Week 2 & 3 Reading: The Ventrolateral Frontal Region Michael Petrides · The lateral frontal cortex is a heterogeneous region comprising several distinct areas that differ both in terms of their cellular architecture (cytoarchitecture) and their connections with other cortical and subcortical areas · The posterior part of the frontal lobe includes several motor and premotor areas that lie mostly on the precentral gyrus · The compact layer of small neurons (layer IV) clearly separating the pyramids of layer III from those of layer V in primary sensory cortex and other isocortical areas is difficult to discern in the motor/premotor areas because these small neurons are intermixed with larger pyramidal neurons, leading to the tradition of referring to these areas as "agranular." - However these areas are not actually agranular · In front of the motor regions are several areas with a distinct layer IV, which is referred to was the granular frontal cortex or the prefrontal cortex . The caudal part of the PFC on the superior and middle frontal gyri is occupied by Area 8 (Attentional Processes); in front is Areas 46 & 9/46, which play a role in memory · The dorsolateral areas are not core language areas; the core language areas are the inferior frontal gyrus and the ventral part of the precentral gyrus; together this is the ventrolateral frontal region . The ventral part of the primary motor cortex (BA 4) represents orofacial musculature; it is hidden in the central sulcus - Immediately in front of this area lies the pars opercularis (BA 44) and in front of BA 44 is BA 45 (the pars triangularis); these two parts make up the Inferior Frontal Gyrus (Broca's Region) - It is hard to know specifically which parts of the IFG are the precise areas for language because lesions are rarely restricted to specific subdivisions of the IFG · Broca's Aphasia is a result of massive damage to the territory of the upper division of the middle cerebral artery and involves not only the cortical structures in the posterior part of the inferior frontal gyrus (i.e., areas 44 and 45) but also the adjacent frontoparietal opercular region and the anterior parts of the insula · The evidence that has best supported this comes from electrical stimulation, which can produce Dysphasic Speech Arrest
· Dysphasic Speech Arrest typically occurs from stimulation of the Pars Opercularis (BA 44), though it can also occur from BA 45 · Stimulation of the ventral precentral region (where the orofacial musculature is represented) also interferes with speech · Such studies have also established the importance of the supplementary motor area, which lies on the posterior part of the dorsomedial surface of the frontal lobe - BOTH vocalization and speech interference can occur from stimulating this area Human 8B GDC 4 6DR 9/464 RA 9/46 4 6V 46 ( Monkey 9 6DC 4 6DF 8B 9/46d BAd 46 10 9/46 BAY 4 454 -44 6VC 458 6VR 44 45A 10 47/12 6YR 47/12 BA