Introduction:
The region of the human brain, that is involved in processing the visual information is the occipital lobe. Occipital lobe is located behind the imaginary parieto-occipital line all the way up to the occipital pole. This lobe houses numerous sulci, namely, the transverse occipital, lateral occipital and lunate sulci. The pattern of sulci and gyri of the occipital region was extensively studied. However, these previous studies concentrated more on the relation of calcarine sulcus with the striate cortex.
The human occipital sulci show variability due to degree of regional cortical folding, sulcal presence or absence, diversity of sulcal origin, sulcal segmentation, differences in sulcal length, etc. The position and extent of human occipital cortical areas have characteristic relationships to occipital sulci and sulcal intersections [1].
The information procured from standard textbooks makes it difficult to establish the relations between sulcal landmarks and identified visual areas with modern functional neuroimaging [2]. An earlier study by Ono [3], revealed the patterns of sulci on the medial surface of the occipital region, namely, the patterns formed by the calcarine and parieto-occipital sulci. In addition to this they also studied the sagittal sulci of the cuneus and lingual gyrus. However, their study did not involve the observation of the sulci on the lateral surface of the occipital region.
Therefore, this study aimed to define the variability of the human occipital sulci with regards to its presence, position, extent and intersections which would help in predicting the focal lesions as well as in performing neuro-surgical procedures.
Materials and Methods
The sulci on the occipital region were observed in 40 formalin-fixed human adult cadaveric cerebral hemispheres (20 right hemispheres and 20 left hemispheres) (of 20 brain specimens) procured from the Department of Anatomy, Kasturba Medical College, MAHE, Manipal. The lepto-meninges along with the blood vessels were carefully removed to expose the occipital sulci. Variable occipital sulci were observed, documented and photographed. In order to avoid any discrepancies related to the description of the occipital sulci, the Duvernoy terminology has been followed in this study (4). They are: POS Parieto-occipital sulcus, CaS Calcarine sulcus, SOS Superior occipital sulcus, SLOS Superior lateral occipital sulcus, ILOS Inferior lateral occipital sulcus, LuS Lunate sulcus.
Results
A total of six sulci were observed in the occipital region both on the medial surface and lateral surface.
1. Calcarine sulcus The CaS is seen both on the medial and lateral parts of the occipital lobe. It extends from below the splenium of corpus callosum up to the occipital pole on the lateral surface. Therefore, the anterior end of the CaS was seen on the medial surface and the posterior end appeared on the lateral surface. Based on the course of the CaS (from anterior end to its posterior end), four types were identified single apex (with one pointed bend), two apices (with two pointed bends), horizontal or S-shaped (Table 1) (Figure 1).
 |
Figure 1: CaS (Calcarine sulcus) - A. Posterior view of left cerebral hemisphere showing CaS extending from the medial surface to the superolateral surface along the occipital pole, arrow indicating single apex; B. Posterior view left cerebral hemisphere showing CaS extending from medial surface to superolateral surface along the occipital pole, arrows indicating two apices; C. Medial surface of right cerebral hemisphere showing S-shaped CaS; D. Medial surface of left cerebral hemisphere showing Horizontal CaS |
 |
Figure 2: Superior view of left cerebral hemisphere showing POS (parieto-occipital sulcus) extending from the medial surface to the superolateral surface - A. Straight POS; B. Y-shaped POS |
| Table 1. Different types of CaS based on its course |
Calcarine sulcus (CaS) (n=40 hemispheres) |
Right cerebral hemisphere (n=20) |
Left cerebral hemisphere (n=20) |
Based on the course |
Single apex |
---- |
8 (40%) |
Two apices |
6 (30%) |
4 (20%) |
Horizontal |
6 (30%) |
3 (15%) |
S-shaped |
9 (45%) |
4 (20%) |
2. Parieto-occipital sulcus The POS intervenes between the cuneus and precuneus, thus separating the occipital and parietal lobes from the medial surface. It cuts the superomedial border of the cerebrum and partly appears on the superolateral surface. With respect to the shape at its termination on the superolateral surface, the POS has been classified into two types, i.e., Straight (without ramification) and Y-shaped (2 rami) (Table 2) (Figure 2). It is noteworthy to know that the POS connected with the CaS in all the 40 cerebral hemispheres.
