What is the smallest intensity of a stimulus that you can detect 50 percent of the time?

Sensation is the process by which physical energy from objects in the world or in the body stimulates the sense organs. The brain interprets and organizes this sensory information in a process called perception. Psychophysics is the study of how the physical properties of stimuli relate to people’s experience of stimuli. Research in psychophysics has revealed much information about the acuity of the senses.

Measuring the Senses

Psychologists assess the acuity of the senses in three ways:

1. Measuring the absolute threshold
2. Measuring the difference threshold
3. Applying signal detection theory

The absolute threshold is the minimum amount of stimulation required for a person to detect the stimulus 50 percent of the time. The difference threshold is the smallest difference in stimulation that can be detected 50 percent of the time. The difference threshold is sometimes called the just noticeable difference (jnd), and it depends on the strength of the stimulus.

Example: If someone were comparing two weak stimuli, such as two very slightly sweet liquids, he’d be able to detect quite a small difference in the amount of sweetness. However, if he were comparing two intense stimuli, such as two extremely sweet liquids, he could detect only a much bigger difference in the amount of sweetness.

Researchers use signal detection theory to predict when a weak signal will be detected. This theory considers the fact that the ability to detect a signal depends not only on the strength of the signal but also on the perceiver’s experience, motivation, expectation, and degree of alertness. Different people respond differently to the same signal, and the same person may detect a particular signal at one time but not another. Furthermore, people can often detect one type of signal in a sensory modality such as hearing or vision but be oblivious to other types of signals in the same sensory modality.

Sensory Adaptation

When people walk into a restaurant, they probably notice food smells right away. However, as they sit in the restaurant, the smells gradually become less noticeable. This phenomenon occurs because of sensory adaptation. Sensory adaptation is the decrease in sensitivity to an unchanging stimulus. The smells don’t disappear—the people just become less sensitive to them.

Development of the Senses

Babies have all the basic sensory abilities and many perceptual skills, but these abilities develop and grow more sensitive over time. Babies can recognize the difference between a human voice and other sounds, and they can locate a sound’s origin. They can recognize the difference between smells and, very early on, can recognize their mother’s particular smell. As for taste, they can differentiate between sweet and salty. Babies also have fairly adept visual abilities. Soon after birth, they can distinguish objects of different colors and sizes. When they are just a few weeks old, they begin to differentiate among contrasts, shadows, and patterns, and they can perceive depth after just a few months.

Sensitive Periods

Even innate perceptual skills need the right environment to develop properly. A lack of certain experiences during sensitive periods of development will impair a person’s ability to perceive the world.

Example: People who were born blind but regain their vision in adulthood usually find the visual world confusing. Since these adults were blind in infancy, they missed the sensory experiences necessary for their visual system to develop fully.

Video transcript

- [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.

What is the minimum intensity of a stimulus?

Why is absolute threshold 50% of the time?

Is the lowest level of stimulation that a person can consciously detect 50 percent of the time the stimulation is present?

What is the lowest level of stimulation that an organism can detect?