Visual Prosthetics

fovea

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Lunate Sulcus Central Sulcus

V1

87 6 5 4 3 2 1

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180 0

905

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horizonta

Figure by MIT OCW.

Present two visual targets Present one visual target and stimulate

Vary contrast or size of the visual stimulus not in the receptive field

Percent target contrast difference

Size diameter difference in minutes

Percent target contrast difference

10090807060

P

erce

nt b

otto

m ta

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403020100

N = 40 trials per point

RF

Contrast series

Eccentricity = 3.4 deg

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

100908070605040

302010

N = 40 trials per point 0

Size series

RF

-15 -10 -5 0 5 10 15

Two visual targets

One visual target paired with electrical stimulation

0 10 20 30 40 50 60 70 80 90

100

Perc

ent c

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ctric

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te20uA

RF

Contrast series

Eccentricity = 3.5 deg Henk, Apr, 03

40uA

4 6 8 10 12 14 16 18 20 22

Percent target contrast

100908070605040302010

00

40uA Electrical stimulation at 200Hz for 80ms

Size series

10 15 20 25 30 35 40

Target size in minutes

One visual target paired with electrical stimulation

2 4 6 8 10 12 14 16

0 10 20 30 40 50 60 70 80 90

100 120uA

90uA 60uA

RF

Eccentricity = 3.4 deg

Percent target contrast

Per

cent

cho

ice

at e

lect

rical

stim

ulat

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site

0 10 20 30 40 50 60 70 80 90

100 Size series

Contrast series

Henk, Jul, 05

5 10 15 20 25

Target size in minutes

One visual target paired with electrical stimulation

2 4 6 8 10 12 14 16

0 10 20 30 40 50 60 70 80 90

100 120uA

90uA 60uA

RF

Eccentricity = 3.4 deg

Percent target contrast

Per

cent

cho

ice

at e

lect

rical

stim

ulat

ion

site

0 10 20 30 40 50 60 70 80 90

100 Size series

Contrast series

Henk, Jul, 05

5 10 15 20 25

Target size in minutes

Using currents of 20 to 120 microamps at eccentricities between 2.5 and 3.5 degrees, the contrast of the visual percept created in monkeys is 6 to 12% and the size is between 15 and 20 minutes of visual angle.

Percent contrast

75% 40% 17% 8%

A - B Contrast = X 100

A + B

Figure by MIT OCW.

Figure by MIT OCW.

5

4

3

21

6FIATLUX

FIATLUX

Figure by MIT OCW.

A. What aspects of vision might be desirable to recover with a prosthetic device?

1. Two dimensional pattern perception for object recognition and reading (what?)

2. Two dimensional spatial vision (where?)

3. Three dimensionl vision (stereopsis, motion parallax, etc.)

4. Three dimensionl spatial localization

5. Motion perception

B. Problems and issues:

1. Type of prosthetic device

2. What brain area should be the target for the device?

3. What is the longevity of any selected prosthetic device?

Illusions

A re-examination of the Hermann grid illusionPeter H. Schiller

The most widely cited theorypurported to explain the illusion:

ON ON

larger response smaller response

Due to antagonistic center/surround organization, the activity of ON-center retinal ganglion cells whose receptive fields fall into the intersections of the grid produces a smaller response than those neurons whose receptive fields fall elsewhere.

The effect is not size dependent

Illusion is less pronounced when display is rotated 45 degrees

The illusion persists with contrast reversal

Differently oriented vertical and horizontal lines reduce illusion

Serrated edges reduce illusion

The center-surround antagonism produced in ON-center ganglion cells is similar

for the four displays but they induce notable differences in the illusory effect

ON

ON ON

ON

Smudges are seen on the left where gray lines are in front and are not seen or the right where white lines are in front.

Color of smudges is defined by which set of lines is in front. On left color lines are in front and smudges have the same color. On right the white lines are in front and the smudges are weakly darker gray.

The illusion is reduced at isoluminancde

Retinal ganglion cell receptive field layout at an eccentricity of 5 degrees

At the eccentricity of 5 degrees the 0.5 by 0.5 degree visual angle area outlined impinges on 365 midget cells and 50 parasol cells. Half of these are ON and half OFF cells. The layout of the ON cells is shown in B and C.

5mm 5 deg of visual angle

Retinal midget cells Retinal parasol cells

0.5 deg of visual angle

Figure by MIT OCW.

Figure by MIT OCW.

Figure by MIT OCW.

Figure by MIT OCW.

Figure by MIT OCW.

Muller-Lyer illusion

A. B.

Limitations and ambiguities in perception:

Figure/ground relationships Face recognition

Figure-ground relationships

a b

Figure-ground relationships

a b

The Road to Salvation

When you get to a fork in the road, take it! YB

The Greek key motif

Basis: The myth of the labyrinth that imprisoned the minotaur

This motif, supposedly symbolic of democracy, graces some of the walls in the US senate building

Summary: 1. Research on visual prosthetics is in its infancy. A great deal of basic research is needed before such a device can become effective.

2. The brain area that holds the best promise for a prosthetic device based electrical microstimulation is V1.

3. There is no unitary explanation for the great many visual illusions extant.

4. The most popular theory explaining the Hermann grid illusion based on the center/surround organization of retinal ganglion cells is incorrect. A more likely theory is the one that assumes that V1 cells are involved.

5. Retinal adaptation processes can explain illusions based on after-effects.

6. Many illusions disappear under isoluminant conditions.

7. There are no viable theories that explain illusions based on figure/ground relationships.