The absolute threshold is an important topic in psychology. Before discussing the absolute threshold, it is better to briefly discuss the detectable range of stimulus.

Every organism has a detectable range for various stimuli such as sight, sound, and smell. For instance, the human ear can only hear sounds between 20 Hz and 2000 Hz. In other words, the human cannot hear sounds below 20 Hz and above 2000 Hz. Such a range of frequencies of sound which can be heard by an organism is called audible frequency range.

The above-noted audible frequency range is for a normal and healthy human being. However, this range may slightly vary with different factors such as physiological and psychological health status, age and so on. For instance, young people may hear a sound with a lower level of intensity, but old people may not be able to hear the sound with the same lower level of intensity. Therefore, the ability of people to hear sound at different levels may slightly vary. This also applies to variation in the ability of different people to see.

Now, let’s move on to the absolute threshold..

Absolute threshold is the lowest level of stimulus that a person can detect 50% of the times.

For instance, a person is asked to hear a sound that is played at its lowest intensity level several times. If the person can detect this sound 50% of the time the sound is played, such the lowest intensity level of sound is called the absolute threshold of sound. Similarly, the absolute threshold for vision would be the lowest level of light that a person can detect 50% of the times.

The absolute threshold can be determined for any stimulus such as light, sound, smell, or touch. However, there are two things to be considered for determining the threshold: 1) it is the lowest intensity level of the stimulus that can be detected by the person, and 2) the person is able to detect it 50% of the times.

As noted, the absolute threshold is simply the lowest detectable level of a stimulus for a person. However, the ability to detect stimulus 50% of the times is set as criteria because there can be individual differences in subjects. For instance, some individuals can detect a specific lowest level of stimulus which cannot be easily detected by many other individuals. Similarly, the subjects may detect a stimulus correctly at some times but may not detect it correctly at other times. Therefore, to have a balanced approach to determining the absolute threshold 50% detectability is considered to deal with the issue of individual differences.

There are a number of factors that influence absolute threshold such as the expectation to detect, motivation to detect, and familiarity with the stimulus.

If a person is expecting to detect a stimulus, he will be able to detect it even in its lowest intensity. For instance, a person is waiting for a text message on his mobile phone, he would be able to readily hear the message tone but if not expecting, he may not hear it or ignore it.

If a person is motivated, he will easily detect the stimulus. For instance, if a kid is asked to hear a sound played at a low volume, and he is promised a chocolate-bar for each time he hears the sound. He will be able to hear it many times because the reward (chocolate) bring motivation in him to hear the sound.

If a person is familiar with a stimulus, he will easily detect it. For instance, the crescent moon is pretty thin and hard to be seen. However, if a person has attempted to see it many times in past, and has become familiar with it, he easily sees it while many may not be able to locate it.

The method used by psychologists to find the absolute threshold of a person is called the method of limits. In this method, a stimulus is presented to a person in ascending or descending order of its intensity. In ascending order, the intensity of the stimulus is increased from low to high in a stepwise manner. In descending order, the intensity of the stimulus is decreased from high to low in a stepwise manner. With either way adopted, the psychologist intends to find the lowest intensity of the stimulus that is detected by the person.

For instance, A sound is played to a person who is asked to hear it. The intensity of a sound is increased from its lowest level to the highest level in a stepwise manner. The lowest intensity level of sound that is detected by the person is noted down on a paper. Then, the sound is played at the noted lowest intensity several times, if the person can detect it 50% of the times the sound was played, it is marked as the absolute threshold of sound for the person.

The same procedure is also carried out to find the absolute threshold of vision. For instance, light is presented in its different intensity levels increasing from lowest to highest. The presented light increases from dimmest intensity to brightest intensity. The lowest level that a person detects 50 % of the times the light was presented, is marked as his absolute threshold of vision for the person.

Measuring the absolute threshold may also involve measuring the DISTANCE of the source of stimulus from the person. This is done for all types of stimuli such as light, sound, and smell. However, for taste and touch, instead of distance other parameters for space-effect may be used.

For determining the distance threshold for vision, light is presented to a person in a dark room at varying distances from the person. Similarly, for finding the distance threshold for sound, a sound is played to a person in a calm environment at varying distances from the person.

The absolute threshold of various stimuli, as found in some experiments, are as below.

• Vision: A candle flame 30 miles away in a dark night.
• Hearing: A ticking of a clock 20 feet away.
• Smell: A drop of perfume in six-room apartments. 
• Taste: A teaspoon of sugar in a gallon of water. 
• Touch: A wing of fly dropped in a centimetre on your cheek.   

The differences threshold is also referred to as Just Noticeable Difference (JND) in psychology.

It is the minimum level of change in a stimulus that a person can detect 50% of the time.

A stimulus can be presented to a person at different levels above the absolute threshold. Each level above absolute threshold is detectable by the person. With the change in each level of stimulus, the person can detect that the level of the presented-stimulus has changed. For instance, with an increase in each level of the stimulus, the person can detect that level of presented-stimulus has increased. The minimum level of change in a stimulus that can be detected by the person is called difference threshold or just noticeable difference (JND).

Let’s explain it with an easy example: Suppose a person is asked to hear a sound and the volume of the sound is raised in a stepwise manner. The minimum change in the volume levels that is detected by the person is called difference threshold or just noticeable difference.

It is important to note that the definition of difference threshold sets criteria that a person can detect the minimum change (in the stimulus) 50% of the times. This means that the experiment for detecting the difference threshold would be repeated several times to ensure that the person can detect the minimum change (in the stimulus) 50% of the time, and then, that minimum detectable change would qualify for being called difference threshold.

It is called just noticeable difference because it is the minimum detectable change in the stimulus.

Sometimes the students are confused by the term absolute threshold and the just noticeable difference. The absolute threshold is the lowest detectable level of stimulus whereas just noticeable difference is the minimum change in a stimulus that is detectable when the level of stimulus changes.

Subliminal stimulation means stimulation that takes place when the stimulus is below the absolute threshold. Since stimuli below the absolute threshold are not detectable by sensory organs, they have no effect on us at all. However, this is not the case. The stimuli below the absolute threshold enter our unconscious mind that we are unaware of, but they do influence our behaviour to an extent. For instance, a recent study of functional magnetic resonance has revealed that subliminal stimulation activates specific regions of the brain and thus may influence our behaviour. However, this calls for further research.

In psychology experiments, subliminal stimuli are presented to the subject in different ways depending on which sensory stimuli is used as well as how they are operationally defined.

Subliminal stimulus is the opposite of super-liminal stimulus which is above the absolute threshold and are detectable by sensory organ.