Sex differences in brain organization: different brains or different strategies?

Gender-dependent variations in optical illusions: evidence from N400 waveforms

OBJECTIVE

The cognitive mechanisms (especially the gender-related ones) underlying optical illusion processing remain elusive. Since the N400 component of event-related potentials (ERPs) is an index of the semantic integration of information processing tasks, the present study focuses on gender-related differences in N400 waveforms elicited during the reasoning process applied to reach a valid conclusion of optical illusions engaging working memory (WM).

APPROACH

Fifty-one healthy participants (28 males, age = 34.25 years ± 10.25, years of education = 16.00 years ± 1.78; and 23 females, age = 33.43 ± 7.93, years of education = 15.56 ± 1.82) were measured. The N400 ERP component was evoked by 39 optical illusions adjusted to induce WM. We compared brain activation patterns while participants maintained conclusions of the optical illusions in WM. The N400 of ERPs was recorded during the WM phase, during which participants were required to draw a logical conclusion regarding the correctness of the optical illusions.

MAIN RESULTS

Analysis revealed that females compared to males exhibited significantly increased N400 amplitudes located at parietal and occipital sites, whereas males exhibited significantly higher N400 amplitudes located at frontal areas. Furthermore, females compared to males demonstrated significantly prolonged latencies of the N400 component located at right frontotemporal abductions.

SIGNIFICANCE

These results suggest that coupling of optical illusions with WM engages distinct gender-related variations of brain semantic processing as reflected by the N400 ERP component. Based on the dual process account, our study gives support to the notion that women tend to employ a more deliberate and slower semantic reasoning than the men who tend to employ an automatic and fast one. Topographically, within the network sub-serving the semantic operation, the posterior brain areas responsible for sensorimotor integration-related processes elicit a greater brain activation among females while the anterior brain areas responsible for control and storage/retrieval operation elicit a greater brain activation among males.

Effect of Visual Complexity in Identification of Tachistoscopic Images

The present study of gender differences in hemispheric processing involved identification of tachistoscopically presented images of varying complexity. A computerized tachistoscopic program was administered to 24 men and 34 women. Time to identify contour and detailed pictures presented to the left or right cerebral hemisphere was recorded. Mean reaction time for contour pictures was significantly faster than for detailed pictures, and mean reaction time to the right hemisphere was significantly faster than that to the left hemisphere. The mean reaction time for men to identify pictures exposed to the left hemisphere was significantly slower than that for exposure to the right hemisphere for women. The mean reaction time for both men and women to identify contour pictures exposed to the right hemisphere was significantly faster than the mean time to identify detailed pictures presented to the left hemisphere. The interaction of gender, hemisphere, and complexity was also significant in that mean reaction times for men to identify detailed pictures presented to the left hemisphere were slower than the times for women to identify contour pictures presented to the right hemisphere. The results are discussed in relation to theories about hemispheres, gender, and differences in picture features.

Illustrating Brain Lateralisation in a Naturalistic Observation of Cell-Phone Use

This article describes an activity designed to increase students' understanding of hemispheric specialisation, specifically the fact that the left hemisphere plays a dominant role in language processing. Because of the contralateral organisation of the auditory system, research has shown a right ear advantage for language processing. The activity involves a naturalistic observation in which students observe individuals using handheld cell (mobile) phones in public locations. The results indicated that more cell phone users listened with the right ear than with the left. Students prepared a written report describing their results, and their understanding of key concepts was assessed before and after the activity. The results demonstrated that the activity increased students' knowledge. Furthermore, students reported that the activity was interesting and enjoyable and they recommended that it be used in future courses.

On the Magnitude of Laterality Effects and Sex Differences in Functional Lateralities

In the last 20 years, the hypothesis that men and women differ in functional lateralities has been used to account for sex-related differences in verbal and spatial skills. However, this hypothesis has not been clearly supported, with some reviewers confirming it (McGlone, 1980 for example), and others rejecting it (Fairweather, 1982 for example). The purpose of the present study was to provide a definite test of this hypothesis and to estimate the magnitude of overall laterality effects by means of a meta-analytic procedure. A total of 396 significance levels from a variety of studies on functional asymmetries utilising auditory, visual, or tactile presentation of verbal or nonverbal stimuli were sampled. Results showed that laterality effects tend to be large and significant but that they are heterogeneous in the visual modality. Homogeneity was generally achieved by a partition of the studies in terms of the specific task used. The results also showed sex differences to be significant in two modalities (visual and auditory). The data indicated the presence of sex differences in favour of men in functional asymmetries. However, it appears that the findings are not resistant to the file drawer problem. The results are discussed with regard to their implications for explanations of individual differences in cognitive abilities. The relation between functional lateralities and anatomical asymmetries is also discussed.

