To give you some idea of what subjects experience
when participating in a psychophysical threshold
experiment.
to gain experience in psychophysical methods and
calculation of threshold under the various methods.
To begin to understand how it is possible to learn
about the operation of human sensory systems.
Theory Of Psychophysical Methods
General Goal and Philosophical Underpinning
Originated by Gustav Fechner - founder, note
German, immense capacity for boredom.
Goal - determine lawful relationship(s)
between dimensions of the physical world and
resulting aspect of sensation and/or perception.
In other words, how does changing something
in a stimulus change what we experience in our
perception.
What do each of these different types of
curves mean about how experiences is related
to perception?
Practical Aspect of the Approach - simplify
the task and judgment of the participant,
control to within an inch of it's life the
physical properties of the stimulus.
That is, know exactly what the stimulus is
that you are presenting to the
participant. Have their response be
something very simple so you know what it
means, such as: I see something, or no, I
don't see something.
Measures (Threshold) or what
are we trying to measure about sensation.
There are many possible measures. Threshold
is only one. Others will be discussed later.
What is a threshold? (Limen is Latin for
threshold) - use room analogy.
Absolute Threshold - Definition: the
lowest sensory level that can be detected
reliably.
Difference Threshold (differential) - Definition:
minimum stimulus change or difference from a
given level that is reliably detectable. also
called Just Noticeable Difference or JND.
Classical Psychophysical Methods
Method of Limits
A set of stimulus intensities are chosen by
the experimeter, say 7. It is important
that the lowest intensity is below the
participants threshold and the highest is above
the participants threshold.
The stimuli are presented from lowest to
highest intensity or reverse. I usually
like to start from the highest intensity first
since it gives participants some idea of the
stimulus they are trying to detect. These
ordered presentations, either from lowest to
highest or reversed, are called
staircases.
After each stimulus, the participant is to
report whether they heard the stimulus or
not. If starting with the greatest
intensity, the participant should hear the
stimulus.
The staircase continues until the participant
no longer responds (there are variations on this
rule). This point where the response
changes is recorded. It is sometimes
called a turnaround.
Then the next staircase is begun. It is
usually a staircase going in the opposite
direction.
The threshold is calculated as average all of
the turnarounds.
Method of Constant Stimuli
This is similar in many ways to the Method of
Limits
A set of stimuli are developed going from
below the threshold to above the
threshold. Again, say 7 stimuli are
chosen.
Then the stimuli are presented in a random
order.
Again, the participants response is whether
the stimulus is detected or not.
Threshold is the stimulus intensity that was
detected 50% of the time.
It is often the case that no stimulus is
detected exactly 50% of the time. Then a
procedure known as linear interpolation is used
to estimate this stimulus value.
Method of Adjustment
Usually a range of stimulus intensities, or
values, are available, often continuous or near
continuous.
The stimulus intensity is controlled by the
participant (either directly or via
experimenter).
The participants task is to move the stimulus
to their threshold. The participant is
often instructed to the stimulus until just can
or just cannot detect the stimulus in an
absolute threshold experiment.
The starting point is chosen randomly and the
procedure is repeated several times.
The threshold is the average of the final
settings.
Limitations of Classical Methods
Can you think of Any?
consider ways that the data may be not
accurate or
ways that the subjects behavior might lead to
an incorrect measure of threshold?