Introduction to vision

7/24/2019 Introduction to vision

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How do our eyes respond to light?

Why do our eyes have two different sets of

receptors rods and cones?


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Figure 3.1 The electromagnetic spectrum, showing thewide range of energy in the environment and the smallrange within this spectrum, called visile light, that we

can see.


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!lectromagnetic spectrum !nergy is descried y wavelength.

"pectrum ranges from short wavelength gamma raysto long wavelength radio waves.

#isile spectrum for humans ranges from $%% to &%%nanometers.

'ost perceived light is reflected light of surfaces.


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Figure 3.( )n image of the cup is focused on the retina,which lines the ac* of the eye. The close+up of theretina on the right shows the receptors and other

neurons that ma*e up the retina.


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The cornea, which is fied, accounts foraout -% of focusing.

The lens, which ad/usts shape for o/ect

distance, accounts for the other (%. )ccommodation results when ciliary muscles are

tightened which causes the lens to thic*en.

0ight rays pass through the lens more

sharply and focus near o/ects on retina.


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Figure 6.2 Cross section of the vertebrate eye

ote how an o/ect in the visual field produces an inverted image on the retina.


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The Eye and Its Connections to the Brain

Pupil-opening in the center of the eye that

allows light to pass through

Lens-focuses the light on the retina

Retina-back surface of the eye that contains

the photoreceptorsThe Fovea-point of central focus on the retina

blind spot-the point where the optic nerve

leaves the eye


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The near point occurs when the lens can no longerad/ust for close o/ects.

2resyopia + elder eye4

5istance of near point increases

5ue to hardening of lens and wea*ening of ciliary muscles

6orrective lenses are needed for close activities, such as

reading


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'yopia or nearsightedness + vision good fornear oects.

7naility to see distant o/ects clearly

7mage is focused in front of retina 6aused y

8efractive myopia + cornea or lens endstoo much light

)ial myopia + eyeall is too long


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Focusing of light y the myopic 9nearsighted: eye.

(a) Distant object out of

focus

(b) No problem with near

object (Nearsighted)

(c) Distant object brought

into focus with lens


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"olutions for myopia

'ove stimulus closer until light is focused on the

retina

5istance when light ecomes focused iscalled the far point.

6orrective lenses can also e used.

0)"7; surgery can also e successful.


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Hyperopia or farsightedness + inaility to seeneary o/ects clearly

Focus point is ehind the retina.


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Transduction First take a look at receptors we nd in the

retina.


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KW 8-5


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KW 8-6


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9a: 8od receptor showing discs in the outer segment.9: 6lose+up of one disc showing one visual pigment

molecule in the memrane.


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8od 8eceptor have outer segments, whichcontain=

#isual pigment molecules, which have twocomponents=

>psin + a large protein

8etinal + a light sensitive molecule

#isual transduction occurs when the retinal

asors one photon. 8etinal changes it shape, called isomeriation.


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Figure 3.@ 9a: 8od receptor showing discs in the outer segment. 9: 6lose+up of one disc showing one visualpigment molecule in the memrane. 9c: 6lose+up showing how the protein opsin in one visual pigmentmolecule crosses the disc memrane seven times. The light+sensitive retinal molecule is attached to the

opsin at the place indicated.


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Figure 3.& 'odel of a visual pigment molecule. The horiontal part of the model shows a tinyportion of the huge opsin molecule near where the retinal is attached. The smaller molecule on

top of the opsin is the light+sensitive retinal. The model on the left shows the retinal moleculeAs

shape efore it asors light. The model on the right shows the retinal moleculeAs shape after it

asors light. This change in shape is one of the steps that results in the generation of an

electrical response in the receptor.


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2rior physical evidence showed that it ta*esone photon to isomerie a pigment molecule.

2urpose of Hecht eperiment + to determinehow many pigment molecules need to e

isomeried for a person to see


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Figure 3.B The oserver in Hecht et al.As 91B$(: eperiment could seea spot of light containing 1%% photons. >f these, C% photons reached

the retina, and & photons were asored y visual pigment molecules.


