Introduction
The exercises you conduct with this simulation will be basically the same as the ones you conducted with the Hermann grid simulation. Therefore, before proceeding further, you should work with that simulation enough so that you feel comfortable with the Hermann grid illusion and the effect of changing various parameters on its appearance.Tips & hints
1. For most of the following exercises, sit so that your eyes are approximately 50 - 60 cm. (20 - 24 in.) from the screen.
2. Don't stare too long at the display! Some of the images you'll generate with this simulation can be quite discomfiting, even disorienting. Negative afterimages can also be a problem. So, give your eyes __ and your brain __ a break every now and then.
3. Remember to note carefully the appearance of the illusion with each change you make in the simulation's parameters. A standard experimental technique used by psychophysical researchers is to have the experimental subjects estimate the "magnitude of the illusion", i.e., the intensity of the illusory spot. This will, of course, be personal & highly subjective, but try to use a scale from 0 (no illusory spots seen) to 10 (illusory spots appear clearly defined at their maximum intensity). You might also consider making sketches of the illusion.
4. Whenever you want to compare your results with the Hermann grid results for the same settings, simply click on the White Gridline radio box and rerun the simulation.Experimental Protocols
For the following exercises, work first with the basic black square-white lines illusion. After you've completed the basic exercises, try the reversed grid (white squares-black lines), then move on to other color combinations.
Basic ExperimentsQuestions
For the following exercises, work first with the basic Scintillating Grid illusion. After you've completed the basic exercises, move on to the exercises in which you change the default settings of the various parameters.
1. Start the simulation and observe its features. Focus your vision on one intersection near the center of the grid and note the appearance of the grid in the periphery of your visual field. What happens if you shift your gaze to another intersection? Can you see an illusory spot when you gaze directly at a particular intersection? What if you continuously scan the grid from side-to-side, top-to-bottom?
2. Vary the size of the grid squares. Does this change the appearance or strength of the illusion? If so, how? Is there an optimal size for the squares that produces the strongest illusion?
3. Use the Reset button to return the grid settings to their default values. Now, vary the width of the grid lines. Does this change the appearance or strength of the illusion? If so, how? Is there an optimal width for the grid lines that produces the strongest illusion? What is the narrowest grid line at which you can still perceive the illusion? The widest?
4. Use the Reset button to return the grid settings to their default values. Try viewing the illusion with your eyes at two or three different distances (say, 30 cm, 60 cm, and 90 cm) from the screen. Be consistent in the distances you use. Does the illusion change with viewing distance?
5. Using the default settings, view the illusion with one eye, first the right, then the left. Does the illusion change when viewed with monocular vision?Advanced Experiments
6. Tilt your head approximately 45o to the right or left and view the grid. Note any effect(s) on the appearance of the illusion.
7. Reset the color settings to black squares with gray gridlines and set line width to 5 and square size to 15. As you'll recall, with the Hermann grid, these settings create the Prandtl illusion, in which dark lines pass diagonally through the squares, connecting the illusory spots in the intersections. Do you observe the Prandtl illusion in the scintillating grid? Is the illusion changed in other ways? Repeat Exercise #6 at these settings. Does the appearance of the illusion change? Experiment to identify combinations of line width and square size for which the illusion is no longer observed.
8. Resize (use your mouse to drag the lower right corner of the window up and to the left) the display to reduce the number of intersections to two. (when you do this, all of the controls will become difficult or impossible to identify, but that doesn't matter for this exercise) Record the strength of the illusion. Now enlarge the window so that there are four intersections visible and record the illusion's strength. Continue resizing the display so that you increase the number of intersections to 16, then 25, then 36, and so on.
9. Try a number of different color combinations and repeat as many of Exercises 1-8 as you wish.
1. With the Hermann grid experiments, you needed to keep your eyes focused on a fixed point in order to see the illusion. Is that true for the scintillating grid illusion? If not, what happens to the appearance of the illusion when you do keep your vision focused on one fixed point?
2. Note that the white circles at the edges of the grid are adjacent to two, rather than four, grid squares. Do those circles show the illusory scintillating spots? How does this result compare with what you observed when working with the Hermann grid? What does this observation suggest about the origin of the illusory spots?
3. Does the illusion appear to be radially symmetrical (i.e., the same in all directions around the point at which your are gazing)?
4. Does the basic illusion change when you observe it with your head tilted at an angle of 45o? If so, does that suggest anything about what part of the visual pathway (i.e., the retina, the lateral geniculate body, or the visual cortex) might be responsible for the illusion?
5. What do the results of your monocular vision experiment suggest about the source of the illusion?
6. Does the strength of the illusion depend on the number of intersections present in the display? Can you suggest an explanation for your observation?
7. What does adding diagonal lines across the grid squares (the Lingelbach illusion) do to the illusion? Is the effect the same as you observed with the Hermann grid?
8. Work with the Jung version of the grid (you'll probably want to use relatively wide (white) lines (10 to 15) and small (black) squares (20 to 30) for this, but experiment around. Compare your results with what you observed in Exercise #1, and with the results of the Jung version of the Hermann grid. Observe the grid while tilting your head 45o to the right or left, and compare your observations with those you made in Exercise #6.
9. Does the color of the squares and/or grid lines affect the illusion? Are the results obtained when the squares are 'lighter' than the grid lines different from those obtained when the squares are 'darker' than the grid lines?
10. If you are color-deficient or color blind, compare your results with those of someone who has full color vision. Or, if you know someone who is color-deficient or color blind, have them conduct some of the above exercises to see if color vision is necessary for the perception of the illusions involved in some of the above exercises.
11. Based on your experiments involving colored versions of the scintillating grid, does it appear that the visual tracts involving the cones and their output are structured __ and function __ pretty much the same as the tracts that involve output from the rods? Do their appear to be any significant differences? Justify your responses. 12. Do you think the Scintillating Grid illusion has any applicability in, or consequences for, art or other visually-based media? How about in environmental engineering?