Lecture 1 Readings – Chapter 1 Introduction
Sensation and Perception:
The ability to detect pressure of a finger or to turn that detection into a private experience is
an example of sensation the ability to detect a stimulus and to turn it into an experience.
Perception can be the act of giving meaning to those detected sensations – how do you
understand the finger that runs down your back?
Everything we feel, think and do depends on sensations and perceptions. To study this it
needs scientific methods:
o Scaling – Measuring Private Experience
o Signal Detection Theory – Measuring Difficult Decision
o Sensory Neuroscience
o Neuroimaging – An image of the Mind
Thresholds and the Dawn of Psychophysics
Gustav Fechner is considered the founder of experimental psychology. Became absorbed
with the relationship between mind and matter. Which left him in the middle of the debate
between dualism and materialism.
o Dualism – the idea that the mind has an existence separate from the material world of
o Materialism – the idea that the only thing that exists is matter, and that all things,
including the mind and consciousness, are the results of interaction between bits of
Fechner proposed to effectively split the difference by imagining that the mind, or
consciousness, is present in all of nature
o This panpsychism – idea that the mind exists as a property of all matter – extended
to animals and inanimate objects.
He thought it should be possible to describe the relationship with the body and mindusing
math and his goal was to formally describe the relationship between sensation (mind) and the
energy (matter) that gave rise to that sensation.
o Called both his methods and his theory psychophysics
He was inspired by Ernst Weber who tested the accuracy of our sense of touch using a
device much like the compass in geometry, and measured the smallest distance between two
points that was required for a person to feel two points instead of one (twopoint touch
Weber asked people to lift one standard weight and one comparison weight that differed from
the standard in incremental amounts.
o Found that the ability of a subject to detect the difference between the standard and
comparison weights depended greatly on the weight of the standard.
o He called the difference required for detecting a change in weight the just noticeable
difference (JND) or difference threshold.
He noticed JNDs change in a systematic way – the smallest change in weight that could be
detected was always close to 1/14 of the standard weight.
o He measured other kinds of stimuli like judging the lengths of two lines, and found
for every measure a constant ratio between the change and what was being changed
could describe the threshold of detectable change quite well.
o The ratio rule holds true except when intensities, size and so on are very small or
very large, nearing the min and max of our senses.
o Fechner called the ratios Weber fractions and made a formula called Weber’s law. They used Weber’s law to create Fechner’s law which is a principle describing the
relationship between stimulus and resulting sensation that says the magnitude of subjective
sensation increases proportionally to the logarithm of the stimulus energy
o It describes the fact that our psychological experience of intensity of light, sound,
smell, taste, or touch increases less quickly than the actual physical stimulus
o This equation provided a mathematical expression that formally demonstrated a
relationship between psyche and physics.
o Fechner invented new ways to measure what people see, hear, and feel.
o Absolute threshold is the minimum amount of stimulation necessary for a person to
detect a stimulus 50% of the time.
How can we measure an absolute threshold in a valid and reliable manner?
o Method of constant stimuli – requires creating many stimuli with different
intensities in order to find the tiniest intensity that can be detected. Participants
respond to each presentation: “yes/no”, “same/different” etc
Intensities would be relatively low, not too far above or below the intensity
where your threshold would be
Multiple times is important because one measure is never enough. You want
to see the pattern of results.
The intensity at which a stimulus would be detected 50% of the time is
chosen as the threshold
• Turns out that no such hard boundary exists because of variability in
the nervous system, stimuli near threshold will be detected
sometimes and missed other times.
o An efficient approach is the method of limits – the experimenter begins with the
same set of stimuli, and instead of a random presentation, tones are presented in
order of increasing or decreasing intensity.
Data from an experiment such as listening to tones, show that there is some
“overshoot” in judgements.
They take the average of these crossover points – when they report hearing it
to not and vice versa – to be the threshold.
o Another measure of threshold is the method of adjustment – like the method of
limits, but the subject is the one who steadily increases or decreases the intensity of
Everyday activities like adjusting the volume of the radio.
It is hard to get people to reliably adjust intensity to the same value across
people and time.
Scaling Methods and Supertasters
Methods of estimation – a psychophysical method in which the participant assigns values
according to perceived magnitudes of the stimuli. Works well when observers are free to
choose their own range of numbers
o Ex: could give observers a series of sugar solutions are ask them to assign numbers
to each sample.
o S.S Stevens invented methods of estimations and measured functions for many
o Even though observers are asked to assign numbers to private experiences, the results
are orderly and lawful, but they are not the same for every type of sensation o The relationship between stimulus intensity and sensation is described by what is
now known as Steven’s power law.
Crossmodality matching – an observer adjusts a stimulus of one sort to match the perceived
magnitude of a stimulus of a completely different sort. Ex: ask a listener to adjust the
brightness of a light until it matches the loudness of a particular tone.
Signal Detection Theory:
Holds that the stimulus you’re trying to detect (“the signal”) us always being detected in the
presence of “noise.
The example with the shower. The noise the water is making is the “noise” and the phone that
rings is the “signal” The task is to detect the signal with the noise.
o Criterion in signal detection theory is an internal threshold that is set by the
observer. If the internal response is above criterion, the observer gives one response
(yes I hear that) and vice versa.
Determined by the observer. Above criterion the signal is present.
Four possible outcomes: false alarm, hit, miss, and correct rejection.
If distributions are on top of each other, you can’t tell noise alone from the
signal and noise. False alarm and hit are just as likely to happen.
By knowing the relationship of hits to false alarm, you can calculate a
sensitivity (dprime) which would be about zero in figure 1.12a
Can’t make yourself more sensitive, all you can do is move the criterion to
the left, you won’t miss