Chapter 4 Notes
Synesthesia - the perceptual experience of one sense that is evoked by another sense.
- for some synesthetes, musical notes evoke the visual sensation of color. Others see
printed letters or numbers in specific consistent colors.
- for those who don't experience synethesia, the prospect of tasting sound or hearing
color may seem unbelievable or the product of some hallucinogenic experience.
- brain imaging Aguirre also shoe that in some synesthetes, areas of the brain is
involved in processing colors are more active when they hear words that evoke color
than when they hear tones that don't evoke color; no such difference are seen among
people in a control group.
Sensation - simple awareness due to the stimulation of a sense organ.
- is it the basic registration of light, sound, pressure, odor, or taste as parts of your body
interact with the physical world.
• After a sensation registers in your central nervous system, perception takes place at
the level of your brain: it is the organization, identification and interpretation of a
sensation in order to form a mental representation.
• You're eyes - the sensory organ - aren't really seeing words; they're simply encoding
different shapes and pattern of ink on a page. You're brain - the perceptual organ - is
transforming those shapes into a coherent mental representation of words and concerts.
• Sensation and perception are related but separate events.
• We possess several more senses besides these five.
Touch for examples encompasses distinct body senses, including sensitivity to pain
and temperature, joint position and balance, and even the state of the gut - perhaps to
sense nausea via the autonomic nervous system.
• Despite the variety of senses they all depends on the process of transduction, which
is the conversion, by sensors in the body, of physical signals from the environment into
neural signals sent to the central nervous system
Gustav Fechner- develop an approach to measure sensation and perception called
psychophysics Psychophysics - methods that measure the strength of a stimulus and the observers
sensitivity to the stimulus.
• The simplest quantitative measurement in psychophysics is the absolute threshold,
the minimal intensity needed to just barely detect a stimulus.
• A threshold is a boundary
• The doorway that separates the inside from the outside of a house is a threshold, as is
the boundary between two psychological states (awareness and unawareness).
• In finding the absolute threshold for sensation, the two states in question are sensing
and not sensing some stimulus.
• To measure the absolutely threshold for detecting a sound, for example, an observer
sits in a soundproof room wearing headphones linked to a computer. The experiemnter
presents a pure tone (the sort of sound made by striking a tuning fork), using the
computer to vary the loudness or the length of time each tone lasts and recording how
often the observer reports hearing the tone under each condition. Investigators typically
define the absolute threshold as the loudness required for the listener to say she or he
has heard the tone on 50% of the trials.
Why can parents identify their own child's cry over other?
• parents can usually detect their own infants cry from the cries of other babies, but it's
probably more useful to be able to differentiate the "I'm hungry" cry from the "I'm cranky"
cry from the "something is biting my toes" cry. In short, the human perceptual system
excels at detecting changes in stimulation rather than the simple onset or offset of
As a way of measuring this difference threshold, Fechner proposed the just noticeable
difference or JND - the minimal change in a stimulus that can just barely be detected.
JND is not a fixed quantity, it is roughly proportional to the magnitude of the standard
Ernst Weber - founded the Weber's law which states that the just noticeable difference
of a stimulus is a constant proportion despite variations in intensity. • When calculating a difference threshold, it is proportional between stimuli that is
important; the measured size of the difference, whether in brightness, loudness or
weight is irrelevant.
Signal detection theory - the response to a stimulus depends both on a persons
sensitivity to the stimulus in the presence of noise and on a persons response criterion.
Dr Gonzalez, uses a very liberal criterion when she reads mammograms. She
recommends a biopsy for every possible case of cancer. This decision strategy
minimizes the possibility of missing a true cancer but leads to many false alarms.
Sensory adaptation - the observation that sensitivity to prolonged stimulation tends to
decline over time as an organism adapts to current conditions.
• 20/29 refers to a measurement associated with a Snellen chart, names after Herman
Snellen, the Dutch ophthalmologist who developed is as a means of assessing visual
acuity, the ability to see fine detail.
Light waves have three properties
0. Length - the length of a light wave determined it's hue, or what humans perceive as
1. Amplitude - the intensity or amplitude of a light wave - how high the peaks are -
determines what we perceive as the brightness of light.
2. Purity - the number of wavelengths that make up the light. Purity corresponds to
what humans perceive as saturation, or the richness of colors. Length, amplitude or
purity are properties of light waves themselves. What humans perceive from those
properties are color, brightness and saturation.
The Human Eye
Light reaches the eyes passes first through a clear, smooth outer tissue called
cornea, which bends the light wave and sends it through the pupil, a hole in the
colored part of the eye. • This colored part is the iris, which is a translucent, doughnut shaped muscle that
controls the size of the pupil and hence the amount of light that can enter the eye.
• Immediately behind the iris, muscle inside the eye control the shape of the lens to
bend the light again and focus it onto the retina, light sensitive tissue lining the back of
Accommodation - the process by which the eye maintains a clear image on the retina.
• If your eyeball are little too long or a little too short, the lens will not focus images
properly in the retina.
• If the eyeball is too long, images are focused in front of the retina, leading to
• If the eyeball is too short, image are focused behind the retina, and the result is
How do eyeglasses actually correct vision?
They can provide an additional lens to help focus light more appropriately.
Phototransduction in the retina
Two types of photoreceptors cells in the retina contain light sensitive pigments that
transduce light into neural impulses.
• Cones detect color, operate under normal daylight conditions, and allow us to
focus on fine detail.
• Rods become active only under lowlight conditions for night vision. Much more
sensitive than cones.
• Because all rods contain the same photopigment, they provide no information
about color, and sense only shades of gray.
• About 120 million rods are distributed more or less evenly around the retina
except in the very centre, the fovea
Fovea - an area of the retina where vision is the clearest and there are no rods at all.
- the absence of rods in the fovea decreases the sharpness of vision in reduced
light, but it can be overcome.
Retina - in contrast to rods, each retina contains only about 6 million cones which are
densely packed in the fovea and much more sparsely distributed over the rest of the
retina. It is thick with cells. • The photoreceptors cells (rods and cones) form the innermost layer. The middle layer
contains bipolar cells, which collect neural signals from the rods and cones and
transmit them to the outermost layer of the retina, where neurons called retinal
ganglion cells (RGCs) organize the signals and send them to the brain.
• The bundled RGC axons - about 1.5 million per eye- form the optic nerve, which
leaves the eye through a hole in the retina called the blind spot, which contains neither
rods nor cones and therefore has no mechanism to sense light.
Receptive Field - the region of the sensory surface that, when stimulated, causes a
change in the firing rate of that neuron
Most receptive fields contain either a central excitatory zone surrounded by a doughnut
shaped inhibitory zone which is called an on-centre cell, or a central inhibitory zone
surrounded by an excitatory zone which is called an off-centre cell.
• The shortest visible wavelength is deep purple and the longest is red.
• As wavelengths increase, the color perceived changes gradually and continuously to
blue, then green, yellow, orange and with the longest visible wavelengths, red.
• This rainbow of hues and accompanying wavelengths is called the visible spectrum.
The Visual Brain
Area V1 - the part of the occipital lobe that contains the primary visual cortex.
Perpetual Constancy - even as expected of sensory signals change, perception remains
Gestalt principles characterize many aspects of human perception. Among the foremost
are the Gestalt perceptual grouping rules, which govern how the feature and the
regions of things fit together. • Simplicity - a basic rule in science is that the simplest explanation is usually the best.