As noted in the previous chapter, in keeping with a tenet of Dr. Theodore Rasmussen (personal communication), the Rolandic cortex is being considered as an independent entity, because of its obvious physiological differences from the more anteriorly situated frontal cortex and its more posteriorly situated parietal cortex. I suppose it is reasonable to include the fact that (Brodmann 1903; Garey 1994) recognized sufficient differences in their morphological characteristics that he assigned different morphological numerical areas to the precentral cortex (Area 4) and the postcentral cortex (Areas 3, 2, and 1). A knowledge of the anatomy and the physiology of the Rolandic cortex aids in the understanding of a number of surgical decisions in dealing with seizure disorders pertaining to it or its immediately adjacent environs.

The physiological concept pertaining to the mammalian cerebral cortex is that of functional units, or columns, of cells arranged perpendicular to the surface of the cortex and probably being something of the order of 0.5–1 mm in diameter. These columns (functional units) are loosely connected to one another. The integration within a column is reflected in the subcortical output of that column. The immediately subcortical U-fibers and the intracortical nerve nets joining the columns to their neighbors provide an intracortical conduction system parallel to the surface of the gyrus. Thus, at least theoretically, a cortical incision, perpendicular to the surface of the cortex, would be less damaging to the physiological role of that cortex than one parallel to the surface (see Colonnier 1966).

Anatomically the afferent and efferent fibers of the cortical columns join and leave, respectively, the precentral and postcentral gyral cortex and traverse the subcortical white matter, forming part of the corona radiata, to reach the genu and posterior limb of the internal capsule. When surgery involves subpial dissection/resection of the cortex adjacent to the pre- and postcentral cortices, then the course of the inflow and outflow of the immediate subcortical connectivity becomes an important surgical anatomical consideration. Figure 8.1 outlines two parasagittal (anteroposterior) sections through the Rolandic region, one just above the Sylvian fissure (Fig. 8.1a) and one just lateral to the interhemispheric fissure (Fig. 8.1b), in other words one in the inferior Rolandic region and one in the superior Rolandic region, respectively.

In the superior part of the Rolandic cortex, the relationship between its surface location and its connectivity is less striking and even somewhat reversed when compared to the inferior Rolandic cortex, as the Rolandic fissure and its associated cortices angle posteriorly in their courses from the inferior Sylvian fissure to the superior interhemispheric fissure. In a series of coronal sections, the difference between its junction with the Sylvian fissure and that of its junction with the interhemispheric fissure is as much as 3–4 cm in an anteroposterior direction. In other words the superior dorsolateral Rolandic cortex at the interhemispheric fissure (IHF) is ~4 cm posterior to the inferior Rolandic cortex at the Sylvian fissure. Given the foregoing and given that the main connectivity of the Rolandic cortex is to the posterior limb of the internal capsule, which is significantly posterior to the inferior Rolandic area, the fibers leaving the inferior Rolandic area must proceed significantly more posteriorly in order to reach the posterior internal capsule than the superior cortex. This is illustrated in Fig. 8.1a. From this figure it can be appreciated that an extension of a straight perpendicular incision inferiorly in the depth of the postcentral gyrus would significantly interfere with that connectivity and thereby with the maintenance of the clinical function represented in its fibers. Such would not be as significant were the incision made in the depth of the precentral sulcus. Similarly, in neither the precentral nor the postcentral sulci near the IHF would such an incision result in such interference in the Rolandic connectivity nor the resulting associated clinical impairments, as the fibers are not sweeping as significantly posteriorly in their subcortical courses. These differences are illustrated in the diagrams in Fig. 8.1a, b, which portray the direction of the course of the fibers of connectivity. This figure depicts similar important surgical anatomical features in subpial resection techniques in cortex adjacent to the so-called eloquent cortex, such as those demonstrated in Figs. 5.​3 and 7.​2 in Sects. 5.​5.​3 and 7.​2.​2, respectively. These latter figures point out the importance of going no deeper than the gray matter in the depths of sulci immediately adjacent to the eloquent cortex. In Fig. 8.1a, b the broad arrows at the bottoms of the pre- and postcentral sulci disclose the path of the fibers, either leaving or proceeding to the Rolandic cortex and going to or coming from the mid- to posterior internal capsule and the thalamus of the motor and sensory fibers, respectively. It is illustrative of the importance of not violating the subcortical white matter immediately below the bottom of the postcentral sulcus. Rather, any continued resection should be directly posterior, as noted in the foregoing, as superficially as possible and directed away from the eloquent cortex.

The foregoing is primarily of importance in the resection of the inferior Rolandic cortex (vide infra, Sect. 8.4), which is probably the commonest operation associated with the Rolandic cortex, as it is used in general neurosurgery probably to a greater extent than in specialized epilepsy surgery. Figure 8.2a represents a cartoon coronal section through the Rolandic cortex, outlining Penfield’s homunculus. Figure 8.2b is an illustration of a coronal section through the same area. The primary reason for this illustration is to bring attention to what I call the “frontal stem,” which is the frontal counterpart to the temporal stem, which is so important in the conduct of an aTLY. It can be appreciated that the frontal stem (FS) in Fig. 8.2b represents an orderly, and indeed intuitively predictable, layering of the fibers subserving the Rolandic opercular cortex (ROC) in which its most inferolateral component is that of the inferomedial ROC embedded in the Sylvian fissure at its junction with the insula (I). Proceeding more superomedially within the FS are the fibers subserving progressively more superior functional components of the ROC within the head, e.g., tongue → jaw → lips → face → eyes, etc. Continuing on in the FS, the components of the head are followed by those of the hand, beginning with the thumb, e.g., thumb → index finger → middle finger → ring and small fingers → hand, etc. It is this junction (see “junction” in Fig. 8.2a, b) between head and thumb within the inferior Rolandic cortex, as well as the subcortical junction of the neuronal efferent outflows between those fibers on either side of the cortical junction, which is of importance in the inferior Rolandic corticectomy (Sect. 8.4).

