Sensation and perception
Chapter 1 – introduction 9-2-
2015
Sensation: the ability to detect a stimulus, and perhaps, to turn that detection
into a private experience.
Perception: the act of giving meaning to a detected sensation.
If our mental life relies on information from our senses, then it follows that the
place for the study of the senses is within the science of human behavior and
human mental life – that is, in psychology.
Method 1: Thresholds:
Method 2 Scaling Methods: Measuring private experience:
Qualia: In philosophy, private conscious experiences of sensation or
perception.
Method 3: Signal detection theory – measuring difficult decisions:
Method 4: Sensory Neuroscience:
Method 5: Neuroimaging – An image of the Mind:
Gustave Fechner (1801-1887): is thought by some to be the true founder of
experimental psychology (Wilhelm Wundt (1832-1920)) Gustave mostly know
from pioneer work relating changes in the physical world to changes in our
psychological experiences.
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 exist is matter, and that all things,
including the mind and consciousness, are the results of interaction between bits
of matter.
Panpsychism: idea that mind exists as a property of all matter – that is, all
matter has consciousness.
Fechner: goal to describe the relationship between sensation (mind) and the
energy (matter) that gave rise to that sensation. He called both his methods and
theory:
Psychophysics: the science of defining quantitative relationships
between physical and psychological (subjective) events.
Ernst Weber: anatomist and physiologist who was interested in touch. He tested
the accuracy of our sense of touch by using a device much like the compass, to
measure the smallest distance between two points that was required for a person
to feel touch on two points. Later Fechner would call the distance between the
points the two-point touch threshold.
Just noticeable difference (JND) or difference threshold: the smallest
detectable difference between two stimuli, or the minimum change in a
stimulus that enables it to be correctly judged as different from a reference
stimulus.
Weber fraction: the constant of proportionality in Weber’s law
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’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 S = k log R
Where S is psychological sensation, equal to the logarithm of physical stimulus
level X constant k.
Absolute threshold: the minimum amount of stimulation necessary for a
person to detect a stimulus 50% of the time (it is not a sharp change in detection
,from never reported to always reported because of the variability in the
nervous system).
Psychophysical Methods:
Method of constant stimuli: A psychophysical method in which many stimuli,
ranging from rarely to almost always perceivably (or rarely to almost always
perceivably different from a reference stimulus), are presented one at a time.
Participants respond to each presentation: “yes/no,” “same/different,” and so on.
Method of limits: A psychophysical method in which the particular dimension of
a stimulus, or the difference between two stimuli, is varied incrementally until the
participant responds differently.
“overshoot” in judgments: 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.
Method of adjustment: A method of limits in which the subject controls the
change in the stimulus.
Scaling Methods
Magnitude estimation: A psychophysical method in which the participant
assigns values according to perceived magnitudes of the stimulus.
Stevens’ 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 S = A*Ib the
sensation (S) is related to the stimulus intensity (I) by an exponent (b).
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 sensations works. In particular, Fechner’s law 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, such as in
electric shock example.
3. Stevens’ power law describes rating data quite well, but notice that rating
data are qualitatively different from the data that supported Weber’s law.
We can record the subjects’ rating 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.
Supertaster: those individuals who experience the most intense taste
sensations; for some stimuli, they are dramatically more intense than for medium
tasters or nontasters. Supertasters also tend to experience more intense oral
burn and oral touch sensations.
Signal Detection Theory
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.
- Signal detection theory exists to help us understand what’s going on when
we make decisions under conditions of uncertainty.
,Criterion: in signal detection theory, an internal threshold that is set by the
observer. If the internal response is above the criterion, the observer gives one
response (“I heard that”). Below criterion, the observer gives another response
(“I hear nothing”).
Decision to react is made automatically, you do not make a conscious
choice.
Four outcomes: Correct rejections = Pr(N/n), Hit = Pr(S/s), Miss = Pr(N/s), False
alarm = Pr(S/n).
Sensitivity (d’): In signal detection theory, 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 stimulus 2 (in case of 0
sensitivity, the curves are completely overlapped).
Receiver operating characteristic (ROC) curve: in studies of signal
detection, the graphical plot of the hit rate (y) as a function of the false alarm (x).
If these are the same, points fall on the diagonal, indicating that the observer
cannot tell the difference between the presences and absence of the signal. As
the observer’s sensitivity increases, the curve bows upward toward the upper left
corner. That point represent a perfect ability to distinguish signal from noise
(100% hits, 0% false alarms).
Fourier Analysis
Joseph Fourier (1768-1830) showed how very complex signals could be
understood as combination of simple sine wave components.
Sine wave: A simple, smoothly oscillation that repeats across space. Higher
frequency sine waves have more oscillations and lower frequencies have fewer
oscillations over a given distance.
1. In hearing, a waveform for which variation as a function of time is a sine
function (pure tone).
2. In vision, a pattern for which variation in a property like brightness or color
as a function of space is a sine function.
The pressure in a sine wave changes continuously (sinusoidally) at one frequency.
Wavelength: the distance required for one full cycle of oscillation for a sine
wave.
Period: for hearing, the time required for a full wavelength of an acoustic sine
wave to pass by a point in space.
Phase: a fraction of a cycle of the sine wave described in degrees (0-360) or
radians (0 π to 2 π ). In hearing, phase can be used to describe fractions of a
period that relate to time.
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.
Spatial frequencies: The number of cycles of a grating per unit of visual angle
(usually specified in cycles per degree). One degree is about the size of a
thumbnail at arm’s length.
Cycles per degree: the number of pairs of dark and bright bars per degree of
visual angle.
Johannes Müller (1801-1885) formulated the doctrine of specific nerve
energies, which says that we are aware only of the activity in our nerves and we
cannot be aware of the world itself. For this reason, what is important is which
nerves are stimulated, not how they are stimulated.
