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Sensation and PerceptionClass IV: Retina & Visual Processing

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The First Steps in Vision:Seeing Stars

2

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A Little Light Physics

Light: A wave; a stream of photons, tiny particles that

each consist of one quantum of energy

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Light can be absorbed, scattered, reflected, transmitted, or

refracted

Absorbed: Energy (e.g., light) that is taken up,and is not transmitted at all

Scattered: Energy that is dispersed in an irregularfashion

When light enters the atmosphere, much of it is absorbed

or scattered and never makes it to the perceiver

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Reflected: Energy that is redirected when itstrikes a surface, usually back to its point of origin

Transmitted: Energy that is passed on through asurface (when it is neither reflected nor absorbedby the surface)

Refracted: Energy that is altered as it passes intoanother medium, (e.g., light entering water fromthe air)

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Eyes That See Light

The human eye is made up of various parts:

Cornea:The transparent window into the eyeball

Aqueous humor: The watery fluid in the anteriorchamber

Crystalline lens: The lens inside the eye, whichallows changing focus

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Eyes That See Light

Pupil: The dark circular opening at the center ofthe iris in the eye, where light enters the eye

Vitreous humor: The transparent fluid that fillsthe vitreous chamber in the posterior part of theeye

Retina: A light-sensitive membrane in the back ofthe eye that contains rods and cones, whichreceive an image from the lens and send it to thebrain through the optic nerve

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Figure 2.2 The human right eye in cross section (viewed fromabove)

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Eyes That See Light

Problems of refraction:

Sometimes the way the lens refracts light causes the point offocus of the image to be either in front of or behind theretina. In these cases, corrective lenses are needed to allowfor normal vision

Myopia: When the light entering the eye is focused in front

of the retina and distant objects cannot be seen sharply;nearsightedness

Hyperopia: When light entering the eye is focused behindthe retina; farsightedness

Astigmatism: A visual defect caused by the unequal curvingof one or more of the refractive surfaces of the eye, usuallythe cornea

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Figure 2.3 Optics of the human eye

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Figure 2.4 Fan chart for astigmatism

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Eyes That See Light

Additional eye problems

Presbyopia:(Old sight). The condition of beingunable to accommodate the lens sufficiently tofocus on nearby objects.

Caused by sclerosis (hardening of lens) andloss of elasticity of the tissue that moves thelens

Cataracts: Opacities in the lens that developthrough age or illness.

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Eyes That See Light

Using the ophthalmoscope, doctors can view the back surface ofpatients eyes, called the fundus

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Eyes That See Light

Blind spot demo

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Retinal Information Processing

Photoreceptors: Cells in the retina that initially

transduce light energy into neural energy

Rods: Photoreceptors that are specialized fornight vision

Respond well in low lighting

Do not process color

Cones: Photoreceptors that are specialized fordaylight vision, fine visual acuity and color

Respond best with lots of light

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Figure 2.8 Rods and cones

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Photoreceptors exhibit dark current.

When there is no light, they are constantlydepolarized by Na+ and Ca2+ currents in theouter segment.

Consequently, they are constantly releasing

neurotransmitter (glutamate). Light causes a reductionin the dark current, and

thus a decreasein transmitter release.

This decrease is the neural signal that theres lighton that spot in the retina.

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Visual Transduction

Light (photons) activates rhodopsin molecules.

Each rhodopsin molecule activates hundreds of transducinmolecules

Each transducin molecule activates hundreds of

phosphodiesterase molecules.

Each phosphodiesterase molecule hydrolyzes (inactivates)hundreds of cGMP molecules.

The drop in active cGMP levels causes a hyperpolarizationof the photoreceptor.

The hyperpolarization reduces transmitter release, thussending a neural signal.

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INTERLUDE

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Figure 2.7 Photomicrograph of the retina

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The distribution of rods and cones is not constant over

the retina

Cones process color; rods do not

This means that you have very poor color vision in yourperiphery. It may seem as if your entire field of viewhas full-resolution color, but it does not

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Figure 2.9 Photoreceptor density across the retina

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DEMONSTRATION: Foveal vs peripheral vision

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Vision scientists measure the size of visual stimuli by

how large an image appears on the retina rather thanby how large the object is

The standard way to measure retinal size is in terms

of degrees of visual angle

The rule of thumb: If you hold your thumb out at armslength, the width of your thumbnail is about 2 degrees

of visual angle

In summary: The visual angle of an object is a functionof both its actual size and distance from the observer

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Figure 2.10 The rule of thumb

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Whistling in the Dark: Dark and Light Adaptation

One of the most remarkable things about the human

visual system is the incredible range of luminancelevels to which we can adjust

THREE mechanisms for dark and light adaptation:

Pupil dilation Photopigment replacement rates

Changing connections between retinal neurons

Pooling of inputs from multiple photoreceptors in

low light parasitic electrical connections between rodsand cones (more later)

Figure 2 17 The possible range of pupil sizes in bright illumination

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Figure 2.17 The possible range of pupil sizes in bright illuminationversus dark

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Neural circuitry of the retina accounts for why we are

not bothered by variations in overall light levels

The amount of photopigment available inphotoreceptors changes over time

The more light entering the retina, the faster the

photopigments are used up, and the fewerphotopigments there are to process more light

The less light entering the retina, the moreslowly photopigments are used up, and the

more photopigments there are to process whatlittle light is there

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The visual system regulates the amount of light entering the eye, and ignores

whatever variation in overall light level is left over In bright light, the pupil contracts, letting in less light

Next, the number of photopigments in the photoreceptorsdecreases over a few minutes

What it means to be light-adapted is that even though there

are more photons coming into the eye, there are fewerphotopigments available to process them, so some of thelight is thrown away

In this sense, the remaining variations in light are ignored,beyond whatever level of luminance the eye is adapted to

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The Man Who Could Not See Stars

Retinitis pigmentosa (RP): A family of hereditary

diseases that involves the progressive death ofphotoreceptors and degeneration of the pigmentepithelium

Many people may not notice the onset of retinitis

pigmentosa at first because it primarily affectsperipheral vision

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Figure 2.5 Fundus of the right eye of a human

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Figure 2.18 Fundus of a patient with retinitis pigmentosa

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The Man Who Could Not See Stars

Visual field tests for someone with normal visioncompared with someone with retinitis pigmentosa

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g

Light passes through several layers of cells before reaching the rodsand cones

The Vertical Pathway: photoreceptor to

bipolar to ganglion cell Light activates a photoreceptor, which then signals the horizontaland bipolar cells that synapse with it

Bipolar cells are connected to amacrine cells and ganglion cells

Ganglion cells have axons that leave the retina through the optic

disc (blind spot)


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P ganglion cells: Small cells. Connect

to the parvocellular pathway Parvocellular pathway is involved in fine

visual acuity, color, and shape processing.

They have poor temporal resolution but goodspatial resolution

M ganglion cells: Large cells. Connectto the magnocellular pathway

Magnocellular pathway is involved in motionprocessing. They have excellent temporalresolution but poor spatial resolution

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Various regions of the retina interact vialateral connections

Horizontal cells:These cells are responsible for lateralinhibition, which creates the centersurround receptive fieldstructure

Amacrine cells:These cells synapse horizontallybetween bipolar cells and retinal ganglion cells

Lateral Inhibition

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Lateral Inhibition

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Receptive field:The region inspace in which

stimuli will activatea neuron

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