Corpus callosum size is linked to dichotic deafness and hemisphericity, not sex or handedness

This article was originally published in a journal published by Elsevier, and the attached copy is provided by Elsevier for the author’s benefit and for the benefit of the author’s institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues that you know, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier’s permissions site at: http://www.elsevier.com/locate/permissionusematerial Brain and Cognition 62 (2006) 1–8 www.elsevier.com/locate/b&c Bruce E. Morton b a,* , co p y Corpus callosum size is linked to dichotic deafness and hemisphericity, not sex or handedness Stein E. Rafto b a Department of Biochemistry and Biophysics, University of Hawaii School of Medicine, 2 Bonito Court, Santa Fe, NM 87508, USA Department of Radiology, Kaiser Permanente Medical Care Program, Moanalua Medical Center, 3288 Moanalua Road, Honolulu, HI 96819, USA al Accepted 10 March 2006 Available online 4 May 2006 Abstract pe rs on Individuals differ in the number of corpus callosum (CC) nerve fibers interconnecting their cerebral hemispheres by about threefold. Early reports suggested that males had smaller CCs than females. This was often interpreted to support the concept that the male brain is more ‘‘lateralized’’ or ‘‘specialized,’’ thus accounting for presumed male predominance in mathematics, as well as for aggressive behavior. Ultimately, meta-analyses of these many reports found no significant overall sex differences in inter-cerebral information carrying capacity. Here, using quantitative MRI, we found the midline CC area of 113 subjects was significantly correlated, not with handedness or sex, but with dichotic deafness, and even more so with redefined hemisphericity, the latter accounting for over 19% of CC variability. That is, both dichotic hearing and right brain-oriented individuals of either sex had significantly larger CCs than dichotically deaf or left brain-oriented persons. Thus, current traditions of brain laterality and gender may benefit from revisions that include redefined hemisphericity.  2006 Elsevier Inc. All rights reserved. Keywords: Asymmetry; Brain; Behavior; Dichotic listening; Cognition; Laterality; Left-brain; Polarity; Right-brain or 's 1. Introduction Au th Together the cerebral hemispheres contain about 23 billion neurons in men and about 19 billion neurons in women (Pakkenberg & Gundersen, 1997). The corpus callosum, which is the major information highway between the hemispheres, contains axons from about 0.2 billion (1%) of these neurons (Aboitiz, Scheibel, Fisher, & Zaidel, 1992). These fibers of passage which compose the corpus callosum originate from neurons from the functionally different right and left hemispheres (Galaburda, 1991). Their axons connect the different areas of the opposite hemispheres in a unique and often asymmetrical manner (Brown, Larson, & Jeeves, 1994). There are many more longitudinal fibers within each hemisphere than there are trans-hemispheric callosal fibers (Schuz & Preissl, 1996). * Corresponding author. E-mail address: bemorton@hawaii.edu (B.E. Morton). 0278-2626/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.bandc.2006.03.001 Individuals differ by nearly threefold in the number of their trans-hemisphere fibers. Subject corpus callosum cross sectional area (CCA) varies from about 3–11 cm2 at sagittal midline (Holloway, Anderson, Defendini, & Harper, 1993; Yazgan, Wexler, Kinsbourne, Peterson, & Leckman, 1995). The great CCA similarities in monozygotic twins indicate that these corpus callosal size differences are under considerable genetic control (Tramo et al., 1998). Across subjects, neither brain size (Mitchell et al., 2003) nor overall density of callosal large and small fiber types were tightly correlated with CCA (Aboitiz et al., 1992). These wide individual differences in trans-hemispheric connectivity have invited speculation: Could left handed individuals have larger CCAs (Witelson, 1985)? Or, in some studies males had smaller CCAs than females (Holloway et al., 1993). Did that support the view that the cerebral hemispheres of males are less interconnected than females to produce ‘‘more lateralized’’ or ‘‘specialized’’ brains with greater apparent math skills, 2 B.E. Morton, S.E. Rafto / Brain and Cognition 62 (2006) 1–8 on al co p y single unilateral executive system (‘‘there can only be one bottom line’’) which is inherently embedded either on the right or left side of the asymmetric cerebral hemispheres in the region of the anterior cingulate cortex (Vogt, Finch, & Olson, 1992). That is, an individual appears to be born either left or right brain-oriented, based upon the right or left local cerebral asymmetric environmental influences within which their unilateral executive must operate. In confirmation of this new concept of hemisphericity, four biophysical methods have been developed which show high inter-method correlations, including the above ‘‘Dichotic Deafness Test’’ (Morton, 2001, 2002). The other three methods were the ‘‘Phased Mirror Tracing’’ (Morton, 2003a); the ‘‘Best Hand Test,’’ a two-hand, line-bisection task (Morton, 2003b); and ‘‘The Hemisometer Test,’’ a visual discrimination based upon wellknown cis or trans brain output from the temporal or nasal sides of the retinae (Morton, unpublished). Use of these procedures resulted in the independent segregation a given population into two unique subgroups that were similar across all four methods. Ultimately, one set of each of the subgroups was discovered to be enriched in right brain-oriented persons, the other with left brain-oriented persons (Morton, 2002). Use of these methods to define hemisphericity resulted in the subsequent creation and calibration of three derivative behavioral preference questionnaires: elements from Zenhausern’s ‘‘Preference Questionnaire’’ (Morton, 2002; Zenhausern, 1978); the ‘‘Polarity Questionnaire’’ (Morton, 2002); and the ‘‘Asymmetry Questionnaire’’ (Morton, 2003c). The combination of these seven highly inter-correlated hemisphericity methods (Morton, 2001, 2002, 2003a, 2003b, 2003c) made it possible to determine the hemisphericity of individual subjects quite reliably (Morton, unpublished). Here, we report a marked association of subject hemisphericity and dichotic deafness with individual corpus callosum size. One hundred and thirteen Caucasian subjects of known hemisphericity were scanned by MRI to determine their corpus callosal cross sectional areas (CCA). These MRI data confirmed our hypothesis that dichotic deafness was significantly inversely correlated with CCA, while handedness and sex were not, in agreement with others (Bishop & Wahlsten, 1997; Luders et al., 2003). Furthermore, it was found that subject hemisphericity was the item most highly correlated with individual CCAs, apparently accounting for more than 19% of CCA variability. Au th or 's pe rs accompanied by a seeming predilection for the ‘‘martial’’ arts? But, what about studies where the average CCA of males was found to be larger than females (Clarke, Kraftsik, Van Der Loos, & Innocente, 1989)? That variations in CCA for handedness or sex are much larger in individuals than are mean population CCA differences appears to have stimulated much research in this area. As a result, literally hundreds of papers reporting very small CCA differences in one direction or the other for sex or handedness have been published (Bishop & Wahlsten, 1997; Luders et al., 2003). This suggests that individual CCA differences must depend upon yet unidentified factors. New speculation has arisen from reports that about half the population are ‘‘dichotically deaf’’ in their minor ear when attempting to report the identity of two different consonant–vowel (CV) syllables sent simultaneously but separately, one to each ear (Morton, 2001, 2002). This dichotic deafness disappeared immediately if syllables were separated by only 90 ms, suggesting a cause other than auditory structural failure for this state-specific deafness (Morton, 2001, 2002). Could having a smaller CCA account for this dichotic deafness? Indeed, we postulate that in part dichotic deafness results from the smaller interhemispheric information transfer capacity in individuals with smaller CCAs. If so, what other properties might individuals with small CCAs have in common? Crucially, groups of dichotic deafness individuals were found to be enriched with left brainoriented persons (Morton, 2002). Could the left or right brain-oriented thinking and behavioral styles of redefined hemisphericity (see below, Morton, 2003a, 2003b, 2003c, 2003d) also be related to individual differences in CCA? For millennia, humanity has sought a second ‘‘either– or’’ dyad beyond sex for individual characterization. Thus far, the left–right distinction of handedness has failed in this regard (McManus, 2002). However, due to discoveries resulting from split-brain surgical procedures (Sperry, 1961), the imagination of popular culture continues to be captivated by the left brain, right brain concept of hemisphericity (Bogen, 1969). Hemisphericity is embodied by the idea of the existence of contrasting