Studying Vision and Edges

• Ganzfield
• Mach Bands

-experimental instrument

-cylinder w/ a fogging solution in it, when participant looks in it it confuses their vision (can't tell if eyes are open, can't walk)

• Strips homogeneous in distribution of amplitude of light waves
• each strip will have a higher amplitude than the band immediately to it's left (brighter)
• at the border b/ bands we see the contrast as exaggerated (border contrast)
• This border contrast creates lateral inhibition

(b) is the real wavelength intensity (amplitude)

(c) is what we perceive

[image]

Done using the horseshoe crab bc they have really large photoreceptors (due to lateral plexus)

• Shine light at photoreceptor 1 and electrode at receptor 1 records activation
• shine light at photoreceptor 1 & 2, 1 sends activation but less bc 2 inhibits it
• increase the light at receptor 2 the more inhibition occurs and less firing
• When eyes get closer to brighter band, the intensity is higher at neighboring receptors (from brightness) so it inhibits more

 Horizontal cells allow lateral inhibition & communication between photo receptors in humans

similar to lateral plexus in horse shoe crab

Computer-controlled display coupled to an eye-movement detector

-keeps image focused on the same photoreceptors (comp moves image to keep it constant on retina)

-constant activation of the same receptor, bleaches it out so photopigments break down and don't have time to rebuild receptor

-loss of vision for edges first then entire stimulus

-Image fades bc of adaptation/habituation

Decrease in sensitivity or response to a continuously presented stimulus

Measured with the stabilized retinal image technique

Corrected for with Involuntary eye movements (reason we don't completely habituate in every day life & lose vision)

Tremors:short quick movements, distance of about 1 cone

Drifts: take place of the distance fo about 12 cones

Microsaccades: rapid (25msec) jerking motion after a drift to bring the object back onto focus at the original fixation point

Purpose of these is to sweep edges back & forth over receptors creating constant change (imp to avoid habituation)

Krauskopf: what happens to habituation part that disappears?

-Showed participant red disk w/ green ring around it: Red disk stabilized, green ring was not stabilized

-@ first bipolar is activated by red and slightly inhibited by green surround so participant sees both

-then red receptor is bleached out and can't rebuild so only green inhibition occurs

-participant sees a large green circle (no blank dot in the middle, similar fill in as the blind spot experiments)

precisions by which we see fine details

-measured by visual angle which depends on the size of stimulus and distance from the eye

-smallest visual angle measured in seconds and larger in degrees (60 sec=1min, 60min=1°)

-studied using static (still) stimulus

first number=distance patient is from stimulus, second number=distance normal person needs to be to detect target

20/10=better than normal

20/20=normal (1 min visual angle)

20/40=bad (2min visual angle)

20/200 (legally blind in most states)

Represents a static measure of acuity that determines the visual angle

Snellen Eye chart

• Reliable but not Valid, Problems include:
• all letters aren't equally difficult to identify
• we can use distinctive features to narrow down options (uses top-down when acuity is meant to measure bottom-up)
• diagnoses good vision better than poor version (less optionst at the top, 1 letter for 20/200)
• People can memorize it

• Identification tasks: what letter is this? (eye chart)
• 20/20, 1 min or 20/40, 2 min
•  Detection tasks:can you see this line? or this dot?
• .5 sec
• Resolution tasks: is this one dot or two? Is this a grating of black and white bars or a gray patch?
• 30 min
• Vernier acuity tasks:is the upper line to the R or L of lower line?
• 1-2 sec
• Dynamic visual acuity tasks: try to identify a letter as it is moving

(the smaller the visual angle needed the easier the task and/or the greater the subject's acuity)

-Foveation: focusing sensation on the fovea (what magincians have you do so they can change things in your peripheral where there is less acuity)

-Pupil size: healthy iris muscles to contract

-Luminance: amount of light entering eye, high enough so that cones are used

Optic disc does not effect acuity

-what you miss w/ a blind spot you get w/ the other eye

-disc is on medial side of retina so L eye loses L visual field and R eye loses R visual field

-involuntary eye movements also ensure you don't lose infor and brain fills it in

Increase in sensitivity in the dark bc we are going into our rod vision

-rods are anatomically constructed to be sensitive to light

-much convergence into diffuse bipolars and parasol ganglion cells

present an adaptation stimulus of high illumination

-take out rod vision for about 1 min and put into cones

Then turn off lights to test in darkness and after some time present test stimulus

