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Signal Detection Theory – Sensory Processing (PSY, BIO)

by Tarry Ahuja, MD
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    00:01 Now, let’s take a look at Signal Detection Theory and we’re going to look at what are some of the different things that impact signal detection.

    00:11 So what we’re talking about when we’re saying signal detection is detecting the presence of a stimulus if there are things going on.

    00:19 So we’ll walk through a lot f examples but first, let’s highlight these four factors.

    00:24 The first is alertness.

    00:25 How alert are you in terms of trying to identify the stimulus? So are you completely focused and alert saying, “I’m looking for a certain stimulus”? Okay.

    00:37 Are you expecting that? Is there some level of expectation? Or are you completely unaware of whether or not the stimulus is going to arrive? Okay.

    00:45 So think of getting slapped; knowing the slap is coming because you’ve just insulted your partner.

    00:54 And you know, here, she’s going to slap you.

    00:56 Let’s say she.

    00:57 She’s going to slap you when you tell her that you’ve decided to now date her sister.

    01:03 The slap is coming.

    01:04 So you’re expecting the slap.

    01:05 So you’re expecting the incoming stimulus versus you getting slap completely unannounced.

    01:10 You’re not ready for it.

    01:11 You aren’t really ready to detect that stimulus.

    01:14 Is there a motivation? Are you motivated to try and detect that stimulus? And experience. Have you ever experience this stimulus before? If there was a previous experience, then you might be tuned to and be behaviorally aware of what to expect.

    01:31 Okay? So look at the analogy of, let’s say, if you’re looking at your child, you’re a mother and your child is now missing, and you’re at a -- you’re at a carnival or a festival and your child has ran off.

    01:48 Now, you are looking for your child and you are completely in tuned to hearing their voice.

    01:57 Okay. So little Timmy has run off and you’re the mother and you are listening to, you’re completely alerted to, little Timmy’s voice.

    02:08 You know that little Timmy likes to hide underneath large trees.

    02:12 So there are some expectations as well.

    02:14 So you’re going to start looking at that expectation and experience together.

    02:18 You’re looking at trees, places that he has done this before.

    02:21 And then there’s motivation.

    02:23 Obviously, as a mother, you’re highly motivated to find your child.

    02:26 So collectively, all these things will elevate your ability to detect the stimulus.

    02:31 Okay? So that’s a kind of stimulation detection theory as a whole.

    02:35 Now, this can be an active versus a passive process and we’re going to walk through some of those scenarios.

    02:42 So, based on -- based on the four factors that we just mentioned; detection and influencing factors, there are four possible outcomes that you can have when looking at signal detection.

    02:55 Now when we’re saying active, we’re referring to you allocating some resources towards that signal detection versus passive when you’re sort of just sitting there and the stimulus is being presented to you, and you’re not actively really doing much other than just detecting it once it’s presented to you.

    03:14 Now, you have a matrix here that we’ve created and here are options, is that the stimulus is present or it’s not there.

    03:21 Okay. So it’s in front of you or it’s not in front of you.

    03:23 And you’re either going to detect it or you’re not going to detect it.

    03:26 Now, if the stimulus is present and the stimulus is not detected, we say that that’s a missed.

    03:32 You did not detect the stimulus even though it was there.

    03:36 If it was missing and you didn’t detect it, you correctly rejected it.

    03:41 Okay? Now, if the stimulus is present and you’ve detected it, that’s a hit.

    03:46 If the stimulus was missing and you detected it, that’s a false alarm.

    03:51 So what we’re going to do is if you take this matrix and have it cut in half, if you look at the scenarios where there was a missed and a correct rejection, we would say that that individual is quite conservative because they’re not actively trying to detect the stimulus.

    04:05 Versus where it says stimulus detected and then we have a hit and false alarm, that person is much more aggressive in trying to find a stimulus, which is why you have a greater success of getting a hit which you also have the chance of getting that false alarm.

    04:19 Okay? The reason we’ve created this matrix is to give you some understanding as to when you’re looking for a stimulus, you’re trying to detect it, you can be quite active or aggressive or you can be a little more passive and conservative, and that will impact your success rate.

    04:38 Now, Now we’re talking about detecting all of this stimulus and responding to it.

    04:44 But once you do that, what happens? Well most of the time, you’re going to respond to it.

    04:48 Now eventually when you respond to enough stimuli, you start to adapt.

    04:53 And what we’re saying here is that you’re going to change over time the way you respond to certain stimulus.

    04:59 So it’s defined as the change over time in the responsiveness of the sensory system to a constant stimulus.

    05:05 And that’s the key term here is constant.

    05:07 So what I’m saying to you is if you are constantly being inundated with the stimulus, over time you will adapt to that.

    05:14 Okay? So I had mentioned this earlier in another lecture and that we’re saying that our brain is actually designed to tune out what we call unimportant information.

    05:23 And that’s a kind of a misnomer because it’s not that it’s not important, it’s just that right now, biologically speaking, it’s really not that relevant to you.

    05:31 And your body likes to detect change and not constant information.

    05:36 So you want to see how is this different than things normally appear.

    05:40 If it’s different, then I’m going to really focus in on and determine what that difference is.

    05:45 So think of the situation of, you know, putting on a watch, and you wear that watch every day.

    05:51 Is it really relevant for your sensory receptors to feel and know that you’re wearing a watch all time or the clothes that you wear? Your body doesn’t really need to know that information all the time.

    06:03 It is tuned that out and it is saying, “I understand I have clothes on hopefully, and that I don’t need to know that all the time.” Think about the last time you had a nice big bowl of spicy soup.

    06:12 You’re eating this soup and the first, first few bites, your tongue is on fire and you’re sweating.

    06:18 What happens by the end of the bowl? You actually start to get used to it.

    06:21 You get adapted to it and you’re kind of like, “This isn’t so bad.” That is your sensory receptors getting adapted to that level of spiciness.

    06:31 Now one group of receptors that actually do not adapt are the nociceptors which are aligned to detect pain.

    06:38 And it’s one of the reasons why it doesn’t matter how many times you get slapped in the face or you get kicked in the leg, you’re going to feel that and you’re going to feel it just as bad over time.


    About the Lecture

    The lecture Signal Detection Theory – Sensory Processing (PSY, BIO) by Tarry Ahuja, MD is from the course Sensing the Environment.


    Included Quiz Questions

    1. Depends on detection of the presence of a stimulus amid background noise.
    2. Doesn’t depend on psychological state.
    3. Doesn’t depend on previous experience with the stimulus.
    4. Depends on proportionate difference between stimuli.
    5. Depends on the linear relationship between actual and perceived stimulus intensity.
    1. Stimulus not detected when the stimulus is absent
    2. Stimulus detected when stimulus is present
    3. Stimulus not detected when stimulus is present
    4. Stimulus detected when stimulus is absent
    5. It is an active process
    1. …nociceptors.
    2. …rods.
    3. …olfactory receptors.
    4. …fungiform papillae.
    5. …cutaneous mechanoreceptors.

    Author of lecture Signal Detection Theory – Sensory Processing (PSY, BIO)

     Tarry Ahuja, MD

    Tarry Ahuja, MD


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