,Doctrine of specific nerve energies: doctrine, formulated by Johannes Müller,
stating that the nature of a sensation depends on which sensory fibers are
stimulated, not on how fibers are stimulated.
Cranial nerves: twelve pair of nerves (one for each side of the body) that
originate in the brain stem and reach sense organs and muscles through
openings in the skull.
In order of location, beginning at the front of the skull:
Three cranial nerves: exclusively dedicated to sensory information.
1. Olfactory (I) nerves: the first pair of cranial nerves. The axons of the
olfactory sensory neurons bundle together after passing through the
cribriform plate to form the olfactory nerve, which conducts impulses from
the olfactory epithelia in the nose to the olfactory bulb.
2. Optic (II) nerves: the second pair of cranial nerves, which arise from the
retina and carry visual information to the thalamus and other parts of the
brain.
3. Vestibulocochlear (VIII): The eight pair of cranial nerves, which connect
the inner ear with the brain, transmitting impulses concerned with hearing
and spatial orientation. The vestibulocochlear nerve is composed of the
cochlear nerve branch and the vestibular nerve branch.
Three more cranial nerves: dedicated to muscles that move the eyes.
4. Oculomotor (III) nerves: the third pair of cranial nerves, which innervate
all the extrinsic muscles of the eye except the lateral rectus and the
superior oblique muscles, and which innervate the elevator muscle of the
upper eyelid, the ciliary muscle, and the sphincter muscle of the pupil.
5. Trochlear (IV) nerves: the fourth pair of cranial nerves, which innervate
the superior oblique muscles of the eyeballs.
6. Abducens (VI) nerves: the sixth pair of cranial nerves, which innervate
the lateral rectus muscle of the eyeballs.
The other six cranial nerves are:
- Exclusively motor: spinal accessory [XI], hypoglossal [XII]
- Both sensory and motor signals: trigeminal [V], facial [VII],
glossopharyngeal [IX], vagus [X].
Polysensory: referring to blending multiple sensory systems.
Herman Ludwig Ferdinand von Helmholtz (1821-1894): one of greatest
scientist of all time. He made many important discoveries in physiology and
perception. Influenced by Müller (but disliked Müllers vitalism: idea that there is
a force in life that is distinct form physical entities vitalism violated the
physical law of conservation of energy).
Helmholtz was the first to effectively measure how fast neurons transmit their
signals. Not all neurons are equal when it comes to speed (human between 165-
330 feet/sec).
Santiago Ramón y Cajal (1852-1934): created drawings of brain neurons. He
named the gap between the axon of one neuron and the dendrite of the next a
synapse (this permits information transfer).
Otto Loewi (1873-1961): demonstrated that neurons communicate with one
another by releasing chemicals called neurotransmitters. There are many
different kinds of neurotransmitters in the brain
Excitatory: some neurons increase the response of the next neuron.
Inhibitory: some neurons decrease the response of the next neuron.
Sir Alan Hodgkin (1914-1998) and Sir Andrew Huxley (1917-2012): made the
first recordings from inside a neuron. They pierce a giant squid axon with an
electrode to measure voltage, and could inject different chemicals inside. They
learned that neural firing is actually electrochemical.
, Na+ goes quickly in the axon, membrane changes in a way that pushes K+
ions out the axon.
Neuroimaging: a set of methods that generate images of the structure and/or
function of the brain. In many cases, these methods allow us to examine the
brain in living, behaving humans.
Electroencephalography (EEG): A technique that, using many electrodes on
the scalp, measures electrical activity from populations of many neurons in the
brain
Event-related potential (ERP): A measure of electrical activity from a
subpopulation of neurons in response to a particular stimuli that requires
averaging EEG recordings.
Magnetoencephalography (MEG): A technique, similar to
electroencephalography, that measures changes in magnetic activity across
populations of many neurons in the brain.
Computed tomography (CT): An imaging technology that uses X-rays to
create images of slices through volumes of material (human body) detector on
one side of the head measures the amount of energy that has been lost on the
way through the head, many positions together can create a three-dimensional
picture of the head.
Magnetic resonance imaging (MRI): An imaging technology that uses the
responses of atoms to strong magnetic fields to form images of structures like the
brain. The method can be adapted to measure activity in the brain as well (using
a powerful magnetic field that influences the ways the atoms in the brain spin, by
pulsing the field it is possible to measure a signal that can indicate the presence
of specific elements in the tissue).
Functional magnetic resonance imaging (fMRI): A variant of magnetic
resonance imaging that makes it possible to measure localized patterns of
activity in the brain. Activated neurons provoke increased blood flow, which can
be quantified by measuring changes in the response of oxygenated and
deoxygenated blood to strong magnetic fields.
Blood oxygen level-dependent (BOLD) signal: the ratio of oxygenated
to deoxygenated hemoglobin that permits the localization of brain neurons
that are most involved in a task.
Positron emission tomography (PET): An imaging technology that enables
us to define location in the brain where neurons are especially active by
measuring the metabolism of brain cells using safe radioactive isotopes.
College - chapter 1: 3-2-
2015
What is perception: a translation of the physical environment into a pattern of
neural activity that can be used by our brain to guide behavior.
- Therefore it is: a useful “representation” of our environment, limited by
the physical properties of our sensors and influence by “higher order”
cognitive processes, such as memory and (verbal) knowledge.
Perception is selective:
- Only limited physical magnitudes that are relevant.
- Only limited sensitivity range of sensors
Perception is influenced by other (cognitive) processes, it is not always “veridical”
(think of to dark shades that are the same but one looks darker because of the
environment).
(Perception bias because we know the light comes from above)
We also “know” that faces are convex.
We study perception because we want to:
