Visual System

Properties of Light

Particles of Energy (Photons): Sense intensity, perceive it as brightness

Waves: Sense wavelength, perceive it as color

• Short wavelength = blue, ultraviolet

• Long wavelength = red

Refraction

When light rays hit something, they start to get bent. The image is sensed upside-down by lenses in the retina and perceived as right-side up 

Cornea

Front biological lens: primary refractive element, 2/3 bending of the light lens

Iris 

Gives the eye its color and regulates the amount of light entering the eye via the size of a small pupil 

Lens

Secondary refractive element: Focuses light on the retina; ciliary muscles alter the shape of the lens as needed, 1/3 bending of the light

Accommodation

The process of adjusting the lens

• Near object: lens bends cornea further

• Far object: lens doesn’t need to bend 

• Decreases with age

Pupil

Light rays go through the pupil to get to the lens

Retina

Back of the eye where the light rays actually hit; contain photoreceptors and associated neuronal circuitry

Extra-ocular Muscles

Control eye movements:

• Voluntary: Saccadic and smooth pursuit

• Reflexive: Image stabilization re-head movements 

Retina Layers 

1. Ganglion Cells: Axons gather together and puncture the back of the eye

   1. Form cranial nerve #2

2. Amacrine Cells

3. Bipolar Cells

4. Horizontal Cells

5. Photoreceptors: Sensitive to light

   1. Up to down

Fovea

Best vision: high-acuity area at the center of the retina, photoreceptors are displaced laterally so that light can impinge more directly on the photoreceptors, highest density of receptors 

Optic Disk

“Blind spot,” exit point for the optic nerve; region of the retina with no receptors and where retinal ganglion cells axon exit the eyes

• Completion: Visual system “fills in” the blind spot—perception compensates for the lack of sensation 

Rods (Scotopic Vision)

Cylinder, blurry vision, scotopic vision

• High sensitivity: one photon of light to activate a rod

• Low acuity

• No color 

• Sensitive to shorter wavelengths

Cones (Photopic Vision)

Predominantly in the fovea, good vision

• Color

• Low sensitivity: Daytime

• 1 to 1 relationship with bipolar and ganglion cells 

• Sensitive to longer wavelengths

Spectral Sensitivity

• Red: most sensitive to long wavelengths

• Green: Sensitive to the mid-range of wavelengths

• Blue: Sensitive to short wavelengths

Transduction

Outer segment: Photoreceptors are depolarized in the dark, and hyperpolarized in the light

• In the dark: 

  • cGMP-gated Na+ channels are open, leading to inward (depolarizing) Na+ current

  • Glutamate released at -40mV

• In the light:

  • Retinal changes shape

  • Opsin dissociates and activates transducin

  • Transducin activates the enzyme phosphodiesterase (PDE), which breaks down cGMP

  • cGMP-Na+ channels close

  • Membrane hyperpolarizes (-70mV)

Transmission

Inner segment:

• Synapses with bipolar and horizontal cells

• Transduction current modulate release of glutamate

Retinal Receptive Fields

How much can you move the light to activate a rod or cone: some range of motion that activates the cell

• Wider motion in fovea

• Smaller motion in the periphery

Ganglion Cell Receptive Fields

Circular, with antagonists’ (opponent) responses between center and surround

• Receptive fields are either “on-center” or “off-center”

  • On-center: Shining a light to trigger hyperpolarizing (inhibitory)

  • Off-center: Excitation

Formation of Opponent RFs

1. The opponent center-surround aspect is created by mutual inhibition among photoreceptors by horizontal cells 

   1. Created by antagonism through the horizontal cells

2. On- and off-center aspects are created by bipolar cells, which show opposite responses to glutamate released by their receptors

   1. Two types of bipolar cells 

Off-center Bipolar Cells

Have metabotropic glutamate receptors; respond best when their receptor releases less glutamate 

On-center Bipolar Cells

Have ionotropic glutamate receptors; respond best when their photoreceptor input releases more glutamate 

Achromatic Vision

• On-and off-center responses create a push-pull system that provides sensory redundancy and decreases sensitivity to common rate changes

  • Ganglion cells with inputs mainly from rods (in peripheral retina) are sensitive to light and dark

  • Ganglion cells with inputs primarily from cones (in the fovea) are sensitive to color contracts

Color Opponent Cells

• Red-green

• Blue-yellow

M Cells

• Peripheral retina (~100,000)

• Large cell bodies and large RFs

• Coarse resolution

• Inputs from 1000s rods and cones—convey light vs dark information

• Rapid adaptation and conduction of APs

• Motion response better than stationary

• Optimized for large-scale, low contrast moving patterns 

P Cells

• Central retina (~1 million)

• Small cell bodies and small RFs

• Fine resolution

• Inputs from ~1 cone—convey color  

• Slow adaptation and conduction of APs

• Stationary targets better than moving

• Optimized for small-scale, high-contrast fine patterns

Retina to Primary Visual Cortex 

Optic nerves exit each eye and project bilaterally to the lateral geniculate nucleus (LGN) of the (visual) thalamus 

• From Temporal: Projects to ipsilateral LGN

  • Views contralateral half of visual field

• From Nasal: Decussate in optic chiasm and project to the contralateral LGN

  • Views ipsilateral half of visual field 

LGN Anatomy

• 6 layers:

  • 1 and 2: magnocellular, large cells, input from M-RGCs

  • 3-6: parvocellular, small cells, input from P-RGCs

• Monocular: Receive input from hemi-retina of only one eye:

  • 1,4,6: Contralateral nasal hemi-retina

  • 2,3,5: Ipsilateral temporal hemi-retina 
    

Primary Visual Cortex

LGN projects to V1 in the occipital lobe

Blobs

Process object color—tends to be more superficial (layers 2 and 3); receive dominant input from P-type cells

Inter-blobs

Process form/orientation; receive input from P-type cells

Dorsal Stream

M-cells; motion/space

• Bypass V1 → MT → MST → Parietal cortex → Prefrontal cortex

Ventral Stream

P-cells; color and form 

• V4 → Inferotemporal cortex → Prefrontal cortex

Akinetopsia (Dorsal Stream)

Selective inability to perceive motion

Optic Ataxia (Dorsal Stream)

Impairment of visually guided reaching

Hemi-neglect Syndrome (Dorsal Stream)

Perceptual unawareness of half of visual space contralateral to the lesion

Face Processing

• V4: First interblob processor; processes color patterning

• Inferotemporal: Second inter-blob processor

Achromatopsia (Ventral Stream, V4 Lesion)

Loss of color vision

Prosopagnosia (Ventral Stream, IT lesion)

Failure to recognize familiar faces