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.