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Psychophysics and Fechner's Law – Sensory Processing (PSY, BIO)

by Tarry Ahuja, MD
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    00:01 Now let’s get into something called psychophysics or Fechner’s law.

    00:06 So Fechner, our good friend Fechner, was actually a student of Weber who generated Weber’s law.

    00:10 So this is sort of a furthering or a deviant of Weber’s law.

    00:14 And it’s imparting some of the information, but there’s a small twist to it.

    00:18 So psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions that they affect.

    00:26 So what it’s doing it’s actually bringing some of the -- it’s bringing some of the onus onto the subject.

    00:32 So the person who’s perceiving the stimulus.

    00:35 So Fechner describes a logarithmic relationship between perceived intensity of a stimulus versus the actual physical stimulus intensity.

    00:41 So a lot of words, let’s break that down into simple English.

    00:44 So what we’re saying is as a subject, what you’re perceiving in terms of change is a little bit different than the actual change that’s happening.

    00:54 So, when I say to you, “Can you detect a change in brightness?” For you arbitrary units, you can say if this level of brightness is unit one, how much until I see a change? I’m going to call that unit two.

    01:08 Now that difference that we’re seeing might not be exact compared to the difference that we’re physically seeing, actually seeing.

    01:16 So we’ve described actually a logarithmic relationship and that logarithm relationship is the Fechner’s law.

    01:22 And so we’ll kind of walkthrough an example of that that helps us.

    01:25 And that’s -- it’s looking at, say, stars.

    01:27 When you’re looking at a star through a microscope and you detect a certain level of brightness, you might look at other stars around.

    01:34 And I say to you, look and find me another star that’s just a bit brighter, like that’s the next closes level of brightness.

    01:44 And you’re going to look in your telescope and you’re going to say, “Oh, the one here to the right.” That one is just a smidgen brighter.

    01:53 Now that smidgen is a personal unit that you’ve arbitrarily assigned and that’s your perception of brightness.

    01:59 But that is actually, if you’re looking at lumens of brightness, the number that we’re going to get is going to be a logarithmic scale of what we’re actually perceiving.

    02:08 Okay. So that relationship is really what you need to understand.

    02:12 So in terms of the MCAT, you’re not going to be put through a Fechner’s law actual mathematical question.

    02:18 Most likely, they're going to want you to know that that relationship exists and that it’s a logarithmic relationship.


    About the Lecture

    The lecture Psychophysics and Fechner's Law – Sensory Processing (PSY, BIO) by Tarry Ahuja, MD is from the course Sensing the Environment.


    Included Quiz Questions

    1. Fechner’s Law
    2. Weber’s Law
    3. Difference Threshold
    4. Absolute Threshold
    5. Signal Detection Theory
    1. …a logarithmic curve.
    2. …a linear relationship.
    3. …random.
    4. …an exponential curve.
    5. …a bell curve.

    Author of lecture Psychophysics and Fechner's Law – Sensory Processing (PSY, BIO)

     Tarry Ahuja, MD

    Tarry Ahuja, MD


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