Problem is that the eyeball is too short or the cornea and lens are not powerful enough to focus even parallel rays of light

-Focus happens behind the retina

-Corrective lens adds to the strength of cornea and lens and allows for the image to be formed on the retina

Overtime, as you get older, you lose the ability to accommodate

Lens isn't flexible anymore

Far point the same, losing ability to see things close up

-lack of radial symmetry in the cornea or lens

--cornea should be spherical like a basketball, but if you have astigmatism it’s more like a football – it’s more curved in one dimension –

-Curviture means optical power – ability to bend light – if you have astigmatism, your ability to bend light is different from one plane to another

If some lines are dark and others are washed out in wheel diagram, you have a stigmatism

-clouding or opacity of the lens in the eye

--Common but not universal in old age

--Happens very gradually – starts out where doctor can see the effect when looking into your eye

--Overtime, the problem cannot be corrected with glasses

--When it gets bad enough, you need medical intervention – surgery

Operation of the cataract happens by sticking a probe into the eye where the lens is (next to the lens)

-Serves as a miniature vacuum cleaner

-Ultrasound breaks up little pieces of the lens, then the pieces are sucked up

-Final step is to put a plastic lens in the eye to replace the cataract

--Smaller than the regular lens so it’s easy to pop into place

Far point = the farthest away distance you can see clearly

Near point = the nearest distance point you can see clearly

Nearsightedness, very common 

-People with myopia – their far point isn’t very far away

-When two far away, parallel points enter the eye, they come into focus but they don’t come together and meet in the retina – this is a problem because you see with a retina (according to Descartes)

--it’s formed somewhere in the v. humor

-No

--overtime, the rays will bend more and make the situation worse

To correct this problem, doctor prescribes corrective lens – “negative lens” – which bends the rays outward in order to make them eventually converge in focus on the retina

-Laser eye beam surgery is also used, usually for myopia, where the cornea is reshaped so it’s not quite as curved and their myopia is cured

Farsightedness

Problem is that the eyeball is too short or the cornea and lens are not powerful enough to focus even parallel rays of light

-Focus happens behind the retina

-Corrective lens adds to the strength of cornea and lens and allows for the image to be formed on the retina

all over the retina was a sort of orange rose color

What is this mystery substance and what is it good for?

--This substance bleaches away when lights hits it

--Retina gradually loses it’s color – its retinal pigment

-Light bleaches this pigment

Big part = opsin, small = retinal (made up of vitamin A – this is why you need vit. A to have good vision)

the opthamoloscope

He proposed you have a piece of glass at a 45 degree angle where light from a lamp bounces from the glass into the patients eye, bounces off the retina, comes forward again through the glass, and into the examiner’s eye 

all over the retina was a sort of orange rose color

What is this mystery substance and what is it good for?

--This substance bleaches away when lights hits it

--Retina gradually loses it’s color – its retinal pigment

Orange rose thing Helmholtz discovered 

Big part = opsin, small = retinal (made up of vitamin A – this is why you need vit. A to have good vision)

-When light hits the retinal, it changes shape and can no longer fit together with the opsin, so it drifts away

Each of the little dots in the picture of the retina are nucleus’ in cell bodies

-Retina is a very complicated structure – has cells in very different layers

--Visual pigment – receptors – are located in the back of the retina

Rod

Pigments absorb only a fraction of light, allowing some to be absorbed and some to pass through

At 500nm, 10% photons absorbed and 90% pass through 

lowest intensity of a stimulus that can just be perceived

◦Gustav Fechner (1801-1887), developed the "classical" psychophysical methods to measure threshold

◦threshold is difficult to measure accurately because it varies from time to time (i.e. distracted, another sound drowns out sound we are measuring)

◾as solution, taking series of measurements and taking average to get threshold

Sensitiviey = 1/threshold 

High threshold = low sensitivity (can't see light unless it's really bright

perception is the mind’s method of making sense of chaos

-Our senses are created in the nervous system – we create them, not the environment

--physically, there is no such thing as sweetness – we create the sensation

1. Environmental stimulus

2. Reflected and transformed in the eyes 

3. Receptor processes

4. Neural processing

5. Perception

6. Recognition

7. Action 

--The aqueous humor serves a very vital purpose, which is to supply metabolic needs such as oxygen, glucose, and getting rid of waste products for the cornea and the lends

---There are no capillaries in the cornea or the lens, because you can’t see through capillaries b/c light is distorted – therefore, the capillaries are in the aqueous humor

