I was happily absorbed in my slightly vegetative stupor on the couch when my roommate walked into the room and starting talking about physics. Ugh, physics, I thought, but I politely listened as she began talking about lenses, specifically how they are related to sight. It is common knowledge that the images we see are inverted on the retina, and then further processed. However, my roommate was discussing experiments done on humans that inverted their vision by 180 degrees and found that, though at first they could not function normally, eventually they adapted. I thought this was fascinating, and wondered what the brain had to do with this process. Unfortunately my roommate’s knowledge was pretty limited, so I decided to do some research of my own.
Research on visual distortion of the retina has been going on for quite some time. Devices have been used that invert the retinal image, so that everything is seen upside down. At first subjects wearing this device will reach for things and miss, or will bump into things as they travel about a space. Eventually, they adapt. What I wanted to know is how do they adapt? What changes take place in the brain that allow them to do so? Is it just simply learning to reach a little farther to grab something, or walk a bit differently to avoid bumping into something? What is happening at the neural level?
What I discovered is that in order to make sense of the incorrect efferent sensory information, the subject must rely on perceptual and proprioceptive feedback (proprioceptive refers to the position sense, which is what allows us to perceive the location of various parts of the body). However, while the subject can adapt to this altered state of vision, it is not accompanied by a return to upright vision. Researchers found that the contralateral posterior parietal cortex is activated when subjects were trying to reach objects while their vision was distorted, and thus has an important role in visuospatial processing. Other studies indicated that there was no evidence of remapping of retinal coordinates. Further research needs to be conducted on neurons to see what (if anything) is occurring at the neural level. However, due to the rapid recovery of normal function after vision was restored, it seems unlikely that any major modifications were made. What I learned from this investigation is that changes seem to be taking place at the processing level that allow subjects to adapt. However, how this occurs still remains to be investigated.
Linden D.E.J., Kallenbach U., Heinecke A., Singer W., Goebel R. 1999. The myth of upright vision. A psychophysical and functional imaging study of adaptation to inverting spectacles. Perception 28(4) 469 – 481.
Harris, C.S. 1965. Perceptual adaptation to inverted, reversed, and displaced vision. Psychological Review 72(6) 419-444.