Which of the following is the smallest change in a stimulus that can be detected 50% of the time?

Sensation and Perception is possibly one of the less appreciated areas in psychology for the majority of high school and undergrad students because of its resemblance to physics and other hard sciences that commonly scare psychology students away. This happens especially when weird terms like Absolute Threshold, Difference Threshold, and Weber’s Law come up. However, these themes are not as difficult as they may sound, and if you still fear psychophysics, read on in this Advanced Placement (AP) Psychology Crash Course Review to clear your doubts and understand what Absolute Threshold, Difference Threshold, and Weber’s Law are all about.

Why is Psychophysics Important Anyway?

If upon studying psychophysics you inevitably end up wondering why this is an important part of psychology, you are not being picky. Wanting to know the relevance of each concept and theory is what all psychology students should seek, considering that psychology is a science that requires a lot of reflection and critical thinking. So indeed, why is the study of Sensation and Perception important for the understanding of animal and human behavior?

First of all, there is the more technical and historical side: psychophysics was one of the first fields of psychology to use rigorous methods of study, giving to psychology a very scientific approach rather than a more philosophical one. This prepared the field for other studies that would treat psychology as a strict science that should be tested, measured and quantified.

Secondly, from a philosophical point of view, psychophysics shows that physical reality and psychological reality are not identical. This was already a discussion in philosophy, but it was psychophysics that first came with scientific proof of that statement. That is because psychophysics quantifies the relationship between physical stimuli and individual perception, building the bridge between the mental world and the material world.

What this all means is that the way we experience and interpret the world establishes our sense of reality. If you’ve ever watched The Matrix, you may remember Morpheus questioning the main character’s reality: “What is real? How do you define real? If you’re talking about what you can feel, what you can smell, what you can taste and see, then real is simply electrical signals interpreted by your brain.”

Which of the following is the smallest change in a stimulus that can be detected 50% of the time?
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Sensation, Perception, and Thresholds

Although pretty scary, that quote from The Matrix helps us understand two concepts that are vital in psychophysics: bottom-up and top-down processing.

Bottom-up processing is when the information acquired in our sense receptors (sight, hearing, taste, touch and smell) goes to our brain to be interpreted. Top-down processing is when our brain uses information that has already been brought by sensory systems to organize our experiences and expectations. Together, they form what is called Sensation and Perception, a field of psychology that makes up for 6-8% of the AP® Psychology exam.

But what is a threshold and how does it relate to Sensation and Perception? Simply put, thresholds refer to limit values, and in this case, it means that our perceptions have limits. Take the intensity of a light bulb, for example. There is a minimum value in its intensity for us to perceive the light. And if you increase its intensity, there is a minimum value for us to perceive any change in the light. These minimum values are called absolute threshold and difference threshold.

Absolute Threshold

An absolute threshold is the smallest amount of stimulation needed for a person to detect that stimulus 50% of the time. This can be applied to all our senses:

  • The minimum intensity of light we can see
  • The lowest volume of a sound we can hear
  • The smallest concentration of particles we can smell
  • The smallest concentration of particles we can taste
  • The lightest touch we can feel

But what is that “50% of the time” part of the definition for? Why not 100% of the time?

That is because our absolute threshold can vary according to external and internal factors like background noise, expectation, motivation and physical condition. It is easier to hear a sound when we are in perfect health, expecting to hear it in a quiet room than when we are tired, unaware of it and in a noisy street.

The affirmation that there is no single absolute threshold is called signal detection theory. Because our perception responses may vary, to find a person’s absolute threshold researchers conduct multiple tests until they find the amount that is perceived 50% of the time.

There is also another factor that influences the absolute threshold: sensory adaptation. Sensory adaptation happens when a stimulus remains the same for a long period of time, and our bodies stop recognizing it.

Think of entering a room where the air conditioner is really loud. In the beginning, the sound of the air conditioner may bother you, but after you’ve been in the room for a while, you’ll stop noticing it. If somebody turns off the air conditioner, you’ll immediately notice the difference, even if you were not aware of its sounds before. This is a biological response that makes total sense because if a stimulus is perceived for an extended period of time and nothing bad happens, then that stimulus is not dangerous and it can be ignored since it’s not worth spending energy to sense and perceive it. That is sensory adaptation.