| Table 2. Different types of POS based on its termination |
Parieto-occipital sulcus (POS) (n=40 hemispheres) |
Right cerebral hemisphere (n=20) |
Left cerebral hemisphere (n=20) |
Shape |
Straight (without ramification) |
14 (70%) |
14 (70%) |
Y-shaped (two rami) |
8 (40%) |
4 (20%) |
3. Superior occipital sulcus The SOS is restricted to the superolateral surface of the occipital lobe. This is a highly variable sulcus. If found in the occipital lobe, it extends in anteroposterior direction, almost parallel to the superomedial border of the cerebral hemisphere. It has also been observed that in 32 cerebral hemispheres (80%), the SOS is a direct continuation of the Intraparietal sulcus (IPS) of the parietal lobe. The SOS presented intersections with the Superior lateral occipital, Intraparietal and Lunate sulci (Table 3) (Figure 3) (Chart 1).
 |
Figure 3: Superolateral surface showing intersections of SOS (Superior occipital sulcus) with: A. IPS (Intraparietal sulcus); B. SLOS (Superior lateral occipital sulcus); C. LuS (Lunate sulcus) |
| Table 3. Sulci on the superolateral surface of the occipital lobe |
Presence of Sulci (n=40 hemispheres) |
Right cerebral hemisphere (n=20) |
Left cerebral hemisphere (n=20) |
Superior occipital sulcus (SOS) |
14 (70%) |
18 (90%) |
Superior lateral occipital sulcus (SLOS) |
14 (70%) |
16 (80%) |
Inferior lateral occipital sulcus (ILOS) |
15 (75%) |
20 (100%) |
Lunate sulcus (LuS) |
8 (40%) |
8 (40%) |
 |
Chart 1: Percentage of intersections of SOS, SLOS, ILOS and LuS with the neighbouring sulci |
4. Lateral occipital sulcus The Lateral occipital sulcus runs horizontally along the continuity of the posterior upturned end of the lateral sulcus (posterior ramus), thus dividing the occipital lobe into superior and inferior occipital gyri. In a few cases two lateral occipital sulci would be seen, which are named separately as Superior lateral occipital and Inferior lateral occipital sulci.
4.a. Superior lateral occipital sulcus The SLOS is a variable sulcus. When found, it is only seen along the superolateral surface of the occipital lobe. It runs in the same direction as the SOS. It presented intersections with the SOS (Table 3) (Figure 4) (Chart 1).
|
Figure 4: Superolateral surface of left cerebral hemisphere showing intersections of SLOS (Superior lateral occipital sulcus) with SOS (Superior occipital sulcus) |
4.b. Inferior lateral occipital sulcus The ILOS is horizontally oriented between the SLOS and Inferior occipital sulcus when both are present. It has interconnections with the Superior temporal sulcus (STS) anteriorly and with the Lunate sulcus (LuS) posteriorly (Table 3) (Figure 5) (Chart 1).
 |
Figure 5:Superolateral surface of left cerebral hemisphere showing intersections of ILOS (Inferior lateral occipital sulcus) with: A. STS (Superior temporal sulcus); B. LuS (Lunate sulcus) |
5. Lunate Sulcus Among all the above sulci, the LuS was found to be highly variable. Its location along the superolateral surface close to the occipital pole is noteworthy. It is oriented transversely or obliquely with a curved lateral convexity. Sometimes, the posterior end of the calcarine sulcus meets the lunate sulcus in a (- shaped manner. The LuS had intersections with CaS, SOS and ILOS (Table 3) (Figure 6) (Chart 1).
M |
Figure 6: Intersections of LuS (Lunate sulcus) with: A. CaS (Calcarine sulcus) (posterior view); B. SOS (Superior occipital sulcus) (superolateral surface of left cerebral hemisphere); C. ILOS (Inferior lateral occipital sulcus) (superolateral surface of left cerebral hemisphere) |
Discussion
The numerous sulci and gyri, tend to increase the surface area for various brain function. Thus the functional abilities of the brain are enhanced without increasing the size of the brain beyond the stipulated dimension. In this regard, the occipital region, which is functionally related to visual area, shows enormous variability in the sulci. The sulci on the medial surface of the occipital region have been extensively studied. However, the sulci on the superolateral surface of the occipital region have created confusion due to considerable morphological variation and discrepancies in nomenclature (same sulcus has been given different names and different sulci have been given similar names). Despite all this, a basic pattern governing the variability can be used to identify most of the sulci consistently. In this study, an attempt has been made to observe and delineate the sulci on the superolateral surface of the occipital lobe. The findings of this study concludes that the POS and the CaS are the most constant sulci and they do not show any variability in their position [5]. Thus they prove to be important anatomical landmarks of the posterior interhemispheric region (occipital lobe) [6]. The other sulci (SOS, SLOS, ILOS, LuS) were highly variable. It has also been observed that the complexity of the POS and CaS, manifested by the presence of multiple types were similar to those observed by Malikovic et al [1] and Ono et al. [3].
