Chapter 1 – Introduction
The ability to detect the pressure of a finger and, perhaps, to turn that detection into a private
experience is an example of sensation.
Perception can be thought of as the act of giving meaning and / or purpose to those detected
sensations.
Methods used to study the senses:
1. Thresholds
a. How sensitive are your senses? What is the loudest you can hear, what is the limit,
the threshold before your ears hurt?
2. Scaling – measuring private experience
a. We have no direct way to experience someone else’s experiences. Some people do
inhabit different sensory worlds.
b. Qualia = in reference to philosophy, private conscious experiences of sensation and
perception.
3. Signal Detection Theory – Measuring difficult decisions
a. Decisions can be measured scientifically by this method.
4. Sensory neuroscience
a. Example: How does a pepper fool your nervous system into thinking that your
tongue is on fire? It studies the ways in which sensory receptors and nerves
undergird our perceptual experience.
5. Neuroimaging – An image of the mind
a. Binocular rivalry. It represents a dissociation of the stimuli, presented to the eyes,
and your private perceptual experience. Modern brain-imaging techniques enable us
to see traces of that experience as it takes place in the brain.
Thresholds and the Dawn of Psychophysics
Dualism = The idea that the mind has an existence separate from the material world of the body.
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 matter.
Panpsychism = The idea that the mind exists as a property of all matter – that is, that all matter has
consciousness. This extended not only to animals, but also to inanimate things.
Fechner thought it should be possible to describe the relation between mind and body using
mathematics. His goal was to formally describe the relationship between sensation (mind) and the
energy (matter) that gave rise to that sensation.
→ Psychophysics = The science of defining quantitative relationships between physical and
psychological (subjective) events.
The two-point touch thresholds = The minimum distance at which two stimuli (e.g. two simultaneous
touches) are just perceptible as separate.
Weber called the difference required for detecting a change in a stimulus the just noticeable
difference (JND) or the difference threshold.
This ratio rule holds true except when intensities, size, and so on are very small or very large, nearing
the minimum and maximum of our senses.
The Weber fraction = The constant of proportionality in Weber’s law.
,The Weber’s law = The principle describing the relationship between stimulus and resulting
sensation that says the just noticeable difference (JND) is a constant fraction of the comparison
stimulus.
Fechner assumed that the smallest detectable change in a stimulus could be considered a unit of the
mind, because this is the smallest bit of change that is perceived.
→ The Fechner’s law = 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 intensity.
An absolute threshold = The minimum amount of stimulation necessary for a person to detect a
stimulus 50% of the time.
The method of constant stimuli = A psychophysical method in which many stimuli, ranging from
rarely to almost always perceivable (or rarely to almost always perceivable different from a reference
stimulus), are presented one at a time. Participants respond to each presentation: ‘’yes/no’’,
‘’same/different’’, and so on.
→ It requires creating many stimuli with different intensities in order to find the tiniest intensity that
can be detected.
The multiple times piece is important, because subtle perceptual judgements (threshold judgements)
are variable:
- The stimulus varies for physical reasons.
- The observer varies.
- Attention waivers and sensory systems fluctuate for all sorts of reasons.
A more efficient approach is the method of limits. The experimenter begins with the same set of
stimuli, in this case, tones that vary in intensity. Instead of random presentations, tones are
represented in order of increasing or decreasing intensity. It usually takes more intensity to report
hearing the tone when intensity is increasing, and it takes more decreases in intensity before a
listener reports that the tone cannot be heard. 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.
The method of adjustment = it is just like the method of limits, except the person being tested is the
one who steadily increases or decreases the intensity of the stimulus. It is not usually used to
measure thresholds.
Scaling Methods
Magnitude estimation = A psychophysical method in which the participants assigns values according
to perceived magnitudes of the stimuli.
,Steven’s power law = A principle describing the relationship between stimulus and resulting
sensation that says the magnitude of subjective sensation is proportional to the stimulus magnitude
raised to an exponent.
The three laws that have been presented:
1. 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.
2. Fechner’s law begins with the same sort of objective measurements as Weber’s, but the law
is actually a calculation based on some assumptions about how sensation works. It assumes
that all JNDs are perceptually equivalent. In fact, this assumption turns out to be incorrect
and leads to some places where the ‘’law’’ is violated (electric shock example).
3. Steven’s power law described rating data quite well, but notice that rating data are
qualitatively different from the data that supported Weber’s law. We can record the
observer’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.
Cross-modality matching = The ability to match the intensities of sensations that come from
different sensory modalities. This ability allows insight into sensory differences. For example, a
listener might adjust the brightness of a light until it matches the loudness of a tone.
→ This method of scaling can show us that different individuals can live in different sensory worlds,
even if they are exposed to the same stimuli.
Signal Detection Theory
→ = A psychophysical theory that quantifies the response of an observer to the presentation of a
signal in the presence of noise. Measures obtained from a series of presentations are sensitivity (d’)
and criterion of the observer.
There is internal noise (the static of your nervous system) and external noise. Sometimes neither
internal nor external noise is much of a problem.
Signal detection theory exists to help us understand what’s going on when we make decisions under
conditions of uncertainty.
There is noise and there is a signal.
Two distributions of responses in your nervous system can be:
1. Noise-alone distribution
2. Signal-plus-noise distribution
You need a criterion level of response.
Criterion = In reference to SDT, an internal threshold that is set by the observer. If the internal
response is above criterion, the observer gives one response. Below criterion, the observer gives
another response.
The decision you make is automatically.
Four possible outcomes:
1. A correct rejection
2. A hit
3. A false alarm
4. A miss
Sensitivity = In reference to SDT, a value that defines the ease with which an observer can tell the
difference between the presence and absence of a stimulus or the difference between Stimulus 1
, and 2. Measured as d’ (d-prime). It is illustrated by the separation between the distributions of your
response to noise alone, and to signal plus noise.
For a fixed d’, all you can do is change the pattern of your errors by shifting the response criterion. If
you don’t want to miss any signals, you move your criterion to the left, but then you have more false
alarms. If you don’t like false alarms, you move the response criterion to the right, but then you
make more misses.
Changing the criterion changes the hits and false alarms in predictable ways. If you plot false alarms
on the x-axis of a graph against hits on the y-axis for different criterion values, you get a curve known
as a receiver operating characteristic (ROC) curve.
→ = In reference to studies of signal detection, the graphical plot of the hit rate as a function of the
false-alarm rate. If these are the same, point fall on the diagonal, indicating that the observer cannot
tell the difference between the presence and absence of the signal. As the observer’s sensitivity
increases, the curve bows upward toward the upper left corner.
Fourier analysis
Simplest kind of sounds is a sine wave (a pure tone). The time taken for one complete cycle of a sine
wave, or for a wavelength to pass a point, is the period of the sine wave. The height of the wave is its
amplitude. The phase of the wave is its position relative to a fixed marker.
Sine wave = A simple, smoothly changing oscillation that repeats across space. Higher-
frequency sine waves have more oscillations, and lower frequencies have fewer oscillations, over a
given distance. 1. In reference to hearing, a waveform for which variation as a function of time is a
sine function. Also called pure tone. 2. In reference to vision, a pattern for which variation in a
property like brightness or color as a function of space is a sine function.
Sine waves are not common, everyday sounds, because few vibrations in the world are so
pure. But, it turns out that all sounds, no matter how complex, can be described as a combination of
sine waves.
The Fourier analysis = A mathematical procedure by which any signal can be separated into
component sine waves at different frequencies. Combining these sine waves will reproduce the
original signal.
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