Sensation and perception 7th edition
- Chapter 1 - introduction
- Welcome to your world through our world
The ability to detect the pressure of your finger as you swipe a screen and turn that detection
into your own private experience is an example of sensation. Perception can be thought of as
the act of converting detected sensations into a representation that offers the ability to perform
an action, derive meaning, or understand events in a broader social context. When someone
touches you, neurons within the skin respond to the pressure and send electrical signals farther
into the nervous system. This elementary process of converting physical signals into neural
responses is called transduction.
- How many senses do we use to perceive?
Interoception is the perception of sensations that come from inside the body, and it is
necessary for basic functions as eating. Could it be considered as a sense? On the one hand;
yes, as it is produced by a neural message of mechanical pressure; on the other hand; no, as
receptors in the stomach do not transfer the
perception of sour or sweet as the taste receptors on
the tongue.
- How accurately do we perceive, and by what
method would we know?
Not everyone likes the same things, some like beans,
others do not. It is really difficult to tell what someone
perceives.
- Thresholds and their measurement by
psychophysics
The study of the senses was a mix of experimental
science and philosophy. Psychophysics emerged as
a term for the quantitative measurement of the
relationship between physical events and their
mental consequences (Fechner was a founder).
o Absolute threshold
One of the most basic psychophysical measures is
the absolute threshold: the minimum intensity of a
stimulus that can be detected.
1. The method of constant stimuli; a technique
to measure an absolute threshold; one requires creating many stimuli with different
intensities to find the intensity that can be detected some proportion of the time. Subtle
perceptual judgments such as threshold judgments are variable, as the stimulus varies
physical, the observer varies, attention varies. Therefor, one measure is almost never
enough. Overall, the intensity at which a stimulus would be detected 50% of the time
would be chosen as your threshold.
,Because of variability in the nervous system, stimuli near threshold will be detected sometimes
and missed other times -> there is no hard boundary. As a result, the function relating the
probability of detection with the stimulus level will be gradual, and we must settle for a
somewhat arbitrary definition of an absolute threshold.
2. The method of constant stimuli is simple to use, but it is an inefficient way to conduct
an experiment because much of the listener’s time is spent with stimuli that are well
above or below threshold. A more efficient approach is the method of limits; with this
method, the experimenter begins with a set of stimuli, which are presented in order of
increasing or decreasing intensity (instead of random). It usually takes more intensity to
report hearing the tone when intensity is increasing, and it takes more decreases in
intensity. An important consideration in the method of limits is to avoid stimuli that differ
by too large a step size, in which case the jump between unperceived and perceived will
be so large that we can only say “the threshold is in there somewhere”.
3. Method of adjustment: this method is similar to method of limits, except that there is
essentially continuous control over the level of the stimulus and the person being tested
is allowed to steadily increase or decrease the intensity of the stimulus.
- Difference threshold
Whereas the absolute threshold assesses our ability to tell something from nothing, the
difference threshold measures how well we can tell something from something else (introduced
by Weber). Weber’s most important findings involved judgments of lifted weights. People had to
lift one standard weight, and one comparison weight that differed from the standard. He then
increased the comparison weight in incremental amounts over the trials. He found that the
ability of a person to detect the difference between the standard and comparison weights
depended greatly on the weight of the standard. With a light standard, people were much better
at detecting a small difference when they lifted a comparison weight. The difference required for
detecting a change in weight is the just noticeable difference (JND) or the difference
threshold. Weber figured there was a constant ratio (the smallest change in weight that could
be detected was always close to 1/40 of the standard weight), also known as Weber fractions or
Weber’s law; states that the size of the JND is a constant proportion (K) of the level of the
stimulus (I). Fechner assumed that the smallest detectable change in a stimulus (delta I) could
be considered a unit of the mind because this is the smallest bit of change that is perceived;
Fechner’s law (S = k log R).
