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Perception and VR

MONT 104S, Fall 2008Lecture 2

The Eye

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Fun with Size and Depth

The NeuronMajor parts of a neuron:

1) Cell body2) Axon3) Terminal4) Synaptic cleft5) Dendrites

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The Eye

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The Lens

The lens focuses images on the retina at the back of the the eye.

Accommodation: A process in which a specialized muscle (the "ciliary muscle") changes the shape of the lens to focus on objects and different distances. (Thin for distant objects, Thick for close objects).

As people get older, the lens loses its elasticity. People need to wear glasses for reading or close-up vision.

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Is the eye built backward?

The retina, the part of the eye that detects light, is at the back of the eye.

The photoreceptors, the cells that are sensitive to light, are at the back of the retina.

This is a result of the way the eye develops.

There are blood vessels and neurons in front of the photoreceptors that the light passes through.

Question: Why don't we see the blood vessels (or neurons)?

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The Eye

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The Optic Disk

Axons (neural fibers) of the retinal ganglion cells leave the eye by way of the Optic Disk.

There are no photoreceptors in this region, so there is a gap in the visual field, known as the blind spot.

The brain fills in the space in the blind spot, so we are unaware of the gap.

Demonstration.

This "filling in" of gaps by the brain also takes place for people who have gaps in vision due to strokes, etc.

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Major Cell Types in Retina

Major cell types:

Photoreceptors

Bipolar Cells

Horizontal Cells

Amacrine Cells

Retinal Ganglion Cells

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Photoreceptors•Photoreceptors sense light through phototransduction.•They are located at the back of the eye, next to the pigment cells which prevent light scattering and are involved in metabolic maintenance of the photoreceptors.•Two major types:

•Rods: Rod shaped, very sensitive to light, low acuity, slow temporal response.•Cones: Cone shaped, lower light sensitivity, high acuity. Three types of pigments that are sensitive to three wavelengths of light (red, green, blue) for color vision.

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The Fovea

•The fovea is a specialized region in the center of the eye.•It measures about 1 mm in diameter•Within this region, cones are closely packed together.•This region gives us the greatest acuity, allowing reading and other fine visual judgments.•There are no rods in the fovea.•Outside the fovea there is a mixture of rods and cones. The ratio of rods to cones increases toward the periphery.•Can see fainter light with your peripheral vision (Try to see the faintest stars with peripheral vision).•If fovea bleached (by looking at a bright light), you cannot read until it recovers.• Demo at end of lecture

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Schematic of the Fovea

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Other retinal cells

Bipolar cells: Provide straight pathway from photoreceptors to retinal ganglion cells.

Horizontal Cells: Present in the layer between the photoreceptors and the bipolar cells. Have long horizontal connections.

Amacrine Cells: Present in the layer between the bipolar cells and the retinal ganglion cells. Also have horizontal connections. There are lots of different shapes.

Retinal Ganglion Cells: Final cells in the retinal pathway. The first cells in the pathway to fire action potentials. They send signals to the LGN by way of their axons, which form the optic nerve.

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Receptive Fields

The receptive field of a neuron is the region of the visual field to which a cell responds.

Receptive fields can be mapped for neurons in the primate visual cortex by recording electrical signals from the cell while showing the animal a visual stimulus (e.g. a bar of light).

The cell will respond by increasing or decreasing the rate of action potentials when the light is in the cell's receptive field.

Show Movie!

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Response of Retinal Ganglion Cells

Retinal Ganglion cells and bipolar cells have receptive fields that exhibit a center-surround structure.

Discussion Question: What is the center-surround structure useful for?

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After-images

When photoreceptors absorb light, they are temporarily "bleached", and rendered less sensitive to light.

If we are exposed to a bright light, such as a camera flash, we will see a dark after-image in the location of the bleaching. The image appears to drift as we move our eyes.

If we bleach the region of the fovea, it becomes difficult to read or perform tasks that require high visual acuity.

Demo.