Ohio State University's news article on the science behind the discovery gives more information.
In order to answer their questions, the researchers needed to determine how the apparent 3D structure of the object depicted in the picture was influenced by changes in the viewing direction. They were particularly interested whether points that appeared to be closest or farthest in depth relative to other neighboring points would remain the same when the picture was observed at different viewing angles. They also wanted to determine how the relative magnitude of the perceived depth in different regions of the picture would be affected when viewed at different angles.
In order to address these issues, the researchers did two types of tasks. In one, they moved a dot around on the computer screen to show which points on the torso appeared to be nearest and which appeared to be the furthest away. The researchers did this hundreds of times to find near points and far points on various parts of the torso.
In a second task, they used a gauge figure (a circle with a needle sticking out) that had to be placed on the torso so it looked to be flat against the surface (The needle had to appear like it was perpendicular to the surface of the torso). This allowed the researchers to determine how viewers perceived the 3D shape of the depicted object.
