Distinct parts of the intraparietal sulcal cortex contribute to sensorimotor integration and visual spatial attentional processing. A detailed examination of the morphological relations of the different segments of the complex intraparietal sulcal region in the human brain in standard stereotaxic space, which is a prerequisite for detailed structure-to-function studies, is not available. This study examined the intraparietal sulcus (IPS) and the related sulcus of Jensen in magnetic resonance imaging brain volumes registered in the Montreal Neurological Institute stereotaxic space. It was demonstrated that the IPS is divided into two branches: the anterior ramus and the posterior ramus of the IPS, often separated by a submerged gyral passage. The sulcus of Jensen emerges between the anterior and posterior rami of the IPS, and its ventral end is positioned between the first and second caudal branches of the superior temporal sulcus. In a small number of brains, the sulcus of Jensen may merge superficially with the first caudal branch of the superior temporal sulcus. The above morphological findings are discussed in relation to previously reported functional neuroimaging findings and provide the basis for future exploration of structure-to-function relations in the posterior parietal region of individual subjects.

It has been demonstrated that the postcentral sulcus, which forms the posterior boundary of the sensorimotor region, is a complex of distinct sulcal segments. Although the general somatotopic arrangement in the human sensorimotor cortex is relatively well known, we do not know whether the different segments of the postcentral sulcus relate in a systematic way to the sensorimotor functional representations. Participants were scanned with functional magnetic resonance imaging while they made movements of different body parts and the location of functional activity was examined on a subject-by-subject basis with respect to the morphological features of the postcentral sulcus. The findings demonstrate that the postcentral sulcus of each subject may be divided into five segments and there is a tight relationship between sensorimotor representations of different body parts and specific segments of the postcentral sulcus. The results also addressed the issue of the transverse postcentral sulcus, a short sulcus that is present within the ventral part of the postcentral gyrus in some brains. It was shown that, when present, this sulcus is functionally related to the oral (mouth and tongue) sensorimotor representation. When this sulcus is not present, the inferior postcentral sulcus which is also related to the oral representation is longer. Thus, the sulcal morphology provides an improved framework for functional assignments in individual subjects.

The morphological structure of the postcentral sulcus and its variability were investigated in 40 structural magnetic resonance images of the human brain registered to the Montreal Neurological Institute (MNI) proportional stereotaxic space. This analysis showed that the postcentral sulcus is not a single sulcus, but rather a complex of sulcal segments separated by gyri which merge its banks at distinct locations. Most of these gyri are submerged deep within the sulcus and can be observed only by examining the depth of the sulcus, although a small proportion may be observed from the surface of the brain. In the majority of the examined cerebral hemispheres (73.75%), the postcentral sulcus is separated into two or three segments, and less frequently into four or five segments (12.5%) or remains continuous (13.75 %). Examination of the in-depth relationship between the postcentral sulcus and the intraparietal sulcus revealed that these two sulci may appear to join on the surface of the brain, but they are in fact always separated by a gyrus in the cortical depth. In 32.5% of the examined hemispheres, a dorso-ventrally oriented sulcus, the transverse postcentral sulcus, is located anterior to the postcentral sulcus on the lower part of the postcentral gyrus. Systematic examination of the morphology of the postcentral sulcus in the proportional stereotaxic space which is used in functional neuroimaging studies is the first step towards the establishment of anatomical-functional correlations in the anterior parietal lobe. 3 IntroductionThe postcentral gyrus, which forms the anterior part of the parietal lobe, is delimited by the central sulcus, anteriorly, and the postcentral sulcus, posteriorly (Figure 1). Pioneer studies using electrical stimulation during brain surgery established that, as in nonhuman primates, the postcentral gyrus of the human brain is the somatosensory cortex and that there is an orderly arrangement of sensory representations of the different parts of the body along its dorsoventral extent (Penfield and Rasmussen, 1952;Woolsey et al., 1979). More recently, the organization of the somatosensory cortex along the postcentral gyrus of the human brain has been examined and confirmed with functional neuroimaging (Nakamura et al., 1998;Boling et al., 1999Boling et al., , 2002Fabri et al., 2005). The postcentral sulcus, which separates the postcentral gyrus from the posterior parietal cortex, is traditionally described as a prominent sulcus extending from the superior aspect of the hemisphere to the lateral fissure (e.g. Ono et al., 1990;Duvernoy et al., 1991). It is sometimes illustrated as a single continuous sulcus (e.g. Smith, 1907;Economo and Koskinas, 1925) or divided into two or three segments which are superior and inferior to each other (e.g. Eberstaller, 1890;Retzius, 1896;Brodmann, 1909;Sarkissov et al., 1955;Ono et al., 1990). The dorsal end of the postcentral sulcus is often depicted as a V-shaped structure consisting of two branches coming together at one point (Brodmann, 1909;Economo and...