# Weber´s Law – Sensory Processing (PSY, BIO)

by Tarry Ahuja, MD
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00:01 So this brings us to something called Weber’s law.

00:05 So Weber took a look at that relationship.

00:07 And what it states is that two stimuli must differ by a constant proportion in order for their difference to be perceived.

00:15 So in English, if we’re starting with paper or if we’re moving to a textbook, it kind of doesn’t matter because you’re going to need a certain proportion of difference for that to be detected.

00:25 Okay. So that delta is difference threshold or the JND.

00:30 And it kind of depends on the medium as well.

00:32 So weight versus light versus sound, you’re going to have differences.

00:37 But the proportion within each medium is consistent.

00:40 Okay? So if you’re starting with 5 pounds and I change that two 5.01 pounds, probably you’re not going to notice that difference.

00:50 But if I go from 5 pounds to 5.5, you might notice that.

00:55 Now if you move to, say, light, and we’re looking at lumens of light, you need a certain difference there.

01:01 And that proportion is going to be different than what we see for weight and the same thing holds true for sound.

01:08 So we can look at these by certain senses and this is an average generalization.

01:12 So it can vary as well.

01:14 In weight, you need roughly a 2% JND and light roughly 8% JND and for sound roughly 3% change for you to detect that difference.

01:25 So, So this is an equation that is for sure going to show up on the MCAT.

01:30 So we should know this and this is Weber’s law.

01:32 We’re going to walk through the three components.

01:34 So Delta I represents the difference threshold, so the JND.

01:37 This means the difference from your stimulus.

01:40 And the eye represents the initial stimulus intensity.

01:43 So what you were starting with, the Delta is the difference and you put those over one another, you’re going to get the K which is Weber constant.

01:51 Okay? Now, let’s go through an example.

01:54 And this is an MCAT style question that’s probably going to pop-up and we’re going to present the passage or scenario and you’re going to have to pull out bits of information to answer the question and you’re going to have to implement Weber’s law here.

02:07 So let’s think of our friend little Timmy here.

02:09 He’s a grocery clerk and he takes people’s bags to their car and helps load them up.

02:15 And our little friend Timmy, he can detect the difference between a 25-pound bag of groceries and a 30-pound bag of groceries.

02:22 So when it’s 25 pounds, he knows that weight.

02:25 And if it jumps up to 30, he can detect that there is a difference.

02:28 Okay. Now my question to you is what’s the JND? Let that marinate for a second.

02:35 So the question I’m going to ask you is, if he’s helping another customer, which one of these bugs would he be able to differentiate by weight? We have two options.

02:43 A 2-pound bag versus a 3-pound bag, would he detect that difference? And the 20-pound bag versus the 22-pound bag.

02:50 Okay. Are you with me? So let’s move on and we’re going to take a look at this example and we’re going to walk through the bits and pieces that you should know.

02:57 So if you were to see this, you should, right away, and I’d ask you, what’s the JND? So what’s the JND? Well, did you come up with this answer? 30-pound bag minus the 20-pound bag, so there is your Delta that we’re trying to figure out, over the initial stimulus which was the 20-pound bag.

03:13 That gets us 5 pounds over 25 pounds, cancel out the units and you’re left with 0.2 or 20% difference.

03:22 So what that means in English is little Timmy can detect a 25% difference in weight.

03:28 And when there is that 25% difference or more, he will say, “Yes, these two bags are different.” So we had two options, 2 and 3-pound bag.

03:37 Can Timmy detect the difference between a 2 and 3-pound bag? So using the relationship that we just figured out at the top.

03:44 We’re going to say the JND in this scenario was what? 3-pound bag minus a 2-pound bag, that’s going to give us our Delta over the original stimulus, which is a 2-pound bag.

03:54 So three minus two is one pound over the two pounds which is .5, which is the 50% difference, which means ding, ding, ding, yes, little Timmy will detect that difference.

04:06 Okay? Now what was our other option? Twenty versus twenty-two pounds.

04:12 Same thing, let’s walk through this.

04:14 JND, 22 pounds minus 20 pounds over the original stimulus which was 20 pounds.

04:21 So that gives us two over twenty, right? And that gives us .1 or a 10% difference.

04:29 Will Timmy be able to tell the difference between 20 and 22? Well, the answer is no because it’s less than the JND that we said we know Timmy can detect which was .2.

### About the Lecture

The lecture Weber´s Law – Sensory Processing (PSY, BIO) by Tarry Ahuja, MD is from the course Sensing the Environment.

### Included Quiz Questions

1. Two sounds should be differ by a constant proportion of 0.3%.
2. Can compare difference in the sound and weight of a phone.
3. Two stimuli differ by an logarithmic proportion.
4. Can tell the difference in intensity between 15 and 16 candles.
5. Two weights should differ by a constant proportion of 1%.
1. Difference thresholds can be measured by Weber’s Law.
2. Difference thresholds described by Fechner’s Law
3. Difference thresholds are dependent on signal transduction theory.
4. Difference thresholds are dependent on alertness.
5. Difference thresholds are dependent on previous exposure to the stimulus.
1. Minimum noticeable difference between two sensory stimuli 50% of the time
2. Minimum sensory threshold to perceive a stimulus 50% of the time
3. Minimum stimulus intensity to perceive light in a dark room
4. Minimum stimulus intensity for unconscious perception
5. Motivation to detect a difference between two stimuli

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