Difference Threshold

A difference threshold is the minimum required difference between two stimuli for a person to notice change 50% of the time (and you already know where that “50% of the time” came from). The difference threshold is also called just noticeable difference, which translates the concept more clearly. Here are a few examples of difference thresholds:

  • The smallest difference in sound for us to perceive a change in the radio’s volume
  • The minimum difference in weight for us to perceive a change between two piles of sand
  • The minimum difference of light intensity for us to perceive a difference between two light bulbs
  • The smallest difference of quantity of salt in a soup for us to perceive a difference in taste
  • The minimum difference of quantity of perfume for us to perceive a difference in something’s smell

You may have already had the experience of turning up the TV or radio volume and not noticing a difference until a certain point. That is the difference threshold concept in action. If you don’t notice the difference, your difference threshold has not been reached yet.

To quantify the difference threshold, psycho-physicist Ernst Weber developed what is known as the Weber’s Law. Weber’s Law states that rather than a constant, absolute amount of change, there must be a constant percentage change for two stimuli to be perceived as different. In other words, the higher the intensity of a stimulus, the more it will need to change so we can notice a difference.

Imagine the TV/radio situation again, and imagine the manufacturers built a bad volume system in which each increase in volume corresponds to a constant increase in absolute (not percentage) amount. You can notice a difference when you go from volume 1 from volume 3, for example, but you don’t perceive the same difference when the volume goes from 40 to 43. According to Weber’s Law, for you to perceive the difference between volume 40 and 43 the same way you perceived the difference between volume 1 and 3 (an increase of 300%), volume 40 would have to go up to 120 (the same increase of 300%).

Sensation and Perception on the Exam

On the exam, you might be asked to define and compare both thresholds, stating how they can be applied to a concrete situation. For example, describe how each one could be used in the development of a new set of speakers and headphones.

Now that you’ve mastered this AP® Psychology Crash Course Review, you could answer that the absolute threshold can be used to set the minimum volume, as it would be pointless to insert a volume that nearly nobody can hear. The difference threshold can be used to set a smart, user-friendly system in which a raise in each volume bar would be perceived as a constant increase in volume because it would be proportional to the previous amount, just as stated in Weber’s Law.

However, it is also possible that you encounter a broader question on the topic of Sensation and Perception, like this one:

Describe the psychological concept of expectancy or set. Discuss a specific example of how each expectancy or set affects each of the following.

  • Human perception

A possible answer to this question is top-down processing, which influences our expectations and ultimately our perceptions. Another answer could refer to signal detection theory because it is easier for us to detect a stimulus if we are expecting it.

Psychophysics can seem intimidating, but you’re surely more confident about it now that you know what sensation and perception are and the difference between absolute threshold and difference threshold. So, what is the importance of studying human senses for you? How do you relate your individual perceptions of reality to the external, material reality? Share in the comments below!

Let’s put everything into practice. Try this AP® Psychology practice question:

Which of the following is the smallest change in a stimulus that can be detected 50% of the time?

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What are the weakest stimuli that can be detected 50% of the time?

An absolute threshold is the smallest level of stimulus that can be detected, usually defined as at least half the time. The term is often used in neuroscience and experimental research and can be applied to any stimulus that can be detected by the human senses including sound, touch, taste, sight, and smell.

Is the smallest difference between to stimuli that is detectable 50% of the time?

The smallest difference between two stimuli that can be detected 50 % of the time it is present is called? Just noticeable difference.

What is the smallest amount of a particular stimulus that can be detected?

The smallest detectable stimulus is called the absolute threshold, while the smallest detectable change in the intensity of a stimulus is called the difference threshold.

What is the smallest detectable difference between two stimuli?

Just noticeable difference (JND): The smallest detectable difference between two stimuli, or the minimum change in a stimulus that can be correctly judged as different from a reference stimulus; also known as difference threshold.