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Frontal Lobe

Higher mental function, personality, emotions, thinking, insight, attention, memory, language, orientation and action, decision making, initiative, spontaneity, impulse control, social and sexual behavior.

The frontal lobes are important for mental life and for how we think, feel, make decisions, and act. They mature only in the mid- to late twenties. It is especially the myelinated tracts that mature slowly. The degree and structure of myelination vary substantially between people partly because of individual experiences, as in the case of mastering an instrument at expert level. As mentioned in the section “Brain Organization” earlier in this chapter, there is a trade-off between the cell count and myelination density because of the limited space of the skull.

Research indicates that the prefrontal cortex is essential for planning, decision making, reasoning, personality, and social behavior. It is found in the anterior part of the brain and extends back to border the motor areas (the premotor and supplementary cortices). The orbitofrontal cortex (OFC), near the eyes, supports emotional functioning, without which normal decision making breaks down.

The primary motor cortex lies posterior in the frontal lobes and controls movements. From exploratory brain stimulations done in conjunction with his Montreal method,[1] Penfield noted that the primary motor cortex is mapped to the body in an upside-down fashion and illustrated it with his so-called motor homunculus (Schott 1993). The body is unevenly represented, with more motor cortex for dexterous areas (hands, tongue) than for others (back, calves, etc.). The motor cortex is cross- wired—the left motor cortex controls the right side of the body, while the right controls the left. Penfield also found two areas in front of the primary motor cortex: the premotor area, specializing in sense-based motor guidance; and the supplementary motor area, specializing in planning movements. These areas collaborate with many others through complex error-correcting feedback loops. The cerebellum, a structure located at the lower posterior end of the brain, is part of this orchestration, and it helps especially with sequences of fine-tuned, skillful movements. It has more neurons than the rest of the brain while occupying only one tenth of the brain’s total volume. The cerebellum accomplishes high neuronal density through granule cells, one of the smallest brain cell types.

Two other important structures involved in motor function are the basal ganglia and the superior colliculus. They lie outside the frontal lobes but are discussed here as part of the motor system. The superior colliculus has a gaze-directing retinotopic map—local neural activity on it can cause our eyes to move to the corresponding point in visual space. But the basal ganglia must allow it. For much of the time, the basal ganglia inhibit the superior colliculus. During movement, the basal ganglia stop doing this and the eyes move corresponding to the localized neural activity on the retinotopic map of the superior colliculus. What could be the reason for such a mechanism? It makes sense to have a trigger-ready visual system to support responsive interaction with the world. Our brains work much in preparation mode to save time when action is required.

The basal ganglia also have another role in movement, as demonstrated through studies of Parkinson cases. Parkinson patients struggle to initiate movements. Such difficulty derives from dying dopamine-producing neurons in the substantia nigra of the basal ganglia. Dopamine is necessary for movement. As the disease progresses, most of these neurons die and finally movement becomes impossible.

  • [1] A surgery procedure that he invented, during which he systematically stimulated the cortex withan electrode and, after noting the responses, decided where to make incisions so as to spare criticalareas from destruction (Penfield and Jasper 1954).
 
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