Eventually, the instructor notes for each chapter will include: learning objectives, suggested activities, elaboration on further explorations, suggested answers for the questions, and more references. For now, these pages contain notes to myself, observations about the material, and to-do lists. I welcome suggestions from beta testers: please email me at rwytten@emory.edu (Bob Wyttenbach).
The Hering (and maybe Scintillating) grid has a straightforward explanation based on center-surround cells. However, distorting the grid (via twist) weakens or eliminates the illusion. Skew has little effect and rotation has none. See Michael Bach web site for linked publications, which may give alternate explanations. Students can explore this with Twisted Grid. Can point them in that direction by assigning or discussing the Bach papers. Or (maybe better) prompt them to make predictions based on the center-surround model: should it be affected by grid skew or twist? Have them test those predictions, along with any predictions about size and color. I am considering a hexagonal Hering grid, where the spots seem weaker (I once considered a variable-spoke array but did not implement it). Could compare 3, 4, and 6 way intersections with hexagons, squares, and triangles. Could add variants (e.g. twist) to the center-surround (students can do this by saving a grid and then uploading to center-surround).