Psychophysics: Sensory and Perceptual Limits. The Human Umwelt

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Psychophysics – Sounds cool right? It is. It really is cool. I am not overstating when I say that psychophysics is associated with our very existence.

The human body has about 10 sensory systems which send in information to the central nervous system (brain and its neural extensions).

Psychophysics deals with the very core of this – how strong or weak should the information be for our senses to actually ‘sense’ it.

We are going to look at the experimental results of research on the sensory and perceptual limits of the human body. This article will only look at the 5 popular senses – hearing, sight, touch, smell, and taste because the research on these senses is well documented, reliable, and replicable.

A simple definition of psychophysics:

The branch of psychology that deals with the relationship between physical stimuli, behavior, and mental phenomena. For example – how bright should your phone screen be for you to actually see what’s on it.

The geek definition of psychophysics

The capture, conversion, and translation of physical phenomena (as studied by physics) into sensory input which then affects our sensation and perception.

Each of our sensory organs has biologically evolved to capture some useful information from the surroundings. We call this information the ‘stimulus’. For a stimulus to reach our sensory systems and become a ‘sensation’, the mechanisms present in our sensory organs have to processes the information. For them to processes, the information needs to be strong enough or in a particular range. It needs to be detected. We’ll talk about this soon. Let us first see what psychophysics investigates.

Psychophysics experimentally studies 2 important aspects of sensation.

  • The threshold at which our senses detect information.
  • The change in intensity of a stimulus which is noticed and discriminated by our senses.

Sensing information means our senses capture it via the senses. Perception means our brains process it in a way that it somehow affects at least one of the following:

  • Cognition
  • Behavior
  • Emotions
  • Instincts
  • Reflexes
  • Memory

Concluding the geek definition of psychophysics:

Psychophysics studies the thresholds and sensitivity of our sensation and perception with respect to stimuli.

Psychophysics studies the boundaries of what humans can sense. It is the study of the Umwelt (the world as it is experienced by a particular organism) – The horizon of what humans can sense and perceive

The context of psychophysics

Do you know how small a slice of the universe we sense with our biological bodies? Our reality is made up of a very small input of information from the universe. Humans have a set of physical properties which make up our reality. Dogs have their own, Cats have their own. Each living creature has a unique set of psychophysical properties which evolution shaped. But humans are different. Humans have science, engineering, the vision to see beyond what is seeable. To go where no man has gone before.

This is where technology comes into play. It lets us manipulate information, read information which exists beyond our sensory capacity. X-rays, a type of electromagnetic radiation which humans cannot see with the eyes turned out to be pretty useful. We build machines (X-ray machines) to look at broken bones. We expanded our Umwelt to include this information. But, only technology would let us do that. The same goes with night goggles, cellphones, radios, etc. Technology made information beyond our threshold available to us in a useful way.

Let us return to the important point. Psychophysics deals with the human body’s capacity to sense. Let’s leave technology out of it and look at psychophysical data for our senses.

The range of seeing and hearing: limits of Vision and Audition

Range of Human Eyes

The visible spectrum of humans normally ranges around 380 to 720 nanometres.  Other species, however, can detect parts of the electromagnetic spectrum. For instance, the vision of insects like bees and butterflies and even some species of birds extends in the ultraviolet range. Certain snakes can detect infrared radiation, which have longer wavelengths than the visible spectrum. For human beings, the perception of color is linked with light of a particular wavelength. Red has longer wavelengths about 750nm, and is at the lower end of the visible spectrum. Green is at the middle of the spectrum, whereas violet is at the upper end, with shorter wavelengths of 380nm.

Violet380–450 nm
Blue450–495 nm
Green495–570 nm
Yellow570–590 nm
Orange590–620 nm
Red620–750 nm

Range of Human Ears

The range of hearing in human being is typically 20 – 20000 Hz . However, this varies as the person gets older. The hearing range of other species greatly differs from that of humans. For example, the audible range of the beluga whale is 1000 to 123000 Hz .

Bats rely on their hearing, using sound to navigate, which is called echolocation. This compensates for their lack of visual prowess. Their ultrasonic calls range in the frequency of 2000 to 110000 Hz. The hearing of household pets like cats and dogs falls approximately in the range of 67- 45000 Hz and 45-64000 Hz respectively.

Range of human hearing and animal hearing
8va stands for the hearing range in octaves. C stands for the musical C note.

Sensory thresholds and sensitivity

Psychophysics involves the study of sensitivity to stimuli around us and the point of intensity at which stimulus is detected, which is called a ‘threshold.’ Mainly, the just noticeable difference or the difference threshold and the absolute threshold are measured.

Just Noticeable differences

The just noticeable difference (jnd) or the difference threshold is the smallest change in a stimulus which is detectable by a person 50% of the time. The concept of difference thresholds has applications in all areas of perception: hearing, touch, sight, taste, and smell.

To make JNDs meaningful, we are going to look at a psychophysical law. Ernst Weber was the first researcher to systematically study this concept and formulated the Weber’s Law based on his research.

Weber’s Law states that the size of the just noticeable difference is a constant proportion of the value of the original stimulus. For the purpose of this article, we don’t need to get into the experimental methods and the calculations. We’ll directly look at the experimental results.

Weber’s proportion for common judgements:

Hearing –

In the case of hearing, most studies place the jnd around 3% in the 100 Hz range, but only 0.5% in the 2000 Hz range.

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Pitch 1/333 (⅓ of 1%) – If any sound is off pitch by ⅓ of 1%, then it will be noticed.

Loudness 1/10 (10%) to detect a change. The just noticeable difference in loudness varies from 3 dB at the threshold of hearing to 0.5 dB for the sounds that are loud.

Weights –

In the case of lifted weights, the Weber fraction or difference threshold is 1/50 (2%) to detect the smallest change between two weights.

Taste –

For taste a ⅕ (20%) change is necessary to detect a change. For example, 1 tsp of sugar will have to be added to a cup of coffee that already has 5 tsps in it to notice the change in sweetness.

Brightness –

The Weber constant for visual brightness is 1/60 (1.67%). Think about it, we adjust our phones brightness and have a preferred sweet spot. A little more or a little less can bother many of us. That is because the Weber constant is a small fraction.

Touch –

The jnd for touch or pressure on skin is 1/80 (1.25%). This shows that we can notice subtle changes in pressure on the skin.

Absolute Thresholds

The other commonly measured threshold is the absolute threshold. Absolute threshold is the smallest level of energy required by an external stimulus to be detectable by the human senses, including vision, hearing, taste, smell, and touch, 50% of the time.

SenseAbsolute Threshold
Vision A candle flame seen at 30 miles on a dark, clear night
HearingThe tick of a watch under quiet conditions at 20 feet
TasteOne teaspoon of sugar in two gallons of water
SmellOne drop of perfume diffused into the entire volume of a six room apartment
TouchWing of a fly falling on your cheek from a distance of 1 centimeter

With this data, we’ll conclude th article. There will be more in the future – How technology has helped us overcome sensory limitations; How humans are adapting and evolving (naturally and technologically). There is a lot more to know about human sensation and perception.

This was just a small slice (article) of a small slice (research findings) of a small slice of information (potential limits of knowledge) we have access to. 🙂

Further Reading

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