Fear-specific leftward bias in gaze direction judgment

Previous studies have shown that humans have a left spatial attention bias in cognition and behaviour. However, whether there exists a leftward perception bias of gaze direction has not been investigated. To address this gap, we conducted three behavioural experiments using a forced-choice gaze direction judgment task. The point of subjective equality (PSE) was employed to measure whether there was a leftward perception bias of gaze direction, and if there was, whether this bias was modulated by face emotion. The results of experiment 1 showed that the PSE of fearful faces was significantly positive as compared to zero and this effect was not found in angry, happy, and neutral faces, indicating that participants were more likely to judge the gaze direction of fearful faces as directed to their left-side space, namely a leftward perception bias. With the response keys counterbalanced between participants, experiment 2a replicated the findings in experiment 1. To further investigate whether the gaze direction perception variation was contributed by emotional or low-level features of faces, experiment 2b and 3 used inverted faces and inverted eyes, respectively. The results revealed similar leftward perception biases of gaze direction in all types of faces, indicating that gaze direction perception was biased by emotional information in faces rather than low-level facial features. Overall, our study demonstrates that there a fear-specific leftward perception bias in processing gaze direction. These findings shed new light on the cerebral lateralization in humans.

Comparison of Interests and Special Abilities of Left- and Right-handers

Cerebral lateralization is associated with differences in brain organization and handedness is seen as its main marker. In order to verify the hypothesis that differences in brain organization tend to be associated with different patterns of interests and abilities, 379 normal left-handed subjects (221 women, aged 18 – 72 years) and 366 normal right-handed subjects (256 women, aged 18 – 72 years) were interviewed about their interests and special abilities. The following interests and special abilities were studied: singing, playing an instrument, dancing, painting, acting, foreign language skills, sports skills, and writing poetry.The results in the left-handers group showed that there is a significantly higher percentage of subjects with a pronounced interest in playing an instrument. In the group of right-handers, the study demonstrated a significantly higher percentage of subjects specifically interested in learning foreign languages. With respect to the abilities, a significantly higher incidence of participants with vocal, instrumental, sports, and foreign language skills was found in the left-handers’ group. Differences in functional cerebral organization between left- and right-handers were suggested as possible explanation for the relationship of handedness with special abilities.

Quantitative Assessment of Handedness and Cerebral Lateralization

Handedness is the most studied human asymmetry due to its connection to various lateralized behaviours and hence many studies have focused on developing a valid assessment. One of the most popular is the Edinburgh Handedness Inventory (EHI), however, its psychometric properties have been questioned and due to its complex response format, various modified versions of EHI are in use. One of them is a 7-item questionnaire produced by Dragovic and Milenkovic (2013), which seeks to reduce a potentially continuous variable to a categorical one by clearly classifying people as right- or left- handed. The purpose of this study was to develop a questionnaire to more accurately quantify mixed-handedness as a continuous variable, and to investigate the correlation between Dragovic’s modified 7-item EHI, new items created for this study, and three performance measures (grooved pegboard, finger-tapping and grip strength/dynamometer). A total of 113 self-reported right-, left- and mixed-handed participants were randomly recruited to complete the questionnaire and behavioural measures. The questionnaire data was submitted to exploratory factor analysis and resulting factor scores were examined for correlations with behavioural tests. Compared to the modified EHI, the questionnaire showed a more continuous grading of handedness. Moreover, the degree of handedness on the questionnaire showed a stronger correlation with all the performance measures than the performance measures had amongst themselves. These findings show that the novel questionnaire with modern-day items can provide an accurate estimate of the degree of mixed-handedness in both right- and left-handed individuals. Future studies should examine these measures on a larger sample of left-handed and ambidextrous people, who tend to be more variable than right-handers in their usage. The questionnaire can also be suitable for studying the relationship between variable handedness and other aspects of brain lateralization.

