Behavioral and Anatomical Correlates of Corpus Callosum SizeChristine Chiarello1, Suzanne Welcome1, Laura K. Halderman2, Stephen Towler4, Ronald Otto3, & Christiana M. Leonard4

University of California, Riverside1, University of Pittsburgh2, CDIC Riverside3, University of Florida, Gainesville4

Introduction

AcknowledgmentThis research was supported by NIDCD grant 5R01DC6957.

ConclusionsMethod

•Corpus callosum size, when adjusted for brain volume, does not differ between sex or handedness groups.

•Instead, sex and handedness may be important moderators of relationship between asymmetry and callosal area.

•Frontal asymmetries were associated with smaller callosa, but only in males.

•In contrast, correlations with behavioral asymmetry or posterior anatomical asymmetries were always positive - greater asymmetry was associated with larger callosa. This is consistent with an inhibitory role for the corpus callosum.

It is known that individuals vary widely in the area of the corpus callosum. Some proposed correlates of this variation include:

• Sex: •Males have larger absolute corpus callosum size, but not when brain volume is controlled (Bishop & Wahlsten, 1997)

•Consistency of Handedness:•Some find no significant differences in callosal area between consistent and non-consistent right-handers (Preuss, 2002; Hines, et al., 1992)•In males only, non-consistent right-handers may have larger callosa, especially isthmus (Wistelson & Goldsmith, 1991)

•Degree of Behavioral Lateralization:•Several dichotic listening studies have found negative associations between left hemisphere asymmetry and callosal area (Westerhausen, et al., 2006; Yazgan, 1995; Hines, et al., 1992)

•Brain Asymmetry•In males, but not females, asymmetry of Sylvian fissure negatively correlated with size of isthmus (Aboitiz, Scheibel, & Zaidel, 1992)

No prior studies of the corpus callosum have investigated both behavioral and anatomical asymmetries. In the current large-scale study, we re-examine sex/handedness differences in callosal area and correlations with behavioral and anatomical asymmetries.

ReferencesAboitiz, F., Scheibel, A.B., & Zaidel, E. (1992) Morphometry of the sylvian fissure and the corpus callosum, with emphasis on sex differences. Brain, 115 (5), 1521-41. Bishop, K.M. & Wahlsten, D. (1997) Sex difference in the human corpus callosum: myth or reality? Neurosci Biobehav Rev, 21 (5), 581-601.Hines, M., Chiu, L., McAdams, L.A., Bentler, P.M., & Lipcamon, J. (1992) Cognition and the corpus callosum: verbal fluency, visuospatial ability, and language lateralization related to midsagittal suface areas of callosal subregions. Behav Neurosci, 106(1), 3-14.Preuss, U.W., Meisenzahl, E.M., Frodl, T., Zetzsche, T. Holder, J., Leinsinger, G., Hergerl, U., Hahn, K., & Moller, H.J. (2002) Handedness and corpus callosum morphology. Psychiatry Res., 116(1-2), 33-42.Westerhausen, R., Woerner, W., Kreuder, F., Schweiger, E., Hugdahl, K., & Wittling, W. (2006) The role of the corpus callosum in dichotic listening: a combined morphological and diffusion tensor imaging study. Neuropsychology, 20 (3), 272-9.Wistelson, S.F. & Goldsmith, C.H. (1991) The relationship of hand preference to anatomy of the corpus callosum in men. Brain Res, 545(1-2), 175-82.Yazgan, M.Y., Wexler, B.E., Kinsbourne, M., Peterson, B., & Leckman, J.F. (1995) Functional significance of individual variations in callosal area. Neuropsychologia, 33(6), 769-79.

PARTICIPANTS:• 100 male, 100 female native English speakers• 18-34 years of age• 103 consistent-handed (exclusive preference for one hand), 97 mixed handed

DVF TASKS:• Lexical Decision• Masked Word Recognition (2 AFC procedure)• Word Naming (administered twice with different stimuli)• Nonword Naming• Semantic (manmade vs natural) Decision• Verb Generation• Category Generation•Composite asymmetry score across all 7 tasks calculated separately for accuracy and reaction time

BRAIN MEASUREMENTS FROM MRI:• Area of corpus callosum: total, rostrum, genu, anterior body, middle body, posterior body, isthmus, and splenium•Asymmetry of posterior (planum temporale and primary Heshl’s gyrus) and frontal (pars opercularis) anatomy

Multiple regression was used to control for brain volume in all analyses

Results

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Corrected Corpus Callosum Area

Composite Accuracy Asymmetry

0.4

9.129.318.9

16.8

22.4

16.5

Male Consistent (N=44)

Male Mixed

(N=56)

Female Consistent

(N=59)

Female Mixed

(N=41)

Callosum Area 5.58 5.71 5.75 5.64

Accuracy Asym .111 -.005 -.081 .005

RT Asym .112 -.060 -.053 .038

Planum Temporale Asym.

.087 -.208 .027 .152

Heschl’s Asym. -.019 .011 -.049 .075

Pars Opercularis Asym .157 -.138 .019 -.008

Groups characterized by consistency of handedness and sex do not differ in callosal area, degree of behavioral asymmetry, or degree of brain asymmetry

Female mixed-handers with greater left-hemisphere advantages have larger callosa when brain size and average accuracy are controlled

For this group, relationships between behavioral asymmetry and area hold for all regions of the callosum except the splenium, and account for between 9 and 29 percent of variance in regional area, as shown below

Relationships between asymmetries and area of corpus callosum differ between groups:

•Mixed-handed females show strongest relationship between behavioral (accuracy) asymmetry and callosal area•No groups show relationship between RT asymmetry and callosal area•Degree of leftward-asymmetry of temporal areas positively associated (pink) with callosal area in some groups•Degree of leftward asymmetry of frontal areas negatively associated (yellow) with callosal area in males only•No relationship between callosal area and either behavioral or anatomical asymmetry in female consistent-handers

Rostrum

Genu

Anterior

Body

Middle

Body

Posterior

Body Isthmus

Splenium

Male Consistent

Male Mixed Female Consistent

Female Mixed

Accuracy Asym NS NS NS Total, genu, rostrum, ant. body, mid. body, post. body, isthmus

RT Asym NS NS NS NS

Planum Temporale Asym.

Rostrum NS NS NS

Heschl’s Asym. Isthmus NS NS Post. body

Pars Opercularis Asym

Ant. Body Isthmus,

SpleniumNS NS