Transcript The Retina and LGN - Play Psych Mun

Psychology 4051
The Retina and LGN
Retino-Geniculate-Cortical Pathway
The Retina
• Located at the back of the posterior chamber,
forms the inner tunic of the eye.
• Surface on which the visual image is focused
The Retina
• A laminar tissue with multiple layers.
– Pigment epithelium, photoreceptor layer, external
limiting membrane, outer nuclear layer, outer
plexiform layer, inner nuclear layer, inner plexiform
layer, ganglion cell layer, optic nerve layer, internal
limiting membrane.
• Transduction takes place in the
photoreceptors.
– Rods and cones
• Light hits the outer segment of rods and cones which
contain photosensitive chemicals (photopigments).
• Rod-rhodopsin cones-iodopsin
• The light changes the molecular properties of the
photopigments, which in turn changes the electrical
state of these cells – this is called transduction.
The Retina
The design of the
retina is unusual.
Light must pass
through 8 layers
Before it hits the
Photoreceptors.
Light
The Retina
• The retina contains a central pit, above which cell
layers are pushed away.
– The fovea: 1mm in diameter
– Packed with cones
fovea
Fovea
•At the center of the retina, high
acuity
•Reduced light distortion
The Retina
• Rods and cones connect
with horizontal and bipolar
cells (collector cells).
• Lateral interaction takes
place at horizontal cells.
• Bipolar cells are connected
to retinal ganglion cells.
• Amacrine cells connect
adjacent bipolar cells.
Rods
• 120 million rods in the human retina
• Concentrated in the periphery of the retina
Rods
• Extremely sensitive to
light.
• Many rods converge
onto a single retinal
ganglion cell (RGC)
Rods – good light sensitivity
• As a result, low levels of
light can be detected
– Responsible for night
vision
• But, spatial resolution is
very poor
– RGCs can not determine
which rods were
stimulated
Cones
• Approximately 6 million cones
• Cones are packed densely in the fovea
Cones
• Cones are
connected to
bipolar cells, then
to retinal ganglion
cells (RGCs)
• Few cones
converge onto a
single retinal
ganglion cell (RGC)
– As a result,
resolution is
excellent
Cones
• Light sensitivity is poor
– Day vision
Cones
• Humans have 3 different cone types, each
with a different photopigment.
• Photopigments are maximally sensitive to
specific wavelengths.
– Short wavelength sensitive
– Mid-wavelength sensitive
– Long wavelength sensitive
Retinal Ganglion Cells
• Retinal ganglion cells possess receptive
fields that are responsive to light
stimulation.
• Receptive field – that area of the retina
over which a ganglion cell is sensitive to
light stimulation (area of the retina that
the RGC monitors).
Receptive Fields
• Their receptive fields are designed in an
antagonistic fashion
• A single receptive field has an on-center/offsurround arrangement or an off-center/onsurround arrangement.
“On-Center” Cells
• On-center cell: Light stimulation of the center of
the receptive field produces depolarization and
an increase in the firing rate of the ganglion cell.
• Stimulation of the surround produces a
hyperpolarization and a decrease in the firing
rate of the cell.
-
+
“Off-Center” Cells
• Off-center cell: Light stimulation of the surround
of the receptive field produces depolarization and
an increase in the firing rate of the ganglion cell.
• Stimulation of the center produces a
hyperpolarization and a decrease in the firing rate
of the cell.
+
-
Retinal Ganglion Cells
• The axons of the RGCs converge and leave the
eye through as the optic nerve.
• They connect to the lateral geniculate nucleus
(LGN).
Retinal Ganglion Cells
• Most RGCs fall into two functional classes, M
and P cells
• M cells – project to the magnocellular layers
of the lateral geniculate nucleus (LGN)
• P cells – project to the parvocellular layers of
the LGN
M Cells
• M cells receive input from a relatively large
number of photoreceptors (mostly input
from rods)
• Good light sensitivity, good temporal
resolution (sensitive to motion)
• Poor spatial resolution
• Large – with broad axons and consequently
faster nerve conduction velocities
• Not color-sensitive
P Cells
• P cells receive input from a relatively small
number of photoreceptors (mostly input from
cones)
• Good spatial resolution
• Poor temporal resolution
• Colour sensitivity
LGN
• A six-layered
thalamic relay
station
LGN
• Cells in the magnocellular layer have large cell
bodies and long, straight axons.
• Cells in the parvocellular layer possess smaller
cell bodies and short, curved axons.
• Layers 1, 4, and 6 receive input from the
contralateral eye.
• Layers 2, 3, and 5 receive input from the
ipsilateral eye.
LGN
• Most parvocellular cells appear to be
responsive to color and have center/surround
receptive fields.