Sylvian fissure morphology and asymmetry in men and women: Bilateral differences in relation to handedness in men

Abstract The anatomy of the sylvian fissure in the human brain was studied to develop reliable criteria for anatomical landmarks of the posterior part of the fissure for use in its definition and measurement; to quantify right‐left asymmetries in segments of the sylvian fissure; to assess whether any anatomical features are associated with hand preference (selected as one index of hemispheric functional asymmetry) and whether structure‐function relationships are similar in men and women. A sample of 67 brain specimens (24 men and 43 women, mean age = 53 years) was studied postmortem (with the aid of dissection) from people who had been tested before death for detailed hand preference. Sylvian fissure anatomy in the human brain is very variable and no agreement exists as to the point of its posterior termination. The posterior ascending ramus, originating at the posterior bifurcation of the fissure, was found to be the continuation of the main limb of the sylvian fissure. Three segments of the sylvian fissure were defined and measured: anterior, horizontal, and vertical. The anterior segment showed no asymmetry; the horizontal segment was twice as large on the left side as on the right; and the vertical segment twice as large on the right. The two asymmetries counterbalanced each other, and overall asymmetry in the posterior region (horizontal plus vertical) was minimal. The basic asymmetry is in the position at which the fissure turns up, resulting in the different extent and position of the surrounding right and left parietal and temporal gyri and associated cytoarchitectonic regions. The possible embryological course of the asymmetry is discussed. Handedness correlated with anatomy of the sylvian fissure in men. In contrast to general expectation, hand preference was associated with a bilateral feature of morphology, and not with less asymmetry in non‐right‐handers. Men having consistent‐right hand preference had longer horizontal segments in both hemispheres compared to men not having consistent‐right‐hand preference. The direction and magnitude of asymmetry did not differ between the two male hand‐preference groups. Since hand preference is an index of other motor and perceptual functions which are asymmetrically represented in the two hemispheres in gyri surrounding the sylvian fissure, it is suggested that anatomy of the sylvian fissure is related to functional asymmetries in men. A sex difference in structure‐function relationship was observed. No association was found between hand preference and sylvian fissure anatomy in women. These results corroborate a similar sex difference in the structure‐function relationship observed for the corpus callosum (Witelson; Brain 112 :799–835, 1989). They are also consistent with neuropsychological findings of sex differences in the localization of cognitive functions in parietotemporal regions, such as differences in degree of functional asymmetry and in the pattern of intrahemispheric representation of cognitive functions. It is suggested that there is a greater dissociation between motoric and perceptual asymmetries in women than in men. © 1992 Wiley‐Liss, Inc.