Gender differences in laterality on a dichotic task: the influence of report strategies

Four experiments were conducted to investigate the influence of report strategies on gender differences in laterality. Ten males and 10 females completed a dichotic consonant-vowels pairs task under four conditions manipulating reporting strategies. Free recall allowed participants to report syllables in any order they chose, whereas order of report control required the report of a pre-specified ear first. In focused attention, they reported only one response from a pre-specified ear. Finally, in ABX discrimination, participants were required to indicate whether a binaural probe was one of the stimuli presented dichotically on the same trial. It was hypothesized that improved control of report strategies would increase the likelihood of detecting significant gender differences in laterality. This was confirmed when results showed no significant gender differences in laterality for the free recall and order of report control conditions, whereas focused attention produced marginal gender differences and clearly significant differences were obtained in the ABX discrimination condition. These findings have implications for interpretations of gender differences in laterality based on strategy effects. Interpretations emphasize the role of attention deployment in the measurement of laterality.

Comparing the Fused Dichotic Words Test and the Intracarotid Amobarbital Procedure in children with epilepsy

The validity of the Fused Dichotic Words Test (FDWT) in predicting the nature of speech representation, as determined by the Intracarotid Amobarbital Procedure (IAP), was examined in a sample of 28 children with epilepsy. Various methods of analysis (difference score, lambda, and lambda(*)), for the FDWT data were calculated and compared. Results showed the validity of the FDWT did not change depending on the method of analysis. The difference scores showed that 18 of the 19 patients with left hemisphere speech obtained right-ear advantages, while the patient with right hemisphere speech showed a left-ear advantage. As a group, patients with left-hemisphere speech obtained a statistically significant right-ear advantage for the lambda and lambda(*) indices, while the patient with right-hemisphere speech showed a left-ear advantage that was also significant for both lambda measures. Patients with bilateral speech, as a group, displayed a non-significant ear advantage. Some of the scores from the left-hemisphere group overlapped with those from patients with bilateral speech representation. Controlling for stimulus dominance effects using the lambda(*) measure did not improve classification accuracy for nature of speech representation based on FDWT scores. Finally, comparison of our data using the laterality index from a similar study, showed scores smaller in magnitude than that found in adults with epilepsy.

Spatial Perception and Mental Rotation Produce Gender Differences in Cerebral Hemovelocity

Abstract We sought to determine if gender differences in cerebral blood flow velocity emerge while persons performed cognitive tasks known to favor men, e.g., tests of spatial abilities. Bilateral measures were obtained simultaneously from the middle cerebral artery (VMCA) by transcranial Doppler sonography while men and women college students performed 31-s thinking tasks. Tests of spatial ability included (1) three spatial visualizing tasks (finding words among sets of letters, locating pictures hidden within a complex scene, and finding embedded geometric patterns), and (2) a mental rotation task. Two nonspatial visualizing control tasks were looking at (1) a list of words and (2) a set of pictures. Women had significantly faster global VMCAs than men during all tasks except looking at pictures. Two tasks (looking at pictures, mental rotation) produced hemispheric asymmetry (right > left) in women only. Gender differences in the number of correct responses occurred for finding words (women > men) and mental rotation (paradoxically, men > women) but not the other tests of spatial abilities. Our study shows that transcranial Doppler sonography provides noninvasive, real-time physiological indices of gender differences in spatial abilities.

Dyslexia, gender, and brain imaging

Future brain imaging studies of dyslexia should have a sufficient number of males and females to detect possible gender differences in the neurological underpinning of this disorder. Detailed knowledge about such differences may clarify our understanding of the structural and functional impairments which lead to the phonological deficits that characterize dyslexia. Functional brain imaging studies have shown that males and females exhibit different patterns of brain activation during phonological processing. Further differences between the brains of males and females have been suggested by studies of normal brain development, morphology, and functional activation during reading. Animal studies have shown that lesions, similar to those seen in postmortem studies of dyslexia, affect rapid auditory processing in males, but not in females. The large body of research on gender differences in brain development, functional organization, and activation during reading tasks urges separation of males and females in dyslexia research in order to minimize variance and to detect subtle, but functionally-relevant, differences. Well-controlled studies, with large numbers of male and female dyslexics, may produce more sensitive and accurate identification of the neurological substrates of dyslexia.

