For species of mammals, larger brains (in absolute terms, not just in relation to body size) tend to have thicker cortices. The smallest mammals, such as shrews, have a neocortical thickness of about 0. 5 mm; the ones with the largest brains, such as humans and fin whales, have thicknesses of 2–4 mm. There is an approximately logarithmic relationship between brain weight and cortical thickness. Magnetic resonance imaging of the brain (MRI) makes it possible to get a measure for the thickness of the human cerebral cortex and relate it to other measures. The thickness of different cortical areas varies but in general, sensory cortex is thinner than motor cortex. One study has found some positive association between the cortical thickness and intelligence. Another study has found that the somatosensory cortex is thicker in migraine sufferers, though it is not known if this is the result of migraine attacks or the cause of them. A later study using a larger patient population reports no change in the cortical thickness in migraine sufferers. A genetic disorder of the cerebral cortex, whereby decreased folding in certain areas results in a microgyrus, where there are four layers instead of six, is in some instances seen to be related to dyslexia.