Band form
Like focal subcortical heterotopia, "band" heterotopia form in the white matter beneath the cortex, but the gray matter is more diffuse and is symmetric between the hemispheres. On imaging, band heterotopia appears as bands of gray matter situated between the lateral ventricle and cerebral cortex and separated from both by a layer of normal appearing white matter. Band heterotopia may be complete, surrounded by simple white matter, or partial. The frontal lobes seem to be more frequently involved when it is partial. Patients with band heterotopia may present at any age with variable developmental delay and seizure disorder, which vary widely in severity. Subcortical band heterotopia, also known as "double cortex" syndrome, refers to a band of subcortical heterotopia neurons, located midway between the ventricles and the cerebral cortex. The disorder is seen primarily in females and typically causes varying degrees of intellectual disability and almost all of them have epilepsy. Approximately two thirds of patients with epilepsy ultimately develop intractable seizures. MRI of the brain in subcortical band heterotopia demonstrates two parallel layers of gray matter: a thin outer ribbon and a thick inner band, separated by a very thin layer of white matter between them. The severity of epilepsy and developmental delay is directly correlated with the degree of migration arrest, as indicated by the thickness of the subcortical band heterotopia. Subcortical band heterotopia is caused by mutations in the microtubule-associated DCX gene.
The nervous system originates as a tiny, simple open tube called the neural tube;  the front of this tube develops into the brain (and retinas of the eye), while the spinal cord develops from the very back end. Neurons begin to form early, but most of them become structural rather than active nerve cells. The brain generally forms from the inside-out, especially in the case of the neocortex. The difficulties arising from this are readily apparent, as each successive layer of cells must travel through the previous layer to reach its destination. Therefore, nervous tissue develops ladders made of radial glial cells that neurons climb, through the previous layers, to reach their proper destination. Some destinations, such as the cerebral cortex, even have "placeholder" neurons that travel up the ladder to form a structure;  when the final neurons germinate, they find a correct placeholder and then the placeholder cell dies. Heterotopia
The complexity of neural development makes it fraught with opportunities for error. Grey matter heterotopia is such an example. It is believed that gray matter heterotopia are caused by arrested migration of neurons to the cerebral cortex; that is, when neurons that are supposed to form part of the cerebral cortex. fail to climb to the end of their ladder correctly and are permanently situated in the wrong location. Gray matter heterotopia are common malformations of cortical development classed as neuronal migration disorders. Heterotopias are classed in two groups: nodular and diffuse. Nodular types are subependymal and subcortical; diffuse types are termed band heterotopias.