Seeing Things 1 Eye and Brain How Your Brain Works - Week 3 Dr. Jan Schnupp...

Seeing Things 1Eye and Brain

How Your Brain Works - Week 3

Dr. Jan Schnuppjan.schnupp@dpag.ox.ac.uk

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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+

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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.