Neurocognitive and behavioral outcome at adolescence of 105 infants born very preterm (<33 weeks) was studied and correlated with neonatal ultrasonographic scans and MRI at age 14-15 years, at University College London Medical School, UK. Of 72 cases completing the study, 40 had abnormal and 15 equivocal MRIs, markers of hypoxic-ischemic damage, especially ventricular dilatation, atrophy of the corpus callosum - predominantly posterior, and white matter abnormalities. In contrast, of 21 controls, one had abnormal and 5 equivocal MRI. MRI detected more abnormalities than neonatal ultrasonography (40 of 72 vs 31 of 72). Tests of functional neurodevelopment and behavior at age 14-15 years showed significantly more reading, adjustment, and neurological impairments in the preterm cases than controls, but only the impaired behavior on the Rutter scale was significantly related to structural brain abnormalities on MRI. Four cases and no controls had epilepsy, and all four had abnormal MRI scans. 
COMMENT. Very preterm infants are at high risk of neurocognitive and behavioral problems and MRI abnormalities in adolescence, and abnormalities in the MRI, notably in the corpus callosum, are significantly correlated with neurobehavioral disorders. MRI identifies more brain lesions than does neonatal ultrasonography.
Callosal and cortical contribution to procedural learning has been studied in eleven adult patients, 6 acallosal and 5 callosotomized, at the University of Montreal, Canada . Epileptic foci in frontal or temporal areas occurred in 7 and post-surgical bilateral prefrontal atrophy in one. Performance of a serial reaction time task, involving bimanual or unimanual key-pressing responses to a sequence of repeated visual stimuli, showed that the anterior part of the corpus callosum is crucial for integration and transfer of a procedural visuomotor skill, and the frontal lobes are important for unilateral procedural learning. Acallosal subjects have explicit knowledge of the procedure but are incapable of learning the skill implicitly when interhemispheric integration is required. In agenesis of the corpus callosum subjects, there is a disconnection deficit and a dissociation of declarative (explicit) and procedural (implicit) memory.
Visuomotor learning, a procedural memory task, refers to a motor skill or cognitive routine acquired through practice. This type of implicit memory, requiring the integrity of the frontal lobes and corpus callosum, is expressed by a reduction of reaction time or errors over trials. In contrast, declarative memory, mediated by a corticothalamocortical circuit, is the ability to store and recall visual pictures, words or events. The anatomy of memory is discussed in Progress in Pediatric Neurology II. PNB Publ, 1994;ppl78-9.