Sensation & Perception
Chapter 1: Introduction
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.
Methods used in the study of the senses:
1. Thresholds: for example, “what is the loudest sound you can hear?”
2. Scaling, measuring private experience. When you say that you hear or taste something,
are those experiences the same as the experiences of the person you’re talking to?
3. Signal detection theory, measuring difficult decisions.
4. Sensory neuroscience
5. Neuro imaging, an image of the mind.
6. Computational models
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.
Threshold’s method
Just noticeable difference (JND) or difference threshold is 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 noticed that JNDs change in a
systematic way. This ratio rule holds true except for extreme stimulus – stimuli so small or
large that they approach the minimum or maximum of our senses.
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. In other words, as the intensity of a physical stimulus
increases, a larger change in that physical stimulus is required to produce a just detectable
difference in sensation.
An absolute threshold is the minimum intensity of a stimulus that can be detected.
How can we measure an absolute threshold in a valid and reliable manner? One method,
known as the method of constant stimuli, requires creating many stimuli with different
intensities in order to find the tiniest intensity that can be detected. You need to repeat the
measure over and over and then average the responses or otherwise describe the pattern of
results. In general, the intensity at which a stimulus would be detected 50% of the time would
be chosen as your threshold.
We might expect the threshold to be a sharp change in detection from never reported to
always reported, but this is not so. It turns out that no such hard boundary exists. As
mentioned, a somewhat arbitrary point on the curve, often 50% detection, is designated as the
threshold.
,Scaling methods
Magnitude estimation = a psychophysical method in which the participant assigns values
according to perceived magnitudes of the stimuli.
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.
Signal detection theory begins with the fact that the stimulus you’re trying to detect is always
being detected in the presence of noise. This is internal noise, the static in your nervous
system. Many neurons in the brain are firing all the time, even when nothing is happening.
When you’re trying to detect a faint sound or flash of light, you must be able to detect it in the
presence of such internal noise. Signal detection theory exists to help us understand what’s
going on when we make decisions under conditions of uncertainty.
In reference to signal detection theory, sensitivity, is a measure 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.
Johannes Muller formulated the doctrine of specific nerve energies, stating that the nature of a
sensation depends on which sensory fibres are stimulated, rather than how they are stimulated.
The central idea of this doctrine is that we cannot be directly aware of the world itself, and we
are only aware of the activity in our nerves.
Cranial nerves = twelve pairs 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.
, A typical neuron
Dendrites receive information from other neurons. Information collected by dendrites is
integrated in the axon hillock, which generates action potentials. The axon conducts action
potentials away from the cell body. Axon terminals synapse with a target cell.
They learned that neuronal firing is electrochemical. Voltage increases along the axon are
caused in the membrane of the neuron that permit positively charged sodium ions (Na+) to
rush very quickly into the axon from outside the cell. Then, very quickly, the membrane
changes again in a way that pushes positively charged potassium ions (K+) out of the axon,
restoring the neuron to its initial resting voltage.
One way to investigate what a neuron encodes is to try to identify the stimulus that
makes it fire the most vigorously.
Neuroimaging
- Electroencephalography (EEG) measures electrical activity through dozens of
electrodes placed on the scalp. EEG can be used to roughly localize whole populations
of neurons and to measure their activities with excellent temporal accuracy.
- Magnetoencephalography (MEG) also provides good measures of neuronal timing
while providing a better idea of where in the brain neurons are most active. MEG
takes advantage of the fact that very small changes in local magnetic fields accompany
the small electrical changes that take place when a neuron fires. MEG has a better
spatial resolution that EEG.
- 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. To
produce MRI images on the brain, that brain is placed in a magnetic field powerful
enough to influence the way the atoms spin. It is possible to measure a signal that
indicates the presence of specific elements in the tissue.
- With Functional magnetic resonance imaging (fMRI) we can see the activity of the
living brain. Here the critical factor is that active brain tissue is hungry brain tissue. It
needs oxygen. The result is that there is a Blood oxygen level-dependent (BOLD)
signal that can be measured. However, the temporal resolution of the method is slow
compared with EEG/ERP.
- Positron emission tomography (PET) is an imaging technique in which a small amount
of a safe, biologically active, radioactive material (a tracer) is introduced into the
participant’s bloodstream, and a specialized camera detects gamma rays emitted from
brain regions where the tracer is being used most.
