PSYC314 Chapter Notes - Chapter 6: Visual Agnosia, Vitreous Body, Color Vision
27 Apr 2018
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Chapter 6: vision
Visual agnosia
• Agnosia is failure to know
• Refers to any inability to perceive or identify a stimulus even though its
details can be detected and the person retains relatively typical intellectual
capacity
• Prosopagnosia – agnosia for specific faces – caused by damage to the visual
cortex following a stroke
The eye:
Sensation and perception:
• Vestibular senses- control movement and balance in space
• The inner ear – auditory information supplying information about head
orientation and movement
• Somatosensory – sense of touch, detects changes in pressure, warmth, cold,
vibration, limb position, and several kinds of events that damage tissue and
produce pain
Sensation:
• Involves the cells of the nervous system that are specialized to detect stimuli
from the environment
• The stimuli occur in specific forms of energy such as light, sound or heat
• Neurons that detect this energy transform it into action potentials that get
sent through the nervous system
Perception:
• The conscious experience and interpretation of information from the senses
and involves neurons in the central nervous system
The stimulus: light:
• Photoreceptors in the eye detect the presence of light stimuli
• For humans visible light is a narrow band of the spectrum of electromagnetic
radiation
• Electromagnetic radiation with a wavelength of between 380-760 nanometer
is visible to us
• The range of wavelengths we call light is simply the part of the continuum
that we humans can detect
• Other animals can detect different ranges of electromagnetic radiation
o Example- honey bees- can detect differences in ultraviolet
radiation reflected by flowers that appear white to us
• 3 dimensions determine the perceived color of light – hue, saturation,
brightness
• light travels at a constant speed of about 300,000 kilometers per second
o if the peak of oscillation of the wave varies, the distance
between the peaks of the waves will vary similarly but in
inverse fashion
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o slower oscillations lead to longer wavelengths
o faster oscillations lead to shorter wavelengths
o wavelengths determine the hue – the visible spectrum displays
the range of hues that our eyes can detect
• light also varies in intensity
o brightness is related to this
o if the intensity of the electromagnetic radiation is increased the
apparent brightness increases too
• saturation
o refers to the relative purity of the light that is being perceived
o if all the radiation is of one wavelength the perceived color is pure or
fully saturated
o if the radiation contains all visible wavelengths it produces no
sensation of hue and appears to be white
o colors with intermediate amounts of saturation consist of different
mixtures of wavelengths
Anatomy of the eye:
• we receive information about the environment from sensory receptors –
specialized neurons that detect a variety of physical events
o specialized neurons unlike the ones for neurotransmitters,
neuromodulators and hormones which are specialized proteins that
bind with certain molecules
o sensory transduction - stimuli are detected by sensory receptors
that alter through various processes the membrane potentials of the
cells- sensory events are transduced (transferred) into changes in the
cells membrane potential
o receptor potentials – electrical changes – affect the release of
neurotransmitters and can modify the pattern of firing with which
these cells form synapses
o many sensory receptors lack axons – a portion of their somatic
membrane forms synapses with the dendrites of other neurons
• retina- for an individual to see, an image bust be focused on the retina
o inner lining of the eye
o this image causes changes in the electrical activity of millions of
sensory receptors in the retina which results in messages being sent
through the optic nerves to the rest of the brain
o the retina is part of the brain and both it and the optic nerve are in the
central nervous system
o the bones and muscles surrounding the eye aid in focusing an image
in the retina
o the eyes are suspended in the orbits – bony pockets in the front of the
skull
o sclera- the tough white outer coat of the eye where the eyes are
attached - holds the eyes in place and moved by 6 extrocular muscles
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➢ the sclera is opaque and does not permit the entry of light into
the eye
➢ normally we cannot look behind our eyeballs and see these
muscles because the conjunctiva hides their attachments to the
eyes
➢ these mucous membranes line the eyelid and fold back to
attach to the eye (preventing a contact lens that has slipped off
the cornea from falling behind the eye)
• cornea:
o transparent
o the amount of light that enters is regulated by the size of the pupil
which is an opening in the iris (the pigmented ring of muscles behind
the cornea)
o the lens situated behind the iris consists of a series of transparent,
onionlike layers
o the shape of the lens can be altered by contraction of the ciliary
muscles
o Accommodation – process where the changes in shape permit the
eye to focus on images near or distant objects on the retina
• After passing through the lens, light transverses the main part of the eye that
is filled with vitreous humor – a clear gelatinous substance
o After passing through the vitreous humor, light falls on the retina
o Rods – receptor cells – about 120million
o Cones- receptor cells – about 6 million – provide us with most of the
visual information about our environment
o Photoreceptors – rods and cones – located in the retina
• Human retina contains 3 main cellular layers
1. The photoreceptive layer
2. The bipolar cell layer
3. Ganglion cell layer
o all receptors are at the back of the retina and the light must pass
through the overlaying layers to get to them
o these layers are transparent
Photoreceptors:
• Cones- receptor cells – about 6 million – provide us with most of the visual
information about our environment
o They are responsible for daytime vision
o Provide us with information about small features in the environment
and are the source of vision of the highest sharpness or activity
o For color vision – our ability to discriminate light of different
wavelengths
• Rods – receptor cells – about 120million
o Do not detect different colors
o Provide vision of poor acuity
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Visual agnosia: agnosia is failure to know, refers to any inability to perceive or identify a stimulus even though its. Perception: the conscious experience and interpretation of information from the senses and involves neurons in the central nervous system. The sclera is opaque and does not permit the entry of light into the eye. Normally we cannot look behind our eyeballs and see these muscles because the conjunctiva hides their attachments to the eyes. Example- the photopigment of human rods rhodopsin consists of rod opsin plus retinal. When the receptors experience darkness they respond by depolarizing. When they experience light they respond by hyperpolarizing. In vertebrate retina photoreceptors provide input to both changes in the membrane potential for audition) become depolarized by stimuli parts rod opsin and retinal. Think of photoreceptors as (cid:498)darkness detectors(cid:499) potentials instead their release of neurotransmitter.
