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A hydranencephalic infant lacking cerebral hemispheres and a normal twin were tested for associative learning. After repeated trials in which two stimuli were temporally paired, test trials were given in which the second stimulus was omitted. Cardiac orienting responses to stimulus omission indicated that learning had taken place in both infants.

In the monkey, the blood-brain barrier and the blood-aqueous and blood-vitreous barriers of the eye can be opened by internal carotid perfusion of solutions of 2 molar urea in a way compatible with survival and, in some few cases, without detectable neurological deficits. Urea presumably acts by osmotically shrinking the endothelial cells of the cerebrovascular vessels and opening their tight junctions. The high incidence of brain necrosis with neurological sequelae after perfusion of urea by the present technique precludes the use of osmotic opening of the blood-brain barrier for pharmacotherapy at this time.