Thresholds in Psychophysics: Exploring Sensory Perception and Its Diverse Applications

Psychophysics, started by Gustav Fechner, is the study of how we sense and perceive physical things. It connects the strength of a physical stimulus to what we experience mentally. Psychophysics focuses on three main questions:

1. What is the smallest stimulus we can detect (stimulus threshold)?
2. What is the smallest difference between two stimuli that we can notice (difference threshold)?
3. How does the strength of a stimulus relate to the strength of our sensation?

The notion of a threshold plays a central role. When studying how we detect and experience sensations like sights, sounds, smells, tastes and touches, researchers rely on the concept of thresholds. The thresholds provide clear boundaries that separate conscious perception from unconscious unawareness. A threshold is a representation of the minimum intensity of stimulation required to induce a sensory response. Thresholds are not philosophical constructs. Rather, they are empirical and provide scientific measurement that enables diverse applications. For instance, establishing aroma detection thresholds in product development helps determine which extreme component may contribute to a perfume’s overall fragrance. 

Types of Thresholds 

Several key types of thresholds have been defined: 

Absolute Threshold: The Boundary of Perception 

The absolute threshold refers to the minimum level of a stimulus that a person can detect 50% of the time. This threshold can apply to any of the senses, such as hearing, taste, vision, smell, and touch. For example, imagine being in a completely silent room. As faint sounds are gradually introduced, you will eventually reach a point where you first become aware of hearing something. That identifiable level of audible sound represents your absolute threshold for hearing at that moment if that is detectable fifty per cent of the time.

For vision, it might be the dimmest light that can be perceived. For taste or smell, it is the lowest concentration of a compound that evokes those sensations. And for touch, it could be the lightest pressure or faintest vibration detectable on the skin. Importantly, the absolute threshold is not a fixed value but varies based on factors like the person’s state of adaptation, attention, and expectations.

Even for the same individual, absolute thresholds fluctuate based on fatigue, age, and environmental conditions like lighting or background noise. Measuring absolute thresholds has applications from product testing to diagnosing sensory deficits. But it only reflects the lower boundary of perception.  Once a stimulus exceeds the absolute threshold, its perceived intensity grows with increasing stimulus magnitude, following the principles of psychophysics. 

Minimum level of stimulation required for a person to detect the presence of a sensation 

The Difference Threshold: Noticing Distinct Sensations 

While the absolute threshold marks the level where a stimulus is first detected, the difference threshold represents the amount a stimulus must change for the difference to be perceived. This is also called the “just noticeable difference” or JND. For example, if you are looking at a dimly lit scene, the difference threshold specifies how much brighter the illumination must become for you to consciously see that the light level changed.

For sounds,  it is the minimum increase in volume needed to detect the sound as louder. The difference threshold is always measured relative to a baseline or reference stimulus level. It represents the sensitivity for discriminating differences in magnitude around that reference point on the psychophysical scale.

Difference thresholds exhibit analogous principles to absolute thresholds. They can vary substantially across people based on factors like age, attentiveness, and perceptual learning. The specific sensory modality also affects how large or small the just noticeable differences are.

Knowing difference thresholds has applications like optimizing multimedia compression codecs and determining minimum differentiable product variations. However,  measuring them is more complex than absolute thresholds, often requiring specialized psychophysical testing procedures. 

Just Noticeable Difference 

Recognition Threshold 

The recognition threshold is the level at which the specific quality or identity of the stimulus becomes apparent. Recognition requires both detection and successful identification against competing qualities. It is the shortest exposure to a stimulus at which recognition of that stimulus can occur.  

Terminal Threshold 

At the other extreme from the absolute threshold lies the terminal threshold, which is the highest stimulus intensity producing no further increase in perceived magnitude. It represents a sensory system’s upper limit, where maximum neural firing rates or receptor saturation occurs. For most environmental stimuli we encounter, the terminal threshold is not typically reached due to the recruitment of new sensations like pain or irritation at very high levels.

Threshold of Consciousness 

The minimum above which stimuli enter awareness is characterized in terms of stimulus intensity, duration, and relevance. The threshold concept has been applied to memory and mood as well as to sensory stimulation. 

