The Visual System:
Neuron to Higher Level Processing

I. The Visual System: - 3 major areas

• Eye

• Lateral Geniculate nucleus (in the thalamus)

• Visual Cortex - (a.k.a. striate cortex)

II. Light -

• What is light? -- one kind of electromagnetic radiation

Light is both a particle and a wavelength.

 

• Photon -

  • travel at a constant speed of about 300,000 km per second (speed of light).

  • the element that is absorbed by the photoreceptors in our eyes.

 

• Wavelength -

 

• Light:

• from 400-700 nm (nanometers--1 billionth of a meter)

 

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• Other animals can perceive longer and shorter wavelengths

 

 

• Light is structured by the environment

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• Ganzfeld-

 

 

 

III. Light Reception-

• Types-

  • Light Gathering -

e.g. single-celled amoeba --

 

e.g. light sensitive area on lower forms of life-

 

• Image-forming -

• pinhole eye of Nautilus (mollusk) -

 

• compound eye (insects) -

 

 

• Vertebrate Eye -

IV. Anatomy of Human Eye -

• From the outside (look in a mirror)

  • Sclera:

  • Pupil:

  • Iris:

  • Cornea:

• On the Inside:
  

• Lens:

 

 

 

 

• accommodation:

 

for far away objects--

 

 

for nearby objects--

 

 

 

• Retina:

 

 

• Photoreceptors absorb light rays and transform them into information transmitted by the neurons to the brain.

 

Process:

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• photopigments: 2 components

1)

 

2)

 

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Types of visual receptors

• Rods:

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• Cones:

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• Fovea:

 

 

 

 

 

 

• Perpiheral Retina -

 

 

 

• Optic Nerve:

 

 

Location of the blind spot

 

V. Duplicity Theory -

• Shape and Distruibution differences:

cones:

 

 

 

rods:

 

 

 

• Dark Adaptation Differences -

 

 

 

 

 

• kink in the graph is due to the difference in rods and cones.

 

cones:

 

 

rods:

 

 

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• Light Adaptation Differences -

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• Spectral sensitivity (to different wavelengths along the spectrum), and color vision differences

cones: (Phototopic)-

 

 

cones:

 

 

 

rods: (Scotopic)-

rods:

 

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Purkinje Shift -

 

 

 

• Acuity -

cones:

 

 

rods:

 

 

 

 

Why the difference?

Convergence:

 

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Results:

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2)

 

 

 

Measuring acuity:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

VI. Vertical Connections in Retina:

Types of cells:

Bipolar:

 

midget bipolar cells --

 

Diffuse bipolar cells --

 

Ganglion cells:

 

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How do this?

 

• use single cell recording technique for one ganglion cell--find area on retina that corresponds to that one ganglion.

• record normal low-level spontaneous level of activity of ganglion

• then turn on light in & around that area and see how it effects the firing of the ganglion cell

 

 

• ganglion cells respond differently depending on what area of their receptive field is stimulated

 

Two types of ganglion cells:

1) on-center, off-surround

 

2) off-center, on surround

 

 

 

 

 

 

 

 

 

e.g. on-center, off-surround

 

 

How does this type of receptive field work?

 

 

 

 

 

 

 

Horizontal and Amacrine Cells:

 

 

Lateral Inhibition:

 

 

 

 

 

 

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• Mach Bands -

 

 

 

 

 

 

 

 

 

 

 

 

 

Explanation:

 

 

 

 

 

 

 

 

 

 

• Also explains Hermann Grid & Scintillating Grid

• Different amount of lateral inhibition cause the intersections to appear darker (dimmer) than the hallway areas.