(2002). However, to differentiate between the ventrolateral region below the principal sulcus and the lateral orbital cortex we will use 12 ventrolateral (12vl) and 12 orbital (12o), respectively, as defined by Preuss and Goldman-Rakic (1991).
The VLPFC also commonly includes a small sulcus—termed the inferior frontal sulcus by Winters et al. (1969) and the infraprincipal dimple or the inferior prefrontal dimple (IPD) by others (Paxinos et al., 2000; Petrides and Pandya, 2002; Petrides et al., 2005)—which varies in its position and depth in M. mulatta. Some schematics of VLPFC depict the IPD as running in a rostral-to-caudal direction and separating area 45A from area 46 (Petrides and Pandya, 2002). However, in our neurophysiological recordings (Romanski and Goldman-Rakic, 2002; Romanski et al., 2005) and in other studies (Petrides et al., 2005), it is depicted as running dorsal to ventral and separating area 45 from area 12/47. It is not always described or visible in studies of other subspecies of macaque monkeys. Thus, there is variability in the position of area 12/47 and area 45 not only between the subspecies of macaques but also within M. mulatta individuals. As explained later, the IPD is the primary location in which auditory responsive neurons and audiovisual responsive cells have been reliably located in several studies, and may be a critical landmark for delineating the functional auditory responsive prefrontal region in macaques. Whether it defines the border of areas 12/47 and 45 is unclear.
In the human brain, areas 44 and 45 have been associated with language processing confirmed by electrical stimulation, PET and functional MRI (fMRI). However, the areas that control vocalization production in Old World monkeys are not as well understood and could include VLPFC, whereas other studies have implicated ventral premotor and the cingulate vocalization area (Petrides et al., 2005; Jürgens, 2009; Coudé et al., 2011).
Cytoarchitectonic Organization of VLPFC
The cytoarchitectonic descriptions here are taken from Preuss and Goldman-Rakic (1991), who described the frontal lobe of the rhesus macaque, M. mulatta, which is the same species that has been examined in most neurophysiology studies of VLPFC. These descriptions are in general agreement with Petrides and Pandya (2002). Area 45 is located ventral to the caudal principal sulcus within the ventral limb of the arcuate and extends onto the cortical surface (Fig. 15.1C–E). It is composed of large pyramidal cells in layer V and deep layer III. Layer IV is thick with densely packed small cells, with some of the larger pyramidal cells from deep layer III and superficial layer V intruding on layer IV. It is densely myelinated. Area 45 is bordered dorsally by area 8a (Preuss and Goldman-Rakic, 1991), which can be distinguished from area 45 by the presence of