Vision

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

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.

Cones

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

Receptive Field – that part of the visual field to which a particular cell in the visual system responds

Ganglion Cell Receptive Field

A circular group of receptors feed into a single ganglion cell

They are wired up such that the receptors in middle of circle have an opposite effect on firing rate than those in edge.

The receptive fields of other ganglion cells are the opposite (- in the middle, + on the edge)

Cortical Receptive Fields – bar shaped instead of circular, respond to lines or edges

Lateral Inhibition – active cell inhibits neighboring cells. Lateral inhibition increases the difference in neuron activity on the light and dark sides of a contour or border. Makes sure our visual system responds to edges.

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

Vision

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

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.

Cones

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

Receptive Field – that part of the visual field to which a particular cell in the visual system responds

Ganglion Cell Receptive Field

A circular group of receptors feed into a single ganglion cell

They are wired up such that the receptors in middle of circle have an opposite effect on firing rate than those in edge.

The receptive fields of other ganglion cells are the opposite (- in the middle, + on the edge)

Cortical Receptive Fields – bar shaped instead of circular, respond to lines or edges

Lateral Inhibition – active cell inhibits neighboring cells. Lateral inhibition increases the difference in neuron activity on the light and dark sides of a contour or border. Makes sure our visual system responds to edges.

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)

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

At Least 2 Routes Thru the
Primary Visual Pathway: