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The Visual System:
From Eye to Cortex
Ch. 6
Outline
(1)
(2)
(3)
(4)
The Eye
The Retina
Visual Transduction by Rhodopsin
From Retina to Primary Visual Cortex
The Eyes
• Structure: review the major structures of
the eye (diagram given in class)
(note that the size of the pupils is a
compromise between sensitivity and acuity)
The Eyes
• ciliary muscles adjust the lenses to focus
visual images sharply on each retina,
regardless of the distance of the image from
eyes; this focusing is called acommodation
The Eyes
• When we look at something near, the ciliary
muscles contract, tension on the lenses is
reduced, and they become more cylindrical
• When we look at distant object, the muscles
relax, tension on the lens is increased, and
the lenses flatten
The Eyes
• Binocular Disparity: unlike most
vertebrates, most mammals have two eyes
on the front of their heads, rather than one
on each side; this cuts down the field of
view, but it insures that most of what is seen
is seen through both eyes
The Eyes
• Because each eye sees things from a slightly
different perspective, there is a difference in the
two retinal images; this binocular disparity is
greater for closer things; the degree of binocular
disparity associated with a particular visual
stimulus helps the brain create a 3-D perception
from two 2-D retinal images thus depth perception
is partially achieved from this (remember, we can
still perceive depth with one eye closed)
The Retina
• Structure: five layers receptor layer;
horizontal cell layer; bipolar layer; amacrine
cell layer; and retinal ganglion cell layer
(diagram in class)
• The receptor is the farthest from light;
therefore, incoming light is distorted by four
layers of neurons before reaching the
receptors
The Retina
• The fovea is a pit about 330 micrometers in
diameter; it is the only part of the retina
capable of mediating high-acuity vision.
This is due, in part, to the fact that axons of
retinal ganglion cells are thinnest over the
fovea and light is distorted less before
reaching the layer of receptors. The fovea is
roughly in the center of your visual field.
The Retina
• The optic disk is where the axons of retinal
ganglion cells penetrate the retina and exit
the eye; the optic disk has no receptors thus
creating a blind spot
The Retina
• We all have a blind spot at the optic disk, due to
the exit of axons from the retinal ganglion cells
• We are normally unaware of our blind spots, even
when looking through one stationary eye because
of completion; the visual system is able to use
visual information gathered from receptors around
the optic disk to complete the visual image
• Completion illustrates the creativity of the visual
system (demonstration in class)
The Retina
• Cone and Rod Vision: in a sense, each of
us has two visual systems; the photopic
system functions in lighted conditions; the
scoptopic system functions in dim light
The Retina
• Cones mediate photopic vision; rods
mediate scotopic vision; this is called the
duplexity theory of vision
• The photopic system mediates high-acuity
vision; the scotopic system mediates lowacuity vision
The Retina
• The photopic system has low sensitivity with few
receptors’ information combined at the next cell
level (low convergence)
• The scotopic system has high sensitivity with
many receptors converging on ganglion cells
(high convergence)
• Only cones in the fovea; rods predominate in the
periphery
The Retina
• Our eyes are in continual motion; they make
a series of fixations (about 3 per second)
connected by saccades (rapid movements
between fixations)
The Retina
• Eye movements keep the visual image in
continual motion on the retina; the
importance of this movement is illustrated
by the fact that stabilized retinal images
disappear; most visual system neurons
respond to change, not to steady input
The Retina
• What we perceive at any instance is the sum
of the input that has been received during
the last few fixations; this temporal
integration of the retinal images explains
why our visual images are detailed, colored,
and wide-angled, despite the small size of
the fovea; it also explains why things don’t
disappear when we blink
Visual Transduction
by Rhodopsin
• Tranduction is the conversion of one form
of energy to another; the first step in visual
transduction is conversion of light to neural
signals by the rods and cones
Visual Transduction
by Rhodopsin
• Transduction by rods is well understood;
there is pink substance in rods called
rhodopsin; light is absorbed by rhodopsin,
and this bleaches it (removes its color); this
bleaching occurs because retinal and opsin,
the two components of rhodopsin, separate
in light
Visual Transduction
by Rhodopsin
• This bleaching reaction hyperpolarizes the rods,
thus the bleaching reaction tranduces light to a
cascade of intracellular events resulting in neural
signal
• When rhodopsin is totally bleached by intense
light, rods lose their ability to absorb light and
transduce; however, in the dark, retinal and opsin
are reunited, and rhodopsin regains its color and
its ability to absorb light and transduce
Visual Transduction
by Rhodopsin
• Rhodopsin is a G-protein linked receptor
that responds to light by initiating a cascade
of intracellular chemical events
• cGMP is an intracellular chemical that
keeps Na+ channels partly open when rods
are in darkness; rods are thus slightly
depolarized
Visual Transduction
by Rhodopsin
• Bleaching rods via exposure to light results
in an intracellular cascade of events that
deactivates cGMP, closing Na+ channels,
which hyperpolarizes the rod and reduces
the release of glutamate
• Rods thus transmit signals through the
neural system via inhibition
From Retina to
Primary Visual Cortex
• This is called the retina-geniculate-striate
pathway (diagram in class)
• The primary visual cortex is striate cortex
after the stripe seen in a Nissl-stained cross
section of lower layer IV; the stripe is
composed of the terminals of axons from
the lateral geniculate nuclei
From Retina to
Primary Visual Cortex
• The retinal ganglion cell axons from the
nasal hemiretinas decussate via the optic
chiasm (they are contralateral); those from
the temporal hemiretinas remain
ipsilateral providing redundancy for the
system
From Retina to
Primary Visual Cortex
• The most important feature of visual cortex
organization is its retinotopic layout; the
surface of the visual cortex is a map of the
retina
From Retina to
Primary Visual Cortex
• The retina-geniculate-striate system
includes two largely independent channels:
The P pathway and the M pathway
From Retina to
Primary Visual Cortex
– Parvocellular layers (P pathway) are found in
the top four layers of each lateral geniculate
nucleus (LGN) and are composed of small body
neurons; they are responsive to color, fine detail
patterns, and react to slow or stationary objects
– Magnocellular layers (M pathway) are found
in the bottom two layers of the LGN, composed
of large body neurons, and are responsive to
rods and movements