Other Thresholds 

Additional thresholds have been proposed for more specialized purposes. For example,  a rejection threshold attempts to pinpoint concentrations where off-flavours become unacceptable to consumers. In signal detection theory, different criteria define thresholds optimized for separating noise from the signal, like setting decision boundaries to maximize hit rates or minimize false alarms. 

Characterizing them reveals the remarkable dynamic ranges that allow us to perceive and adapt to the vastly differing intensity levels constantly happening in our sensory worlds. 

• Detection (absolute) Threshold: The point at which the substance is differentiated from the background
• Recognition threshold: The point at which the substance is currently named
• Difference threshold: (just-noticeable-difference, JND) Point at which the change in concentration is noted
• Terminal threshold: The point at which no further intensity increase is found with increasing concentration
• Consumer rejection threshold: The point at which a consumer preference occurs for a sample not containing the substance

Factors influencing Threshold

Methods of Threshold Measurement 

When measuring sensory detection thresholds, researchers have several practical methods at their disposal. One widely used approach is the ascending forced-choice method of limits (ASTM E-679). In this technique, the concentration of the stimulus is raised step-wise until it can be detected. At each concentration level, the participant receives three samples:

One containing the target stimulus and two blanks. They must identify the odd sample in this forced-choice task. Individual thresholds are calculated as the geometric mean of the highest missed and lowest detected concentrations. The group threshold is then the geometric mean of the individuals. Another method is to have panellists rate the degree of difference between a sample containing the stimulus and a blank control sample.

As concentrations increase, the ratings diverge from the control ratings. Statistical analyses determine the threshold concentration at which this divergence becomes significant. Adaptive staircase procedures can also efficiently home in on threshold values. Here, stimulus levels are adjusted up or down based on detection/non-detection in the previous trial.

This focuses on sampling around the critical threshold region. Variations use transformation rules, incorporate forced-choice elements, or require strings of correct/incorrect responses before level changes. In some cases,  suprathreshold scaling methods like intensity or descriptive profiling can serve as an initial screening step. Moderate negative correlations have been found between thresholds and suprathreshold ratings, allowing ratings at carefully chosen levels to estimate sensitivity. For measuring the potencies of aroma compounds, gas chromatography coupled with olfactometry  (GCO) has proven useful.

Thresholds are measured and then combined with concentrations in products to calculate odour unit ratios indicating likely sensory impact. Alternatively, GCO can present a standardized dilution series to directly assess panellist sensitivity. A classic approach is the Scoville method for measuring the pungency of chilli pepper extracts. Originally based on dilutions until the “bite” was no longer detected, modern Scoville measures often use recognition thresholds or rating scales anchored with fixed reference samples.

Application  

Threshold has wide applications in everyday real life, from the action potential of the neurons inside the body to professional cooking, product development, recipe formulation, and designing of human spaces & environment and in fact, it has applicability even in clinical settings for screening and diagnostics. 

For instance, absolute taste and aroma detection thresholds help in precise ingredient control, and understanding consumer rejection thresholds for off-flavours helps to ensure final products remain acceptable. By pinpointing these sensory boundaries, food scientists and chefs can finely optimize recipes and products to meet desired sensory targets. On the other hand, 

knowledge of terminal thresholds suggests maximum tolerable levels for noise, lighting, etc. in workspaces and thus contributes in designing of human spaces. In clinical setups understanding of thresholds helps in screening for potential deficits or neural/ sensory impairments. Reduced thresholds may notify conditions like hearing loss, numbness, anosmia,  etc. Threshold testing provides an objective, quantitative method for monitoring patients and evaluating intervention efficacy over time. 

Even at an individual and non-professional level understanding of the threshold can be relevant or important. For instance, if someone is attempting to lose weight by exercising and dieting, they can lose hope if physical changes take time to manifest and need them to cross a certain threshold for visual awareness of those changes. Understanding thresholds can help one become more sensitive to small differences or at least patient enough to notice absolute changes. This can help set more reasonable expectations and maintain motivation. Without the ability to recognize subtle progress, one may decide to give up or abandon their efforts in the process itself. 

Enhancing real-world systems that are in sync with human sensory reality is possible through studying sensory thresholds, which transforms fundamental psychophysical principles into something perceivable & tangible for the identification of perceptual abilities and limitations. Their uses open doors for modifying different human behaviours and examining our sensory functionality.