- Scaling methods
Fechner’s law underscores a very fundamental principle of perception: mind and matter are
related, but mental representations are not direct copies of physical properties. Through
magnitude estimation, observers are free to rate the experience. The relationship between
stimulus intensity and sensation is now known as: Stevens’s power law (S = aI^b), where the
sensation (S) is related to the stimulus intensity (I) by an exponent (b); a is a constant that
corrects for the units that are used.
➔ Comparing the different laws
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. In
, particular, Fechner’s law assumes that all JNDs are perceptually equivalent. In fact, this
assumption turns out sometimes to be incorrect and leads to instances where the “law”
is violated.
3. Stevens’s power law describes rating data quite well, but notice that rating data are
qualitatively different from the data that support 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.
A useful variant of the scaling method shows that different individuals can live in different
sensory worlds, even if they are exposed to the same stimuli, this method is called cross-
modality matching; an observer adjusts a stimulus of one sort to match the perceived
magnitude of a stimulus of a completely different sort.
Example: adjusting the brightness of a light to the loudness of a pitch
This does not work for taste due to the PROP molecule.
- Measuring discrimination: Signal Detection Theory
Important as it deals with the fact that thresholds are not absolute. The signal detection theory
states that the stimulus one is trying to detect (the signal) is always being detected in the
presence of ‘noise’. Near one’s threshold, it will be hard to tell a real stimulus from a random
surge of noise. Signal detection theory exists to help us understand what’s going on when we
make decisions under conditions of uncertainty. The problem is that there is no way of knowing
at any given moment whether you’re hearing noise alone or signal plus noise. The best thing to
do is to decide on a criterion level of response. Thus, in signal detection theory, a criterion is a
value that is somehow determined
by the observer. A response above
the criterion will be taken as
evidence that a signal is present, a
response below that level will be
treated as noise. There are four
possible outcomes: correct
rejection; hit; false alarm; or miss.
By knowing the relationship of hits to
false alarms, you can calculate a sensitivity measure known as d’.
The table shows a receiver operating characteristic (ROC) curve.
- Measuring the time course of perception
When a stimulus is strongly present, its presentation can be short
(Bloch’s law). The processing of a stimulus continues after the
physical stimulus ends. Therefor we cannot use the stimulus duration
as the measure of time. The time course measure would be time from
the start of the stimulus to the start of the mask (stimulus onset
asynchrony – SOA). How long it takes to respond to a stimulus is
called the simple reaction time. This can vary with the sensory
system; detecting a thermal stimulus relies on a slow physical
process of heat flow between the skin and the surface being touched.
, - Sensory neuroscience
Physiologist Müller formulated the doctrine of specific nerve energies as an answer, where the
central idea is that since we are only aware of the activity in our nerves, what is most important
must be which nerves are stimulated and not how they are stimulated. It is not the stimulus per
se that elicits a response (vision) but the nerves that are stimulated (the optic nerve). The paths
from the outside world to the brain are cranial nerves, that pass through the skull in small
openings.
3 pairs of nerves are exclusively dedicated to sensory information:
1. Olfactory
2. Optic
3. Vestibulocochlear: serves 2 sensory modalities; the vestibular sensations that support
our sense of
equilibrium and
hearing
3 pairs are dedicated to
muscles that move the
eyes:
1. Oculomotor
2. Trochlear
3. Abducens
Taste input comes from 3
different cranial nerves:
1. Facial
2. Glossopharyngeal
3. Vagus
There are warmth and cold
nerve fibers that respond to
increases and decreases in
temperature on the skin.
Some areas of the
brainstem and cerebral
cortex are also dedicated to particular tasks. There are the primary sensory areas that process
the first stimulation, further and more complex sensory processing involves different cortical
areas -> secondary sensory areas. As processing extends beyond primary areas, cortex often
becomes polysensory; information from more than one sense is being combined in some
manner.