left-brain, rightbrain cognitive and behavioral styles between individuals. However, no quantitative standards were ever developed to confirm the many controversial claims made regarding the hemisphericity of individuals or groups. Also, the misleading concept that individual hemisphericity can lie on a gradient ranging from extreme right to extreme left obscured experimental results. This idea, plus other confounding elements led to conflict, confusion, doubt, and ultimately to the collapse of the field (Beaumont, Young, & McManus, 1984). Recently, the concept of hemisphericity has been redefined to surmount its earlier un-testability and other difficulties that led to the demise of the original idea of hemisphericity among academic psychologists. This redefinition states that the behavioral laterality of hemisphericity is inevitable because it results from unavoidable existence a 2. Methods 2.1. Subjects Subjects (n = 113) were volunteers from the University of Hawaii community, ranging in age from 18 to 74 years, 45.2 years median age, ±14.2 years SD. Of these, 48% were female, 11% claimed left-handedness, and all were Cauca- 3 B.E. Morton, S.E. Rafto / Brain and Cognition 62 (2006) 1–8 co p on rs 2.3. Dichotic deafness test al 2.2. Subject hemisphericity calibrations The six following independent hemisphericity methods, are described in detail elsewhere in this journal: Dichotic Deafness Test (Morton, 2001, 2002; see below), Phased Mirror Tracing (Morton, 2003a), Best Hand Test (Morton, 2003b), Zenhausern’s Preference Questionnaire (1978: Morton, 2002), Polarity Questionnaire (Morton, 2002), and Asymmetry Questionnaire (Morton, 2003c). They were earlier used to determine individual hemisphericity (here, mean number of methods/subject was 5.1). The Best Hand Test alone has also been used to investigate the mean hemisphericity of academic and professional populations (n = 1048; Morton, 2003d). 2.3.2. MRI corpus callosum cross sectional area determinations MRI assessments employed a General Electric Signa 1.5 Tesla MRI instrument. A midsagittal plane setup calibration protocol was run for 3 min using a T1 weighted spin echo sequence (TR = 400 ms, TE = 1/Fr) to image 5 mm slices from the midline plane and two adjoining sagittal planes 6 mm on either side. Whole-head photographic images were prepared from these three planes, and additionally, a 2.3· enlargement of the most medial plane, centering on the corpus callosum. These four exposures were printed on a single 35 · 43 cm film sheet for each subject. Sagittal corpus callosal cross sectional areas were determined by tracing the corpus callosal outline of the 2.3· midline enlargement upon computer printer paper (Weyehauser 1180, 20 lb stock) of predetermined weight per unit area. The 812  11 in. pages varied in weight by ±0.6%. Corpus callosal cutouts weighed on a microbalance varied in weight by ±35%. These data were converted to absolute corpus callosum cross sectional areas by use of predetermined magnification and paper weight constants. Subject corpus callosal areas ranged from 4.5 to 10.1 cm2. Clearly, there can be no individual variation in brain size comparable to that of the CCA. In fact, only a weak linear relationship of CCA with cerebral volume has been noted (r2 = .15) by Mitchell et al. (2003). Others have also noted a small relationship between brain volume and CCA (Bishop & Wahlsten, 1997; Jancke, Staiger, Schlaug, Huang, & Steinmetz, 1997), while some have found none at all (Carter, Botvinick, & Cohen, 1999; Kertesz, Polk, Howell, & Black, 1987). It has been argued that comparing the ratio of CCA to whole brain size is inappropriate because it can create a false impression of sex differences in the corpus callosum (Bishop & Wahlsten, 1997). For these and other reasons, corpus callosum size was assumed to be essentially independent of brain size (not determined here), and age (Mitchell et al., 2003; Pfefferbaum, Sullivan, Swan, Carmelli, 2000). Regarding handedness, conflicting reports of the effect of handedness on overall CCA exist (Kertesz et al., 1987; Mitchell et al., 2003; O’Kusky et al., 1988; Westerhausen et al., 2004; Witelson, 1985, 1989), with most reports finding handedness significance not referring to overall CCA but only to anatomical subsections within it (Clarke & Zaidel, 1994). y sian. There were 51% right brain-oriented persons (R-bop)s, composed of 47% right brain-oriented females (R-bof)s and 53% right brain-oriented males (R-bom)s. The 49% left brain-oriented persons (L-bop)s consisted of 47% left brain-oriented females (L-bof)s and 