The constant sulci
CaS Even though it is considered to be the most constant sulcus, positron emission tomography studies have shown wide variation in its location [7]. On the medial surface, the CaS intersects with the POS at the cuneal point. Based on the relationship of cuneal point with the occipital pole, three parts of CaS has been identified as anterior, middle and posterior (retrocalcarine sulcus) [1, 4]. Similar parts were identified in this study. Generally, it is said that the CaS reaches the occipital pole and intersect with the LuS. However, a study revealed that the CaS was restricted only onto the medial surface and hence did not reach the superolateral surface to intersect with the LuS [8]. In the present study, only 30% of the cerebral hemispheres showed the extension of CaS onto the superolateral surface and thus intersected with the LuS. In addition to this, our study also observed four types of CaS based on its course One apex, two apices, horizontal and S-shaped (Table 4).
| Table 4. Types of CaS based on its course |
CaS |
Malikovoc et al (2011) (n=30 hemispheres) |
Present study (n=40 hemispheres) |
Single apex |
63.3% (19 hemispheres) |
20% (8 hemispheres) |
Two apices |
10% (3 hemispheres) |
25% (10 hemispheres) |
Horizontal |
20% (6 hemispheres) |
22.5% (9 hemispheres) |
S-shaped |
6.7% (2 hemispheres) |
32.5% (13 hemispheres) |
The CaS is an important landmark for the estimation of visual area and hence variations in the architectonic areas may be due to different degrees of folding. Since the length and depth have already been assessed in the earlier cadaveric and MRI studies [9, 10], the present study has only looked into its presence, location and termination.
POS It is one of the most constant sulci beginning from the medial surface, cutting the superolateral border and extending onto the upper part of the superolateral surface. In a study conducted by Mandal et al. [10] on 53 brain specimens, many of the POS failed to reach the superolateral surface. Also, based on the termination of the POS, they classified into two categories single rami and two rami [9]. A third type of termination has also been defined by Malikovic et al. as T-shape form [1]. In the present study, in all the hemispheres, the POS extended onto the superolateral surface and only two specimens showed termination of POS as two rami (Table 5).
| Table 5. Types of POS based on its termination |
POS |
Malikovoc et al (2011) (n=30 hemispheres) |
Mandal et al (2014) (n=106 hemispheres) |
Present study (n=40 hemispheres) |
Straight (without ramification) |
36.7% (11 hemispheres) |
98.11% (104 hemispheres) |
70% (28 hemispheres) |
Y-shaped (two rami) |
53.3% (16 hemispheres) |
1.88% (2 hemispheres) |
30% (12 hemispheres) |
T-shaped |
10% (3 hemispheres) |
------ |
------ |
The variable sulci
SOS, SLOS & ILOS As described earlier, the SOS is highly variable and hence very few studies were involved in identifying the SOS. Among SLOS & ILOS, the ILOS was found frequently and considered to be constant among the two lateral occipital sulci.
LuS The LuS is a very highly variable sulcus. The LuS has been extensively studied in both humans and nonhuman primates. These studies have confirmed that in nonhuman primates, the LuS forms a constant border between the occipital and temporal lobes. However, in humans, the LuS is highly variable and more posteriorly placed [11]. In the present study, the lunate sulcus, when present was found along the superolateral surface, close to the occipital pole (Table 6).The LuS has been generally described as a curved sulcus with convexity laterally. But a study conducted by Mandal et al. [10], has shown various shapes (convex, straight vertical, straight horizontal, Y-shaped, f/S-shaped, Arch shaped and dividing into rami) of LuS. In our study, all the LuS were convex shaped. As mentioned earlier, the LuS is not always present. A case has been reported on the unilateral absence of LuS [12]. The presence of the LuS has been depicted in table 6. A few research studies have commented that the LuS is not a single sulcus but a combination of fragments of several neighbouring sulci [14].