Laterality in Responses to Acoustic Stimuli in Giant Pandas

Cerebral lateralization is a common feature present in many vertebrates and is often observed in response to various sensory stimuli. Numerous studies have proposed that some vertebrate species have a right hemisphere or left hemisphere dominance in response to specific types of acoustic stimuli. We investigated lateralization of eight giant pandas (Ailuropoda melanoleuca) by using a head turning paradigm and twenty-eight acoustic stimuli with different emotional valences which included twenty-four conspecific and four non-conspecific acoustic stimuli (white noise, thunder, and vocalization of a predator). There was no significant difference in auditory laterality in responses to conspecific or non-conspecific sounds. However, the left cerebral hemisphere processed the positive stimuli, whereas neither of the two hemispheres exhibited a preference for processing the negative stimuli. Furthermore, the right hemisphere was faster than the left hemisphere in processing emotional stimuli and conspecific stimuli. These findings demonstrate that giant pandas exhibit lateralization in response to different acoustic stimuli, which provides evidence of hemispheric asymmetry in this species.

Δυσλεξία και εγκεφαλική ασυμμετρία: ανασκόπηση μελετών ανατομικής και λειτουργικής απεικόνισης

Developmental dyslexia is a learning disorder that is marked by reading achievement that falls substantially below that expected given the individual's chronological age, measured intelligence, and age-appropriate education (ICD-10, WHO, 1993). It is estimated to affect 5-17% of the student population. It is a disorder that is neurological in nature (Habib, 2000. Σίμος, Μουζάκη, & Παπανικολάου, 2004), but which requires educational intervention. Recently, there has been considerable progress in the investigation of the neurobiological underpinnings of dyslexia. Yet, even today Orton’s 1925 hypothesis, postulating that individuals with dyslexia have not adequately developedthe typical left cerebral lateralization for language and are symmetrical or right-hemispheric dominant for language, remains timely and is under investigation. The present narrative review presents research studies that have investigated the inadequate lateralization hypothesis using anatomical and functional brain imaging. Findings show, for the largest part, that individuals with dyslexia do indeed show anatomical and functional differences in the language networks of the left hemisphere, compared to typical readers. In conclusion, inadequate lateralization is an important neurobiological factor for the development of dyslexia, albeit it is not the only one.

Lateralization correlates with individual differences in inhibitory control in zebrafish

Individual fitness often depends on the ability to inhibit behaviours not adapted to a given situation. However, inhibitory control can vary greatly between individuals of the same species. We investigated a mechanism that might maintain this variability in zebrafish ( Danio rerio ). We demonstrate that inhibitory control correlates with cerebral lateralization, the tendency to process information with one brain hemisphere or the other. Individuals that preferentially observed a social stimulus with the right eye and thus processed social information with the left brain hemisphere, inhibited foraging behaviour more efficiently. Therefore, selective pressures that maintain lateralization variability in populations might provide indirect selection for variability in inhibitory control. Our study suggests that individual cognitive differences may result from complex multi-trait selection mechanisms.

Cerebral laterality for writing in right- and left- handers: A functional transcranial Doppler ultrasound study

The cerebral lateralization of written language has received very limited research attention in comparison to the wealth of studies on the cerebral lateralization of oral language. The purpose of the present study was thus to further our understanding of written language lateralization, by elucidating on the relative contribution of language and motor areas. This was achieved by comparing written word generation with a writing task that has similar visuomotor demands, but does not include language (i.e., the repeated drawing of symbols). We assessed cerebral laterality in 54 left- and right-handed participants by means of functional transcranial Doppler ultrasound (fTCD), a non-invasive, perfusion-sensitive neuroimaging technique. Our findings suggest that, in right-handers, the linguistic aspect of the written word generation task recruited left-hemispheric areas during writing, similarly to oral language production. In left-handers, we failed to observe the same effect, possibly due to the great variability in cerebral laterality patterns within this group or alternatively due to the attentional demands of the symbol copying task. Future work could investigate such demands using both simple and complex stimuli in the copying condition.