53% left brain-oriented males (L-bom)s. The Committee of Human Studies of the University of Hawaii Institutional Review Board had earlier approved all appropriate elements of this unfunded research. Au th or 's pe 2.3.1. Dichotic deafness task: (Morton, 2001, 2002) ‘‘Tonal and Speech Materials for Auditory Perceptual Assessment:’’ Disc 1.0 (1992) was purchased from the Long Beach Research foundation through Richard Wilson at the Veterans Affairs Medical Center, Mountain Home, TN 37684 (Wilson and Leigh, 1996). Bands five and six of this disc were used to measure minor ear deafness during simultaneous and 90-ms separated consonant–vowel syllable presentations, as described for the Dichotic Deafness Test (Morton, 2001). Based upon their laterality index (LI) scores, where LI = (# of correct symbols for the major ear # of correct symbols for the minor ear)/(# of correct symbols for the major ear + # of correct symbols for the minor ear), subjects were sorted into two groups, earlier defined by a natural separation in minor ear scores (Morton, 2001). One was the ‘‘dichotically deaf’’ group, where the subject’s minor ear correctly reported simultaneously delivered consonant–vowel (CV) syllables less than 40% as well as the major ear did. This group of subjects had high LI scores and was enriched in left brain-oriented individuals. The other group was the ‘‘dichotically hearing’’ group where the minor ear correctly reported CV syllables more than 40% as well as the major ear. These subjects had low LI scores and were enriched in right brain-oriented individuals (Morton, 2002). Based upon standard dichotic listening calculations, the dichotically deaf group would contain individuals with strong right or left ear advantage scores, while the dichotically hearing group would contain those subjects with only moderate ear advantages (Kimura, 1967). 2.3.3. Statistical analysis The Statistica 6.0 package was used to assess the strength of these data and their associations with the various hemisphericity methods. CCA values for each of the 113 subjects, precalibrated for hemisity as described above, were assessed for the following four groups: right brain-oriented males (n = 30), right brain-oriented females (n = 28), left brain-oriented males (n = 29), and left brain-oriented females (n = 26) using ANOVA. Sex, hemisphericity, and handedness group means, standard deviations, and standard errors were determined and plotted. Performance results of the individuals of these groups regarding the 4 B.E. Morton, S.E. Rafto / Brain and Cognition 62 (2006) 1–8 various instruments used to assess hemisphericity were compared with the group CCA means, as indicated in the last table. co p on al Fig. 1. Effect of sex and hemisphericity upon corpus callosal area. Abbreviations: R-bom (right brain-oriented male), R-bof (right brainoriented female), L-bom (left brain-oriented male), and L-bof (left brainoriented female). Scale: whiskers (SD), outer box (SEM), and inner box (Mean, cm2). The group mean difference in corpus callosal midline cross sectional area between right brain-oriented males (n = 30) and left brain-oriented females (n = 26) was 14% (0.89 cm2, r2 = .194, p = .000). Fig. 2 provides MRI images illustrating the largest CCAs (right brainoriented female, 10.1 cm2; right brain-oriented male, 9.2 cm2) and smallest CCAs (left brain-oriented male, 4.8 cm2; left brain-oriented female, 4.5 cm2) of subjects from this group of 113 subjects. Table 3 presents correlations between CCA and several variables. CCA was significantly inversely correlated with hemisphericity (r = .32, p = .000, n = 120) and dichotic deafness (r = .22, p = .04, n = 87), while it was not for sex and handedness. In addition, CCA was significantly inversely correlated with two other biophysical measures or 's pe rs As indicated in Table 1, the mean CCA for the group (n = 113) was 6.75 ± 0.09 SEM. When the individuals of this group were sorted according to sex, the CCA mean of the males was slightly (>3%), but not significantly larger (r = .11, p = .229) than that of the females. Similarly, when sorting by handedness, the mean CCA of left handed individuals was also larger (<3%), but not significantly (r = .06, p = .552). If instead, the subjects were sorted into dichotic deafness and dichotic hearing categories, the mean CCA of the dichotically deaf, left brain-orientation enriched group was indeed significantly smaller ( 6%) than that of the right brain-orientation enriched dichotically hearing group (r = .22, p = .046). Further, the segregation of individuals by hemisphericity into