The Hermann Grid Visual Illusions

Introduction

This simulation allows you to view one of the classic visual illusions, the Hermann Grid Illusion. Although the image that produces the illusion is quite simple, consisting simply of a grid of black (or white) squares separated by white (or black) lines, the simplicity is misleading. This illusion is still employed today, sometimes in modified form, by scientists attempting to develop an understanding of how the essentially on-off response of rods and cones to photons of light gets translated into conscious perception of visual images. Because the mechanistic basis for the Hermann grid illusion is believed to lie in the way retinal receptive fields respond to stimulation of their centers and lateral surrounds, working with this illusion will help give you a better understanding of retinal receptive fields and how they function. And, in the bigger picture, understanding the visual system helps us understand many other sensory modalities as well.

One of the benefits of this particular simulation is that it lends itself to hypothesis testing, because it's easy to test predictions that you make based on a hypothesis and its assumptions. This is, of course, how science works:

Repeatable observations è proposed explanation ( = the hypothesis) è testable predictions developed from the hypothesis è experimental test(s) of the predictions è support or refutation of the hypothesis, depending on whether the predictions are confirmed or not.

I encourage you to make some predictions based on the Simultaneous Contrast Hypothesis, then use the simulation to run some experiments and see if your predictions bear out under your experimental scrutiny.

This simulation is intended to be a fun, enjoyable learning experience. Therefore, I want you to play around with it, going where your thoughts take you. However, don’t lose sight of the fact that serious research in the field of perceptual psychology has, and still is, being done with figures that are based on the Hermann grid. Actually, in a very real sense you will be conducting your own original research project when you work with this simulation.

What Does The Hermann Grid Simulation Allow You To Do?

This simulation allows you to work with the basic Hermann grid simulation, both in its standard form (black squares with white gridlines) and in it 'reversed' form (white squares with black gridlines). However, the color of the squares and the grid lines can be varied independently, as can the size of the squares and the width of the grid lines.

Finally, this simulation allows you to work with three modified versions of the basic grid. These produce some surprising changes in the appearance of the illusion and have been used in a number of research protocols over the years:

1. Lingelbach et al. 1985 – this version combines the Ehrenstein figure (i.e., a series of intersecting diagonal lines that cross the field of vision; Ehrenstein, 1954) with the basic Hermann grid.

2. Dombrowski, 1942 – this version uses grid squares with rounded corners (this may be difficult to see on some monitors, particularly when using small grid squares).

3. Jung, 1973 – in this version, each intersection is bounded by a square that is the same color as the grid squares.

These modified Hermann grids are particularly useful for conducting experiments designed to test the Simultaneous Contrast Hypothesis. As you will see, some of them yield data that seem inconsistent with this most widely accepted explanation for the Hermann Grid illusion.

What Does The Hermann Grid Simulation Display?

When you initiate a simulation run, you will see a grid consisting of black squares separated by white grid lines. Below the grid are two slider/textfield combinations that let you control the size of the grid squares and the width of the grid lines that separate them. As with other simulations you've already performed, you may change the value of these two parameters either by moving the slider or by typing the desired value in the textfield. On the right side, top to bottom, you will see (i) checkboxes that let you change the color of the squares and the grid lines independently of each other, (ii) checkboxes that let you simulate alternate versions of the basic Herman grid that other authors have developed for use in their research, and (iii) the usual control buttons.