vertical horizonta fovea Figure by MIT OCW.

Transcription

1 Visual Prosthetics

2 Lunate Sulcus Central Sulcus vertical 270 horizonta fovea V1 Figure by MIT OCW.

3 Present two visual targets Present one visual target and stimulate Vary contrast or size of the visual stimulus not in the receptive field

4 Percent bottom target choice N = 40 trials per point Contrast series Percent target contrast difference Size diameter difference in minutes RF Eccentricity = 3.4 deg N = 40 trials per point Two visual targets Size series RF

5 One visual target paired with electrical stimulation Percent choice at electrical stimulation site uA 40uA Contrast series RF Eccentricity = 3.5 deg Henk, Apr, Percent target contrast 40uA Size series Electrical stimulation at 200Hz for 80ms Target size in minutes

6 One visual target paired with electrical stimulation Percent choice at electrical stimulation site uA uA 60uA Henk, Jul, 05 Contrast series Percent target contrast RF Eccentricity = 3.4 deg 100 Size series Target size in minutes 25

7 One visual target paired with electrical stimulation Percent choice at electrical stimulation site uA uA 60uA Henk, Jul, 05 Contrast series Percent target contrast RF Eccentricity = 3.4 deg 100 Size series Target size in minutes 25

8 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.

9 Percent contrast 75% 40% 17% 8% A - B Contrast = X 100 A + B

10 Figure by MIT OCW.

11 Figure by MIT OCW.

12 FIAT LUX FIAT LUX Figure by MIT OCW.

13 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?

14 Illusions

15 A re-examination of the Hermann grid illusion Peter H. Schiller

16 The most widely cited theory purported 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.

17 The effect is not size dependent

18 Illusion is less pronounced when display is rotated 45 degrees

19 The illusion persists with contrast reversal

20 Differently oriented vertical and horizontal lines reduce illusion

21 Serrated edges reduce illusion

22 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

23 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.

24 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.

25 The illusion is reduced at isoluminancde

26 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

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30 Figure by MIT OCW.

31 Figure by MIT OCW.

32 Figure by MIT OCW.

33 Figure by MIT OCW.

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40 Muller-Lyer illusion A. B. Figure by MIT OCW.

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42 Limitations and ambiguities in perception: Figure/ground relationships Face recognition

43 Figure-ground relationships a b

44 Figure-ground relationships a b

45 The Road to Salvation When you get to a fork in the road, take it! YB

46 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

47 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.