forebrain hypoplasia MP:0000784 MPheno.ontology hypoplastic forebrain prosencephalon hypoplasia underdevelopment or reduced size, usually due to a reduced number of cells, in the anterior of the three primary divisions of the developing chordate brain or the corresponding part of the adult brain (in vertebrates, includes especially the cerebral hemispheres, the thalamus, and the hypothalamus and especially in higher vertebrates is the main control center for sensory and associative information processing, visceral functions, and voluntary motor functions) decreased forebrain size