- [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.