Area 8 is concerned with turning the eyes and head contralaterally. Area 44 of the dominant hemisphere (Broca area) and the contiguous part of area 4 are “centers” of motor speech and related functions of the lips, tongue, larynx, and pharynx. Left-sided lesions cause a distinctive articulatory and language syndrome, and bilateral lesions in these areas cause paralysis of articulation, phonation, and deglutition. The medial-orbital gyri and anterior parts of the cingulate and insular gyri, which are the frontal components of the limbic system, take part in the control of respiration, blood pressure, peristalsis, and other autonomic functions. The most anterior parts of the frontal lobes (areas 9 to 12 and 45 to 47), sometimes referred to as the prefrontal areas, are particularly well developed in human beings but have imprecisely determined functions. They are not, strictly speaking, parts of the motor cortex in the sense that electrical stimulation evokes no observable movement (the prefrontal cortex is said to be inexcitable). Yet these areas are involved in the initiation of planned action and executive control of all mental operations, including emotional expression.
The frontal agranular cortex (areas 4 and 6) and more specifically, pyramidal cells of layer V of the preand postcentral convolutions provide most of the cerebral efferent motor system that forms the pyramidal, or corticospinal, tract (see Figs. 3-2 and 3-3). Another massive projection from these regions is the frontopontocerebellar tract. In addition, there are several parallel fiber systems that pass from frontal cortex to the caudate and putamen, subthalamic and red nuclei, brainstem reticular formation, substantia nigra, and inferior olive, as well as to the ventrolateral, mediodorsal, and dorsolateral nuclei of the thalamus. Areas 8 and 6 are connected with the ocular and other brainstem motor nuclei and with identical areas of the other cerebral hemisphere through the corpus callosum. A tract, the fronto-occipital fasciculus, connects the frontal with the occipital lobe and the uncinate bundle connects the orbital part of the frontal lobe with the temporal lobe.
The granular frontal cortex has a rich system of connections both with lower levels of the brain (medial and ventral nuclei and pulvinar of the thalamus) and with virtually all other parts of the cerebral cortex, including its limbic and paralimbic parts. As to its limbic connections, the frontal lobe is unique among cerebrocortical areas in that electrical stimulation of the orbitofrontal cortex and cingulate gyrus has manifest effects on respiratory, circulatory, and other vegetative functions. These parts of the frontal cortex also receive major afferent projections from other parts of the limbic system (Papez circuit), presumably to mediate the emotional responses to sensory experiences; they, in turn, project to other parts of the limbic and paralimbic cortices (hippocampus, parahippocampus, anterior pole of the temporal lobe), amygdala, and midbrain reticular formation. Chapter 24 describes these frontal–limbic connections in greater detail.
Most of the popular notions relating to the function of the frontal lobes are oversimplified. In the frontal lobe are presumed to reside the mechanisms that govern personality, character, motivation, and our unique capacities for abstract thinking, introspection, and planning. These qualities and traits do not lend themselves to easy definition and study and certainly not to discrete localization. Most are too subtle to isolate or even to measure accurately. Except for the more posterior frontal mechanisms subserving motility, motor speech, and certain behaviors relating to impulse (conation), neurologists recognize that the other features of frontal lobe disease are more abstruse.