Cortical substrates of perceptual stability during eye movements.

We are usually unaware of retinal image motion resulting from our own movement. For instance, during slow-tracking eye movements, the world around us remains perceptually stable despite the retinal image slip induced by the eye movement. This example of perceptual invariance is achieved by subtracting an internal reference signal, reflecting the eye movement, from the retinal motion signal. If the two cancel each other, visual structures, which do not move, will also be perceived as nonmoving. If, however, the reference signal is too small or too large, a false eye-movement-induced motion of the external world will be perceived. We have exploited our ability to manipulate the size of the reference signal in an attempt to reveal the structures in visual cortex, encoding the perception of self-induced visual motion rather than the retinal motion signal. Using EEG and lately also MEG recordings in human subjects and single-unit recordings in monkeys, we have been able to show that our ability to perceive the world as stationary despite eye-movement-induced retinal image slip is based on "late" parts of the cortical hierarchy of motion processing, sparing the early stages up to cortical area MT and, among others, involving cortex at the junction between the parietal and temporal lobes close to the parieto-insular-vestibular cortex. Lesions of this network in humans render the visual system unable to compensate for the visual consequences of eye movements, giving rise to severe dizziness, whenever the eyes move smoothly.