The human cerebral cortex represents, in some ways, the pinnacle of evolution. In addition to containing networks of neurons related to the initiation of movement and to sensation from the body and the special sensory organs, the cortex is the substrate for functions that include comprehension, cognition, communication, reasoning, problem-solving, abstraction, imagining, and planning.
The frontal lobes contain phylogenetically “new” parts of the cortex, and serve as an “executive” part of the cortex. They participate in higher order functions that include reasoning and abstraction; planning and initiating of activity; monitoring and shaping of behavior to ensure adaptive actions; inhibiting maladaptive behavior; prioritizing and sequencing actions; problem solving; and coordinating elementary motor and sensory functions into a coherent and goal-directed stream of behavior.
Damage to the frontal lobes (as can occur, eg, with brain tumors or head trauma) can produce profound behavioral changes. Several syndromes are especially common: Following damage to the dorsolateral part of the frontal lobes (the convexity), patients tend to become indifferent, abulic, or apathetic (mute and motionless in some cases). Following damage to the orbitofrontal area of the cortex, there is a syndrome of disinhibition, in which the patient appears labile and irritable. These patients are inattentive and distractible, with impaired judgment and loss of the usual inhibitions and social graces. Damage to the medial part of the frontal lobes can produce a syndrome of akinesia (lack of spontaneous movements) and apathy. Injury to the basal part of the frontal lobes can also result in impairment of memory. These frontal lobe syndromes are more frequently seen in patients with bilateral lesions.
Language is the comprehension and communication of abstract ideas. This cortical function is separate from the neural mechanisms related to primary visual, auditory, and motor function.
The ability to think of the right words, to program and coordinate the sequence of muscle contractions necessary to produce intelligible sounds, and to assemble words into meaningful sentences depends on Broca's area (areas 44 and 45) within the inferior frontal gyrus, located just anterior to the motor cortex controlling the lips and tongue.
The ability to comprehend language, including speech, is dependent on Wernicke's area. This area is located in the posterior part of the superior temporal gyrus within the auditory association cortex (area 22).
The arcuate fasciculus provides a crucial are-shaped pathway within the hemisphere white matter, connecting Wernicke's and Broca's areas (Fig 21–1). Because the arcuate fasciculus connects the speech comprehension area (Wernicke's area) with the area responsible for production of speech (Broca's area), damage to this white matter tract produces impairment of repetition.
Central speech areas of the dominant cerebral hemisphere. Notice that Broca's and Wernicke's areas are interconnected via fibers that travel in the arcuate fasciculus, subjacent to the cortex.