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8esults showed= a person can see a light if seven rod receptors are

activated simultaneously.

a rod receptor can e activated y the

isomeriation of /ust one visual pigment molecule.


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5ifferences etween rods and cones "hape

8ods + large and cylindrical

6ones + small and tapered 5istriution on retina

Fovea consists solely of cones.

2eripheral retina has oth rods and cones.

'ore rods than cones in periphery.

umer + aout 1(% million rods and C million cones


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Figure 6.2 Cross section of the vertebrate eye

ote how an o/ect in the visual field produces an inverted image on the retina.


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Figure 3.1( The distriution of rods and cones in theretina. The eye on the left indicates locations in degreesrelative to the fovea. These locations are repeatedalong the ottom of the chart on the right. The verticalrown ar near (% degrees indicates the place on theretina where there are no receptors ecause this iswhere the ganglion cells leave the eye to form the optic

nerve.


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Dlind spot + place where optic nerve leaves the eye

We donAt see it ecause=

one eye covers the lind spot of the other.

it is located at edge of the visual field.

the rain fills in4 the spot.


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Figure 3.1$ There are no receptors at the place where the opticnerve leaves the eye. This enales the receptorAs ganglion cell fiersto flow into the optic nerve. The asence of receptors in this area

creates the lind spot.


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'acular degeneration Fovea and small surrounding area are destroyed

6reates a lind spot4 on retina

'ost common in older individuals

Wet4 causes vision loss due to anormal loodvessel growth

5ry4 results from atrophy of the retinal pigment,which causes vision loss through loss ofphotoreceptors


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What aout cones and color vision? "eparate lecture on

color vision.

rods

Cones: short, medium, long


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'ethod used in all eperiments= >server is light adapted

0ight is turned off

>nce the oserver is dar* adapted, she ad/usts theintensity of a test light until she can /ust see it.


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Figure 3.1- #iewing conditions for a dar* adaptationeperiment. The image of the fiation point falls on thefovea, and the image of the test light falls in the

peripheral retina.


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!periment for rods and cones= >server loo*s at fiation point ut pays attention

to a test light to the side.

8esults show a dar* adaptation curve=

"ensitivity increases in two stages. "tage one ta*es place for three to four minutes.

Then sensitivity levels off for seven to ten minutes+ the rod+cone rea*.

"tage two shows increased sensitivity for another(% to 3% minutes.


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Figure 3.1B Three dar* adaptation curves. The red line is the two+stage dar* adaptation curve, with an initial cone ranch and a laterrod ranch. The green line is the cone adaptation curve. The lac*

curve is the rod adaptation curve.


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2rocess needed for transduction= 8etinal molecule changes shape

>psin molecule separates

The retina shows pigment leaching.

8etinal and opsin must recomine to respond to light.

6one pigment regenerates in si minutes.

8od pigment ta*es over 3% minutes to regenerate.


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8ods and cones send signals vertically through ipolar cells.

ganglion cells.

ganglion aons.

"ignals are sent horiontally etween receptors y horiontal cells.

etween ipolar and etween ganglion cells yamacrine cells.


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1(@ million rods and cones converge to 1million ganglion cells.

Higher convergence of rods than cones )verage of 1(% rods to one ganglion cell

)verage of si cones to one ganglion cell 6ones in fovea have one to one relation to ganglion

cells


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8ods are more sensitive to light than cones. 8ods ta*e less light to respond

8ods have greater convergence which results insummation of the inputs of many rods into ganglion

cells increasing the li*elihood of response. Trade+off is that rods cannot distinguish detail


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Figure 3.(@ The wiring of the rods 9left: and the cones 9right:. The dotand arrow aove each receptor represents a spot4 of light thatstimulates the receptor. The numers represent the numer ofresponse units generated y the rods and the cones in response to a

spot of intensity of (.%.


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)ll+cone foveal vision results in high visualacuity

>ne+to+one wiring leads to aility todiscriminate details

Trade+off is that cones need more light torespond than rods


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Figure 3.(- eural circuits for the rods 9left: and the cones 9right:.The receptors are eing stimulated y two spots of light.

Cones pick

up on

location.

Rods do

not pick

up onlocation.


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ods Cones

Introduction to vision

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