It is normal to think of vision as beginning with the formation of an image on the back of the eye, which in turn stimulates a cascade of nerve impulses that "feed forward" to send signals deep into the brain. It is in the brain's visual cortex that these signals are interpreted. Signals in the visual cortex also travel in the opposite, "feedback," direction, but much less is known about the function of these signals. A recent study by graduate students Tamara Watson and Joel Pearson and their supervisor Dr. Colin Clifford at the University of Sydney suggests that these feedback signals carry information about what we expect to see and that these signals act to constrain our interpretation of incoming visual information.
The separation between our eyes gives us two slightly different views of the world. Ordinarily, our brain will fuse these two views to add depth to our visual world. However, if the two eyes' images are so different that they cannot be fused, we experience "binocular rivalry" a process in which one eye's image is perceived and the other's is suppressed. Every few seconds, perception switches spontaneously between the two images. While binocular rivalry is rarely encountered in the normal visual environment, it provides a useful means of probing the workings of the visual parts of our brain: although an experimental visual stimulus is artificial, the brain is functioning in its usual way. Watson, Pearson, and Clifford used binocular rivalry to demonstrate the importance of feedback in our interpretation of the visual image.