Frontal lobe functions and dysfunctions.

Abstract

Frontal lobe syndromes with personality change are well known for about 150 years but mysteries of their underlying anatomophysiological mechanisms have started to unfold in recent years. The frontal lobe accommodates an extension of the primary motor (precentral) cortex, the premotor region and the interhemispherically located supplementary motor region, both of them forming area 6 and its subdivisions. The anterior and inferior frontal structures (and also the eye field, area 8) belong to the prefrontal region. The prefrontal cortex is also involved in motor action: in its initiation, planning, designing and sequencing rather than its mere execution. This requires drive and initiative to move, their deficits resulting in moderate to extreme apathy. Forces of motivation are likely to be mediated via the fronto-orbital cortex (known as an extension of the temporo-limbic system) which also exerts inhibitory influences on prefrontal functioning, and disinhibitory frontal lobe syndromes (from tactlessness to aggression) are the result of frontobasal impairment. A special form of frontal-motor cortex disconnection may occur in childhood: in the rare Rett syndrome and, with limitation to the prefrontal cortex, in the common Attention Deficit Hyperactivity Disorder (ADHD). The initiation of movement is based upon the constant influx of afferent-sensory messages from posterior cerebral regions and strong mesiodorsal thalamic connections, providing information for each following initiatory prefrontal motor impulse which depends on such information from the past moment. This "Working Memory" appears to be the keystone of mental and motor prefrontal function. Becoming unnecessary in the next moments, the afferent signals undergo immediate extinction due to inhibitory fronto-orbital impulses. It is assumed that the classical absence attack with generalized spike-waves of frontal onset causes a "suspension of the working memory" and thus permits immediate postictal recuperation of frontal functions. The gap between afferent and efferent signals in the working memory is likely to permit neurocognitive-processes and, in Homo sapiens, a widened gap is thought to provide a basis for reflection and cogitation uniquely human properties.