Experimental neuropsychology approaches to gender differences in cerebral dominance.

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   Experimental psychologists have devised two basic techniques for studying hemispheric dominance inexpensively in normal subjects without invasive or painful manipulations.  The first of these techniques is called dichotic listening.  Dichotic listening literally (etymologically) means listening to stimuli presented simultaneously to each ear -through headphones.  The left ear sends 80% of its nerve fibers to the right hemisphere and 20% to the left.  The left ear sends 80% of its fibers to the right hemisphere and 20% to the left.   I reviewed the scientific literature on gender differences in normal people in ear asymmetries in dichotic listening.   I found 33 relevant reports.  Most of the studies used verbal stimuli,  which always produce a slight average right ear advantage,  surely due to the left hemisphere's specialization for processing language.   About a third of the studies were on children.   Twenty of the studies reported that boys or men had a greater right ear advantage, only one study found women to have a greater right ear advantage,  and the rest found no difference.  This very strongly suggests that with respect to this type of brain activity,  there is a sex difference -though it is of a small magnitude.  The male sex seems to be more hemispherically functionally lateralized.   Boys and men use their left hemisphere more exclusively to process verbal stimuli under these (very artificial) experimental conditions.   They also seem to use their right hemisphere more exclusively to process non-verbal stimuli.

Several studies have shown that dichotic listening is very much dependent upon cognitive style.  For example,  the right ear advantage in verbal dichotic listening tasks has been shown to increase significantly as a function of head turning during the task,  field independence (the ability to visually disembed jumbled figures),  self-reported strategy (ex: non-verbal for visual processing of geometrical forms),  and so on.  All of these characteristics or strategies, favoring ear advantages, were found to be more typical of men than women.  At this point,  what is needed is a positron emission tomography study demonstrating that ear advantages really do reflect greater metabolic engagement of the supposedly specialized hemisphere in men.

The second major technique of experimental neuropsychology is tachistoscopy.  Tachistoscopy literally (etymologically) means viewing of very brief stimuli.  The technique allows researchers to present a stimulus to the right or left of a centrally fixated point for a few milliseconds (thousandths of a second).   The visual pathway of the brain is such that the right visual field projects to the left hemisphere and the left field to the right hemisphere.   I reviewed the scientific literature for reports of gender differences in field asymmetries (or absence of such gender effects).  Most of these experiments consisted of spatial discrimination tasks.  Only a few studies investigated children  -probably because it is difficult to get young children to reliably fixate a central point during such a task.  I found 25 such reports.  Fifteen concluded to more field asymmetry in boys or men, only one report found more field asymmetry in women, and the rest found no gender difference.   It seems that boys and men are more hemispherically functionally lateralized in this type of brain processing as well.   This gender difference seems particularly credible for spatial processing,  but the less numerous studies including verbal discrimination tasks also found that men were more hemispherically functionally lateralized.

The exploitation of the tachistoscopic technique for supporting inferences about sex differences in brain organization is also complicated by matters of cognitive style.  A group of Italian neuropsychologists published a most eloquent demonstration of this several years ago.  They were interested in determining whether there was a sex difference in hemispheric dominance on a task of visual discrimination.  They designed a study requiring subjects to determine whether two stimuli (pictures) in a given visual field were identical or different.   The stimuli resembled  letters.  Women obtained a right field advantage (typically inferred to involve the left hemisphere)  and men obtained a left field advantage  (typically inferred to involve the right hemisphere).   It turns out that the field effects were more due to the way in which men and women carried out the task, probably more than to any inherent differences in brain hemispheres. The researchers observed that the women spontaneously tended to use a “verbal” strategy and the men a “visual” strategy.  In a subsequent experiment, irrespective of gender, subjects who were instructed to use a verbal strategy consisting of viewing the pictures as alphabetical letters got a right field advantage.   Accordingly,  subjects got a left field advantage,  whether they were men or women,  when they  used a non-verbal "perceptual" strategy.    In a task like this one,  a non-verbal perceptual strategy is more efficient,  giving to men not only a field effect but also an overall performance advantage in the first experiment.   But on other tasks,  women's preference (often noted and documented          -including in navigational situations and mazes as we saw in the previous section) for more verbal cognitive problem-solving strategies  give them the advantage.  These matters of cognitive style can give a false impression of superiority of one sex over the other.   They can also lead researchers who are too naïve into believing in the existence of brain differences and even hemispheric asymmetries which don't exist.  In my own extensive research in the domain of tachistoscopic approaches to neuropsychology I have often obtained complex interactions involving gender  -in the absence of main effects,   suggesting to me that cognitive styles are probably at work -in addition to basic intrinsic differences in hemispheric specialization.