Executive Functioning and the Frontal Lobes

Executive Functioning and the Frontal Lobes

Most researchers now agree that the study of frontal lobes repre-sents an important aspect of attempts to understand of humans’ higher mental functions, such as planning, decision-making, rea-soning and judgment, which are often referred to as executive processes.

The frontal lobes of the human brain comprise all the tis-sue anterior to the central sulcus. Four major subdivisions of the frontal lobes have been suggested: 1) the motor area; 2) the pre-motor area; 3) the prefrontal area, and 4) the basomedial portion of the lobes (Walsh, 1994).

Two prefrontal areas, namely, the orbitomedial prefrontal cortex and the dorsolateral prefrontal cortex, have been targeted in much of the research concerning executive processes. The func-tional role of the frontal system in providing executive control to behavior is probably related to the extensive reciprocal anatomical connectivity between the frontal lobes and other brain regions involved in information processing. Numerous afferent and ef-ferent connections of the frontal lobe have been demonstrated. The extracortical and transcortical connections of the frontal lobes are exceedingly complex, especially where the prefrontal cortex is involved (Kolb and Wishaw, 1990). In general, brain connectivity encompasses three major types: cortical–cortical, thalamic–cortical, and subcortical–cortical (Figure 15.6). For a detailed review of brain connectivity, the reader is referred to the works of Pandya and Yeterian (1984) and Goldman-Rakic (1988).

Cortical–cortical connections of the frontal lobes take on a number of distinct forms (Kolb and Wishaw, 1990). First, as pre-viously mentioned, connections within the frontal lobes them-selves involve projections from tertiary cortex in the prefrontal areas to the premotor cortex and to the motor cortex. Secondly, there are reciprocal connections between the prefrontal cortex (Brodmann’s areas 8, 9 and 46) and the temporal, auditory and visual association regions, as well as the medial temporal lobes. Thirdly, there is another set of reciprocal connections connecting the prefrontal areas and the anterior and medial temporal regions. Fourthly, there are connections between prefrontal areas and the limbic system, including a reciprocal connection between the amygdala and the frontal lobe.

Thalamic–cortical connections include projections to the prefrontal lobe from the pulvinar, anterior nuclei and dorsomedial nucleus of the thalamus. In addition, via the dorsomedial nucleus, information from limbic areas and the hypothalamus is relayed to the frontal lobes for processing of emotions and internal states.

Subcortical–cortical connections include projections from the frontal cortex to various subcortical structures includ-ing the caudate nucleus, superior colliculus and hypothalamus. Particular attention has been paid to the interconnectivity of the frontal lobe and basal ganglia via the corticostriate projection system. Lesions in either area are associated with similar cogni-tive impairments, such as decreased cognitive flexibility or set switching (Eslinger and Grattan, 1993

Examination of the connectivity of the frontal lobes has revealed that the pattern of connectivity is best viewed from within the context of the parallel and distributed anatomical network (Figure 15.7).