On the reliability and validity of noninvasive laterality measures

The purpose of the present study was to quantify the reliability and validity of laterality effects obtained with noninvasive measures. A meta-analytic approach was used with 88 significance levels pertaining to reliability data and 11 significance levels concerning the validity of the measures. Results showed that reliability is affected by a number of procedural factors. In general, reliability was found to be at a moderate level. The validity of laterality measures was found to be significant but low. These findings suggest that more empirical work is needed to investigate and to improve the validity and reliability of the tasks used in the assessment of laterality effects.

Effect of visual complexity in identification of tachistoscopic images

The present study of gender differences in hemispheric processing involved identification of tachistoscopically presented images of varying complexity. A computerized tachistoscopic program was administered to 24 men and 34 women. Time to identify contour and detailed pictures presented to the left or right cerebral hemisphere was recorded. Mean reaction time for contour pictures was significantly faster than for detailed pictures, and mean reaction time to the right hemisphere was significantly faster than that to the left hemisphere. The mean reaction time for men to identify pictures exposed to the left hemisphere was significantly slower than that for exposure to the right hemisphere for women. The mean reaction time for both men and women to identify contour pictures exposed to the right hemisphere was significantly faster than the mean time to identify detailed pictures presented to the left hemisphere. The interaction of gender, hemisphere, and complexity was also significant in that mean reaction times for men to identify detailed pictures presented to the left hemisphere were slower than the times for women to identify contour pictures presented to the right hemisphere. The results are discussed in relation to theories about hemispheres, gender, and differences in picture features.

Right-, left-dominance and ambidexterity in grasp reflex in human newborn: importance of left brain in cerebral lateralization

The grasp reflex was studied in human newborn without familial sinistrality. Of 60 females, 26 (43.3%) were right-handed and 34 (56.7%) ambidextrous. Of 62 males, 20 (32.3%) were right-handed, 39 (62.9%) ambidextrous, and 3 (4.8%) left-handed. There was a nonsignificant preponderance of right-dominance in females and a significant preponderance of nonright-handedness in males. In right-handers, the mean right minus left (R-L) grasp-reflex showed a positive linear correlation with the grasp-reflex from the right and left hands, with a higher correlation for the right hand. In ambidexters, the R-L grasp reflex did not show any significant correlation with the grasp reflex from the right and left hands. The mean grasp-reflex from right and left were found to be significantly smaller in ambidextrous males and females then right-handed males and females, with a much higher significance for the right hand. It was concluded that females tended to have a more pronounced reflex lateralization than males. The results also indicated that the left brain may be more important than the right brain for the development of a spinocerebral motor lateralization in humans.

Influence of Stronger Attentional Manipulations on the Processing of Dichotic Inputs in Right-Handers

Recent studies have shown that a pronounced right-ear advantage can occur within dichotic tasks when persons of a particular sex are directed to focus their attention onto the right ear. To examine the effects of attentional bias on laterality more precisely, we provided subjects a greater number of focused-ear trials. Analysis indicated a very strong right-ear advantage in men and women under the right-focus condition as well as a significant left-ear advantage for both sexes with directed left-focus. These findings suggest that sustained ear-attendance exerts a powerful influence on dichotic performance.

Influence of stronger attentional manipulations on the processing of dichotic inputs in right-handers

Recent studies have shown that a pronounced right-ear advantage can occur within dichotic tasks when persons of a particular sex are directed to focus their attention onto the right ear. To examine the effects of attentional bias on laterality more precisely, we provided subjects a greater number of focused-ear trials. Analysis indicated a very strong right-ear advantage in men and women under the right-focus condition as well as a significant left-ear advantage for both sexes with directed left-focus. These findings suggest that sustained ear-attendance exerts a powerful influence on dichotic performance.

Asymmetrical processing of dichotic inputs in undergraduates across sex, handedness, ear-side, and experimental instructions

Past studies have shown that some female as well as left-handed students do not demonstrate a right-ear advantage (REA) for verbal materials, suggesting that linguistic functions may not be handled in one hemisphere exclusively. To examine these laterality effects more closely, 96 undergraduates were equally divided by sex and hand dominance. Moreover, experimental instructions as to which ear to focus on were provided in a dichotic listening procedure, with left-ear attendance alternating with right-ear across four counterbalanced blocks of 60 trials each. Analysis indicated a major interaction of sex x ear x instructions, with men showing a right-ear advantage when attending to that respective side, highlighting the importance of experimental demands on dichotic performance. The right-ear advantage in right-handed persons was uninfluenced by these instructions, suggesting more pronounced asymmetries in this group.

