The purchase of such an electronic LCD may require written permission by the patient's optometrist. It was found that the flicker rates of 9 to 11 Hz that were employed in the TBIS device are slow enough and the stimulus weak enough to avoid any risk of epileptic seizure. In principle alternating occlusion may interfere with normal visual input. Strictly speaking, alternating occlusion is a form of visual deprivation and as such may have negative effects during a critical period of development. Experiments on kittens that were published 1965 by Hubel and Wiesel showed that continuously submitting kittens at a young age to a protocol under which the eyes were alternatingly occluded on a day-by-day basis over a prolonged period of time led to changes in the visual cortex, in particular a disruption in cellular connections that would normally enable binocular vision. Experiments with rearing young kittens under rapidly alternating monocular occlusion using constantly-worn electronic goggles showed that cortical binocularity was reduced if the flicker rate was 2 Hz or lower, and that binocularity developed normally if it was 2.5 or 3 or 5 Hz.