- [Voiceover] The absolute threshold of sensation
is the minimum intensity of a stimulus
that is needed to detect a particular stimulus
50% of the time.
So it is the lowest level of any stimulus
that we can generally detect.
And the 50% of the time clause here
is actually really important.
And this is for a number of reasons.
One is individual differences.
Simply put, at really low levels of a stimulus,
some subjects can detect it while others cannot.
There also might be differences
within an individual.
Think about maybe a time when your friend
asks if you heard a sound, and you think
maybe you did, but you're not entirely sure.
Well that's what this 50% helps us take into account.
So rather than asking, "What is the absolute
"lowest sound a person is ever capable of hearing?"
We want to know what is the lowest level of sound
or light that a person can reliably hear or see.
And let me try to graph this
to make it look a little clearer.
So on the X-axis we'll put Intensity,
and we'll have lower intensity on one side,
and higher intensity on the other.
And so for things like sound,
a lower intensity would be a very quiet sound,
or as a higher intensity sound would be
a much louder one.
And then on the Y-axis I'm going to put
Percentage of Correct Detections.
And what that's referring to is actually something
you may have experienced yourself
when you were in elementary school.
One thing that nurses can do in elementary school
are these sound tests.
And the way that they did them back when I was in school,
was that they'd put these large headphones on your head,
and then they'd play tones of different intensities
into each of your ears.
And the only thing that you as a listener had to do was
raise your hand, either your left hand or your right hand
when you heard a tone in either your left or right ear.
And from the sounds they played,
there were probably some that I correctly detected
100% of the time.
I always heard them.
I always raised my hand.
But it is possible that I did not
correctly identify all of the sounds 100% of the time.
And that's what this Y-axis is referring to.
And so imagine that this is you,
and you have these headphones on,
and that you're going through these same tests,
and sometimes a sound is presented,
and other times it isn't,
but all the sounds are of different intensity.
Well if you were to graph the responses,
you might wind up with something like this,
where the higher the tone is,
you're more likely to identify it.
But as it gets lower,
you're less reliable in your judgement.
And that 50% mark would be our
absolute threshold of sensation.
Before we move on, I do want to mention
that the absolute threshold of sensation
is different from a concept that has been
talked about in other videos.
And that's the difference threshold,
or the just noticeable difference.
And what the difference threshold is referring to,
is the smallest difference in stimulation
that can be detected 50% of the time.
So imagine that you have a light
with a dimmer switch on it,
and it's really sensitive.
And imagine that you're rotating that switch
really slowly, and so the light is gradually
increasing in intensity.
The difference threshold would be the point
between your starting point,
and when you were able to perceive
that the light was brighter.
And I want you to keep this in mind,
because I think that these concepts
are pretty easy to mix up.
And while they are related to each other,
they are actually referring to different things.
Looking back at our graph here,
you might get the impression that
the absolute threshold
would be some kind of fixed, unchanging number.
But it actually turns out
that it can be influenced by a number of different factors.
In particular, it can be influenced
by a number of different psychological states.
So let's think about this in terms of detecting
a text message when your phone is on vibrate,
and either it's in your bag,
or maybe it's on a desk nearby.
What are some things that might influence
whether or not you will detect
that your phone is buzzing?
Well one thing that might influence
whether or not you hear it is whether or not
you are expecting a text.
Also, our experience with that particular phone
could influence it.
Is it a new phone?
Or are you really familiar with the particular
sound that it makes when it vibrates
against a wooden table, or against your cloth bag?
How motivated we are can also have a huge effect
on whether or not we detect something.
So let's say that I've texted someone
who I'm interested in to see whether or not
they want to go on a date.
So in this case, not only am I expecting a text,
but I'm also really motivated to get it,
because it's important for me to see the response.
Another thing that might influence
whether or not I hear the text message is alertness,
meaning that I'm much more likely
to notice it when I'm awake,
as opposed to when I'm drowsy
and half asleep on the couch.
And before we move on,
I also want to take a moment to talk about
a term that's used to describe stimuli
that we cannot detect 50% of the time,
and that's subliminal.
And maybe you've heard about
subliminal messages before,
and that's what this is referring to.
It's referring to stimuli that is below
our absolute threshold of sensation.