Chapter 1 - Introduction
Welcome to Our World
Sensation and Perception
• Only your own sensory experience is directly accessible to you.
• The ability to detect the pressure of a finger and, perhaps, turn that detection into a private experience is
an example of sensation.
• Perception can be thought of as the act of giving meaning to those detected sensations.
• Everything we feel, think and do depends on sensations and perceptions.
• We approach study of sensation and perception as a scientific pursuit, therefore it needs scientific meth-
ods that will be discussed in the this chapter.
Method 1: Thresholds
Method 2: Scaling - Measuring Private Experiences
• When you say that “hear” or “taste” something, are those experiences = qualia (singular quale) the
same experiences of the person you’re talking to?
• No answer, however we can demonstrate that different people do, in some cases inhabit different
Method 3: Signal Detection Theory - Measuring Difficult Decisions
Method 4: Sensory Neuroscience
• Perception of the world depends on the activity of our sensory nerves at least as much as it depends on
the world itself.
Method 5: Neuroimaging - An Image of the Mind
• Binocular rivalry: two images would vie to dominate your perception. You would not see the two
images together. -> Represents a dissociation of the stimuli presented to eyes, and your private per-
• Brain imaging techniques help see the experience in the brain.
Thresholds and the Dawn of Psychophysics
• Study of senses was always a mix of experimental science and philosophy.
• Gustav Fechner: sometimes considered to be the true founder of experimental psychology, even that ti-
tle is usually given to Wilhelm Wundt, who began his work sometime later.
• Philosophical debate between dualism and materialism.
• Dualists hold that the mind has an existence separate from the material world of the body.
• Materialists hold that that the mind is not separate. 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 mat-
• Mind is what the brain does.
• Fechner proposed to effectively split the difference by imagining that the mind, or consciousness, is
present in all of nature = panpsychism= mind exists as a property of all matter = have a consciousness,
extended not only to animals, but to inanimate things as well. • Fechner’s goal was to formally describe the relationship between sensation (mind) and the energy (mat-
ter/body) that gave rise to sensation.
• He called both his methods and his theory psychophysics. (psycho- mind, physics - matter)
• Weber tested the accuracy of our sense of touch using a device much like the compass one might use
when learning geometry. He used this device to measure the smallest distance between two points that
was required for a person to feel two points instead of one. Fechner called it the two-point touch
• Weber 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.
• When the standard was relatively light, people were much better at the detecting the small differ-
ence when they lifted the comparison weight.
• When the standard was heavier, people needed a bigger difference before they could detect the
• He called the difference required for detecting a change in the wight the just noticeable difference
• Another term for JND, the smallest change in a stimulus that can be detected is the the difference
• Weber noticed that JNDs change in a systematic way. The smallest change in weight that could be de-
tected was always close to one-fortieth of the standard weight. Thus a 1-gram change could be detected
when the standard weighed 40 grams. (10-gram for a 400 standard weight).
• For judging the length of 2 lines for which the ratio was 1:100.
• For virtually every measure (time, brightness. pitch), a constant ratio between the change and and what
was being changed could describe the threshold of detectable change quite well.
• This ratio holds true except when intensities, size and so on are very small, or very large nearing the
minimum and maximum of our senses = Weber fractions.
• Fechner also gave Weber’s observations a mathematical formula. Weber’s law: the size of the de-
tectable difference (triangle I) is a constant proportion (K) of the level of the stimulus (I).
• Fechner found what he was looking for: a way to describe the relationship between mind and matter in
Weber’s observations. He assumed that the smallest detectable change in a stimulus (triangle I) could be
considered a unit of the mind because this is the smallest bit of change that it perceived.
• He extended Weber’s law to Fechner’s Law:
S = k log R
• S is the psychological sensation, which is equal to the logarithm of the physical stimulus level (log
R) multiplied by a constant (k).
• This equation describes the fact that our psychological experience of the intensity of light, sound,
smell, taste, or touch increases less quickly than the actual physical stimulus increases. -> A rela-
tionship between psyche and physics.
• We typically make a distinction between units of physical entities (light, sound) and measures of peo-
• An absolute threshold is the minimum intensity of a stimulus that can be detected. (What is the faintest
sound you can hear for example). Psychophysical Methods
• How can we measure absolute threshold in a valid and reliable manner?
