2.1 Introduction

Eloquent cortical areas are areas of the cortex that if removed may result in loss of linguistic ability, motor function, or sensory perception. These areas commonly include the precentral gyrus (primary motor cortex), postcentral gyrus (primary sensory cortex), supplementary motor area (speech and motor function), perisylvian area (language), medial occipital lobe (primary visual cortex), and medial temporal lobe (memory). Eloquent cortical area will depend also on whether the area is in the dominant hemisphere, as in the case of speech areas. Although the whole cortex may be regarded as eloquent if we consider function, we use the term eloquent to distinguish specific areas of the brain that carry a higher risk of morbidity and disability in the postoperative period.

The localization of function and certain anatomical cortical regions, such as Broca area, is variable among individuals and the surgeon depends upon cortical stimulation and cortical mapping to correlate function and anatomy with certainty. ¹ Localization of function cannot depend only on anatomical landmarks. However, knowledge of the anatomy of the sulci and gyri of the brain provides the surgeon with several key elements to plan procedures. ^{2 ,​ 3 ,​ 4 ,​ 5} First, understanding the relation of the tumors with the sulci and gyri is helpful in planning the craniotomy for tumor resection. ² Second, tumor location and extensions are often correlated with the anatomy of the gyri, as tumors are often located in a specific gyrus or lobe, and tumors are known to extend depending on the cytoarchitecture of the area where they originated. ⁴ Examples are tumors extending in the limbic lobe and tumors commonly spreading from the opercula to the insula. Third, there is a relationship between brain structure and brain function that allows the surgeon to plan in advance which intraoperative monitoring may be necessary for specific brain regions. ^{1 ,​ 6 ,​ 7 ,​ 8}

The cerebrum is commonly divided into five lobes: frontal, temporal, parietal, occipital, and insula. Yasargil ⁴ proposed a division into seven cerebral lobes: frontal, central (precentral, postcentral, and paracentral gyri), parietal, occipital, temporal, insular, and limbic. Yasargil’s division was a surgical conception of the cerebrum, taking in consideration function and the embryological aspects of cortical organization. We follow Yasargil’s division, since this separates the central lobe in a distinct lobe, highlighting its importance as the primary sensory–motor area. We discuss the anatomy of the opercula and the insula as we review the anatomy of the Sylvian fissure. We review the anatomy of the cortical arteries as they relate to the sulci and gyri.

Although there is great variation in the anatomy of the sulci and gyri among individuals, there is a common pattern in the organization of the sulci and gyri of the cerebrum that can be recognized and studied. ^{3 ,​ 4 ,​ 5} Only four sulci are consistently uninterrupted: the Sylvian fissure, the collateral sulcus, the callosal sulcus, and the parieto-occipital sulcus. The central sulcus and the calcarine sulcus are uninterrupted in 92% of the cases. ^{4 ,​ 5} Because most of the sulci are interrupted, the anatomical boundaries of the gyri are not always clearly demarcated. Often, we consider a gyrus as areas of the brain consisting of several gyri, as in the case of the paracentral and medial frontal gyri. One gyrus may be continuous in another surface of the hemisphere: the inferior temporal gyrus (both lateral and basal surface of the temporal lobe) with the parahippocampal gyrus (both medial and basal surface of the temporal lobe).

2.2 Central Lobe

The central lobe is formed by the precentral and postcentral gyri on the lateral surface and by the paracentral lobule on the medial surface of the hemisphere. ^{3 ,​ 4 ,​ 8}

