• photoreceptors transduce incoming light

• ganglion cells send signals along to the brain

The Retina has layers of cells

• 2 types of photoreceptors: rods and cones

• rods are very sensitive - useful in dim light

•

Two kinds of Photoreceptors

• Rods and cones are distributed differently across the retina

The Retina

• visual acuity (ability to see detail) depends on cones - thus acuity varies across the visual field

The Retina

• Why don’t you notice your blind spot?

The Retina

• Why don’t you notice your blind spot?– Blindspots don’t overlap!

– Your brain “fills in” the missing information

– The specific information in the blindspot isn’t much more missing than the rest of the periphery!

The Retina

• three types of cones: short, medium, and long

• different absorptions enable color vision

The Retina

• each ganglion cell integrates information from a particular spot on the retina called its receptive field

Neurons “collect” information

Receptive Fields

Stimulus is in receptive

fieldStimulus is

near receptive

fieldStimulus is outside

receptive field

Action potentials

• Ganglion cells project to the brain via the optic nerve

• information is projected to contralateral cortex!

Visual Pathways

• the retina is mapped onto primary visual cortex

• called a retinotopic or spatiotopic map

Visual Pathways

Stimulus Cortical Activity

• signals are separated according to the type of information

Visual Pathways

Dorsal Dorsal “Where” “Where” Pathway: Pathway:

Motion and Motion and LocationLocation

Ventral “What” Pathway: Color and FormVentral “What” Pathway: Color and Form

Primary Primary Visual Cortex Visual Cortex

(V1)(V1)

Seeing

Seeing

• Discussion of Gregory’s Article on Visual Illusions - Tuesday Oct 23

Seeing

• Seeing is the process of converting electromagnetic radiation into a conscious mental event

Seeing

• visual system faces many challenges– too much information– too little information– conflicting information– ambiguous information

Seeing

• too much information

Seeing

• too little information

Seeing

• Conflicting Information

Seeing

• Ambiguous information

Seeing

• We will consider two aspects of vision to explore how the brain overcomes these challenges:

– Seeing depth– Seeing in color