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Seeing Things 1 Eye and Brain How Your Brain Works - Week 3 Dr. Jan Schnupp [email protected] HowYourBrainWorks.net
Seeing Things 1Eye and Brain
How Your Brain Works - Week 3
Dr. Jan [email protected]
HowYourBrainWorks.net
Light Wavelength
Optics of the Eye
BA
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+
-
--
--
++
+
-
--
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Photo-receptors
HorizontalCell
BipolarCell
RetinalGanglionCell
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 WheelYellow-Blue
Red Green
Why Colour Vision Does
Not Work Well in Poor
Light
surround
centre
Cone mosaics
• Cone mosaics for four different individuals
Colour blindnessRed-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.
