EFFECTS OF HANDEDNESS AND GENDER ON THE SURFACE AREA OF THE HUMAN CORPUS CALLOSUM: A PRELIMINARY STUDY USING MAGNETIC RESONANCE IMAGING

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James B. Pinkston, Department of Psychology

Introduction

Several studies have found differences in the human corpus callosum across gender and handedness (Aboitiz, Scheibel, Fisher, & Zaidel, 1992; Burke & Yeo, 1994; Cowell, eta!., 1993; de Lacoste-Utamsing & Holloway, 1982; Denenberg, eta!., 1991; Habib, eta!., 1991; Johnson & Bigler, eta!., 1994; Johnson & Farnworth, eta!., 1994; Laissy, et al .. 1993; Potter & Graves, 1988; Steinmetz, eta!., 1992; Witelson, 1989; Witelson, 1991; Witelson & Goldsmith, 1991). A myriad of others claim that these differences do not exist (Byne, eta!., 1988; Clarke, Kraftsik, VanDer Laos, & Innocenti, 1989; Hines, eta!., 1992; Kertesz, Polk, Howell, & Black, 1987; Laissy, et al., 1993; O’Kusky, eta!., 1988; Oppenheim, Lee, Nass, & Gazzaniga, 1987; Piccirilli, Giancarlo, & Sciarma, 1989; Pujol, Vendrell, Junque, MartiVilaltra, & Capdevila, 1993; Raine, eta!., 1990; Steinmetz, eta!., 1992; Weis, Kimbacher, Wenger, & Neuhold, 1993; Weis, Weber, Wenger, & Kimbacher, 1989; Witelson, 1985). These discrepancies may be attributable, in part, to the different methods used in these studies. While several aspects of the corpus callosum are agreed upon, methods used to study it differ (Driesen & Raz, in press).

This study was undertaken in an effort to delineate between some of the differences across current corpus callosum studies on gender and handedness. To this end, the author has attempted to replicate facets of several current studies. Such elements include correcting for subject head size and utilizing both writing-hand and hand consistency usage definitions of handedness as a measure of subject lateralization.

Method

Subjects

Forty-four (21 female, 23 male) normal volunteers ages 18-26 were dministered a handedness questionnaire and received magnetic resonance scans. All subjects were college students. Nineteen of the subjects were left-handed (LH), 25 were right-handed (RH), 18 were consistentlyhanded (CH) and 26 were non-consistently handed (NCH). All subjects were screened for any history of alcohol or drug use as well as psychiatric conditions and brain insult. Individuals not meeting these criteria were excluded. All subjects gave informed consent and were part of a larger normative study. Subjects were grouped according to gender, preferred writinghand, and consistency of hand use. Handedness was determined from a twelve item questionnaire based on the Edinburgh Handedness Inventory (Oldfield, 1971) involving such activities as writing, brushing one’s teeth, and kicking (Steinmetz, et al., 1992). Individuals scoring two or more items with the hand opposite their writing hand were classified as non-consistently handed (Steinmetz, et al., 1992; Witelson, 1989).

Materials

All MR images were performed at 1.5 Tesla with a quadriture head coil using standard clinical protocol on a GE Signa scanner (General Electric Co. Milwaukee WI) at LDS Hospital as part of a larger normative data base. All sagittal scans (scans taken at a plane perpendicular to the floor and bisecting the subject’s head from front to back) were T1 weighted, 500/1112 (TR/TEl excitations) with a 256 X 192 acquisition matrix and a field of view of 24 em. Each image was 5 mm. thick with a 1 mm. skip between images, 13 images per subject. The midsagittal (MS) image (the central image which clearly exposes the corpus callosum) was selected using the cerebral aqueduct and the cribriform plate as defining characteristics. (Johnson & Farnworth, et al., 1994)

