# 8.2: Psychophysical Methods


##### Learning Objectives

Differentiate between experiments measuring absolute thresholds, difference thresholds, and magnitude estimations

Describe at least two different methods of estimating a threshold

Understand what a subliminal message is

Explain Weber’s law (also called Weber-Fechner law)

Brief Overview

Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they produce. Psychophysics has been described as the scientific study of the relation between stimulus and sensation or, more completely, as the analysis of perceptual processes by studying the effect on a subject's experience or behaviour of systematically varying the properties of a stimulus along one or more physical dimensions.

The sensitivity of a given sensory system to the relevant stimuli can be expressed as an absolute threshold. Absolute threshold refers to the minimum amount of stimulus energy that must be present for the stimulus to be detected 50% of the time. Another way to think about this is by considering how dim a light can be or how soft a sound can be and still be detected half of the time. The sensitivity of our sensory receptors can be quite amazing. It has been estimated that on a clear night, the rods (sensitive sensory cells in the back of the eye) can detect a candle flame 30 miles away (Okawa & Sampath, 2007). Under quiet conditions, the hair cells (receptor cells of the inner ear) can detect the tick of a clock 20 feet away (Galanter, 1962) see figure 8.4.1 below.

It is also possible for us to get messages that are presented below the threshold for conscious awareness—these are called subliminal messages. A message below the absolute threshold is not strong enough to excite sensory receptors that send nerve impulses to the brain; when this occurs that threshold is said to be subliminal: We receive it, but we are not consciously aware of it. Over the years there has been a great deal of speculation about the use of subliminal messages in advertising, rock music, and self-help audio programs. Research evidence shows that in laboratory settings, people can process and respond to information outside of awareness. But this does not mean that we obey these messages or are heavily influenced by these messages; in fact, hidden messages have little effect on behavior outside the laboratory (Kunst-Wilson & Zajonc, 1980; Rensink, 2004; Nelson, 2008; Radel, Sarrazin, Legrain, & Gobancé, 2009; Loersch, Durso, & Petty, 2013).

Methods for estimating thresholds

When we design experiments, we have to decide how we’re going to approach a threshold estimation. Here are three common techniques

• Method of Limits. The experimenter can increase the stimulus intensity (or intensity difference) until the observer detects the stimulus (or the change). For example, turn up the volume until the observer first detects the sound. This is intuitive, but it is also subject to bias — the estimated threshold is likely to be different, for example, if we start high and work down versus starting low and working up.
• Method of Adjustment. This is very much like the Method of Limits, except that the experimenter gives the observer control of the stimulus adjustment with the instructions to: “adjust the stimulus until it’s clearly visible” or “adjust the color of the patch until it matches the test patch.”
• Method of Constant Stimuli. This is the most reliable, but also the most time-consuming. You decide ahead of time what levels you are going to measure, do each one a fixed number of times, and record percent correct (or the number of detections) for each level. If you randomize the order, you can get rid of bias.

Absolute thresholds are generally measured under rigidly controlled conditions in situations that are optimal for sensitivity. Sometimes we are more interested in how much difference in stimuli is required to detect a difference between them. This is known as the just noticeable difference (jnd) or difference threshold. Unlike the absolute threshold, the difference threshold changes depending on the stimulus intensity. As an example, imagine yourself in a very dark movie theater. If an audience member were to receive a text message on their cell phone which caused their screen to light up, chances are that many people would notice the change in illumination in the theater. However, if the same thing happened in a brightly lit arena during a basketball game, very few people would notice. The cell phone brightness does not change, but its ability to be detected as a change in illumination varies dramatically between the two contexts. Ernst Weber proposed this theory of change in difference threshold in the 1830s, and it has become known as Weber’s law.

Weber’s law is approximately true for many aspects of our senses — for brightness perception, visual contrast perception, loudness perception, and visual distance estimation, our sensitivity to change decreases as the stimulus gets bigger or stronger. However, there are many senses for which the opposite is true: our sensitivity increases as the stimulus increases. With electric shock, for example, a small increase in the size of the shock is much more noticeable when the shock is large than when it is small. A psychophysical researcher named Stanley Smith Stevens asked people to estimate the magnitude of their sensations for many different kinds of stimuli at different intensities, and then tried to fit lines through the data to predict people’s sensory experiences (Stevens, 1967). What he discovered was that most senses could be described by a power law of the form P ∝Sn (where P is the perceived magnitude, ∝ means “is proportional to”, S is the physical stimulus magnitude, and n is a positive number). If n is greater than 1, then the slope (rate of change of perception) is getting larger as the stimulus gets larger, and sensitivity increases as stimulus intensity increases. A function like this is described as being expansive or supra-linear. If n is less than 1, then the slope decreases as the stimulus gets larger (the function “rolls over”). These sensations are described as being compressive. Weber’s Law is only (approximately) true for compressive (sublinear) functions; Stevens’ Power Law is useful for describing a wider range of senses see Fig 8.4.2 below.

Both Stevens’ Power Law and Weber’s Law are only approximately true. They are useful for describing, in broad strokes, how our perception of a stimulus depends on its intensity or size. They are rarely accurate for describing perception of stimuli that are near the absolute detection threshold. Still, they are useful for describing how people will expectably react to normal every-day stimuli.

Sensation and Perception Summary:

The world as we experience it is most often multimodal, involving combinations of our senses into one perceptual experience. The combination of senses that allow us to enjoy aspects of our everyday life requires our senses to be integrated. Interestingly, we actually respond more strongly to multimodal stimuli compared to the sum of each single modality together, an effect called the superadditive effect of multisensory integration. This can explain how you’re still able to understand what friends are saying to you at a loud concert, as long as you are able to get visual cues from watching them speak. If you were having a quiet conversation at a café, you likely wouldn’t need these additional cues. Because we are able to process multimodal sensory stimuli, and the results of those processes are qualitatively different from those of unimodal stimuli, it’s a fair assumption that the brain is doing something qualitatively different when they’re being processed. There has been a growing body of evidence since the mid-1990’s on the neural correlates of multisensory integration.

## References

Galanter, E. (1962). Contemporary Psychophysics. In R. Brown, E.Galanter, E. H. Hess, & G. Mandler (Eds.), New directions in psychology. New York, NY: Holt, Rinehart & Winston.

Kunst-Wilson, W. R., & Zajonc, R. B. (1980). Affective discrimination of stimuli that cannot be recognized. Science, 207, 557–558.

Nelson, M. R. (2008). The hidden persuaders: Then and now. Journal of Advertising, 37(1), 113–126.

Okawa, H., & Sampath, A. P. (2007). Optimization of single-photon response transmission at the rod-to-rod bipolar synapse. Physiology, 22, 279–286.

Radel, R., Sarrazin, P., Legrain, P., & Gobancé, L. (2009). Subliminal priming of motivational orientation in educational settings: Effect on academic performance moderated by mindfulness. Journal of Research in Personality, 43(4), 1–18.

Rensink, R. A. (2004). Visual sensing without seeing. Psychological Science, 15, 27–32.

Steingrimsson, R.; Luce, R. D. (2006). "Empirical evaluation of a model of global psychophysical judgments: III. A form for the psychophysical function and intensity filtering". Journal of Mathematical Psychology. 50: 15–29. doi:10.1016/j.jmp.2005.11.005

Stevens, S. S. (1957). On the psychophysical law. Psychological Review 64(3):153—181. PMID 13441853

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