Decrease in sensitivity in the light because we are going into cone vision

-cones are not anatomically constructed to be sensitive to light

-little convergence onto midget bipolars and midget ganglion

Show an adaptation stimulus which is done in low illumination to take out the cone vision and put into rod vision (takes about 30 minutes)

Then turn on lights to test in high illumination and after some time show the test stimulus

-small spot of light to measure threshold

Assess relationship between time in high illumination and threshold for intensity of light that is barely detected

-Light adaptation takes about 1 minute

Kink occurs because pupil dilation is another factor

-dilation allows cones to detect slightly less illumination immediately

[image]

rods

-monochromat (one color): people who see only gray bc they only have rods, no cones

-hard to find participants bc its rare (1/33,000)

-Shine light 20° outside of foves

-not perfect, some cones still present but mostly rods

cones

• Dark adaptation: Rod are highly sensitive to light
•  adaptation stimulus has high illumination
• delay is due to rhodopsin's being broken down during adaptation stimulus
• when lights are turned off rhodopsin needs time to restabilize and you stay in cone vision
•  Light adaptation: Cones have poor light sensitivity
• illumination in adpatation stimulus is too low for cones to detect
• photopigments in cones doesn't bleach on in adaptation stimulus and don't need time to restabilize

Putting on red goggles when leaving a dark room

-red wave length is 670, too long for rods to detect so they won't be bleached out in light

-don't need to spend time readjusting to dark

a change in the shape of the lens to bring light to the retina

Emmetropia: normal accomodation

Accomodation table

When ciliary contract they push towards lens, when they relax they pull away from lens

Far point: farthest point we can see clearly

-can't see farther bc we can't thin our lens any more

-farther away an object is the close together the rays are entering the eye so the thinner the lens must be to get them all the way to back of eye

Near point: closest point we can clearly see

-can't see closer bc we can't thicken our lens any more

-closer objects have rays more diverging so the thicker the lens must be to get them focused in time to reach back of eye

Longer eyeball means you need to get the rays farther back by thinning the lens more than normal

-for far away objects this means you can't thin the lens enough so you can't see as far as most people

-closer near point (better) and far point (worse)

Myopia: near sighted

Need to bend light faster so it is focused in time to reach the retina

-need to make lens thicker then most people to bend the light faster

-for close items can't make the lens thick enough so you can't see as close up as most people (farther near point)

Hypermetropia: far sighted

Thin lens: converges light slower, it can travel a farther distance

-needed if eye is longer then normal

Thick lens: converges light faster, travels a shorter distance

-needed if eye is shorter than normal

Concave needed for myopia (nearsighted)

-spreads the light out before it reaches eye

-lens doesn't have to thin as much bc rays are farther apart to start with

Convex needed for hypermetropia (farsighted)

-brings the light closer together before reaching eye

-lens doesn't have to be as thick to get rays focused in time to reach back of eye

1. Version movements

-constant agnle of the line of sight b/ 2 eyes

-eyes move in same directions

-e.g., tracking an object or reading

2. Vergence movements

-changing angle of line of sight b/ 2 eyes

-eyes move in opposite directions

-convergence or divergence causes this

-slow eye movements that rarely exceed 10º/sec

Saccades

(speed, parts)

-Voluntary version movements (used in reading)

-rapid, jumpy movement

-saccadic movement take 20-100 msec

-speed =600º/sec (really fast)

Includes a fixation pause (200 msec)

-where foveation occurs and when we see

-we don't actually see when eyes are moving during saccade (or we would see a blur

• Ballistic: destination is predetermined @ onset
• every saccade shows a similar pattern
• entails top-down processing
• Brain controls at approximately 6-9 letters/spaces per saccade
• Perceptual span (range of letters/spaces that we perceve at a fixation pause)
• 4 spaces to the left and 15 to the right

Saccade doesn't stop (fixation pause) on blank spaces or words w/ little information and past predictable words (e.g., 'a'/'the')

If distortion in the text is detected the next saccade is shorter (only if error is within the perceptual span of 4 letters to the L and 15 to the R)

Saccade movements don't make your eyes tired

-tiring is due to low levels of light

-bring reading closer to face

-eyes converge

-vergence movements are tiring for your eyes

Voluntary version eye movements to track a moving object

-slow and smooth (unlike saccades)

30-100 degress/second

-you can be trained to do this faster (athletes @ 150 degrees/second)

underpursuit: objects appear blurry if they are moving too fast for our eyes to pursue

Tremors: about 1 cone movement; continuous movement

Eye drifts: about 12 cones

Microsaccades: about 25 msec, jerks back to fixation point