Stimulus which activates visual receptors: light waves in the visible spectrum


Types of Electromagnetic Energy

Transparent Cornea

Green Muscular Iris & Black Pupil Hole


 Rods vs Cones

 ~120 million rods/eye

 more in periphery

 very sensitive (low threshold)

 ~100 rods share same optic nerve fiber to brain 

 night vision (scotopic vision)

 ~6 million cones /eye

 most in center, especially in the fovea 

 need bright light to reach threshold

 have 1-to-1 lines to brain- good for detail vision or “acuity” 

 3 different types - provide color vision

Layers of the Retina

Optic Nerve


Turning Light Waves Into Electrical Messages:

 Rods & cones have molecules of light sensitive photopigments (11-cis-retinal+an opsin) embedded in cell membrane.

 Linked to G-proteins like metabotropic neurotransmitter receptors, except they receive light!


Those Odd Rods (& Cones) 

 They don’t produce action potentials.

 In the dark they have open Na+ channels, constant depolarization & transmitter release!

 Chemical changes in response to light CLOSE Na+ channels, causing hyperpolarization & LESS release of transmitter (glutamate)!

 Rods signal bipolar cells by releasing less transmitter.


 3 different types, absorbing different ranges of wavelengths


Types of Color “Blindness” or deficencies


How Do We See Color Under Unusual Lighting Conditions?
 Retinex Theory

 Cortex uses input from various parts of the retina to determine illumination conditions, then generates color perceptions based on the relative brightness of different areas.


Visual Fields

 Each half of your brain sees the opposite half of your visual world


The Richness of Vision

The visual system shows hierarchial organization, functional segregation, and parallel processing


At Least 2 Routes Thru the 
Primary Visual Pathway:

 From periphery of retina to the large ganglion & geniculate cells (magnocellular) - sensitive to location & depth and to movement

 From fovea of retina to the small (parvocellular) ganglion & geniculate cells - shape analysis

 Also a Mixed pathway for color& brightness

Many Regions of Cortex Involved in Visual Processing

 Primary visual cortex is just the first level of cortical processing

 Secondary “visual cortex” seems to have separate regions devoted to color, location, movement, shape, etc.

 Parietal & temporal association cortex also involved with higher level processing.

The Dorsal Stream 

 From primary visual cortex dorsally thru prestriate cortex to posterior parietal lobe

 Perceiving and remembering/imagining the location & dimensions of visual stimuli

 This allows visual/motor coordination whenever we interact with objects, visual search or scanning movements of eyes

Ventral Stream of Processing

 From primary visual cortex ventrally thru prestriate cortex to inferior temporal lobe

 Recognizing, describing what objects or patterns are

 Color processing including remembering/imagining color of things

 Object perception- damage to inferior temporal cortex causes visual agnosia

A Middle Stream?

 The border between parietal, occipital and temporal lobes seems necessary for movement perception.

 This area receives input not only from visual cortex but also from superior colliculus and pulvinar of thalamus.



Visual Agnosia (not recognizing)

 Because different visual attributes seem to be processed in several different brain areas, different areas of brain damage lead to different kinds of visual agnosia (object agnosia, color agnosia, movement agnosia)

 Prosopagnosia- can’t recognize individual faces (or similar members of other complex classes of visual stimuli)

One Goal of visual system research:Visual Prosthetic Devices