Sensory Thresholds and Psychophysics: Absolute and Difference Thresholds

Sensory Thresholds and the Absolute Threshold

• Definition of Sensory Threshold: All human senses possess a sensory threshold, which is a specific level below which an individual does not perceive anything, even if an external stimulus is physically present.

• Formal Definition of Absolute Threshold: The absolute threshold is formally defined as the minimum amount of stimulus energy that must be present for the stimulus to be detected 50% of the time.

• Conceptual Understanding: A practical way to conceptualize this is by determining how dim a light can be or how soft a sound can be while still being detectable half of the time.

• Sensitivity of Sensory Receptors: Human sensory receptors exhibit remarkable sensitivity. Estimated absolute thresholds for various senses include:     * Vision: On a clear night, the most sensitive sensory cells at the back of the eye can detect the flame of a single candle from a distance of $50\,km$.     * Hearing: In a quiet room, the hair receptors in the inner ear are capable of detecting the ticking of a clock or watch from a distance of $6\,m$.     * Taste: Humans can detect the presence of a single teaspoon of sugar dissolved in $4\,dm^3$ (four litres) of water.         * Contextual Comparison: For comparison, a single $600\,ml$ serving of Coca-Cola contains $16$ teaspoons of sugar. If translated to the same volume as the threshold test ($4\,dm^3$), a proportional amount of Coca-Cola would contain approximately $140$ teaspoons of sugar.     * Smell: Although smell is considered a relatively weak system in humans, it remains highly sensitive; an individual can smell a single drop of perfume diffused throughout a large house.

Factors Influencing Sensory Detection and Variation

• Sensory Adaptation: Humans often believe scents (like perfume) are weaker than they are because we rapidly adapt to them. The system filters out constant stimuli, similar to how one becomes accustomed to the hum of a fan or the physical pressure of sitting in a chair until the sensation is no longer consciously perceived.

• Variability of Thresholds: Absolute thresholds are not truly static; they vary between different people and change depending on the situation. The primary reason for this variation is "noise."

• The Concept of Noise: Noise refers to any irrelevant information being processed by the sensory system. It is categorized into two types:     * External Noise: Ambient factors such as background light or the humming of a fan that interfere with the detection of a specific signal or change in light.     * Internal Noise: This stems from biological and psychological factors:         * Biological: Neurons fire randomly all the time. A signal must be strong enough to produce more frequent action potentials than this random background firing to be detected.         * Psychological: Factors such as expectations, motivation, stress levels, and fatigue can all cause thresholds to vary.

Difference Threshold and the Just Noticeable Difference (JND)

• The Principle of Sensation: Sensation is not a passive process. While absolute thresholds are measured in optimal, controlled conditions, real-world interest often lies in detecting differences between stimuli.

• Definition of Difference Threshold (JND): The difference threshold, commonly referred to as the JND (Just Noticeable Difference), is the minimum amount of difference between two stimuli required to reliably detect that the two stimuli are actually different from one another.

• Contextual Dependency: Unlike the absolute threshold (which is theoretically fixed), the difference threshold changes substantially based on the intensity of the original stimulus.

• The Movie Theatre/Library Thought Experiment:     * If a cell phone screen lights up in a very dark movie theatre, the change in illumination is highly noticeable to many people.     * If the same cell phone screen lights up in a brightly lit library, very few people would notice the change.     * Though the brightness of the phone is identical in both scenarios, the ability to detect that change varies dramatically based on the surrounding context and initial intensity of light.

Ernst Weber and the Weber Fraction

• Historical Discovery: In the 1830s, Ernst Weber discovered that the difference threshold is not a fixed physical amount but rather a constant fraction of the original stimulus.

• Weber Fraction Definition: This is the ratio of change in intensity required to produce a JND compared to the previous/original intensity of the stimulus.

• Predictability: Weber was the first to demonstrate that subjective sensory experiences and objective sensory stimulation could be predicted from one another mathematically.

• Mathematical Examples of the Weber Fraction:     * Case 1: An object weighs $100\,g$. It is noticed to be heavier once it reaches $103\,g$.         * $JND = 103\,g - 100\,g = 3\,g$         * $\text{Weber Fraction} = \frac{3\,g}{100\,g} = 0.03$     * Case 2: A second object weighs $1000\,g$. It is noticed to be heavier once it reaches $1030\,g$.         * $JND = 1030\,g - 1000\,g = 30\,g$         * $\text{Weber Fraction} = \frac{30\,g}{1000\,g} = 0.03$

• Consistency: The Weber fraction remains constant for a specific sensory modality (e.g., judging weight) regardless of the starting intensity/weight, though the fraction itself varies between different senses and individuals.

Fechner’s Law and Stevens’s Modification

• Gustav Fechner: Weber's brother-in-law, Fechner, expanded on the JND to estimate the psychological experience of a stimulus. He argued that the reported intensity of a sensation is based on the actual stimulus energy present.

• Fechner’s Law (Logarithmic Relationship): Fechner found a logarithmic relationship between physical intensity and subjective experience.     * Rule: For subjective intensity to increase arithmetically, the physical intensity must increase geometrically.     * Low Levels of Stimulation: At low intensities, only a small physical increase is required to produce a noticeable increase in subjective experience.     * High Levels of Stimulation: At high intensities, a very large increase in physical stimulation is required to produce an equivalent increase in subjective experience.

• S.S. Stevens: Stevens modified Fechner’s work because the original law did not apply perfectly to all senses and stimuli. While technically different, Stevens's law is conceptually very similar to Fechner's.

Graphical Representation and Conclusion

• Graphical Illustration: In a visual representation of these laws:     * A dim stimulus requires a very small increase in intensity or brightness to reach one JND (one sensation unit).     * A strong stimulus requires a significantly larger increase in stimulus intensity to achieve that same increase of one JND sensation unit.

• Major Takeaway: The laws of Weber, Fechner, and Stevens demonstrate that sensation bears an orderly and predictable relationship to physical stimulation.

• Psychological Reality: Despite this predictability, psychological experience is not a direct "photograph" of external reality; it is processed and filtered by the sensory system.