Stratton's inversion experiment

Stratton described two theories that held an inverted image on the retina to be necessary for seeing things upright. The projection theory traces light rays back into space, and the eye-movement theory ties up and down to the eye movements needed to bring a point into clear view.

His question was whether an inverted retinal image is a necessary condition of seeing things upright. His method was to replace the normal inverted image with an upright one and watch what happened.

He read the first report at the Third International Congress for Psychology in Munich in August 1896, and it appeared in the November 1896 issue of Psychological Review.

Two convex lenses of equal power, set apart by the sum of their focal distances, make a tube through which everything looks inverted. The image on the retina is as if the whole scene had been turned through 180 degrees around the line of sight.

All light not passing through the lenses had to be shut out, so the device was fitted to the face with black linings and pads.

Large thick lenses were too heavy to wear for long. Stratton put a pair of double convex lenses at each end of a short tube, which gave a clear visual field of 45 degrees and was light enough to wear without discomfort.

He used only one eye. Without automatic convergence of two tubes, lining up the two images strained his eyes too much, so the left lens was covered with dull black paper.

For the 1897 experiment the padded case was replaced by a thin cloth-lined plaster cast of the region around the eyes, held on by tapes, with the lens tube fitted in front of the right eye.

In the 1896 trial Stratton bound on the apparatus at 3 o'clock one afternoon and wore it until 10 that night. He wore it again the next day from 9:30 in the morning until about 10 at night, and for two hours on the morning of the third day. The net time was about 21 and a half hours.

The 1897 experiment ran from noon on the first day until noon on the eighth day. After subtracting the time his eyes were blindfolded, he had worn the lenses for about 87 hours.

He wore the lenses for 9 hours on the first day, 12 hours on each of the next two days, and 12 hours 45 minutes on day 4 and 12 hours 40 minutes on day 5. Whenever the glasses were off, his eyes were thoroughly blindfolded.

Stratton was the only observer in both experiments. In 1897 he also walked through the village every evening except the first, with a companion to guide him when needed.

At first the room and everything in it seemed upside down. His hands, stretched out from below, appeared to enter the view from above, and the images seemed misplaced or false rather than real.

Movements were awkward. When he saw an object near one hand and wanted to grasp it with that hand, he moved the other hand, then saw the mistake and corrected it.

With repeated experience he began to use the sight of his feet to expect what they would touch. He could eventually feel his feet strike the floor in the place where he saw it, which was the opposite side of the field from where he first placed those sensations.

By the third day of the 1896 trial the parts of the changing view had been pieced together into a whole, and he could anticipate the order in which objects would enter view when he turned his head.

On the first day of the 1897 trial, he felt a depression in the upper abdomen, akin to mild nausea, which was gone by half past seven that evening.

On the third day of 1897 he repeatedly had to hold back involuntary hand movements toward the wrong tap at the wash-bowl. By the fourth day his hands often moved to the soap or the basin without any correction, and for the first time he preferred to keep the glasses on at 9 in the evening.

Sounds followed the seen position of their source when it was in sight. When he walked and felt his feet strike the floor he saw in the old upper part of the field, the sound of his steps seemed to come from where he felt them strike, a shift of 180 degrees from the old direction.

Stratton wrote that if an inverted retinal image were absolutely necessary for upright vision, it was hard to see how the scene as a whole could even temporarily have looked upright when the image was not inverted. Things that could be seen did repeatedly appear right side up.

The feeling that the field was upside down stayed through most of the 1896 trial. When he gave full attention to outer objects they often seemed normal, and the oddity seemed to lie in himself, as if his head and shoulders were inverted.

He judged the 1896 trial too short to show the final outcome, and he built the longer 1897 experiment partly to settle questions it left open, such as dizziness and the localization of sounds.

Later research on long-term wearing of reversing mirrors and prism goggles was done at Innsbruck by Theodor Erismann (1883 to 1961) and Ivo Kohler (1915 to 1985). It is regarded as a milestone in research on adaptation and after-effects.