know there is a mathematical equation for it, which I linked in here, but no actual fixed percentage, or one that scales for all.
• In class, when it comes to Weber's Law, my teacher says, "We perceive differences on a logarithmic, not a linear scale." (or something like that) What does it mean? I mean, how does it have to do with the linear graph at since my teacher uses the word "logarithmic"?
• All it means is that, the relation between our perceived differences in the stimuli, and that between the energies of those stimuli is logarithmic. For some sensations (like sound), by increasing the stimuli 10 times (from a 100 to a thousand), you would perceive the change of 10 decibels rather than 900 as you would otherwise expect. Here the power is 1, but it can be varied, such that a 10 times increase would produce say square-root(10) times change in the perceived sensation.
(1 vote)
• Did you misspeak at ? I think you have the ratio of the incremental threshold to the background intensity written, but you said it the other way around.

Also, are the JND and incremental threshold the same?
(1 vote)
• Sorry for being extremely late, but for those who will see this in the future, yes JND and incremental threshold are the SAME.
• Hello, what would be an example of "baseline stimuli" that would need to be surpassed when trying calculate "K" in real world situations?

I kind of inferred that "a" in ∆I/ a+ I = K , could be gravity if we are looking at it in the context of lifting a weight. Or even air when looking at it in the context of sound/hearing something.

Please let me know if this is a correct understanding.

Thank you!
• The sensory neurons that Ronald mentions, are they afferent neurons?
Also, what kind of receptor is being used (i.e. position or touch mechanoreceptors)?
(1 vote)
• How is Fechner's law different than Webber's Law?
(1 vote)
• Actually, Fechner was the other individual who helped characterize the "law" that you saw in this video. The "Fechner's law" or "Weber's law" is actually called the "Weber-Fechner law".