• One method is known as the method of constant stimuli. The methods requires creating many
stimuli with different intensities in order to find the tiniest intensity that can be detected. (many
stimuli ranging from rarely to almost always perceivable are presented one at a time, and partici-
pants are asked to respond to each with a “yes/no”, “same/different” etc.
• In general the intensity at which a stimulus would bet detected 50% of the time is chosen as the thresh-
• Because of variability in the nervous system, stimuli near threshold will be detected sometimes, and
missed other times.
• As a result, the function relating the probability of detection with the stimulus level will be more gradu-
al and we must settle for a somewhat arbitrary definition of an absolute threshold.
• Another more efficient approach is the method of limits. The particular dimension of a stimulus, or the
difference between two stimuli is varied incrementally until the participant responds differently.
• We take the average of these crossover points - when listeners shift from reporting hearing the tone
to not hearing the tone, and vice versa - to be the threshold.
• Method of adjustment, is just the like the method of limits except the subject is the one who steadily
increases or decreases the intensity of the stimulus.
• Easiest to understand, however it is hard to get people to reliably adjust intensity to the same value
across people and time.
Scaling Methods and Supertasters
• Moving beyond, absolute and difference thresholds, suppose we wanted to know about the magnitude of
• Magnitude estimation: participants assign values according to perceived magnitudes of the stimuli.
• Works well when observers are free to choose their own range of numbers.
• We might also present a stimuli at an intermediate level, assign it a specific value (example 10) and
ask participants to scale estimated magnitudes sensibly above or below the standard of 10.
• S.S. Stevens invented the magnitude estimation, results of participants assigning to numbers to private
experiences were orderly and lawful, however not same for every sensation
• The relationship between stimulus intensity and sensation is described by what is now known as
Steven’s power law:
S = aIb
• States that the sensation (S) is related to the stimulus intensity (I) by an exponent (b).
• So for example, experienced sensation might rise with intensity squared (I x I). If the exponent is
less than 1, this means that the sensation grows less rapidly than the stimulus. This is what Fech-
ner’s and Weber’s law would predict. -> However this relationship is true over only a moderate
range of sizes. Ex: electric shock = I3.5 so a 4-fold increase in electrical current is experienced as a
128-fold increase in pain!
• Comparisons between the three laws: • Weber’s Law involves a clear objective measurement. we know how much we varied the stimulus,
and either the observers can tell that the stimulus changed or they cannot.
• Fechner’s Law begins with the same sort of objective measurements as Weber’s, but the law is actu-
ally a calculation based on the assumptions about how sensation works. In particular, this law as-
sumes that all JNDs are perceptually equivalent, which is not correct.
• Steven’s power law describes rating data quite well, but notice that rating data are qualitatively dif-
ferent from the data that supported Weber’s Law. We can record the subject’s ratings and we can
check whether those ratings are reasonable and consistent, but there is no way to know whether they
are objectively right or wrong.
• A useful variant of the scaling method can show us that different individuals can live in different senso-
ry worlds, even if they are exposed to the same stimuli.
• The method is cross-modality matching. An observer adjusts a stimulus of one sort to match the per-
ceived magnitude of a stimulus of a completely different sort.
• Ex: ask to listener to adjust brightness of a light until it matches the loudness of a particular tone.
• Produces same pattern of matches across individuals, however still does not give us insight into
someone’s private experiences.
• However this is not the case with the sense of taste. There is a molecule called propylthiouracil (PROP)
that some people experience as very bitter while others experience it as tasteless. -> Genetic basis for
• Using the cross-modality matching, we do not find the agreement that was found when matching
light and sound.
• Some people - nontasters- match the taste of PROP to very weak sensations like the sound of a
watch or a whisper.
• A group of supertasters assert that the bitterness is similar in intensity to the brightness of sun or
the most intense pain ever experienced.
• Medium tasters match PROP to weaker stimuli such as the smell of frying bacon, or a mild
• This example shows that scaling methods can be used to quantify what appear to be real differences in
individual’s taste experiences.
Signal Detection Theory
• Returning to thresholds, and to the fact that they are not absolute. An important way to think about this
is the signal detection theory (psychophysical theory) which holds that the stimulus you are trying to
detect, is always being detected in the presence of “noise” = internal noise = the static in