| Table 6. Presence of LuS |
Presence of LuS |
Ono et al. (1990) |
Allen & colleagues (2006) |
Iaria et al. (2007) (MRI study) |
Singh et al (12) (2016) |
Present study |
Right cerebral hemisphere |
60% |
26.4% |
45% |
40% |
40% |
Left cerebral hemisphere |
64% |
32.7% |
50% |
46.7% |
40% |
Certain anatomical anomalies in situations of polymicrogyria, pachygyria and lissencephalia are associated with the pattern of gyri and relative thickness of gyri [15]. However, our study did not look into the gyri of the occipital lobe in depth. With regards to the CaS, it must be noted that the anatomical alterations in the shape, depth, dimensions vary considerably in various diseases like retinal degeneration [16]. A proper knowledge regarding the orientation of LuS and CaS is essential for the radiologists to locate various visual functional areas in an MRI. In locating the lesions at different visual functional areas, the clinicians can correlate the effects of the lesion at these areas. A well-defined knowledge is also important for the neurosurgeons operating on the posterior lobe tumors.The variable occipital sulci observed in this study may be used as anatomical landmarks for functionally defined visual areas and during less invasive neurosurgical procedures in cases of focal lesions within the occipital lobe.
References
- Malikovic A, Vucetic B, Milisavljevic M, Tosevski J, Sazdanovic P, Milojevic B, Malobabic S. Occipital sulci of the human brain: Variability and morphometry.
Anat Sci Int. 2012; 87:61-70.
- Parent A, Carpenter MB. Carpenters human neuroanatomy. 9th ed. USA: Lippincott Williams & Wilkins; 1996.
- Ono M, Kubik S, Abernathey CD. Atlas of the cerebral sulci. 1st ed. New York: Thieme Medical Publishers; 1990.
- Duvernoy H. The human brain: Surface, three-dimensional sectional anatomy and MRI, and blood supply. 2nd ed. Wien: Springer-Verlag; 1999.
- Iaria G, Petrides M. Occipital sulci of the human brain; variability and probability maps.
J Comp Neurol. 2007; 501(2):243-59.
- Kubik S, Szarvas B. Anatomy of the calcarine sulcus. In: Yasargil MG, ed. Microneurosurgery III A: AVM of the Brain. Stuttgartd: George Thieme Verlag Medical Publishers; 1987; 350-68.
- Endo S, Toyama H, Kimura Y, Ishii K, Senda M, Kiyosawa M, et al. Mapping visual field with positron emission tomography by mathematical modeling of the retinotopic organization in the calcarine cortex.
IEEE Trans Med Imaging. 1997; 16(3):252-60.
- Velmurugan M, Bhaskaralingam S, Ravi C. A study on the position of the lunate sulcus.
Anatomical Adjuncts. 1990; 1(9):89.
- Glissen E, Iba-Zizen MT, Stievenart JL, Lopez A, Trad M, Cabanis EA, et al. Is the length of the calcarine sulcus associated with the size of human visual cortex? A morphometric study with magnetic resonance tomography.
J Hirnforsch. 1995; 36(4):451-9.
- Mandal L, Kumar Mandal S, Dutta S, Ghosh S, Singh R, Chakraborty SS. Variation of the major sulci of the occipital lobe - A morphological study.
Al Ameen J Med Sci. 2014; 7(2):141-5.
- Allen JS, Bruss J, Damasio H. Looking for the Lunate sulcus: A magnetic resonance imaging study in modern humans.
Anat Rec A Discov Mol Cell Evol Biol. 2006; 288(8):867-76.
- Das S, Paul S. Unilateral absence of lunate sulcus: an anatomical perspective.
Rom J Morphol Embryol. 2008; 49(2):257-8.
- Singh GG, Subhash K, Gopichand PVV, Usha C. Morphological Study of Sulci and Gyri of Occipital Lobe of Cerebral Hemisphere of Human Brain.
Int J Pharma Res Health Sci. 2016; 4(6):1454-61.
- Holloway RL, Clarke RJ, Tobia PV. Posterior lunate sulcus in Australopithecus africanus: was Dart right?
Comptes rendus Palιvol. 2004; 3(4):287-93.
- Tallinen T, Chung JY, Biggins JS, Mahadevan L. Gyrification from constrained cortical expansion.
Proc Natl Acad Sci U S A. 2014; 111(5):12667-72.
- Kitajima M, Korogi Y, Hirai T, Hamatake S, Ikushima I, Sugahara T, Shigematsu Y, Takahashi M, Mukuno K. MR changes in the calcarine area resulting from retinal degeneration.
Am J Neuroradiol 1997; 18(7):1291-5.
|
