PSYB65 Chapter 9: Brain&Behaviour Ch 9 Vision

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Published on 14 Nov 2015
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Brain & Behaviour
Ch. 9 How Do We Sense, Perceive, and See the World?
Clinical Focus 9.1 Migraines & a Case of Blindsight
D.Bs recurring headaches began at about age 14.
A visual “aura” warned of a headache’s approach: an oval-shaped area of flashing
(scintillating) light appeared just to the left of center in his field of vision; the oval
enlarged over time but then D.B was blind in the region of the oval.
The oval was described as an opaque white area surrounded by a rim of colour.
D.B suffered from severe “migraine”, a recurrent headache usually localized to one side
of the head.
Migraines vary in severity, frequency, and duration and are often accompanied by
nausea & vomiting.
Migraine is perhaps the most common of all neurological disorders, afflicting
some 5-20% of the population at some time in their lives.
Auras” may be auditory or tactile as well as visual and may result in an inability to
move or to talk.
After an aura passes, most people suffer a severe headache caused by a dilation of
cerebral blood vessels.
The headache is usually localized to one side of the head, just as the aura is on
one side of the visual field; left untreated migraines may last for hours to even
days.
A collection of abnormal blood vessels at the back of the right occipital lobe causes migraine
attacks (a most unusual cause).
To relieve the pain when there is no drug treatment effective, sometimes getting the malformed
blood vessels surgically removed; the operation relieve pain and generally improves life quality,
but sometimes a part of the right occipital lobe is deprived of blood and dies which results in
being blind in the left half of the visual field; unable to see anything to the left of the midline.
D.B couldn’t identify objects in his blind area but he could accurately “guess” if a light had
blinked on there and even where the light was located; this phenomenon is called “blindsight”.
9.1 Nature of Sensation & Perception
We may believe that we see, hear, touch, smell, and taste real things in a real world. In fact, the
only input our brains receive from the “real” world is a series of action potentials passed along
the neurons of our various sensory pathways.
We experience visual and body sensations as being fundamentally different from one another, the
nerve impulses coursing in the neurons of these two sensory systems are very similar, as are the
neurons themselves.
We realize that our senses can deceive us—two people can look at the same optical illusion and
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see very different images, and a person dreaming does not normally think that the dream images
are real, that you often do not think that a picture of you looks like you.
Sensory Receptors
Sensory receptors are specialized cells that transduce (convert) sensory energy – light for
example, into neural activity.
Sensory receptors are designed to respond only to a narrow band of energy – within each
modality’s energy spectrum.
Each sensory system’s receptors are specialized to filter a difference form of energy:
For vision, light energy is converted into chemical energy in the photoreceptors of the
retina, and the chemical energy is in turn converted into action potentials.
In the auditory system, air-pressure waves are converted first into mechanical energy,
which activates the auditory receptors that produce action potentials.
In the somatosensory system, mechanical energy activates receptor cells that are sensitive
to touch, pressure, or pain. Somatosensory receptors in turn generate action potentials.
For taste and olfaction, various chemical molecules carried by the air or contained in
food fit themselves into receptors of various shapes to activate action potentials.
*Vision begins in the photoreceptors – the rods and cones*
For each species and its individual members, sensory systems filter the sensory world to produce
an idiosyncratic representation of reality.
Example: elephants and bats can hear and produce sounds far below and above the range
in which humans hear and dogs have “superhuman” powers: they can detect odors, hear
the low range sounds of elephants, and see in the dark.
An animal’s perception of the world depends on the complexity and organization of its
nervous system
Receptive Fields
Every sensory-receptor organ and cell has a “receptive field”, a specific part of the world to which
it responds.
If you fix your eyes on a point directly in front of you, for example, what you see of the
world is the scope of your eyes receptive field.
If you close one eye, the visual world shrinks, and what the remaining eye sees is the
receptive field for that eye.
Receptive field: region of the visual world that stimulates a receptor cell or neuron.
Each photoreceptor cell within the eye points in a slightly different direction and so has a unique
receptive field.
You can grasp the conceptual utility of the receptive field by considering that the brain
uses information from the receptive field of each sensory receptor to identify sensory
information but also to contrast the information that each receptor field is providing.
Receptive fields are sample sensory information but also help locate sensory events in space.
Because the receptive fields of adjacent sensory receptors may overlap, their relatively
different responses to events help us localize sensations.
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The spatial dimensions of sensory information produce cortical patterns and maps of the
sensory world that form, for each of us, our sensory reality.
Our sensory systems are organized to tell us both what is happening in the world around us and
what we ourselves are doing.
When you move, you change the perceived properties of objects in the world, and you
experience sensations that have little to do with the external world.
When we run, visual stimuli appear to stream by us, a stimulus configuration called optic
flow: streaming of visual stimuli that accompanies an observer’s forward movement
through space.
When you move past a sound source, you hear an auditory flow, changes in the intensity
of the sound that take place because of your changing location.
Auditory flow: change in sound heard as a person moves past a sound source or as a
sound source moves past a person
Optic flow and auditory flow are useful in telling us how fast we are going, whether we are
going in a straight line or up or down, and whether it is we who are moving or an object in
the world that is moving.
Receptor Density & Sensitivity
Receptor density is particularly important in determining the sensitivity of a sensory system.
For example, the tactile receptors on the fingers are numerous compared with those on the
arm.
This difference explains why the fingers can discriminate touch well and the arm cannot
do so as well.
Our sensory systems use different receptors to enhance sensitivity under different conditions.
For example, the visual system uses different sets of receptors to respond to light and color.
Color photoreceptors are small and densely packed to make sensitive color
discriminations in bright light.
The receptors for black–white vision are larger and more scattered, but their sensitivity to
light—say, a lighted match at a distance of 2 miles on a dark night—is truly remarkable.
Variations in receptor density in the human auditory-receptor organ may explain such abilities as
perfect pitch displayed by some musicians.
Neural Relays
Receptors are common to each sensory system; all receptors connect to the cortex through a
sequence of three or four intervening neurons.
The visual and somatosensory systems have three, and the auditory system has four.
Information can be modified at different stages in the relay, allowing the sensory system to
mediate different responses.
Neural relays also allow sensory systems to interact.
There is no straight-through, point-to-point correspondence between one neural relay and the
next; rather, there is a recoding of activity in each successive relay.
Sensory neural relays are central to the hierarchy of motor responses in the brain.
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