2.2.1 Lateral Surface

The central lobe on the lateral surface of the hemisphere includes the precentral and postcentral gyri, divided by the central sulcus (Fig. 2‑1). The central lobe is one of the most important eloquent area of the brain, as it corresponds to the primary motor (precentral gyrus) and sensory (postcentral gyrus) area of the cortex. The anterior and posterior limits of the central lobule are the precentral and postcentral sulci, respectively. The central sulcus originates at the medial hemisphere and runs on the lateral surface from a posterior to an anterior direction toward the Sylvian fissure (Fig. 2‑1). The central sulcus usually does not reach the Sylvian fissure and it is separated from the fissure by a continuation of the precentral gyrus with the postcentral gyrus, called subcentral gyrus. Parallel to the central sulcus there are two interrupted sulci, one anterior (the precentral sulcus) and another posterior (the postcentral sulcus). The central sulcus is usually continuous and has a sinusoidal shape with three curves (Fig. 2‑2). The first curve is near the midline and here the sulcus has its convexity facing anteriorly. Then it curves again, making the middle genu, with its convexity facing posteriorly. Finally, the third curve has its convexity facing anteriorly. The precentral gyrus has the shape of an inverted Greek letter omega (ʊ) at the level of the second curve of the central sulcus, where the convexity of the sulcus is facing posteriorly (Fig. 2‑2). The omega on the precentral gyrus is where the motor representation of the hand is located. ⁶ The omega is easily seen on the CT or magnetic resonance imaging (MRI) scans because deeply inside the central sulcus there are two parallel sulci that run toward the base of the central sulcus on the superior and inferior aspects of the omega, giving its shape even in deeper cuts ⁶ (Fig. 2‑2a, b). The omega is also called the central knob. Another important anatomical relationship in this area is that the posterior part of the superior frontal sulcus ends at the level of the omega. After its third curve, the central sulcus continues inferiorly toward the Sylvian fissure in a sinusoidal line. ^{9 ,​ 10 ,​ 11} The part of the precentral gyrus in front of the last segment of the central sulcus is where the motor representation of the tongue is usually located. ⁹ Also, characteristic is the bifurcation of the superior end of the postcentral sulcus with the marginal ramus of the cingulate gyrus located between this bifurcation ¹⁰ (Fig. 2‑2).

2.2.2 Medial Surface

On the medial surface of the hemisphere the central lobule has a quadrangular shape and its gyri are called the paracentral gyrus or lobule (Fig. 2‑3). This quadrangular shape is given by the limits of the paracentral gyrus: the cingulate sulcus inferiorly, the paracentral sulcus or ramus anteriorly, and the marginal ramus posteriorly. The paracentral sulcus has an upward direction and it is a sulcus that originates from the cingulate sulcus at the level of the middle of the corpus callosum. The marginal ramus is the posterior part of the cingulate sulcus as it curves upward at the level of the splenium of the corpus callosum. The most posterior part of the marginal ramus near the lateral surface is located at the level of the postcentral gyrus. The marginal ramus can be identified in the MRI in the middle of the bifurcation of the postcentral sulcus. The paracentral gyrus includes the continuation of the precentral and postcentral gyri on the medial surface. The supplementary motor area is an area that does not have clear boundaries, but it includes the paracentral gyrus anterior to the precentral gyrus and the posterior part of the superior frontal gyrus. ¹² Stimulation in this area may cause complex postural movement, arrest of movement, or speech arrest. The supplementary area syndrome consists of reversible contralateral weakness and mutism following resection of the dominant supplementary motor area. ¹²

2.3 Frontal Lobe

The frontal lobe includes the superior, middle, and inferior frontal gyri on the lateral surface; the orbital and rectus gyrus on the inferior surface; and the medial frontal gyrus on the medial surface of the hemisphere.

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1 The Early History of Intraoperative Brain Mapping 3 Anatomy of Eloquent White Matter Tracts 9 Awake Craniotomy Operating Room Setup and Surgical Instruments 14 Surgery Around the Command and Control Axis: The Default Mode, Control, and Frontal Aslant Systems 12 Motor Mapping (Rolandic, Pre-Rolandic, and Insular Cortex) 21 Emergence of Deep Learning Methods for Deep Brain Stimulation–Evoked Functional Connectomics

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