- Neural connections
Neurons that are stimulated will produce an action potential that will propagate up the neuron’s
axon. The action potential reaches the axon terminals, where the neuron makes contact with
- Chapter 1 - introduction
- Welcome to your world through our world
The ability to detect the pressure of your finger as you swipe a screen and turn that detection
into your own private experience is an example of sensation. Perception can be thought of as
the act of converting detected sensations into a representation that offers the ability to perform
an action, derive meaning, or understand events in a broader social context. When someone
touches you, neurons within the skin respond to the pressure and send electrical signals farther
into the nervous system. This elementary process of converting physical signals into neural
responses is called transduction.
- How many senses do we use to perceive?
Interoception is the perception of sensations that come from inside the body, and it is
necessary for basic functions as eating. Could it be considered as a sense? On the one hand;
yes, as it is produced by a neural message of mechanical pressure; on the other hand; no, as
receptors in the stomach do not transfer the
perception of sour or sweet as the taste receptors on
the tongue.
- How accurately do we perceive, and by what
method would we know?
Not everyone likes the same things, some like beans,
others do not. It is really difficult to tell what someone
perceives.
- Thresholds and their measurement by
psychophysics
The study of the senses was a mix of experimental
science and philosophy. Psychophysics emerged as
a term for the quantitative measurement of the
relationship between physical events and their
mental consequences (Fechner was a founder).
o Absolute threshold
One of the most basic psychophysical measures is
the absolute threshold: the minimum intensity of a
stimulus that can be detected.
1. The method of constant stimuli; a technique
to measure an absolute threshold; one requires creating many stimuli with different
intensities to find the intensity that can be detected some proportion of the time. Subtle
perceptual judgments such as threshold judgments are variable, as the stimulus varies
physical, the observer varies, attention varies. Therefor, one measure is almost never
enough. Overall, the intensity at which a stimulus would be detected 50% of the time
would be chosen as your threshold.
,Because of variability in the nervous system, stimuli near threshold will be detected sometimes
and missed other times -> there is no hard boundary. As a result, the function relating the
probability of detection with the stimulus level will be gradual, and we must settle for a
somewhat arbitrary definition of an absolute threshold.
2. The method of constant stimuli is simple to use, but it is an inefficient way to conduct
an experiment because much of the listener’s time is spent with stimuli that are well
above or below threshold. A more efficient approach is the method of limits; with this
method, the experimenter begins with a set of stimuli, which are presented in order of
increasing or decreasing intensity (instead of random). It usually takes more intensity to
report hearing the tone when intensity is increasing, and it takes more decreases in
intensity. An important consideration in the method of limits is to avoid stimuli that differ
by too large a step size, in which case the jump between unperceived and perceived will
be so large that we can only say “the threshold is in there somewhere”.
3. Method of adjustment: this method is similar to method of limits, except that there is
essentially continuous control over the level of the stimulus and the person being tested
is allowed to steadily increase or decrease the intensity of the stimulus.
- Difference threshold
Whereas the absolute threshold assesses our ability to tell something from nothing, the
difference threshold measures how well we can tell something from something else (introduced
by Weber). Weber’s most important findings involved judgments of lifted weights. People had to
lift one standard weight, and one comparison weight that differed from the standard. He then
increased the comparison weight in incremental amounts over the trials. He found that the
ability of a person to detect the difference between the standard and comparison weights
depended greatly on the weight of the standard. With a light standard, people were much better
at detecting a small difference when they lifted a comparison weight. The difference required for
detecting a change in weight is the just noticeable difference (JND) or the difference
threshold. Weber figured there was a constant ratio (the smallest change in weight that could
be detected was always close to 1/40 of the standard weight), also known as Weber fractions or
Weber’s law; states that the size of the JND is a constant proportion (K) of the level of the
stimulus (I). Fechner assumed that the smallest detectable change in a stimulus (delta I) could
be considered a unit of the mind because this is the smallest bit of change that is perceived;
Fechner’s law (S = k log R).
