In each HEMISPHERE, the CEREBRAL CORTEX is divided into four lobes, named after the cranial bones (see SKULL) overlying the cortex. If present in a given species, the CENTRAL SULCUS is the anatomical landmark that defines the posterior extent of the frontal lobe. In species with LISSENCEPHALIC brains, the boundary of frontal cortex must be defined functionally as the boundary between PRIMARY MOTOR CORTEX and PRIMARY SOMATOSENSORY CORTEX.
As with the other lobes of the brain, within the frontal lobes are areas of primary, secondary and association cortex. In the human brain, primary motor cortex is anterior to the central sulcus in the PRECENTRAL GYRUS. Secondary motor cortex (PREMOTOR CORTEX, SUPPLEMENTARY MOTOR GORTEX) occupies the cortex adjacent and rostral to primary motor cortex. The most anterior and the largest portion of the human frontal lobe is higher-order association cortex (PREFRONTAL CORTEX and LIMBIC CORTEX), which can be divided into many functionally distinct sub-regions and includes, for example, BROCA’S AREA. LIMBIC ASSOCIATION CORTEX, which includes ORBITOFRONTAL CORTEX, occupies the ventral and medial aspect of the frontal lobe, but also extends to other cortical lobes, all of which is sometimes referred to collectively as the LIMBIC LOBE. However, as the definition of the four cortical lobes is based on structural landmarks, in contrast to the definition of the limbic lobe, which is based on connectivity and function, it is not, strictly speaking, equivalent to a lobe. There is a hierarchical organization of projections within the frontal lobe, with prefrontal and limbic association areas receiving projections from areas of sensory association cortex in the other lobes.
Limbic association cortex projects to prefrontal cortex, which projects to secondary motor cortex, which in turn projects to primary motor cortex.
The frontal lobes control motor output. This is most obviously true of primary motor cortex, which projects directly to the SPINAL CORD (though the bulk of cortical motor out put makes synaptic contact with subcortical brain tissue—the BASAL GANGLIA, PONS and MEDULLA for example). It is also true of secondary motor cortex, which is active in response preparation: lesions here impair bi-manual co-ordination, response sequencing and other complex motor acts. The functions of the prefrontal cortex are more difficult to define, in part because prefrontal cortex is large and heterogeneous with different functions being associated with different portions of prefrontal cortex. The influence of prefrontal cortex on motor output is perhaps better characterized as response, rather than motor, control. Broca’s area is involved in SPEECH PRODUCTION. DORSOLATERAL PREFRONTAL CORTEX is involved in WORKING MEMORY (which may or may not have a spatial component, different areas of prefrontal cortex being implicated accordingly), response planning and response flexibility. Patients with frontal lobe lesions often show disinhibition of responses. Deficits on the WISCONSIN CARD-SORT TEST are typical following damage to prefrontal cortex, characterized by an increase in perseverative responses (see PERSEVERATION). Limbic association cortex is thought to be part of the system wherein information relating to MOTIVATION can influence response output, with damage to orbitofrontal cortex being associated with changes in emotional reactivity and personality (see EMOTION; SOMATIC MARKER HYPOTHESIS). Nevertheless, the definition of the frontal lobe is structural rather than functional and so it is not reasonable to talk about the function of the frontal lobes in anything other than very general terms.
