s&p exam 3

What is light

• The region of electromagnetic radiation spectrum that humans can typically see (400 to 700 nm)

• Wave-like and particle-like properties

Transverse wave

• Stadium-style

• Up and down

• Electromagnetic energy

Longitudinal wave

Oscillations are parallel to the direction the wave travels

Low energy photons

• Travel a long distance over the course of each oscillation, so they have long wavelengths

• Red

The four F’s of evolutionary survival

• Fighting

• Fleeing

• Feeding

• Reproduction

What are the evolutionary advantages of being sensitive to electromagnetic energy (versus some other energy form)

• Electromagnetic energy travels quickly, thereby conveying (almost) immediate information about distant objects, food, predators, potential mates, etc.

• Electromagnetic energy tends to move in straight lines, thereby preserving information about the shape of objects, food, predators, potential mates, etc..

What are the evolutionary advantages of being sensitive to light, per se (versus some other part of the E.M. spectrum)?

• Light is “bouncy”. Unlike longer-wave energy, which passes through many opaque objects, light can be reflected (‘bounced’) off of objects, making them visible. Light’s a better messenger.

• On our planet, light is plentiful. By contrast, shorter-wave energy tends to be absorbed by our atmosphere (i.e., by oxygen and nitrogen), and is less plentiful.

Summary of evolutionary advantages to using light as the “messenger”.

• Plenty fast.

• Plenty straight.

• Plenty bouncy.

• PolAnd just-plain-old, PLENTY!

Polarization

• Some eyes are sensitive to the manner in which light is polarized --that is-- to the plane of polarization (plane of oscillation). Bow-&-Arrow Demo

• Some birds and insects have eyes that can pick-up polarization information.

• Humans, other mammals, and other so-called “higher vertebrates” are blind to polarization information.

What’s happening with the polarized filters?

• On a given pair of filters, one filter blocks certain

planes of polarization, and “passes” others.

• The other filter does the same for a complimentary

set of planes.

• By properly orienting two polarized filters, you can

block all the light.

What’s happening with the “smoked” filters?

• On the first pair, your filter partially attenuates all planes of polarization equally.

• Your second filter does the same. It attenuates whatever is remaining from the first filter. Again, all planes of polarization are equally attenuated.

• Unlike polarized filters, orientation makes no difference for smoked filters.

Point of smoked vs polarization filter demos

• If I placed just one of the filters before your eye, you could NOT determine whether (A) the light was reduced equally across all planes of polarization, or (B) the reduction was polarization-plane specific.

• This is because, as a mammal, you are “polarization blind”.

• Not all animals are polarization blind.

• In principle, sensitivity to the plane of polarization could guide migration patterns in birds.

luminance-by-polarization profile for polarized filters.

luminance-by-polarization profile for “smoked” filters.

study

The eye is said to have two optical components:

The Cornea & The Lens

The cornea

• Image formation on the retina depends on this

• It refracts (bends) light

• Is convex, it makes incoming light rays converge to a focal point

• The refractive power is measured in dipopters

Diopter

= 1 / focal length in meters

Focal length

is the distance from the

optical device, to the focal poi

Crystalline lens

• Unlike the cornea, can change shape, providing additional optical power of 10 to 30 diopters

• Variability in its shape is called accommodation

Together, the cornea and the lens provide refraction ranging from __ and _ diopters --at least in healthy, young eyes.

50 - 70

Myopic

• Near-sighted

• The eye ball is “too long”, given the refraction

Hyperopic

• Far-sighted

• The eyeball is “too short”, given the refraction

Presbyopic

• “Old Sight”

• Caused by sclerosis (hardening) of crystalline lens, i.e., reduced refraction

• Corrected with bifocals

How is image size measured on the retina?

• Size (or retinal subtense) is expressed in degrees of

visual angle.

• If a 1-centimeter long stimulus is viewed from 57.29

centimeters away, the stimulus subtends 1 degree of visual angle on the retina.

• One degree of visual angle corresponds ~300 micrometers (millionths of a meter) on the retina.

• Visual Angle = Arctan (stimulus size / viewing distance)

• “Rule of thumb” - your thumb nail subtends approximately 1 degree of visual angle, when your arm is fully extended.

Blind spot

• a perceptual phenomenon that arises from the optic disc (an anatomical structure on the nasal side of each retina).

• it is vulgar to call the optic disk ‘the blind spot’.

• approximately 6 degrees of visual angle in diameter, so the optic disk subtends about 1,800 micrometers on the retina.

Retina

• retina originates from the same tissue that spawns all neurons.

• Like other brain cells, cells here cannot be repaired.

• Location of phototransduction

• Photoreceptors face the wrong way

• The most central part contains only cones. Cone density decreases with eccentricity, and in the far periphery, only rods are found

__ outnumber _ by approximately 100 million to 5 million in each eye

Rods, cones

photoreceptor density (overall) decreases with ____. So, vision is less sharp in the periphery.

eccentricity

For vertebrates, light increments lead to _______, and light decrements lead to ________: Invertabrates, vice-versa.

• Hyperpolarization

• Depolarization

____ in rods responds best to 500 nm wavelengths

Opsin

The three different cone-opsins respond best to ___, ___,, and ___ nm wavelengths

440, 530, 560

Principle of univariance

All wavelength information is lost at photo-transduciton.

• Any cone absorb any wavelength.

Be able to identify three reasons why it is inaccurate

(“Vulgar”) to refer to our three cone types as red, green,

and blue.

• 3) Newton said so

• “The rays are NOT colored.”

• 2) The Peaks don’t match perception

  • 440 nm looks violet, not blue!

  • 530 nm looks green…OK…but,

  • 560 nm looks yellow, not red!

• 1) Univariance

Three layers of the retina

• Photoreceptors;

• Middle-layer (horizontal, bipolar, amacrine);

• Ganglion cells.

Ganglion cells

• Involved in extracting meaningful visual information

• the most meaningful information for an organism would be the identification of object boundaries and edges.

• specialize in detecting those luminance boundaries, and reporting them to later portions of the visual system.

LGN Projections

• The temporal projections stay on “their side”, but the nasal projections criss-cross at the optic chiasm.

• After the optic chiasm, the optic nerve is called the “optic tract”.

• There is a left and a right optic tract, each projects to its respective Lateral Geniculate Nucleus (LGN).

• The LGN has six layers, neatly organized.

• For a given LGN, ipsilateral (‘same side’) inputs project to layers 2, 3, and 5.

• For a given LGN, contralateral (‘opposite side’) inputs project to layers 1, 4, and 6.

• Neighboring areas of the LGN correspond to neighboring areas on the retina.

• The LGN is therefore said to be “retinotopic”.

Color Opponency in LGN

1. Many cells in the LGN exhibit center-surround antagonism based on color.

2. Some of these cells are red-green opponent, and others are blue-yellow opponent.

3. As we have seen before, other LGN cells are simply luminance opponent (not color opponent)

4. Color opponent LGN cells help to identify color-based edges. (Example: red berries on green leaves.)

5.Cells in the LGN project via that “Optic Radiations” to the primary visual cortex