Lateralization of functions in the animal brain

Symmetric interlimb transfer of newly acquired skilled movements

In this study, we aimed to examine features of interlimb generalization or "transfer" of newly acquired motor skills, with a broader goal of better understanding the mechanisms mediating skill learning. Right-handed participants (n = 36) learned a motor task that required them to make very rapid but accurate reaches to one of eight randomly presented targets, thus bettering the typical speed-accuracy tradeoff. Subjects were divided into an "RL" group that first trained with the right arm and was then tested on the left and an "LR" group that trained with the left arm and was subsequently tested on the right. We found significant interlimb transfer in both groups. Remarkably, we also observed that participants learned faster with their left arm compared with the right. We hypothesized that this could be due to a previously suggested left arm/right hemisphere advantage for movements under variable task conditions. To corroborate this, we recruited two additional groups of participants (n = 22) that practiced the same task under a single target condition. This removal of task level variability eliminated learning rate differences between the arms, yet interlimb transfer remained robust and symmetric, as in the first experiment. Additionally, the strategy used to reduce errors during learning, albeit heterogeneous across subjects particularly in our second experiment, was adopted by the untrained arm. These findings may be best explained as the outcome of the operation of cognitive strategies during the early stages of motor skill learning.NEW & NOTEWORTHY How newly acquired motor skills generalize across effectors is not well understood. Here, we show that newly learned skilled actions transfer symmetrically across the arms and that task-level variability influences learning rate but not transfer magnitude or direction. Interestingly, strategies developed during learning with one arm transfer to the untrained arm. This likely reflects the outcome of learning driven by cognitive mechanisms during the initial stages of motor skill acquisition.

Behavior training reverses asymmetry in hippocampal transcriptome of the cav3.2 knockout mice

Homozygous Ca_v3.2 knockout mice, which are defective in the pore-forming subunit of a low voltage activated T-type calcium channel, have been documented to show impaired maintenance of late-phase long-term potentiation (L-LTP) and defective retrieval of context-associated fear memory. To investigate the role of Ca_v3.2 in global gene expression, we performed a microarray transcriptome study on the hippocampi of the Ca_v3.2^-/- mice and their wild-type littermates, either naïve (untrained) or trace fear conditioned. We found a significant left-right asymmetric effect on the hippocampal transcriptome caused by the Ca_v3.2 knockout. Between the naive Ca_v3.2^-/- and the naive wild-type mice, 3522 differentially expressed genes (DEGs) were found in the left hippocampus, but only 4 DEGs were found in the right hippocampus. Remarkably, the effect of Ca_v3.2 knockout was partially reversed by trace fear conditioning. The number of DEGs in the left hippocampus was reduced to 6 in the Ca_v3.2 knockout mice after trace fear conditioning, compared with the wild-type naïve mice. To our knowledge, these results demonstrate for the first time the asymmetric effects of the Ca_v3.2 and its partial reversal by behavior training on the hippocampal transcriptome.

Lateralization of resting state networks and relationship to age and gender

Brain lateralization is a widely studied topic, however there has been little work focused on lateralization of intrinsic networks (regions showing similar patterns of covariation among voxels) in the resting brain. In this study, we evaluate resting state network lateralization in an age and gender-balanced functional magnetic resonance imaging (fMRI) dataset comprising over 600 healthy subjects ranging in age from 12 to 71. After establishing sample-wide network lateralization properties, we continue with an investigation of age and gender effects on network lateralization. All data was gathered on the same scanner and preprocessed using an automated pipeline (Scott et al., 2011). Networks were extracted via group independent component analysis (gICA) (Calhoun et al., 2001). Twenty-eight resting state networks discussed in previous (Allen et al., 2011) work were re-analyzed with a focus on lateralization. We calculated homotopic voxelwise measures of laterality in addition to a global lateralization measure, called the laterality cofactor, for each network. As expected, many of the intrinsic brain networks were lateralized. For example, the visual network was strongly right lateralized, auditory network and default mode networks were mostly left lateralized. Attentional and frontal networks included nodes that were left lateralized and other nodes that were right lateralized. Age was strongly related to lateralization in multiple regions including sensorimotor network regions precentral gyrus, postcentral gyrus and supramarginal gyrus; and visual network regions lingual gyrus; attentional network regions inferior parietal lobule, superior parietal lobule and middle temporal gyrus; and frontal network regions including the inferior frontal gyrus. Gender showed significant effects mainly in two regions, including visual and frontal networks. For example, the inferior frontal gyrus was more right lateralized in males. Significant effects of age were found in sensorimotor and visual networks on the global measure. In summary, we report a large-sample of lateralization study that finds intrinsic functional brain networks to be highly lateralized, with regions that are strongly related to gender and age locally, and with age a strong factor in lateralization, and gender exhibiting a trend-level effect on global measures of laterality.

