This document contains summaries of the relevant chapters of Sensation and Perception for the course 'Perception' at the RUG. Each of the chapters relevant for the course (1, 2, 3, 4, 5, 6, 9, 10, 11, 12, Appendix) are individually summarised.
Perceiving refers to specialized sensory cells converting physical features of the environment into
electrochemical signals that motivate physiological, behavioral, emotional and cognitive responses and
can give rise to awareness. Sensation refers to the initial steps in the process: converting physical
features into electrochemical signals. Perception refers to the later steps in the process, wherein the
sensory signals are used to form mental representations for further use. There is not really a sharp
dividing line between sensation and perception.
The Perceptual Process
Events that are perceived are referred to as stimuli. A distal stimulus is the thing in the world and the
physical phenomena they produce while the proximal stimulus is the physical phenomenon that acts on
sensory neurons. For example, a phone is the distal stimulus and the sound it makes that you hear is the
proximal stimulus. Specialized neurons that convert proximal stimuli into neural signals are called
sensory receptors. Speed and accuracy of perception are enhanced by the perceiver’s knowledge of the
event and expectations (top-down information), in contrast to bottom-up information contained in the
neural signals from receptors.
The main questions in exploring perception are how does the proximal stimulus carry information about
the thing that is perceived? How is the proximal stimulus transduced (transformed into neural signals)?
How do these neural signals form a neural code (neural representation)? What is the relationship
between perceptual experience and the distal stimulus? The last question is a part of psychophysics.
Senses
Traditionally there are five senses: vision, audition, touch, smell and taste. Five additional senses include
proprioception, pain perception, thermoreception, balance and body movement.
● Vision: light via photoreceptors in the eye
● Audition: sound via hair cells in the cochlea
● Tactile perception: mechanical forces through mechanoreceptors in the skin
● Proprioception: mechanical forces through nerves in muscles, tendons and joints
● Pain perception: mechanical forces, molecules and temperature through nociceptors
● Thermoreception: heat through thermoreceptors
● Balance: gravity through hair cells in the inner ear
● Body movement: acceleration through hair cells in the inner ear
● Olfaction: molecules through olfactory receptors in the nose
● Gustation: molecules through taste receptors in the mouth
Senses evolve as a result of natural selection. For example we see the spectrum of wavelengths because
they are the most abundant wavelengths emitted by the sun and are especially well-transmitted in
seawater where we evolved.
,Psychophysics
The study of the relationship between stimuli and subjective perceptual experiences. By investigating the
thresholds of perceptual experiences and the scaling of these experiences.
The absolute threshold is the minimum intensity of a physical stimulus that can be detected by an
observer. Tested by exposing subjects to repeated stimuli using the method of adjustment, method of
constant stimuli or the staircase method.
- Method of adjustment: starting at a high intensity and tuning it down until the subject can no
longer detect it.
- Method of constant stimuli: providing the subject with stimuli of known intensities at random.
The frequency of detection of each stimulus is then analyzed.
- Staircase method: if the subject indicates they can perceive the stimulus, the next stimulus will
be of a bit lower intensity. If they cannot sense this one, the intensity goes up again. And so on.
What can be seen is a psychometric function relating perceptual experience to the intensity of a
stimulus. Typically this is in the form of an S shape. The staircase method is often used to measure
hearing loss.
The difference threshold (or just noticeable difference JND) is the minimum difference between two
stimuli that allows an observer to perceive the two as different. Weber’s law states that the JND
increases as the intensity of the standard stimulus/background increases. For example the addition of
adding 100g to a 100g weight is noticed easier than adding 100g to a 10kg weight. He made an equation
of JND = kI where k is the weber fraction and varies between senses and I is the intensity of the standard
stimulus.
Psychophysical scaling is the process of measuring how changes in perceived intensity relate to changes
in the stimulus intensity.
- Fechner’s law uses Weber's law to make a new equation about how the perceived intensity of a
stimulus changes as its physical intensity changes. For low-intensity stimuli, a small increase in
intensity causes a greater increase in perceived intensity.
- Stevens’s power law involves magnitude estimation: estimating perceived intensity directly.
Basically states that for different perceptual dimensions there are different relationships
between stimulus intensity and perceived intensity. Loudness and brightness relate to Fechner’s
law, but for electric shock there is a different curve. The power law is S=cl to the power of n
where n is different for each perceptual dimension. When the exponent is less than 1 it produces
a decelerating curve à la Fechner’s law, if it is greater than 1 → accelerating curve.
Neuroscience
According to the neuron doctrine, perception depends on the combined activity of many specialized
neurons, each of which responds to specific aspects of a stimulus called trigger features.
,Neurons convey information about the physical stimulus by its firing rate. Most neurons have a baseline
firing rate. Changes in firing rates can result from a wide variety of changes in the proximal stimuli. The
firing rate is thus the neural code.
The thalamus is the most important subcortical structure involved in perception. Most neural signals
from sensory organs pass through the thalamus on the way to the cortex.
Cognitive Neuropsychology
Based on the ideas of Broca that certain brain regions correspond to certain functions. Depends on the
notion of modularity - the human mind and brain consist of a set of modules each of which carries out
one or more specific functions. Functional and anatomical modularity are not the same thing.
Dissociation is a pattern of brain damage where damage to a region is associated with damage in one
function but not another. Double dissociation is a pattern of brain damage where damage to a region
causes impairment in function A but not in function B. In another patient damage to another region is
associated with loss of function B but not A.
Functional Neuroimaging
- EEG and MEG: based on measuring the electrical and magnetic fields associated with brain
activity. Have low spatial resolution but high temporal precision.
- PET: based on changes in blood flow
- fMRI: based on oxygenated blood flow patterns
- Diffuse optical tomography: new type of brain imaging which also senses changes in blood
oxygen levels using optical techniques (IR).
The difficulties of perception are illustrated by self-driving cars. To make an algorithm for how we
perceive traffic proves difficult. Self-driving cars make use of lidar (light and radar beams) which provides
information about shape, distance, motion and size. They then use a normal camera to sense the color
of traffic lights.
, Appendix: Noise and Signal Detection Theory
Internal variability refers to changes in metabolic processes
in the brain that cause you to have varying perceptions. For
example when you’re in bed and you hear the rain outside
one moment but it drowns out the next moment. Internal
variability is the reason for the S-shape of the psychometric
function →
The psychometric function is the curve that relates a
measure of perceptual experience to the intensity of the
stimulus.
All neurons exhibit noise: random variation in the number of
action potentials produced by neurons in response to a fixed sensory stimulus. The baseline firing rate of
a neuron is also subject to this variability. This variability due to noise results in an S shaped curve in the
psychometric function.
Signal Detection Theory
Noise in neural activity makes it difficult to be certain about perceptual judgments. Perceivers have to
choose some decision criterion N, some number of spikes above which they’ll report having detected a
stimulus. However, one participant may say ‘yes’ about hearing a stimulus when they hear even the
faintest noise, while another participant may only say ‘yes’ if they are absolutely sure they have heard a
sound. Their measured threshold thus also reflects their decision-making bias and not only their
perceptual sensitivity. Signal detection theory is a framework that measures how people make decisions
in the aim of measuring perceptual sensitivity apart from decision-making style.
A typical SDT experiment works as follows: a person is
exposed to auditory stimuli and indicates ‘yes’ or ‘no’.
They can either have a hit, a miss, a correct rejection
or false alarm. The hit rate can be plotted against the
false alarm rate on a receiver operating characteristic
graph. The quality of the person’s performance in a SD
experiment is judged by where their point falls on the
plot. Closer to the top left of the graph is better
performance.
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