Planum Temporale

The planum temporale is an area of the human brain. It is the cortical area just posterior to the auditory cortex (Heschl's gyrus) within the Sylvian fissure. It is a triangular region which forms the heart of Wernicke's area, one of the most important functional areas for language.

The planum temporale shows a significant asymmetry. In 65% of all individuals the left planum temporale appears to be more developed, while the right planum temporale is more developed in only 10%. In some people’s brains, the planum temporale is more than five times larger on the left than on the right, making it the most asymmetrical structure in the brain. Evidence for this asymmetry has also been seen in great apes.

This greater size of the left planum temporale compared with the right is already present in the fetus, where it can be observed starting from the 31st week of gestation. This observation strengthens the hypothesis of a genetic predisposition for brain asymmetry, however the effect of fetal experience has not been ruled out.

The planum temporale seems to be symmetrical in dyslexics, which may indicate that their low specialization in the left hemisphere is a cause of their disability. This symmetry also holds for stutterers, although it is also possible to see a larger right planum temporale in stutters. It is thought that this bias for right hemisphere could be interrupting or impeding information flow between Wernickes and Broca's, which are on the left hemisphere.

However, more and more research is suggesting that the apparent asymmetries in this region are the result of old techniques and criteria used to identify the planum temporale. When new imaging is used that takes into account asymmetries in the curvatures of lateral fissures, the hemispheric asymmetries of the planum temporale become negligible. This newer imaging would indicate that the size of the region would not explain the higher faculties of language in the left hemisphere, but would instead require an analysis of the neural circuitry.