In an effort to keep the effects of human fallacy to a minimum, all images were kept as digital data throughout the entire process. Sagittal images were imported from the Signa computer directly to an Apple computer. Here each midsagittal slice was selected according to the above criteria and subject head tilt variability was corrected for by aligning all of the midsagittal slices to the same horizontal plane. This was done in Adobe Photoshop 3.0 and by using the anterior corpus callosum and posterior corpus callosum as reference points. Images were then imported into NIH IMAGE 1.56 (Rasband, 1993) and magnified to 4:1. After controlling for image contrast, the midsagittal internal skull surface area (MISS) (Johnson & Farnworth, et al., 1994; Laissy, et al., 1993) was obtained by manually tracing the interior of the skull following along the innertable, foramen magnum, clivus, selar diaphragm, and the jugum sphenoidale (Laissy, et al., 1993) with the use of a Kensington Turbo Mouse and then recorded. The total midsagittal corpus callosum area was then obtained in a like manner from the enlarged, contrast adjusted image. Next, the total corpus callosum area was divided into six subregions using Witelson’s criteria (except that area 1, the rostrum, was considered as part of the genu) with the use of preprogrammed macros in the NIH IMAGE (Rasband, 1993) program. The total area, as well as that of the six subregions, were then recorded.

All tracings were performed twice and the average of the two values was then used. The identities of the slices were coded so as to keep the rater blind to the gender, handedness, and handedness consistency associated with each image.

Statistical Analysis

As this is a preliminary study, an investigative approach to the statistical analysis was used. To correct for subject head size variability, the total midsagittal corpus callosum areas, as well as that of its six subregions (genu, rostral midbody, anterior midbody, posterior midbody, isthmus, and splenium), were divided by the individual’s MISS. All values, corrected as well as uncorrected, were then subjected to analysis of variance tests using gender, handedness (as defined by the subject’s writinghand), and handedness consistency as the independent variables. This was accomplished by importing the resultant values Into a SAS data file. All subsequent analysis was then performed using SAS.

Results

As a result of the preliminary and investigatory nature of this study an alpha level of 0.05 was chosen. Consequently, significant differences where found across genders in the total uncorrected corpus callosum area as well as the uncorrected subregions of rostral midbody and anterior midbody. A significant interaction between handedness consistency and gender was also found in the area of the corrected splenium. Here, non-consistently handed males show a larger corrected area than consistently handed males, and just the opposite relationship is seen between the consistently and non-consistently handed females.

Results

This study found essentially two sets of results when the traced areas were compared. The uncorrected areas revealed larger total, rosmidbody and anterior midbody areas for the males. This is likely the result of males having larger brains. However, while the corrective measure used, midsagittal internal skull surface area, eliminated these findings, it failed to support other studies which assert that women have larger corpus callosum than men when corrected for head or brain size (Burke & Yeo, 1994; Johnson & Farnworth, et al., 1994; Laissy, et al., 1993; Steinmetz, et al., 1992; de Lacoste-Utamsing & Holloway, 1982). Though significant (p = 0.0 12) the correlation (r =.38) between MISS and total corpus callosum may not strong enough to warrant its use here as a corrective measure. Perhaps a more precise corrective measure would be more powerful, or maybe the correlation of subject corpus callosum size to midsagittal head size is simply not strong enough to merit much reliability. The second set of results include the interaction between handedness consistency and gender within the corrected splenium. Having corrected for MISS, non-consistent males exhibit larger areas in the splenium than consistent males. This is not surprising, and supports the excitatory theory’s consensus on laterality and greater bihemispheric representation in nonconsistent individuals. The opposite finding for females (consistent females larger than nonconsistent females) is perplexing however. These results are similar to those of Denenberg, et al., (1991), and Habib, et al., {1991) where they found the same, or similar, relationship for both males and females in the posterior midbody and the isthmus. The questions arise as to whether or not the sexes share the same lateralization tendencies, if inhibition is playing a role, or if there is a sexual dimorphism at work.

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Acknowledgements

This research was supported in part by the Deseret Foundation at LDS Hospital, Salt Lake City, Utah, the College of Family, Home, and Social Sciences, the Honors Department, and the Office of Research and Creative Work at Brigham Young University.

The assistance of Erin D. Bigler, Ph.D., Claudia]. Clayton, Ph.D., Sterling C. Johnson, Marc A. Steed, and Tracy]. Abildskov is gratefully acknowledged.