- Scaling methods
Fechner’s law underscores a very fundamental principle of perception: mind and matter are
related, but mental representations are not direct copies of physical properties. Through
magnitude estimation, observers are free to rate the experience. The relationship between
stimulus intensity and sensation is now known as: Stevens’s power law (S = aI^b), where the
sensation (S) is related to the stimulus intensity (I) by an exponent (b); a is a constant that
corrects for the units that are used.
➔ Comparing the different laws
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. In
, particular, Fechner’s law assumes that all JNDs are perceptually equivalent. In fact, this
assumption turns out sometimes to be incorrect and leads to instances where the “law”
is violated.
3. Stevens’s power law describes rating data quite well, but notice that rating data are
qualitatively different from the data that support 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.
A useful variant of the scaling method shows that different individuals can live in different
sensory worlds, even if they are exposed to the same stimuli, this method is called cross-
modality matching; an observer adjusts a stimulus of one sort to match the perceived
magnitude of a stimulus of a completely different sort.
Example: adjusting the brightness of a light to the loudness of a pitch
This does not work for taste due to the PROP molecule.
- Measuring discrimination: Signal Detection Theory
Important as it deals with the fact that thresholds are not absolute. The signal detection theory
states that the stimulus one is trying to detect (the signal) is always being detected in the
presence of ‘noise’. Near one’s threshold, it will be hard to tell a real stimulus from a random
surge of noise. Signal detection theory exists to help us understand what’s going on when we
make decisions under conditions of uncertainty. The problem is that there is no way of knowing
at any given moment whether you’re hearing noise alone or signal plus noise. The best thing to
do is to decide on a criterion level of response. Thus, in signal detection theory, a criterion is a
value that is somehow determined
by the observer. A response above
the criterion will be taken as
evidence that a signal is present, a
response below that level will be
treated as noise. There are four
possible outcomes: correct
rejection; hit; false alarm; or miss.
By knowing the relationship of hits to
false alarms, you can calculate a sensitivity measure known as d’.
The table shows a receiver operating characteristic (ROC) curve.
- Measuring the time course of perception
When a stimulus is strongly present, its presentation can be short
(Bloch’s law). The processing of a stimulus continues after the
physical stimulus ends. Therefor we cannot use the stimulus duration
as the measure of time. The time course measure would be time from
the start of the stimulus to the start of the mask (stimulus onset
asynchrony – SOA). How long it takes to respond to a stimulus is
called the simple reaction time. This can vary with the sensory
system; detecting a thermal stimulus relies on a slow physical
process of heat flow between the skin and the surface being touched.
, - Sensory neuroscience
Physiologist Müller formulated the doctrine of specific nerve energies as an answer, where the
central idea is that since we are only aware of the activity in our nerves, what is most important
must be which nerves are stimulated and not how they are stimulated. It is not the stimulus per
se that elicits a response (vision) but the nerves that are stimulated (the optic nerve). The paths
from the outside world to the brain are cranial nerves, that pass through the skull in small
openings.
3 pairs of nerves are exclusively dedicated to sensory information:
1. Olfactory
2. Optic
3. Vestibulocochlear: serves 2 sensory modalities; the vestibular sensations that support
our sense of
equilibrium and
hearing
3 pairs are dedicated to
muscles that move the
eyes:
1. Oculomotor
2. Trochlear
3. Abducens
Taste input comes from 3
different cranial nerves:
1. Facial
2. Glossopharyngeal
3. Vagus
There are warmth and cold
nerve fibers that respond to
increases and decreases in
temperature on the skin.
Some areas of the
brainstem and cerebral
cortex are also dedicated to particular tasks. There are the primary sensory areas that process
the first stimulation, further and more complex sensory processing involves different cortical
areas -> secondary sensory areas. As processing extends beyond primary areas, cortex often
becomes polysensory; information from more than one sense is being combined in some
manner.
- Neural connections
Neurons that are stimulated will produce an action potential that will propagate up the neuron’s
axon. The action potential reaches the axon terminals, where the neuron makes contact with
