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Steroid fluctuations modify functional cerebral asymmetries: the hypothesis of progesterone-mediated interhemispheric decoupling

This study examines the modulation of functional cerebral asymmetries by gonadal hormones in three distinct groups. Young, normally cycling women performed a prototypical left (lexical decision) and two prototypical right-hemispheric tasks (figural comparison and face discrimination) during the low steroid menses and the high steroid midluteal phase. Saliva progesterone levels were measured with radioimmunoassay (RIA). Parallel to younger females, young men, and postmenopausal women were tested at matching time intervals. Results revealed significant interactions between cycle phase and visual half-field in the accuracy of all three tasks for the younger women; stronger lateralization patterns occurring during menses, while a more bilateral or at least less asymmetric cerebral organization predominated the midluteal phase, when highest levels of progesterone appear. Progesterone seemed to have a significant influence on lateralization in the figural comparison task, with high hormone levels enhancing the performance of the left hemisphere (for this task subdominant), thereby decreasing asymmetry. After menopause, when the levels of gonadal hormones are lower and more stable, the lateralization patterns for all three tasks were similar to those of men and normally cycling women during menses. These results make it likely that steroids and especially progesterone are able to reduce cerebral asymmetries. We hypothesize that progesterone attenuates the effect of glutamate on non-NMDA receptors. This could diminish cortico-cortical transmission which is mostly dependent on a glutamate-induced initial EPSP in pyramidal neurons which receive transcallosal input. The reduction in callosal transfer could then suppress the functional asymmetries.

Large-scale network organization in the avian forebrain: a connectivity matrix and theoretical analysis

Many species of birds, including pigeons, possess demonstrable cognitive capacities, and some are capable of cognitive feats matching those of apes. Since mammalian cortex is laminar while the avian telencephalon is nucleated, it is natural to ask whether the brains of these two cognitively capable taxa, despite their apparent anatomical dissimilarities, might exhibit common principles of organization on some level. Complementing recent investigations of macro-scale brain connectivity in mammals, including humans and macaques, we here present the first large-scale “wiring diagram” for the forebrain of a bird. Using graph theory, we show that the pigeon telencephalon is organized along similar lines to that of a mammal. Both are modular, small-world networks with a connective core of hub nodes that includes prefrontal-like and hippocampal structures. These hub nodes are, topologically speaking, the most central regions of the pigeons brain, as well as being the most richly connected, implying a crucial role in information flow. Overall, our analysis suggests that indeed, despite the absence of cortical layers and close to 300 million years of separate evolution, the connectivity of the avian brain conforms to the same organizational principles as the mammalian brain.

A left-sided visuospatial bias in birds

Document S1. Supplemental Experimental Procedures.xDownload (.03 MB ) Document S1. Supplemental Experimental Procedures.

Menstrual cycle affects functional cerebral asymmetries.

Cerebral lateralization in females is probably modulated by the menstrual cycle such that a part of hemispheric asymmetries are diminished with an increase of the steroids estrogen and progesterone during the follicular and luteal phase and enhanced with steroid decreases during the menstrual phase. However, previous data were contradictory with regard to the hemispheric side of modulation and could not analyze which steroid is mainly responsible for cycle dependent lateralization shifts. Therefore, in the present study estrogen and progesterone changes were assessed separately and related to changes in cerebral asymmetry. Plasma levels of estrogen and progesterone were measured once during luteal and once during menstrual cycle phase while 20 females subjects performed a verbal and a figural lateralized matching task. The results showed a significant cycle phase x lateralization interaction for the right hemisphere dominated figural comparison task but not for the left hemispheric lexical condition. Although the lateralization was modulated by the menstrual cycle, a within-subject regression analysis demonstrated that the asymmetry shift was not under direct influence of estrogen or progesterone. Thus, the present study provides further empirical support for cycle dependent alterations in lateralization but makes it unlikely that this effect is directly caused by estrogen or progesterone plasma level variations.

Visual memory lateralization in pigeons.

Previous experiments employing simple visual discrimination tasks have revealed a cerebral lateralization in the visual system of pigeons with a dominance of the left hemisphere. Until now, visual memory lateralization in birds has not been investigated. To study possible asymmetries of visual memory functions, a simultaneous instrumental discrimination procedure was used. The animals were trained to discriminate 100 different visual patterns from a further 625 similar stimuli. Retention tests were conducted under binocular and monocular conditions. When the subjects looked monocularly, retention performance was significantly higher with the right eye (left hemisphere) than with the left eye (right hemisphere). The results suggest that the lateralization of the pigeons visual system depends at least partly on an asymmetry in visual memory capacity.