Another anatomical feature of the Rolandic cortex pertains to the variability of the positions of the homunculi that can occur. The surgeon should be aware of this variability, but given that these operations are, or should be, conducted under local anesthesia, they are rarely of significance other than this awareness. One variation is that which can exist between the representation of the motor and sensory homunculi across the Rolandic fissure. In the common neuroanatomical textbooks, there is usually no reference to potential asymmetry that may exist between the two homunculi, with respect to the pre- and postcentral gyri. That is to say, the reader of the text book usually comes away with the notion that the two homunculi are perfectly symmetrical in that the motor and sensory representations of each part of the body across the Rolandic fissure are juxtaposed in the precentral and postcentral gyri, as illustrated in Fig. 8.3a. In general, this is the case or very close to the case regarding symmetry, with any asymmetry amounting to simply a differential of a few millimeters. However, on occasion the asymmetry may amount to several millimeters, such as illustrated in Fig. 8.3b, and if the surgeon is not aware of this possibility, it may lead to a search for a presumed error of some kind, e.g., such as faulty stimulation, presumed errors in interpretation of responses, etc.

The only instance where this asymmetry could be of significance is in the resection of the inferior Rolandic cortex. The superior border of this resection is the junction between the inferiorly situated head area of the homunculus and the more superiorly situated thumb area. This junction is the most important border of the operative procedure of inferior Rolandic corticectomy. The junction of the sensory homunculus in the postcentral gyrus cannot be used as the superior border, based on the assumption that the motor homunculus is symmetrically located anteriorly across the Rolandic fissure. For example, if the asymmetry was such as outlined in Fig. 8.3b and the border of the corticectomy was at the junction identified in the postcentral gyrus, then the thumb area of the precentral gyrus would either be partially removed or completely removed with the clinical postoperative counterpart of a paresis or paralysis of the hand. This depicts the importance of utilizing the junction of the motor homunculus rather than that of the sensory homunculus for identifying the superior border of an “inferior Rolandic corticectomy.” This circumstance would only be a potential problem if the motor thumb area could not be found, which, as noted earlier, is not necessarily rare. In the final analysis, any concern at all can, of course, be simply precluded by the operation being conducted under local anesthesia with continual clinical monitoring of thumb function.

Penfield and Rasmussen (1968, p. 47) noted that: “The relative size of the motor face, arm, and leg areas varies considerably. The face area may be large and extend to within 4 cm. of the midline, or it may be so small that thumb responses are elicited 2 cm. above the fissure of Sylvius. A similar variability exists for the leg area. It may be entirely located on the mesial surface of the hemisphere or it may extend well out onto the lateral surface for a distance of up to 3 cm.” This exemplifies the extreme variability of the human motor cortical homunculus along the precentral gyrus, which Penfield and Rasmussen concluded was a representation in which “scientific accuracy is impossible” (p.56). This is illustrated in Fig. 8.3c. This variability is one that I have also observed (vide infra, Sect. 8.7).

Before leaving the physiology and anatomy of the Rolandic cortex, it should be noted that, unless fMRI eventually can clearly outline the Rolandic cortex, the surgeon must be aware that only through local anesthesia in the awake patient with electrocortical stimulation can the Rolandic fissure be confidently identified. I would agree with Penfield who stated “there is no way of determining the central fissure in a living patient except by stimulation because of the great variability in cortical pattern, especially in pathologic conditions.” (1938, p. 242)

Gradually increasing experience, particularly in the hands of Dr.’s Penfield and Rasmussen of the Montreal school, with Rolandic epilepsy led to the evolution of the awareness that the inferior Rolandic cortex usually could be sacrificed without the production of an unwanted neurological deficit. The evolution resulted in the now accepted fact that the entire Rolandic cortex in the suprasylvian region, below the thumb area, can be removed without deficit. This has been intuitively attributed to the fact that much of the head musculature, e.g., face, pharynx, larynx, and neck, has bilateral cortical innervation whereby the loss of one of the two innervations usually can be tolerated without the production of a permanent deficit (Penfield and Rasmussen 1950, 1968; Penfield and Jasper 1954; Penfield and Roberts 1959; Rasmussen 1975a, b; Lehman et al. 1994). This perhaps might have been predicted from the knowledge that in the majority of strokes from occlusion of the middle cerebral artery, the monoplegic upper limb is not accompanied by a permanent significant deficit in the head and neck. When minimal dysarthria or facial paresis occurs, they often disappear within a few days. This is not dissimilar to uncommon such deficits following the inferior Rolandic corticectomy.