---Aqueous humor is constantly circling – old, dirty a.h. filters out of the lens as new, fresh humor filters in

main purpose of this gel-like substance is to keep the eye in shape

AH doesn’t function properly

-Drainage system can get clogged up – inter-ocular pressure increase, which puts the retina at the most risk

--Visual impairment then happens, known as glaucoma

literally means "pit," in the center of the retina 

-when you look at something, you look at it with your fovea 

Descartes said that when you see things, you see them clearly – he then reasoned that the receptors for vision must be somewhere in the eye, where an image is produced of the environment

-in his image, he created a diagram where he found a “retinal image” – the conclusion he drew from this experiment were that the receptors for vision must be located inside the retina

--optical center of the eye

--has the property where any rays of light passing through the nodal point do not bend toward the retinal image – does not bend before, through or anywhere toward the retinal image

-L shaped line is all cones

you’re seeing with your cones first because they’re more sensitive – then you’re seeing with your rods because they’re becoming more sensitive

Rods see black and white while cones mediate color 

-In curve example, dots on the rod line represent black and white while the opposite is true with L-shaped line 

--absolute threshold with rods is lower than that of cones

--you have better acuity with your cones

-activity of rods summates into one second-order cell, so threshold is low

--as opposed to cones which have more private lines 

Second-order cell with rods doesn’t know which rod is sending the message, so you sort of get a blob of light, however with the case of the cones, each one has a second order cell which can tell the difference between one order of stimulation and another, which is why they have better acuity 

Because you have three different types of cones, each with a different visual pigment

--the brain repairs the signals from each of the cones

--the absorption spectrum has a broad range – the brain can figure out the wavelength of the light as color

Pages 33-34 

these cells are the only way for messages to get out of the retina – they need to get to these cells if they’re going to eventually reach the brain

-Ganglian cells are true neurons, just like brain cells are

A typical neuron has a cell body, dendrites (whose job is to receive info. From other cells and convey it to the cell body), an axon (defining property of a neuron which carries messages away from the cell body and can go a long distance)

-

Message cross synapses of neurons through these

--when the neuron is at rest, the transmitters are resting in the vesicles – but when activated, the vescicles rupture and transmitter molecules are released into the synapse

-neurons are the same way as Congress (polarized – r’s and d’s hang out separately) – negative charges stay inside the neuron and positive charges outside

-If the neuron is going to get anything done, the positive and negative charges have to mingle – this process is called depolarization

-Excitatory - charge becomes less negative 

If several depolarizations happen, the total amount of depolarization can reach a dashed line – this is called an action potential 

-explosive event, like a sneeze

-only neurons have them - critical defining property 

-have a life of their own - travel on their own power without getting stronger or weaker; they are all the same size 

Could be expressed in spikes/second or action potentials/second 

-Determined by both excitation and inhibition - some are messages are excitatory and others inhibitory 

The more excitation, the higher the firing rate 

Spontaneous Activity 

Each optic nerve is the axon of a ganglian cell 

-In each optic nerve there are millions of ganglian cells 

the area of retina, stimulation of which determines the firing rate of the cell

A given ganglian cell doesn’t respond to just any place of light shined on the retina – only one particular area of the retina – this is called the receptive field

Receptive field = area of the retina, stimulation of which can stimulate firing of the ganglian cell

Middle has excitatory effect while surrounding area has inhibitory 

If light is shined outside both these areas, ganglion cell will have spontaneous activity (action potentials with no stimulus) 

The firing rate of multiple receptors can influence firing rate of ganglian cell 

Ganglian cell, for the service of your brain, is trying to interpret light in the context of its surroundings – specific light as it compares to its surrounding light

Square looks darker when surrounded by white

-the one surrounded by black is not receiving any surrounding inhibition, so the ganglian cell looking at the square is going to fire a lot

-The surrounding cells are more stimulated by the big white circle

----In both cases, the center is stimulated the same amount but the surround has different degrees of excitation and inhibition

Brain is fooled because it’s perceiving an object and its surroundings – surroundings are affecting the brightness of the object

--main determinant of an object’s brightness is the ratio of the intensity of light from the object : intensity of light from its surroundings

You are perceiving the relative amount of light – this is good because of the survival aspect of the visual system

the brightness of an object, viewed in its natural environment, doesn’t change much as overall illumination changes

-a piece of paper will look brighter outside with the sun than it will inside, but regardless it will look white

Brightness constancy allows you to recognize objects – if you’re a frog, brightness constancy will greatly add to its ability to see a snake hiding in the grass