Differential visual field-interhemispheric transfer: can it explain sex and handedness differences in lateralization?

104 men and women were tested for visual field-hemispheric transfer of spatial information on a dot-localization task. Right-handed subjects showed significant improvement when stimuli were presented to the left visual field of the right hemisphere (LVF-RH) after practice on the same task presented to the right visual field of the left hemisphere (RVF-LH) first. No improvement was found when the task was presented in the reverse order (LVF-RH first followed by RVF-LH). It was concluded that, for right-handers, transfer of spatial information to the right hemisphere is facilitated while transfer to the left hemisphere is inhibited. Left-handed subjects demonstrated no significant improvement in either condition, suggesting inhibition or lack of transfer of spatial information in either direction. No sex differences were found in either right-handed or left-handed subjects. The findings suggest that there may be different mechanisms underlying the similarities in functional lateralization of women and left-handers.

Tactual laterality effects and the processing of spatial characteristics: dichaptic exploration of forms by first and second grade children

Tactual laterality effects (T.L.E.) were examined in first- and second-grade children when performing a dichaptic task. Our procedure, a modified version of Witelson's standard dichaptic task, emphasized the type of errors made, so that we could construct a weighted index (W.I.) for scoring rather than a strictly dichotomous one. The same data were compared using the two types of scoring indexes stated above. T.L.E. occurred consistently with the W.I. scores only, both in two independent groups or in the same group at a year's interval, suggesting (1) that a strictly dichotomous index might lead to loss of information, (2) that the results are reliable at least at the group level. Results are discussed in terms of possible applications of such a procedure to investigating the relationship between performance (spatial processing) and brain organization (hemispheric balance).

Toward a model of dichotic listening performance

Dichotic listening performance for different classes of speech sounds was examined under conditions of controlled attention. Consideration of the complex of target item and competing item demonstrated that, in general, targets were more accurately identified when the competing item shared no relevant features with it and less accurately identified when the competing item shared place, voice, or manner with the target item. Nasals as well as stops demonstrated a significant right-ear advantage (REA). False alarm rates were very similar for left and right attentional conditions, whereas intrusions from the right ear while attending to the left were far more common than intrusions from the left while attending to the right. Attention is viewed as serving to select the stimuli that will be reported, but at a late stage, and only after the right ear perceptual advantage has had its effect. A model of dichotic listening performance is proposed in which both the ease of localizing the item and the strength of evidence for the presence of the item are relevant factors.

Sex differences in the lateralization of language discrimination vs language production

Two tachistoscopic studies employing normal males and females were done to investigate the inconsistency of obtained sex differences on visual laterality tasks to the extent that this inconsistency is task specific. Type of linguistic task (discrimination vs production) and control of attention were manipulated. Results showed that on language discrimination, the sexes were fairly equally lateralized, while on production tasks, females showed greater laterality differences. The attempt to control for attention resulted in higher field differences, but especially so for females. It is suggested that more consideration of task demands in laterality experiments should help clarify the nature of sex differences in laterality.

Sex differences in the cognitive effects of unilateral brain damage: comparison of stroke patients and normal control subjects

Our original paper on the influence of the sex of the patients on the cognitive effects of unilateral brain damage mainly described the differences found between the various lesion groups (left/right, male/female) and said little about comparisons between these groups and our control subjects. Such comparisons are examined here; they confirm that the major sex differences after such brain damage appear in tasks intended to involve nonverbal processing. This evidence supports the hypothesis that women, to a greater extent than men, may employ verbal, left hemisphere processing to solve ostensibly nonverbal problems.

Relationships between speed, accuracy of performance and hemispheric superiorities for visuo-spatial pattern processing in the two sexes

The aim of this study was to investigate the relationship between visual field differences in tachistoscopic recognition of visuo-spatial patterns and overall proficiency in the two sexes. The results demonstrated a left visual field advantage in males but not in females. A female subgroup showed opposite rather than lacking visual field superiorities without affecting speed and accuracy of performance. These data warn against explaining differences in performance in the two sexes in terms of different hemispheric asymmetries.