right or left brain-orientation categories resulted in an even larger difference. The mean CCA of left brain-oriented subjects was smaller ( 10%) than that of right brain-oriented subjects by 0.67 cm2 (r = .38, r2 = .144, p = .000). However, still greater group CCA separations occurred after the subjects were sorted both by hemisphericity and sex, as shown in Fig. 1. There it may be seen that the mean CCA of right brain-oriented individuals, regardless of sex, was substantially greater than that of left-brained males or females, and vise versa. This occurred even while the much smaller sex-specific differences in CCA were retained. Thus (Table 2) the mean CCAs of the right brain-oriented males (n = 30) which was 7.16 ± 0.15 SEM cm2 and right brainoriented females (n = 28) which was 6.98 ± 0.18 SEM cm2 were significantly separated from those of the left brain-oriented males (n = 29) at 6.53 ± 0.17 SEM cm2 and the left brain-oriented females (n = 26) at 6.27 ± 0.16 SEM cm2. y 3. Results th Table 1 Mean corpus callosal cross sectional areas, cm2 cm2 SEM 6.75 6.85 0.09 0.12 6.64 0.13 Left-handers vs. Comparison Right-handers 6.91 0.23 6.74 0.10 Dichotically hearing vs. Comparison Dichotically deaf 7.06 0.15 Group means 2 Au Subjects (range 4.7–10.1 cm ) Males vs. Comparison Females Right brain-oriented persons vs. Comparison Left brain-oriented persons * Significant difference. 6.66 0.13 7.07 0.12 6.41 0.12 D cm2 (D%) r p n 113 59 113 54 0.21(3) .11 .229 0.18 (3) .06 .552 14 111 99 0.40 (6) .22 .046* 37 84 47 0.67 (10) .38 .000* 58 113 55 5 B.E. Morton, S.E. Rafto / Brain and Cognition 62 (2006) 1–8 Table 2 Resolution of mean corpus callosal cross sectional areas of the four hemisphericity types, cm2 2 Subjects (range 4.7–10.1 cm ) Right brain-oriented males (R-boms) Right brain-oriented females (R-bofs) Left brain-oriented males (L-boms) Left brain-oriented females (L-bofs) cm2 SEM 6.75 7.16 6.98 6.53 6.27 0.09 0.15 0.18 0.17 0.16 D cm2 (D%) N 113 30 28 29 26 0.08 (3) 0.26 (8) .2910 .2742 58 55 R-bofs–L-boms R-boms–L-boms 0.45 (7) 0.63 (10) .0477* .0079* 57 59 R-bofs–L-bofs R-boms–L-bofs 0.71 (11) 0.89 (14) .0034* .0002* 54 56 co p R-boms–R-bofs L-boms–L-bofs Significant. th or 's pe rs on al * p y Group means Au Fig. 2. Hemisphericity vs. sex: size-range of corpus callosal areas. Largest CCAs of the subject group (n = 113): (1) Right brain-oriented female (R-bof), 10.1 cm2. (2) Right brain-oriented male (R-bom), CCA 9.2 cm2. Smallest CCAs: (3) Left brain oriented-male (L-bom), 4.8 cm2. (4) Left brain orientedfemale (L-bof), 4.5 cm2. of hemisphericity: The Best Hand Task (Morton, 2003b) (r = .36, p = .000, n = 116) and Mirror Tracing Task (Morton, 2003a) (r = .30, p = .003, n = 94). Further, CCA was significantly inversely correlated with the Polarity Questionnaire (Morton, 2002) (r = .28, p = .004, n = 105) but not with the Asymmetry Questionnaire (Morton, 2003c) or Zenhausern’s Preference Questionnaire (Morton, 2002). 4. Discussion Evidence is presented here that the nearly threefold individual differences in corpus callosal cross sectional area (CCA) appears to be related to the individual and group differences in ‘‘Dichotic deafness’’ (Morton, 2001, 2003a, 2003b, 2003c, 2003d) and those of ‘‘Redefined hemisphericity’’ (Morton, 2003a, 2003b, 2003c, 2003d). 6 B.E. Morton, S.E. Rafto / Brain and Cognition 62 (2006) 1–8 Hemisphericity Dichotic Deafness Test Sex Handedness .35 .22* .13 .10 .120 .050 .017 .011 .000 .038 .161 .268 113 87 113 113 Best Hand Task Mirror Tracing Task Polarity Questionnaire Asymmetry Questionnaire Zenhausern’s Preference Questionnaire .36* .30* .28* .15 .06 .132 .099 .079 .022 .004 .000 .003 .004 .143 .538 113 94 105 98 93 * Significant, p < .05. Au th or 's pe rs Dichotic listening was popularized by Kimura (1967) who found that when two different auditory stimuli are presented simultaneously via stereo headphones, a person usually manifests a ‘‘right ear advantage,’’ due to left hemisphere language specialization (and also due to a contralateral auditory tract advantage). What has only recently been recognized is that dichotic listening data can additionally be used to sort people, regardless of which their advantaged ear was, into two groups. That is, dichotic deafness is a phenomenon where dichotic consonant–vowel syllable stimuli are poorly reported from the non-favored ear by a large subgroup of normal individuals, but only under conditions of simultaneous dissonant auditory stimulus (Morton, 2001, 2002). Importantly, it was observed that dichotically deaf individuals tended to be left brainoriented in their hemisphericity (Morton, 2002). ‘‘Redefined’’ hemisphericity is a behavioral laterality syndrome in which normal individuals are categorized as either right brain- or left brain-oriented in their cognitive and behavior styles (Morton, 2002, 2003a, 2003b, 2003c, 2003d). That is, either they fall into the right brain style, briefly characterized as bold, intense, talkative, big picture oriented ‘‘lumpers,’’ or into the left brain style of cautious, sensitive, quiet, important detail-oriented ‘‘splitters.’’ The first goal of this research was to determine whether by the use of MRI a difference in CCA might be found between dichotically deaf and dichotically hearing individuals. Some earlier research, attempting to tie CCA to dichotic listening ear advantage, found an inverse correlation (Hines, Chiu, McAdams, Bentler, & Lipcamon, 1993; Hines et al., 1992; Yazgan et al., 1995). Others did not (Janke & Steinmetz, 1994; Kertesz et al., 1987). Interestingly, it was observed that in individuals with right speech dominance the CCA was about 2.3% larger than those with language on the usual left side (O’Kusky et al., 1988). Further, because of the relationship of dichotic deafness and hemisphericity (Morton, 2002), as a second goal we also wished to compare the CCAs of right and left brain hemisphericity types. We hypothesized that left brain-oriented subjects would have smaller CCAs as a group. The mean CCA for the entire group (n = 113) was 6.75 cm2. This was reasonably close to the 6.31 cm2 mean y n co p p * r al r2 CCA vs. value for the controls of 11 independent studies in a meta-analysis of corpus callosum size (Woodruff, McManus, & David, 1995). When the individuals of our group were sorted according to sex, the CCA mean of the males was slightly (>3%), but not significantly larger than that of the females, as commonly reported (Bishop & Wahlsten, 1997). Similarly, when sorting by handedness, the mean CCA of left handed individuals was also larger (<3%), but not significantly (Witelson, 1985). As hypothesized, when the subjects were sorted into dichotic deafness and dichotic hearing categories, the mean CCA of the dichotically deaf group (6.66 cm2) was indeed significantly smaller ( 6%) than that of the dichotically hearing group (7.06 cm2). However, an even larger difference between groups resulted when individuals were segregated into right brain (7.07 cm2) or left brain (6.41 cm2)-orientation hemisphericity categories. As expected, the mean CCA of the left brain-oriented subjects was smaller than that of the right brain-oriented ones by a substantial 0.66 cm2 ( 10%). To our knowledge, this is the greatest mean CCA separation between subject groups reported within large (n > 100) normal populations, and with a r2 of .144, appears to account for over 14% of the variance of CCA. However, even larger group CCA separations occurred after subjects were sorted by both hemisphericity and sex. Then, the mean CCA of right brain-oriented individuals, both males and females, was substantially greater than that of left-brained males and females, and vise versa. This occurred even while the much smaller non significant sexspecific differences in CCA were retained. Thus, for right brain-oriented males and females, mean CCAs were 7.16 and 6.98 cm2, respectively. These high values were significantly separated from the lower ones of left brain-oriented males and females which were 6.53 and 6.27 cm2, respectively. The group mean difference in corpus callosal midline cross sectional area between right brain-oriented males and left brain-oriented females now stands at 14% (0.89 cm2, r2 = .194), thus accounting for or over 19% CCA variance, a new record. Regarding correlations, CCA was significantly inversely associated with hemisphericity, accounting for about 12% of CCA variance, and also with dichotic deafness. CCA was not significantly correlated with sex or handedness. In addition, CCA was significantly inversely correlated with two other biophysical measures of hemisphericity: The Best Hand Task (Morton, 2003b), accounting for over 13% of CCA variance, and the Mirror Tracing Task (Morton, 2003a), accounting for almost 9% of CCA variance. Further, CCA was significantly inversely correlated with the Polarity Questionnaire (Morton, 2002), accounting for almost 8% of CCA variance, but not with the Asymmetry Questionnaire (Morton, 2003c) or Zenhausern’s Preference Questionnaire (Morton, 2002; Zenhausern, 1978), an earlier generation hemisphericity measure. Since high correlations for CCA and ear advantage in dichotic listening were