Sensory Thresholds and the Laws of Sensation

Defined as the minimum amount of stimulus energy needed for detection 50% of the time.
Answers the question: How dim can a light be, or how soft can a sound be, and still be detected half of the time?
Sensitivity of sensory receptors can be quite amazing.

Vision: A candle flame can be detected from 50 kilometers away on a clear night.
Hearing: The ticking of a clock can be detected from 6 meters away in a quiet room.
Taste: One teaspoon of sugar can be detected in four liters of water.
- Comparison: A 600ml serving of Coca-Cola contains 16 teaspoons of sugar, which translates to approximately 104 teaspoons in four liters.
Smell: Just one drop of perfume can be smelled throughout a large house.
- Scents like perfumes lead to rapid adaptation, and we filter them out; however, they are still pretty strong.

External Noise: Irrelevant information being processed by the system, such as ambient light or the humming of a fan; this makes it difficult to pick up a particular signal.
Internal Noise: Neurons fire randomly all the time; this random firing can make it difficult to detect a signal because the signal has to be stronger than the random background firing.
Psychological Factors: Expectations, motivation, stress levels, and fatigue can vary thresholds.

Highlights the principle that sensation is not a passive process.
Absolute thresholds are typically measured under incredibly well-controlled conditions ideal for sensitivity.

Focus shifts to how much difference between stimuli is required to reliably detect a difference rather than the minimum amount of energy that can be detected.
Unlike the absolute threshold, the difference threshold changes substantially depending on the stimulus intensity.

In a dark movie theater, a cell phone lighting up is easily noticeable.
In a brightly lit library, the same cell phone brightness is less likely to be noticed.
The cell phone's brightness doesn't change, but the ability to detect that change in illumination varies dramatically between the two contexts.
The just noticeable difference is not the same in the dark theater and in the brightly lit library.

Ernst Weber discovered in the 1830s that the difference threshold is not fixed, but rather it's a constant fraction of the original stimulus.
Weber's law can be expressed as a fraction.
Weber was the first to show that not only were subjective sensory experience and objective sensory stimulation related, but they can be predicted from each other mathematically.
The ratio of change in intensity required to produce a JND compared to the previous intensity of the stimulus can be expressed as a fraction.

Ratio of the change in intensity required to produce a JND compared to the previous intensity of the stimulus, expressed as a fraction.
- Weber Fraction = \frac{JND}{Original \, Intensity}

Example 1: Object weighs 100 grams, and the JND is noticed at 103 grams.
- The JND is 3 grams.
- The Weber fraction is \frac{3}{100} = 0.03
Example 2: Object weighs 1,000 grams, and the JND is noticed at 1,030 grams.
- The JND is 30 grams.
- The Weber fraction is \frac{30}{1000} = 0.03
The Weber fraction is constant in this case because we are judging weight in both cases.
The exact Weber fraction varies depending on the sensory stimuli being used and the individual.
It does not depend on the starting weight of the object.

Gustave Fechner extended Weber's law to estimate the psychological experience of a stimulus.
Argued that the subjective intensity of sensation is based on the amount of stimulus energy present.
Found a logarithmic relationship between physical stimulus and subjective intensity; In other words, in order for subjective intensity to increase arithmetically, physical intensity must increase geometrically.
At low levels of stimulation, a small increase in stimulation is needed to produce an increase in subjective experience.
At high levels of stimulation, a large increase in physical stimulation is needed to produce an increase in subjective experience.

Fechner's law works most of the time, but it was eventually modified by SS Stevens because it did not quite apply to all stimuli and all senses.
Despite the slight modification, these two laws are conceptually very similar to one another.

A dim stimulus requires only a small increase in intensity to reach a just noticeable difference.
A strong stimulus requires a very large increase in intensity to achieve one JND.

Weber's, Fechner's, and Stevens' laws indicate that sensation bears an orderly, predictable relation to physical stimulation.
Psychological experience, however, is definitely not a photograph of external reality.