Lateralization of visually controlled behavior in pigeons

Pigeons were trained binocularly on two different successive pattern discrimination tasks. After reaching criterion they performed the tasks with either the left or the right eye occluded or with both eyes unobstructed. The number of responses emitted was greater when the right rather than the left eye was uncovered. Also the percent correct discrimination scores were better when seeing with the right eye than with the left one. Some pigeons originally acquired the discrimination monocularly, generally using the right eye, although both eyes were uncovered. The results are discussed in relation to brain structures that may mediate this performance.

Right ear advantage for conspecific calls in adults and subadults, but not infants, California sea lions (Zalophus californianus): hemispheric specialization for communication?

This paper explores functional hemispheric asymmetries in the perception of auditory signals in a marine mammal species, the sea lion. Using a head‐orienting task toward sounds we found a right ear – left hemisphere – advantage for conspecific calls in adult and subadult California sea lions (Zalophus californianus) that was absent in infants. Non‐conspecific sounds did not elicit lateralized reactions in any age group. These findings show that maturational steps regarding communication in the brain of pinnipeds are similar to those described in primates. Such a result in a semi‐aquatic species distant from primates on the phylogenetic tree speaks for a stability and an ancient emergence of the left hemispheric specialization for vocal communication. The origin of what seems to be a widespread brain feature might be searched in the temporal and spectral communicative sounds characteristics rather than in its semantic value.

Morphological asymmetries of the tectum opticum in the pigeon

Abstract Pigeons are visually lateralized with a dominance of the right eye. Due to the virtually complete decussation of the optic nerves in birds, a right eye superiority probably depends on a left brain hemisphere dominance. The aim of the present study was to analyze whether morphological asymmetries in the cross-sectional area of perikarya can be found within the retina and the optic tectum. With an image-analyzing system the cross-sectional areas of the somata of retinal ganglion cells and tectal neurons were measured in the left and the right side under blind conditions. The results reveal significant morphological left-right differences, with cells in superficial layers 2–12 being larger on the left side while neurons in laminae 13–15 have larger somata in the right tectum. No retinal asymmetries could be revealed. Since pigeon embryos keep their head turned to the right within the egg, such that the right eye is stimulated by light shining through the shell, it is possible that the morphological asymmetries at the tectal level are induced by left-right differences in prehatching photic stimulation. This embryonic sensory asymmetry might lead to a higher activity level of right eye ganglion cells and to a larger amount of released neurotrophins in the left tectum. This in turn could exert the morphological effects on soma sizes in the superficial retinorecipient layers.

Limb preferences in non-human vertebrates.

There is considerable debate about whether population-level asymmetries in limb preferences are uniquely human or are a common feature among vertebrates. In the present article the results of studies investigating limb preferences in all non-extinct vertebrate orders are systematically analysed by employing cladographic comparisons. These studies analysed 119 different species, with 61 (51.26%) showing evidence for population-level asymmetries, 20 (16.81%) showing evidence for individual-level asymmetries and 38 (31.93%) showing no evidence for asymmetry. The cladographic comparison revealed that research in several key taxa in particular (e.g., Chondrichtyes, Crocodylia, Atlantogenata and Palaeognathae) would have important implications for our understanding of the evolution of vertebrate limb preferences. Furthermore, the findings of the present study support the position that population-level asymmetries in limb preferences as such represent a common vertebrate feature. Looking into the details, however, some important differences from human handedness become visible: Non-human limb preferences typically show a less-skewed lateralisation pattern and there are larger numbers of individuals without a preference in most species compared to humans. Moreover, limb preferences in non-human animals are often less task-invariant than human handedness and are more frequently modulated by external factors and individual characteristics.

Transcallosal inhibition across the menstrual cycle: A TMS study

OBJECTIVE To determine if there are steroid-dependent changes in transcallosal transfer during the menstrual cycle in normal women. METHODS We tested 13 normally cycling women during the menstrual, follicular and midluteal phases. Blood levels of estradiol (E) and progesterone (P) were determined by radioimmunoassay. Ipsilateral tonic voluntary muscle activity suppression, called ipsilateral silent period (iSP), was evoked by applying transcranial magnetic stimulation (TMS) over the left motor cortex and by measuring the EMG of the ipsilateral first dorsal interosseus (FDI) muscle. Both iSP-duration and transcallosal conduction times were measured and related to cycle phase and steroid levels. RESULTS Duration of iSPs varied over the cycle with largest differences between follicular and midluteal phases. During the midluteal phase high levels of P were significantly related to short iSPs. This relation also applied to E levels and iSPs during the follicular phase. CONCLUSIONS Our study shows for the first time that the transcallosal transfer is modulated by E and P and changes over the menstrual cycle. SIGNIFICANCE It is suggested that gonadal steroid hormones affect the interhemispheric interaction and change the functional cerebral organization sex dependently via its neuromodulatory properties on GABAergic and glutamatergic neurons.