Speech Production and Comprehension
Our knowledge of the physiology of language has been obtained primarily by observing the effects of brain lesions on people’s verbal behavior
Verbal behavior is a lateralized function
In over 95 percent of right-handed people the left hemisphere is dominant for speech
In over 70 percent of left-handed people the left hemisphere is dominant for speech
The left hemisphere is more specialized for the analysis of sequences of stimuli occurring one after the other. The right hemisphere is more specialized for the analysis of space and geometrical shapes and forms. Since speech is certainly sequential involving sequences of words which are composed of sequences of sound, it makes sense for the left hemisphere to specialize in speech

Speech Production
Being able to talk requires several abilities
First the person must have something to talk about, if it is something current it involves our perceptions, if it is something that happened in the past it involves our memories
Since both perceptions of current events and memories of events that occurred in the past involve brain mechanisms in the posterior part of the cerebral hemispheres these regions are largely responsible for us having something to say
The conversion of perceptions, memories, and thoughts into speech makes use of neural mechanisms located in the frontal lobes
Therefore, Broca’s area in the inferior left frontal lobe and Wernicke’s area in the left temporal lobe are the primary speech areas of the brain

Speech Comprehension
Comprehension of speech obviously begins in the auditory system which detects and analyzes sounds
Recognizing a spoken word is a complex perceptual task that relies on memories of sequences of sound.  This task appears to be accomplished by neural circuits in the middle and posterior portion of the superior temporal gyrus of the left hemisphere.  This area has come to be known as Wernicke’s area
 

Speech Difficulties

Aphasia – difficulty in producing or comprehending speech not produced by deafness or a simple motor deficit; caused by brain damage
Broca’s aphasia – extreme difficulty in speech articulation
Wernicke’s aphasia – poor speech comprehension accompanied by fluent but meaningless speech
Apraxia – Impairment in the ability to program movement of the tongue, lips, and throat required to produces the proper sequence of speech sounds