What to notice: The letters in the word “LIFE” appear to tilt left and right. The letters are actually vertical, even though they are made up of little tilted line segments.
Press the button to put red vertical lines on the display. This way you can convince yourself that the letters are indeed aligned.
Brief Comment: The image is my reconstruction of Figure 1 from “A New Visual Illusion of Direction,” written by James Fraser in 1908.
The 100th anniversary of Fraser’s paper is worth commemorating. Many of the illustrations in the paper—like the one above—are a staple in books on illusions.
Fraser worked with two strands of fiber, one black and one white. When the two fibers are twisted together, the resulting cord looks like a series of black and white line segments, all inclined at a similar angle. Fraser referred to the line segments as “units of direction” that could make lines appear to tilt one way or the other (see the image above, for example).
The “units of direction” can also make a collection of circles appear as a spiral. In the example below (from the original paper), the image looks like a spiral, but if you click on the button to place red lines on top of the twisted cords, you can see that the image is composed of individual twisted cords that form circles.
The important point is that the twisted cords can be thought of as a global object (i.e., lines and letters) composed of local features (the line segments). The illusion occurs because the visual system receives different stories from these two sources of information: in the word “LIFE,” the global object (a series of lines) says straight, while the local features (line segments) say tilt. The visual system must create a reasonable percept from the conflicting stories.
There have been many studies that have examined how a global percept is influenced by local features. One of the most revealing is Michael Morgan and Bernard Moulden’s 1986 paper, “The Munsterburg Illusion and twisted cords,” published in Vision Research. Morgan and Moulden digitally filtered a twisted cord image to produce a new image in which the tilt of the line is physically present. That is, if you remove some of the information from the original image, you can measure the tilt in the new image with a ruler.
In some ways, then, twisted cord displays disagree with “reality” only if we are tied to the idea that the line (or the circles) are what is important for vision. If you want to read more about this, I strongly recommend Michael Morgan’s chapter on visual illusions in the book Unsolved Mysteries of the Mind, edited by Vicki Bruce (here is a Google sample from the chapter).
A complete pdf of Fraser’s original paper can be downloaded at this link (the pdf file is 3.5 Mb).