Posterior Sylvian Fissure Research Articles

Auditory evoked magnetic fields to tones and pseudowords in healthy children and adults.

Neuromagnetic responses to tones and pseudowords were measured with a 24-channel magnetometer in nine adults and in 23 children, the latter aged 0.3-15 years. Both stimulus types elicited substantially similar responses in all subjects. At 0.9-s interstimulus interval (ISI), the adult response was a stable P1m-N1m-P2m-N2m sequence peaking at 50, 100, 200, and 250 ms, respectively. A biphasic P1m-N1,2m response with peaks at 100 and 260 ms occurred in children up to 12 years of age. At longer ISIs (1.2-2.4 s), an adult-type N1m response appeared in most children. N1m amplitude suppression at short ISIs was stronger in children than in adults and may reflect a longer refractory period of the N1 generator neurons during early childhood than later in life. Peak latencies of P1m, N1m, and N2m decreased with age, most rapidly < 7 years of age. All deflections originated in nearby cortical areas within the posterior sylvian fissure, and may serve as functional landmarks for that anatomic area.

Distinctive electrophysiological characteristics of functionally discrete brain areas: a tenable approach to functional localization.

The direct cortical response (DCR), an electrical potential recorded in the immediate vicinity of a surface cortical stimulus, shows a configuration in the primary sensory areas of animals that is different from the one observed in association cortex. This suggested the possibility that systematic study of the DCR in the human brain might reveal a profile of configurations in which the form of the response provides functional information about the gyri being tested. Studies were carried out in subhuman primates and in patients undergoing surgery for tumors, occult vascular malformations, and epilepsy. In the animals, DCR's from somatosensory, motor, and association cortex are distinguishable; however, there are no differences in configuration between motor and premotor responses, or between association responses from prefrontal and parietal cortex. In patients with epilepsy due to nonspace-occupying pathology, the responses did not show distinguishing features. In contrast, in the patients with tumors or occult vascular malformations, DCR's from somatosensory, motor, and premotor cortex could be readily distinguished from each other. Responses along the mid and posterior sylvian fissure of the dominant hemisphere also had distinctive features, but more data are needed before the significance of this finding with respect to language function can be assessed. The accumulating results suggest that analysis of DCR's may prove to be a useful method for functional localization in individuals with focal space-taking pathology.

Human cortical asymmetries determined with 3D MR technology

A method is described for obtaining clear 3D magnetic resonance (MR) images of the cortical surface of the brain in living human subjects. By combining volume composite and depth encoded images, we have obtained surface coordinate data that resulted in highly repeatable measurements of sulcal lengths and cortical surface areas in eight normal adult volunteers. Sulcal lengths were determined for specific parts of the Sylvian fissure, central sulcus and frontal operculum. Additionally, angles were computed between the anterior and posterior limbs of the pars triangularis and the ascending and horizontal limbs of the posterior Sylvian fissure. The cortical surface areas enclosed by these limbs were also computed. Finally, thirteen non-metric cortical features (e.g., petalias) were scored from the 3D MR images. All measurements were compared in right and left hemispheres. In addition to corroborating cortical asymmetries reported in the literature, we observed previously unrecognized directional asymmetries in the length of the anterior limb of the pars triangularis, length of the ascending limb of the posterior Sylvian fissure, and position of the lateral end of the central sulcus. We attribute the finding of three new directional asymmetries for the human cortex, as well as the high repeatability of our measurements, to the sensitivity and accuracy of the 3D MR imaging technology that has recently become available.

Total surface of temporoparietal intrasylvian cortex: Diverging left-right asymmetries

Measurements of the total convoluted surface area of cortex buried in the temporoparietal transition of the Sylvian fissure were performed using “three-dimensional” magnetic resonance morphometry in 10 randomly selected human cadaver brains. The measurements comprised the planum temporale, which covers the superior temporal gyrus posterior to Heschl's first transverse gyrus, and the caudally adjacent walls of the descending and ascending rami of the Sylvian fissure. Also, the total convoluted surface of the planum temporale was compared with its superficially exposed surface as determined by photographical planimetry after brain section, so that the index of cortical folding of the planum temporale could be estimated. The following results were obtained: Cortical folding of the planum temporale did not differ significantly between left and right hemispheres ( p > .6). The total and the superficially exposed surface of the planum temporale showed significant leftward asymmetry ( p ⩽ .0078). In contrast, the total surface of cortex buried in the caudal segments of the Sylvian fissure posterior to the planum exhibited significant rightward asymmetry ( p ⩽ .016). The combined cortical surface area buried in the whole posterior Sylvian fissure caudal to the first transverse gyrus did not show significant left-right asymmetry ( p > 0.3). The data suggest divergent lateral asymmetries in the posterior intrasylvian region with excesses of superior temporal cortex in left hemispheres and of supramarginal cortex in right hemispheres.

Sulcus topography of the parietal opercular region: An anatomic and MR study

The study describes the sulcal and gyral topography, variability, and left-right asymmetry of the parietal operculum. Eighty postmortem hemispheres as well as sagittal magnetic resonance images from 20 healthy volunteers (40 hemispheres) were evaluated. Four different types of parietal opercular sulcus topography were recognized. Most frequently, and conforming with the anatomic “textbook pattern”, the inferior postcentral sulcus (POCS) is the sulcus anterior to the posterior ascending ramus (PAR) of the Sylvian fissure (type 1). Variations were the following: lack of a PAR (type 2), interposition of an intermediate opercular sulcus and gyrus between PAR and POCS (type 3), and direct transition of PAR into POCS with subsequent lack of a classical supramarginal gyrus (type 4). Inconstancy of the sulcal standard arrangement was especially pronounced among left hemispheres, where the patterns differed from type 1 in one third of the cases. Types 2 and 3 were significantly more frequent in left hemispheres, whereas type 4 occurred significantly more frequently in right hemispheres. Upon intraindividual left-right comparison, a remarkable 38% of the brains showed gross asymmetry of the parietal opercular sulcus patterns, characterized by a left type 2 or 3 and/or a right type 4; another 5% exhibited a reverse type of asymmetry. The findings supplement previous data on gross variability and left-right asymmetry of the posterior Sylvian fissure and its lower bank. They indicate that the Sylvian fissure is an unreliable landmark with respect to inferior parietal structures especially in left hemispheres. Individual mapping of perisylvian topography may contribute to studies on structural-functional relationship.

Cerebral Cortical Localization

The Talairach stereotactic proportional grid method for telencephalic localization was applied to magnetic resonance imaging of 30 brain hemispheres of healthy adult volunteers. Using sagittal images, outlines of the following characteristic principal sulci were identified on the brain surface to validate the method: callosal sulcus, parietooccipital sulcus, marginal sulcus, superomedial portion of central and postcentral sulcus, and posterior sylvian fissure. The maximal variation zones of sulcus location obtained by superimposition of the individual grid data on a standard proportional grid were similar to those originally reported by Talairach from pneumoencephalographic studies. However, the central sulcus, which was only studied postmortem by Talairach, was located 0.5-1 cm more posteriorly in the present study. Magnetic resonance also provided more detailed data on the considerable variation of the terminal parts of the sylvian fissure and on the well-known left-right asymmetries. The left sylvian fissure extended farther posteriorly in 11 of 15 brains. With the exception of this posterior perisylvian region, the proportional grid proved to be valid for indirect cerebral cortical localization in the regions studied.