A threshold in psychology is the minimum level of stimulation required for your senses or brain to detect, notice, or respond to a stimulus.
There are two main sensory thresholds you need to know.
1. Absolute Threshold
The absolute threshold is the smallest amount of stimulation needed to detect a stimulus 50% of the time.
This means that if a stimulus is presented many times under ideal conditions, you will notice it about half of the time.
Examples
- Hearing the ticking of a watch in a quiet room.
- Seeing a candle flame from about 30 miles away on a clear, dark night.
- Smelling a faint perfume.
- Feeling a light touch on your skin.
- Taste a tiny amount of sugar in water.
Key Point
The stimulus must be strong enough to activate the sensory receptors and send nerve impulses to the brain.
Why Is It Called "50% of the Time"?
People do not detect weak stimuli perfectly every time.
Sometimes you notice the stimulus.
Sometimes you do not.
Scientists define the absolute threshold as the point at which detection occurs 50% of the time because it provides a reliable scientific measure.
2. Difference Threshold (Just Noticeable Difference - JND)
The difference threshold, also called the Just Noticeable Difference (JND), is the smallest change in a stimulus that a person can detect 50% of the time.
Instead of detecting whether something is there, you are detecting whether something has changed.
Examples
Weight
Holding a 5-pound weight, adding 1 pound is easy to notice.
Holding a 100-pound weight, adding 1 pound is much harder to notice.
Brightness
In a dark movie theater, a cellphone screen lighting up is immediately noticeable.
In a brightly lit basketball arena, the same phone screen is barely noticeable.
The phone's brightness did not change.
The environment changed.
Sound
If someone whispers in a quiet library, you notice immediately.
If someone whispers during a loud concert, you probably will not hear them.
Temperature
Adding a little warm water to cool water is noticeable.
Adding the same amount of warm water to already hot water may not be noticeable.
Weber's Law
The difference threshold follows Ernst Weber's Law.
Weber discovered that the amount of change needed to notice a difference depends on the original strength of the stimulus.
Rather than a fixed amount, the change must be a constant proportion of the original stimulus.
Examples
| Original Weight | Change Needed to Notice |
|---|---|
| 5 pounds | Small change |
| 50 pounds | Larger change |
| 100 pounds | Even larger change |
As the original stimulus becomes stronger, a greater change is needed before you notice the difference.
Subliminal Messages
Sometimes a stimulus is below the absolute threshold.
This is called a subliminal stimulus.
A subliminal stimulus is too weak to be consciously detected, but it may still be processed by the brain.
Examples include:
- Very brief images
- Extremely quiet sounds
- Hidden words flashed on a screen
Do Subliminal Messages Control Our Behavior?
Research shows that people can process some information outside of conscious awareness.
However, studies also show that subliminal messages:
- Have very small effects
- Do not control people's behavior
- Do not make people obey hidden commands like "zombies"
For example, a hidden advertisement is unlikely to make someone buy a product they were not already interested in.
Signal Detection Theory
Your ability to detect a stimulus depends on more than just its strength.
It is also affected by:
- Attention
- Expectations
- Motivation
- Fatigue
- Distractions
- Background noise
Example
Imagine a quiet room.
If you are listening carefully for a ticking clock, you are much more likely to hear it.
Now imagine a noisy party.
The clock is ticking just as loudly, but you probably will not hear it because of all the background noise.
Signal Detection Theory explains why our ability to detect signals changes depending on both the environment and our mental state.
Physiological Threshold
A physiological threshold is reached when a stimulus is strong enough to activate sensory receptors and produce nerve impulses that travel to the brain.
Once this threshold is reached, the brain has the opportunity to process the information consciously.
Everyday Examples of Thresholds
| Situation | Type of Threshold |
|---|---|
| Hearing a faint alarm clock | Absolute threshold |
| Seeing a dim flashlight | Absolute threshold |
| Feeling a feather touch your arm | Absolute threshold |
| Noticing someone turned the TV volume up slightly | Difference threshold |
| Detecting that coffee tastes sweeter | Difference threshold |
| Realizing your backpack became heavier | Difference threshold |
Quick Comparison
| Absolute Threshold | Difference Threshold (JND) |
|---|---|
| Detects whether a stimulus is present | Detects whether a stimulus has changed |
| Smallest detectable stimulus | Smallest detectable change |
| Measured at 50% detection | Measured at 50% detection of change |
| Example: Hearing a faint ticking watch | Example: Noticing the TV volume increase |
Key Terms to Remember
- Threshold: The minimum level of stimulation needed to produce a response.
- Absolute Threshold: The weakest stimulus that can be detected 50% of the time.
- Difference Threshold (JND): The smallest change in a stimulus that can be detected 50% of the time.
- Weber's Law: The size of the noticeable change depends on the original intensity of the stimulus.
- Subliminal Stimulus: A stimulus presented below conscious awareness.
- Signal Detection Theory: Detection depends on both the stimulus and factors such as attention, expectations, motivation, and background noise.
Summary
Thresholds help psychologists understand how our senses work. The absolute threshold is the minimum amount of stimulation needed to detect something, while the difference threshold (JND) is the minimum change needed to notice that something has changed. These thresholds are influenced not only by the strength of the stimulus but also by factors such as attention, expectations, and the surrounding environment, as explained by Signal Detection Theory. Weber's Law further shows that the amount of change required to notice a difference increases as the original stimulus becomes stronger.
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