Seeing Things 1
Eye and Brain
How Your Brain Works - Week 3
Dr. Jan Schnupp
jan.schnupp@dpag.ox.ac.uk
HowYourBrainWorks.net
Light Wavelength
A
Optics of
the Eye
B
Eye and Retina
The Blind Spot
Retinotopy
•
Adapted from drawings by Ramon y Cajal
The Optic
Pathway:
eye,
optic nerve,
optic chiasm,
optic tract,
thalamus,
optic
radiation,
visual cortex
Rene Descartes, Retinotopy and
the Seat of the Soul
Photoreceptors
• The human eye has ca. 10 million rods and ca 120
million cones
Phototransduction
• Light activated rhodopsin (R) activates G-protein (G)
which in turn activates phosphodiesterase (PDE) which
cleaves cGMP which closes cGMP-gated Na+ channel.
• What does any of this have to do with carrots?
Absorption
Spectra
• The three different types of cones and the rods have
slightly different opsins which are sensitive to different
wavelengths.
• “Trichromacy” theory.
Sensitivity of Receptors
Rod and Cone Distribution
Retinal Wiring
• Photoreceptors
• Horizontal cells
• Bipolar cells
• Amacrine cells
• Retinal ganglion cells
Centre –Surround Receptive Fields
Photoreceptors
Horizontal
Cell
-
-
++ + +
-- - -
Bipolar
Cell
Retinal
Ganglion
Cell
On-centre and
off-centre
Receptive
Fields
• Lateral inhibition
provided by
photoreceptor
ribbon synapses,
horizontal cell
synapses and
amacrine cells
Lateral inhibition for contrast (edge)
detection
RGC
receptive
fields as
“spatial
frequency
filters”
Difference of Gaussians Model of Retinal
Ganglion Cells
• The centre-surround structure of Retinal Ganglion Cells
turns them into “spatial frequency filters”. Larger RGC
receptive fields are tuned to “coarsely grained” structure in
the visual scene, while smaller RFs are tuned to fine grain
structure.
Convolving a Penny with
DoGs
• The picture of an American cent (left)
seen through large (middle) or small
(right) difference of Gaussian receptive
fields.
The Fovea
Eye muscles
Eye movements
•
•
Eye-movement traces while a subject explores a picture of the bust of Nefertiti.
From "Eye Movements and Vision" by A. L. Yarbus; Plenum Press, New York; 1967
Dan Simmons’ visual attention task
• Count the number of passes of the white team
Colour opponency
Colour Opponency
The Colour Wheel
Yellow-Blue
Red Green
Why Colour
Vision Does
Not Work
Well in Poor
Light
surround
centre
Cone mosaics
• Cone
mosaics for
four different
individuals
Colour blindness
Red-green channel broken
Blue-yellow
channel
broken
M cells and
P cells
• 90% P cells
• 5% M cells
• 5% non-M non-P
Projections to
the Lateral
Geniculate
Nucleus
The Stepping Feet Illusion
• http://www.michaelbach.de/ot/mot_feet_lin
/index.html
• What is going on here?
• M cells are colour blind, but very sensitive
to brightness (luminance) contrast.
• P cells are R-G opponent
• Non-M non-P cells are Y-B opponent
• Only M cells project to the motion
processing streams in the brain.