Is the handedness gene on the X chromosome? Comment on Jones and Martin (2000).

G. V. Jones and M. Martin (2000) argued, contrary to M. C. Corballis (1997), that a gene for handedness might plausibly be located in homologous, noncombining regions of the X and Y chromosomes. The specific model they proposed is unlikely to be correct, but a case can be made for an X-linked gene that has no homologue on the Y chromosome and that is subjected to X-inactivation in females. An X-linked gene predicts no overall sex difference in the incidence of left-handedness; the slight preponderance of left-handers among males might then be attributed to a higher incidence of pathologically induced left-handedness.     

Split-Brain: What We Know Now and Why This is Important for Understanding Consciousness

Neuropsychology Review - Recently, the discussion regarding the consequences of cutting the corpus callosum (“split-brain”) has regained momentum (Corballis, Corballis, Berlucchi,...     

Space,time, and language

Cognition is heavily grounded in space. As animals that move in space, we travel both physically and mentally in space and time, reliving past events, imagining future ones, and even constructing imaginary scenarios that play out in stories. Mental exploration of space is extraordinarily flexible, allowing us to zoom, adopt different vantage points, mentally rotate, and attach objects and sense impressions to create events, whether remembered, planned, or simply invented. The properties of spatiotemporal cognition depend on a hippocampal–entorhinal circuit of place cells, grid cells and border cells, with combinations of grid-cell modules generating a vast number of potential spatial remappings. The generativity of language, often considered one of its defining properties, may therefore derive not from the nature of language itself, but rather from the generativity of spatiotemporal scenarios, with language having evolved as a means of sharing them. Much our understanding of the hippocampal–entorhinal circuit is derived from neurophysiological recording in the rat brain, implying that the spatiotemporal cognition underpinning language has a long evolutionary history.     

An investigation of the line motion effect in a callosotomy patient

When a line is flashed instantaneously between two markers it can appear to propagate from one marker to the other. This illusion is known as the line motion effect. We investigated this effect in the two hemispheres of a callosotomy ("split-brain") patient. We found that both hemispheres perceived the line motion effect, and that flashing one of the markers biased the direction of motion away from that marker regardless of which hemisphere received the stimulus. In contrast, matching the width of the line to the width of one of the markers biased the direction of motion away from the marker only when it appeared in the left visual hemifield. This suggests that multiple mechanisms can contribute to the line motion effect, and that some of these mechanisms rely on different neural structures.     

Anterior-posterior beta asymmetries in dyslexia during lexical decisions

High-density EEG was recorded in 12 compensated dyslexics, 6 classified as dysphonetic and 6 as dyseidetic, and in 12 matched controls while they carried out a lexical-decision task. Relative to normal controls, dysphonetics showed higher beta power in anterior relative to posterior regions, while dyseidetics showed higher beta power in posterior relative to anterior regions. Further, controls (but not dyslexics) showed a positive correlation between performance on the task and the ratios of both left-to-right and anterior-to-posterior beta asymmetry. According to the dual-route theory of reading, there are two strategies that can be used in lexical decision: A visual strategy involving visual word identification and direct access to a visual lexicon, and a phonological strategy involving grapheme-to-phoneme conversion and access to a speech output lexicon. Our results therefore suggest compensation through weakness rather than strength, with phonological dyslexics focusing on a grapheme-to-phoneme strategy and dyseidetics focusing on visual word identification.     

Visuospatial processing and the right-hemisphere interpreter

Popular views of hemispheric asymmetry hold that the left hemisphere is specialized for linguistic and cognitive processes and fine motor control, whereas the right is specialized for visuospatial processing. Although this dichotomy contains more than a grain of truth, it is an oversimplification. Experiments with split-brain patients have demonstrated that the left hemisphere retains relatively sophisticated visuospatial abilities, and that the asymmetries that favor the right hemisphere are subtler than those that favor the left. A consideration of the constructive nature of visual perception, and the organization of the visual system in the two hemispheres suggests that asymmetries are likely to arise relatively late in visual processing in areas that represent both sides of visual space. I present evidence in favor of the view that the right hemisphere can be considered more "visually intelligent" than the left, and postulate the existence of a "right-hemisphere interpreter" dedicated to constructing a representation of the visual world.