The absolute threshold is a fundamental concept in psychology. Before discussing the absolute threshold in detail, it is important to briefly explain the detectable range of stimuli.

Every organism possesses a specific detectable range for various stimuli, including sight, sound, and smell. For example, the human auditory system is capable of perceiving sounds within a frequency range of approximately 20 Hz to 2000 Hz. In other words, sounds falling below 20 Hz or above 2000 Hz are generally not detectable by the human ear. This range of sound frequencies that can be perceived by an organism is referred to as the audible frequency range.

It is important to note that this audible frequency range is for a normal and healthy individual. However, this range may vary slightly depending on several factors, including physiological and psychological health, age, and other individual differences.

For instance, younger individuals are typically able to detect sounds at lower intensity levels, whereas older individuals may not be able to hear sounds at the same lower level of intensity. Consequently, the ability to perceive auditory stimuli at varying intensity levels differs among individuals. Consequently, the ability to perceive auditory stimuli at varying intensity levels differs among individuals. Similar variations also apply to the ability of different people to see (visual perception).

With this background in mind, we can now proceed to the concept of the absolute threshold.

The absolute threshold is defined as the lowest level of a stimulus that an individual can detect 50% of the time.

For example, consider a situation in which a person is asked to detect a sound presented at a very low intensity level across multiple trials. If the individual is able to detect the sound in 50% of those presentations, that specific intensity level is identified as the absolute threshold for hearing. Similarly, in the context of vision, the absolute threshold refers to the minimum level of light intensity that a person can detect 50% of the time.

The concept of the absolute threshold can be applied to various types of stimuli, including light, sound, smell, and touch. Two essential criteria must be met when determining the absolute threshold:

1. The stimulus must be presented at the lowest intensity level detectable by the individual.
2. The individual must be able to detect the stimulus in 50% of the trials.

As noted above, the absolute threshold represents the minimum detectable level of a stimulus. However, the ability to detect a stimulus 50% of the time is set as a criterion because there can be individual differences in sensory sensitivity among subjects. This standard accounts for individual differences in sensory sensitivity. For instance, some individuals may be capable of detecting extremely low levels of a stimulus that others cannot. Additionally, even the same individual may not consistently detect the stimulus across all trials. Therefore, establishing detection at 50% of the time provides a balanced and standardized measure.

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

Expectation plays a significant role in sensory detection. When individuals anticipate a stimulus, they are more likely to detect it, even at low intensity levels. For example, a person waiting for a message notification is more likely to notice the alert tone compared to when they are not expecting it.

Motivation also enhances perceptual sensitivity. Individuals who are motivated – such as when a reward is involved – tend to detect stimuli more readily. For instance, a child promised a reward for identifying a faint sound may demonstrate increased sensitivity to that sound.

Familiarity with a stimulus further facilitates detection. Repeated exposure to a stimulus can improve an individual’s ability to recognize it. For example, although the crescent moon is often faint and difficult to observe, individuals who have frequently attempted to locate it are more likely to detect it compared to those without prior experience.

The method commonly used by psychologists to determine an individual’s absolute threshold is known as the method of limits. In this approach, a stimulus is presented to a participant in either ascending or descending order of its intensity.

In the ascending method, the intensity of the stimulus is gradually increased from a very low (often undetectable) level to a higher level in a stepwise manner. In contrast, in the descending method, the stimulus intensity is progressively reduced from a clearly detectable level to a lower level. In both approaches, the objective is to identify the lowest intensity at which the stimulus is reliably detected by the individual.

For example, in an auditory experiment, a sound may be presented to a participant who is instructed to indicate when it is heard. The intensity of the sound is increased incrementally from its lowest level. The point at which the participant first detects the sound is recorded. This intensity level is then presented repeatedly. If the participant is able to detect the sound in approximately 50% of the trials, it is established as the individual’s absolute threshold for hearing.

A similar procedure is applied in visual perception studies. Light is presented at varying levels of intensity, increasing from the dimmest to the brightest. The lowest intensity at which the participant detects the light in 50% of the trials is identified as the absolute threshold for vision.

In addition to intensity, the measurement of the absolute threshold may also involve the distance between the stimulus source and the observer. This approach is applicable to sensory modalities such as vision, hearing, and smell. However, for taste and touch, spatial parameters other than distance may be used.

For instance, in determining the distance threshold for vision, a light source may be presented in a dark environment at varying distances from the observer. Similarly, for auditory threshold measurement, a sound may be presented in a quiet setting at different distances.

Empirical studies have reported approximate absolute thresholds for various sensory modalities, including the following:

• Vision: Detecting a candle flame from approximately 30 miles away on a dark night.
• Hearing: Hearing the ticking of a clock from about 20 feet away.
• Smell: Detecting a drop of perfume in a six-room apartment.
• Taste: Detecting a teaspoon of sugar dissolved in a gallon of water.
• Touch: Sensing the wing of a fly falling from a height of about one centimeter onto the cheek.

The difference threshold, commonly referred to as the Just Noticeable Difference (JND), is a key concept in psychology. It is defined as the minimum change in a stimulus that an individual can detect 50% of the time.

A stimulus can be presented at varying levels above the absolute threshold, and each of these levels is generally detectable. As the intensity of the stimulus changes, an individual may perceive that a difference has occurred. The smallest detectable change in the intensity of a stimulus is known as the difference threshold or JND.

For example, consider an auditory task in which a participant is asked to listen to a sound whose volume is gradually increased in a stepwise manner. The smallest increase in volume that the participant can reliably detect is identified as the difference threshold or just noticeable difference.

It is important to emphasize that the definition of the difference threshold includes the criterion of detectability at least 50% of the time. To establish this, the procedure is typically repeated across multiple trials to ensure consistency and reliability in the individual’s responses.

The term just noticeable difference reflects the idea that the detected change is the smallest possible variation that can be perceived.

Students often confuse the concepts of absolute threshold and just noticeable difference. The absolute threshold refers to the lowest level of a stimulus that can be detected, whereas the just noticeable difference refers to the smallest detectable change in the intensity of a stimulus.

Subliminal stimulation refers to sensory input that occurs below the level of the absolute threshold. Since stimuli below the absolute threshold are not consciously detectable by sensory organs, it may seem that they have no effect on us. However, research suggests that this assumption is not entirely accurate.

Stimuli below the absolute threshold may enter our unconscious mind without our awareness, and they can influence our behavior to some extent. For example, studies utilizing functional magnetic resonance imaging (fMRI) have demonstrated that subliminal stimuli can activate specific regions of the brain, indicating that some level of processing occurs even in the absence of conscious perception. However, this area remains a subject of ongoing research.

In experimental psychology, subliminal stimuli are presented using various techniques, depending on the sensory modality involved and the operational definitions adopted by researchers.

A subliminal stimulus is conceptually contrasted with a supraliminal stimulus, which is presented above the absolute threshold and is therefore consciously detectable by the sensory organs.