Principles of Perceptual Measurement
The first chapter in most academic textbooks forms the foundation for material to be
presented in later chapters and The Fundamentals of Sensory Perception is not
different in this respect. Much of what you learn in this chapter will be applied to specific
sensory systems in later chapters. In this chapter, you will come to appreciate the
various psychophysical methods that have been developed and how they are used to
understand our sensory perceptual systems. As such, the focus in this chapter is on
gaining an understanding of the basic relationship between our perceptual experience
and the physical stimulus that acts upon a sensory system to produce the perception.
You will learn about classical methods of psychophysics, including the methods of
adjustment, limits, and contant stimuli, highlighting the work of Fechner and Weber.
Weber’s law deals with how much additional stimulus intensity is needed for a subject to
just notice a difference in the stimulus intensity. For example, if you had 10 pennies in
each hand, how many more pennies would you need to add to your left hand before the
weight of the pennies was noticably more than the weight of the 10 pennies in your right
Modern methods of psychophysics are introduced with the work of Stanley Stevens.
In this context you learn of magnitude estimation in which a direct rating is provided by a
subject based on his or her sensory experience. This led Stevens to develop the power
law, which relates sensory experience to the intensity of a stimulus raised to a certain
power and the value of this power depends on the actual sensory input. You will also
learn that some sensory experiences can be directly linked to stimulus intensity and
these are known as prothetic sensations whereas others, known as metathetic
sensations, cannot be directly linked.
The final part of the chapter examines signal detection theory. This procedural
advancement in psychophysics allows researchers to determine the sensitivity of an
individual to a particular stimulus by examining experimental trials when the stimulus is
present for detection and when it is not present for detection, which was not done with
the early methods of psychophysics because the stimulus was always present on each
This chapter will help you 2
develop a timeline of the key researchers and their contributions to the field of
psychophysics and the study of sensory perception.
understand how researchers quantify the subjective experience of sensory
perception using difference and absolute thresholds and relate each to examples
found in the environment.
draw and use psychometric functions as a means of understanding the
relationship between psychophysical variables and to distinguish between
use examples to differentiate between the different experimental methods used
in sensory perception research and to understand the advantages and
disadvantages of each method.
apply psychophysical equations of magnitude estimation and scaling to calculate
understand that not all sensory experiences can be quantified using the same
principles or laws. You will learn to identify prothetic and metathetic sensory
experiences and learn the scaling techniques used for each.
consider the human factors that influence our perception of stimuli in our
environment and understand how these factors can affect how we respond to our
environment. You will also learn to apply this knowledge to develop strategies in
guiding your own life and behaviour.
Indicate whether the statement is true or false.
____ 1. There is one Weber fraction that applies to all sensory systems.
____ 2. A standard stimulus is known as a modulus. 3
____ 3. When using Stevens method of estimation, the subject is able to develop their
____ 4. The difference threshold is also known as the just noticeable difference.
____ 5. Like William James, Stanley Stevens believed that sensations cannot be
____ 6. Multi-dimensional scaling is only used in the study of sensation and
____ 7. In signal detection theory, the concept of noise can include things that are
internal and external to the perceiver.
____ 8. Sensation arises from perception.
Identify the choice that best completes the statement or answers the question.
1. The relation between the perceived magnitude of a stimulus and the physical
intensity of that stimulus is known as a _______________.
a) logarithmic function
b) cross-modal comparison
2. The slope of a linear function _______________.
a) decreases over its entire range
b) remains constant
c) progressively increases
d) a linear function has no slope
3. The field of psychophysics was established in 1860 by _______________.