The stability of tachistoscopic measures of hemispheric specialization

Modulation of rhythmic visual stimulation on left-right attentional asymmetry

The rhythmic visual stimulation (RVS)-induced oscillatory brain responses, namely steady-state visual evoked potentials (SSVEPs), have been widely used as a biomarker in studies of neural processing based on the assumption that they would not affect cognition. However, recent studies have suggested that the generation of SSVEPs might be attributed to neural entrainment and thus could impact brain functions. But their neural and behavioral effects are yet to be explored. No study has reported the SSVEP influence on functional cerebral asymmetry (FCA). We propose a novel lateralized visual discrimination paradigm to test the SSVEP effects on visuospatial selective attention by FCA analyses. Thirty-eight participants covertly shifted their attention to a target triangle appearing in either the lower-left or -right visual field (LVF or RVF), and judged its orientation. Meanwhile, participants were exposed to a series of task-independent RVSs at different frequencies, including 0 (no RVS), 10, 15, and 40-Hz. As a result, it showed that target discrimination accuracy and reaction time (RT) varied significantly across RVS frequency. Furthermore, attentional asymmetries differed for the 40-Hz condition relative to the 10-Hz condition as indexed by enhanced RT bias to the right visual field, and larger Pd EEG component for attentional suppression. Our results demonstrated that RVSs had frequency-specific effects on left-right attentional asymmetries in both behavior and neural activities. These findings provided new insights into the functional role of SSVEP on FCAs.

Stressing schizotypy: the modulating role of stress-relieving behaviours and intellectual capacity on functional hemispheric asymmetry

Relative cognitive impairments are common along the schizophrenia spectrum reflecting potential psychopathological markers. Yet stress, a vulnerability marker in schizophrenia (including its spectrum), is likewise related to cognitive impairments. We investigated whether one such cognitive marker (attenuated functional hemispheric asymmetry) during stressful life periods might be linked to individuals' schizotypal features or rather to individuals' stress-related experiences and behaviours. A total of 58 students performed a left hemisphere dominant (lateralised lexical decisions) and right hemisphere dominant (sex decisions on composite faces) task. In order to account for individual differences in stress sensitivity we separated participants into groups of high or low cognitive reserve according to their average current marks. In addition, participants filled in questionnaires on schizotypy (short O-LIFE), perceived stress, stress response, and a newly adapted questionnaire that enquired about potential stress compensation behaviour (elevated substance use). The most important finding was that enhanced substance use and cognitive disorganisation contributed to a right and left hemisphere shift in language dominance, respectively. We discuss that (i) former reports on right hemisphere shifts in language dominance with positive schizotypy might be explained by an associated higher substance use and (ii) cognitive disorganisation relates to unstable cognitive functioning that depend on individuals' life circumstances, contributing to published reports on inconsistent laterality-schizotypy relationships.

Reliability of non-verbal laterality effects in the visual modality

The present experiment investigated the reliability and magnitude of laterality effects in a non-verbal task in the visual modality. The use of a bilateral discrimination task in which participants indicated whether a centrally presented probe stimulus matched either of the bilaterally presented targets was presumed to provide control over attention deployment. This led to the prediction that a reliable left visual field advantage (LVFA) would be obtained. A total of 40 right-handed undergraduate students completed the bilateral discrimination task twice in a test-retest design. Although relatively large test-retest correlations suggested that the laterality effect was quite reliable, a significant LVFA was obtained in the first testing session, and a right visual field advantage in the second one. This finding parallels results obtained in previous work with non-verbal tasks and supports the notion that practice affects the direction of laterality effects. The discussion examines alternative explanations with emphasis on practice effects and possible attentional factors. A possible shift in the bivariate distribution of laterality scores is used as a tentative explanation of the apparent contradiction between the high test-retest reliability and the shift in laterality with practice.

Cerebral Asymmetries for Language: Evidence for Structural-Behavioral Correlations

The current investigation tested 20 male right-handers in 5 divided visual field lexical tasks. Asymmetries in Heschl's gyrus, planum temporale, and planum parietale were measured using structural magnetic resonance imaging. Composite task asymmetries were positively correlated with asymmetry of the planum temporale only. There was also an association between the consistency of anatomical and behavioral asymmetries: Individuals who departed the most from the modal pattern of cortical asymmetry across regions also tended to show the greatest variability in asymmetry across tasks. Hence, individual differences in language laterality tasks may be affected by variation in asymmetry of posterior language structures. Additionally, when typical anatomical asymmetries fail to co-occur, there may be a less strictly regulated distribution of function across hemispheres.

Sex-related engagement of the hemispheres in visuospatial processing

Hemispheric asymmetries in visuospatial functions were investigated in women and men with a tachistoscopic task using lateralized presentation of "Necker" cubes. A lexical decision task served as control. In either of the two tasks the procedure involved a bilateral presentation of correct and distorted items and responses had to be given by means of a nonverbal decision task. The analysis of hits in the cube task showed a right visual field advantage for women and a left visual field advantage for men, but women had more false alarms in the right visual field, whereas men showed the reverse pattern again. The lexical decision task showed similar response patterns for men and women. The distinct response behavior supports the hypothesis that tasks allowing for various strategies to choose from obviously are better suited to elicit a differential engagement of the two hemispheres in women and men.