Sex Differences in Visual Field Lateralization: Where are they?

Christine Chiarello1, Laura K. Halderman1, Suzanne Welcome1, Janelle Julagay1 & Christiana M. Leonard2

University of California, Riverside1, University of Florida, Gainesville2

Introduction

AcknowledgmentThis research was supported by NIDCD grant 5R01DC6957.

Conclusions

MethodThis large-scale investigation provided little evidence for sex differences

in the lateralization of language processing. Across a range of lexical tasks with varying degrees of asymmetry, no across-the-board sex differences were observed. If there are sex differences in language lateralization, these are highly task- and measure-specific, and account for little systematic variance. These results do not support the view that women have a more bilateral organization for language, and confirm Boles’ (1995) earlier findings with different tasks. Occasional reports in the literature of sex X VF interactions may be due to inadequate sample sizes that are unrepresentative of the wider population.

It is frequently claimed that women have a more bilateral organization for language as compared to men. However, experimental support for this claim has been mixed. Although sex differences are sometimes obtained in divided visual field experiments, they rarely replicate, and a recent large-scale study found little evidence for sex differences in functional lateralization (Boles, 2005). In that study, across six laterality tasks (both verbal and nonverbal material) sex accounted for less than 1% of the variance in lateral asymmetry.

The current study, part of the Biological Substrates for Language Project, affords the opportunity to explore this issue in another large-scale investigation. In this project 200 individuals were tested in eight divided visual field tasks exploring different aspects of lexical processing. This multi-task approach enables a powerful test of the hypothesis that women have more bilateral language lateralization. This position would be supported if women show reduced asymmetries for all or most of the tasks. We employed three analytic approaches to address this issue. First, we used analysis of variance to investigate the reliability of sex X visual field interactions across our tasks. Second, multiple regression was used to examine whether sex was a significant predictor of asymmetry in each of our tasks. Finally, we computed the cross-task asymmetry correlations separately for males and females, in order to determine whether lexical asymmetries would be more strongly intercorrelated for males than females.

References

Cross-Task Asymmetry Correlations:

• Accuracy: out of 34 possible task correlations, males demonstrated stronger cross-task correlations for 12 task comparisons, and females demonstrated stronger cross-task correlations for 12 other task comparisons.

• RT: out of 34 possible task correlations, males demonstrated stronger cross-task correlations for 10 task comparisons, and females demonstrated stronger cross-task correlations for 9 task comparisons.

• There is no evidence for sex differences in the strength of cross-task asymmetry correlations.

Multiple Regression:

• Hierarchical regressions were performed for each DVF task on Accuracy and RT Asymmetry Scores with the predictors Sex, Handedness score, Reading Subtests, VIQ, PIQ, and the asymmetry scores for the other tasks.

• Accuracy: when Sex was entered as the first predictor, it accounted for significant variance in only two tasks - Category Generation (19.6%) and Nonword Naming (2%), p < .05. Of this variance only 2.7% was unique to Sex, and only for Category Generation, p < .01.

• RT: when Sex was entered as the first predictor, it accounted for no significant variance for any of the tasks. Sex accounted for unique variance only for Semantic Decision (2.7%), p < .05.

Analysis of Variance:

• Accuracy: there was no Sex X Visual Field interaction, but the Sex X Task X Visual Field Interaction was reliable, F(7,1385) = 3.05, p<.01. Separate analyses by Task indicated that males had stronger asymmetries for Nonword Naming, F(1,198)=4.68, p<.05 and Category Generation, F(1,198)=4.28, p <.05.

• RT: there was no Sex X Visual Field interaction, nor was Sex X Task X Visual Field reliable (Fs < 1). Separate analyses by Task indicated no Sex X Visual Field interactions for any task.

Boles, D.B. (2005). A large-sample study of sex differences in functional cerebral lateralization. Journal of Clinical and Experimental

Neuropsychology, 27, 759-768.

PARTICIPANTS:• 100 male, 100 female native English speakers• 18-34 years of age• 28 (14%) are not right-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

PROCEDURE:• DVF tasks administered across 4 sessions• RT and Accuracy (% correct) recorded

Results

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LexicalDecision

Masked WordRecognition

WordNaming1

WordNaming2

NonwordNaming

SemanticDecision

VerbGeneration

CategoryGeneration

Task

Accuracy Asymmetry

MalesFemales

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LexicalDecision

Masked WordRecognition

WordNaming1

WordNaming2

NonwordNaming

SemanticDecision

VerbGeneration

CategoryGeneration

Task

RT Asymmetry

MalesFemales

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Accuracy (above) and RT (below) Asymmetry Scores for males and females by task.

Positive scores indicate a RVF/LH advantage.

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