reported by others (Yazgan et al., 1995; Gootjes et al., 2006), possibly that on Table 3 Correlations of corpus callosal cross sectional area with other variables 7 B.E. Morton, S.E. Rafto / Brain and Cognition 62 (2006) 1–8 on al co p y A major question arises from these results. For what biological reason should highly lateralized (with small CCAs) and little lateralized (large CCAs) human subgroups exist? What possible reason should right brain-oriented individuals have up to three times more interhemispheric connections than left brainers? Clearly, the issue is other than that of intelligence, since many of our subjects in each of the four categories (RM, RF, LM, and LF) were competent, competitive members on the graduate school faculty of a research university. However, as reported earlier, of the 15 professions assessed at the university, a marked sorting of hemisphericity was observed so that so-called ‘‘big picture’’ professions, such as in astronomy, architecture, and mechanical engineering professions were highly enriched with right brain-oriented practitioners. In contrast, professions dealing with sub visual or abstract level of thinking, such as microbiology, biochemistry, and particle physics were significantly enriched by left brain-oriented professionals. Could it be that this natural selection was based upon the need for global, distributed brain computations by astronomers that requires intense interaction between both hemispheres, vs. the locally wired, predominantly left hemisphere brain operations of particle physicists that require less trans-hemisphere communication (Brown & Kosslyn, 1993; Fink et al., 1996)? Nevertheless, although this discussion illustrates some of the professional consequences of hemisphericity, the teleology behind it remains undescribed. Au th or 's pe rs their procedure could be modified into an additional method to identify individual hemisphericity. These dichotic listening results are consistent with Kimura’s view (1967) that only information presented to the minor ear has to be transferred over the corpus callosum. These outcomes contradict several currently held beliefs about sex and the brain: First, the hemisphericity results lay bare the underlying basis of the previous controversy about gender and laterality. The confusion occurred because in all earlier CCA studies the hemisphericity of the subjects was unknown. This caused an unwitting confounding of the results for subjects sorted only by sex or handedness with hemisphericity, a major factor influencing CCA. This error brings into question the common view that the male brain is more specialized due to its higher laterality (McGlone, 1980). Rather, the CCA data strongly suggest that left brain-oriented individuals of either sex are more lateralized as a class than males are. Correspondingly, right brain individuals of either sex are less lateralized and more broadly generalized as a class than females are, thus contradicting another sexual stereotype. Second, these findings appear to end the controversy about which sex has the larger corpus callosum (Luders et al., 2003). There appears to be no significant difference between the two sexes in either its mean CCA, its range, or in IQ. Rather, the two largest CCAs of individuals from among our 113 subjects were possessed by a right brained female and by a right brain male (10.1 and 9.2 cm2, respectively). Conversely, the two smallest CCAs were 4.8 cm2 for an left brained male and 4.5 cm2 for an left brained female. All four of these individuals held doctoral degrees and professorial status. Third, lack of awareness that hemisphericity contributes to CCA makes it probable that the European studies reporting mean CCAs for males to be larger (Clarke et al., 1989) and American–Australian studies, showing larger female mean CCAs (Holloway et al., 1993) were both correct. Their disagreements could well be based upon regional population differences in hemisphericity, an important but uninvestigated topic. Fourth, it is becoming clear that members of either sex with the same hemisphericity have more behavioral traits in common than do same sex individuals of the opposite hemisphericity. This is strongly supported by data from biophysical method-based derivative preference questionnaires (Morton, 2003a, 2003b, 2003c, 2003d). It would appear that several hemisphericity traits are presently being misidentified as male or female sex traits. That is, men in general do not ‘‘hide in their caves of silence’’ (Gray, 1992; Tannen, 1990). 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