What area of the cortex controls speech?
Linguistic intelligence
Speech production is process by which a thought in the brain is converted into an understandable auditory form. This is a multistage mechanism that involves many different areas of the brain. The first stage is planning, where the brain constructs words and sentences that turn the thought into an understandable form. This occurs primarily in the inferior frontal cortex, specifically in an area known as Broca's area. Next, the brain must plan how to physically create the sounds necessary for speech by linking the planned speech with known sounds, or phonemes. While the location of these associations is not known, it is known that the supplementary motor area plays a key role in this step. Finally, the brain must signal for the words to actually be spoken. This is carried out by the premotor cortex and the motor cortex.
In most cases, speech production is controlled by the left hemisphere. In a series of studies, Wilder Penfield, among others, probed the brains of both right-handed (generally left-hemisphere dominant) and left-handed (generally right-hemisphere dominant) patients. They discovered that, regardless of handedness, the left hemisphere was almost always the speech controlling side. However, it has been discovered that in cases of neural stress (hemorrhage, stroke, etc.) the right hemisphere has the ability to take control of speech functions.
Conduction aphasia
Recent research has pointed to a different explanation for conduction aphasia, similar to Wernicke's, which is based on newer models suggesting language is facilitated by "cortically based, anatomically distributed, modular networks." Anderson et al. describe an experiment in which electrical stimulation of the left posterior superior temporal cortex in a human subject induced symptoms consistent with conduction aphasia, indicating that a deep brain disconnection is not necessary. While this study does not completely discredit the disconnection hypothesis, but does point to a system in which transmission of spoken language information involves more than just the arcuate fasciculus. Regardless of the role that the arcuate fasciculus plays in the disorder, the cortical component cannot be denied.