Experimental Psychology
PSY 433
Appendix A – Experimental Psychology:
A Historical Sketch
Origins in Philosophy
 Mind-body problem – are the mind and body
the same or different?
 If they are different substances, how do they
interact or communicate?
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Dualism – mind (soul) is not governed by
physical laws but possesses free will.
Descartes – mutual interaction.
Animals do not possess souls and can be
studied because they are physical.
Rene Descartes (1596-1650)
Physiology Changed Philosophy
 Localization of cerebral function by
physiologists showed that the brain is the
organ of the mind.
 Mental states were shown to affect the body.

Trauma, mesmeric trance, mental suggestion.
 Huxley’s “Epiphenomenalism” – mental states
have no causal efficacy, like paint on a stone
(neurophysiology is the stone, mind is the
paint).
British Empiricism
 Locke, Berkeley, Hume, Hartley
 Mind may follow laws and thus be modeled
just as the physical world is.
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Elements (ideas)
Forces (associations between ideas)
 Tabula rasa – mind is a blank slate written
upon by experience.
 Mental activity may be mechanical:

Mind as a machine
Application of Scientific Method
 Philosophy uses different methods than
psychology:

Anecdote, reflection, logic
 Experimental psychology emerged out of the
study of sensation, applying laws of physics
and chemistry.

Now called psychophysics
 “Application of scientific method to the
problem of mind” created experimental psych.
Helmholtz (1821-1894)
 Used experimental methods to study vision
and audition.
 Reaction times were used to determine the
speed of neural impulses.

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Test response-times for stimuli from the
shoulder and from the ankle.
Nerve impulses are slow – 50 meters per sec.
 Reaction times vary considerably across
individuals and across trials – how is precise
measurement possible?
Weber (1795-1878)
 Weber studied perceptions of weight and
tried to relate these to actual physical weight.

Weight is an objective physical property of
objects.
 The greater the weight, the greater the
difference between it and a heavier weight
must be in order to be detectable.
 Weber’s Law -- Just-noticeable difference
(JND) is a constant across a sensory
modality.
Just Noticeable Difference (JND)
 How much must a stimulus change in order for
a person to sense the change.

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This amount is called the just noticeable
difference (JND)
The actual size of the JND aries with the size of
the weights being compared.
 JND can be expressed as a ratio:
DR
k
R
where R is stimulus magnitude and k is a
constant and DR means the change in R
(D usually means change in science)
Fechner (1801-1887)
 Tried to relate physical properties to
psychological sensations:

Related the objective to the subjective.
 Fechner’s Law – each JND corresponds to
one subjective unit of measure on a rating
scale

This relationship can be described
mathematically.
 Credited with founding psychophysics.
Fechner’s Law
 Fechner called Weber’s finding about the JND
“Weber’s Law.”
 Fechner’s formula describes how the subjective
sensation is related to increases in stimulus
size:
S  k log R
where S is sensation, k is Weber’s constant and
R is the magnitude of a stimulus
 He also used catch trials to study guessing.
Relationship of JND to Stimulus
S.S. Stevens modified
Fechner’s Log Law to a
Power Function in the
early 1950’s.
Wundt & Ebbinghaus
 Wundt (1832-1920) organized psychology
and helped to establish it as an independent
discipline.

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Wrote “Principles of Physiological Psychology”
Did not believe higher mental processes
(memory, thought, creativity) could be studied
experimentally.
 Ebbinghaus (1850-1909) demonstrated that
memory could be studied experimentally.
Stucturalism vs Functionalism
 Structuralism – focused on the contents of
mind.
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Sensations, images (ideas), affections
Used introspection to identify basic elements.
Introspection proved to be an unreliable
method.
 Functionalism – focused on the adaptive
function of psychological processes within a
context.

Not much experimental work done.
Behaviorism
 Rejected structuralism and functionalism.
 Both referred to mentalistic contents of mind
that could not be directly observed.
 Emphasized focus on relating behavior to
evoking stimuli and contexts.
 Radical behaviorists:
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Watson
Skinner
 Now nearly all experimental psychologists are
behaviorists to some extent.
Gestalt Psychology
 Reaction against structuralism.
 Whole is greater than the sum of its parts.

Complex mental phenomena cannot be
understood by examining elements.
 Wertheimer’s demonstration of shape
constancy seemed incompatible with
structuralism.
 Influential in cognitive psychology.
The Cognitive Revolution (1950present)
 Using scientific methods to study mental
processes that are linked to observable
behaviors
 The mind actively acquires information,
and stores, retrieves, and uses knowledge
 Influenced by the computer analogy and
information processing theory.
Cognitive Neuroscience
 Psychophysiology – intersection between
psychology and physiology.
 Neuroscientists team with psychologists using
imaging techniques (PET, fMRI) to study
cognitive activity.

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Such results must be interpreted with caution
Observing that activity is occurring does not
necessarily tell you what kind of activity is
happening.
Specialization
 Today psychologists tend to identify more
with areas of interest than with schools of
thought (behaviorism, gestalt).

Specialization is the mark of a maturing
science.
 Experimental psychology is one of 54
divisions in the APA (Division 3).
 Other societies: Psychonomic Society, APS,
Society for Cognitive Neuroscience, society
for Research in Child Development (SRCD).