Effects of an early experience of reward through maternal contact or its denial on laterality of protein expression in the developing rat hippocampus

Laterality is a basic characteristic of the brain which is detectable early in life. Although early experiences affect laterality of the mature brain, there are no reports on their immediate neurochemical effects during neonatal life, which could provide evidence as to the mechanisms leading to the lateralized brain. In order to address this issue, we determined the differential protein expression profile of the left and right hippocampus of 13-day-old rat control (CTR) pups, as well as following exposure to an early experience involving either receipt (RER) or denial (DER) of the expected reward of maternal contact. Proteomic analysis was performed by 2-dimensional polyacrylamide gel electrophoresis (PAGE) followed by mass spectroscopy. The majority of proteins found to be differentially expressed either between the three experimental groups (DER, RER, CTR) or between the left and right hemisphere were cytoskeletal (34%), enzymes of energy metabolism (32%), and heat shock proteins (17%). In all three groups more proteins were up-regulated in the left compared to the right hippocampus. Tubulins were found to be most often up-regulated, always in the left hippocampus. The differential expression of β-tubulin, β-actin, dihydropyrimidinase like protein 1, glial fibrillary acidic protein (GFAP) and Heat Shock protein 70 revealed by the proteomic analysis was in general confirmed by Western blots. Exposure to the early experience affected brain asymmetry: In the RER pups the ratio of proteins up-regulated in the left hippocampus to those in the right was 1.8, while the respective ratio was 3.6 in the CTR and 3.4 in the DER. Our results could contribute to the elucidation of the cellular mechanisms mediating the effects of early experiences on the vulnerability for psychopathology, since proteins shown in our study to be differentially expressed (e.g. tubulins, dihydropyrimidinase like proteins, 14-3-3 protein, GFAP, ATP synthase, α-internexin) have also been identified in proteomic analyses of post-mortem brains from psychiatric patients.

Gender differences in locomotor and stereotypic behavior associated with l-carnitine treatment in mice

BACKGROUND

The carnitines exert neuroprotective and neuromodulatory actions, and carnitine supplementation increases locomotor activity (LMA) in experimental animals.

METHODS

We measured 13 indexes of LMA and 3 indexes of stereotypic activity (STA) in adult male and female caged mice. In a randomized 4-week trial, 10 males and 10 females received 50 mg/kg body weight PO l-carnitine, and another 10 males and 10 females received placebo.

RESULTS

Compared with placebo-treated females, placebo-treated males had a greater number of stereotypies (NSTs), stereotypy counts (STCs), stereotypy time (STT), and right front time (RFT), but smaller total distance traveled (TDT), margin distance (MD), number of vertical movements (NVMs), and left rear time (LRT). Compared with placebo-treated males, carnitine-treated males had greater horizontal activity (HA), movement time (MT), NVM, STT, TDT, STC, MD, LRT, and clockwise revolutions (CRs), but smaller left front time (LFT) and RFT. Compared with placebo-treated females, carnitine-treated females had greater NST, STC, STT, LFT, and RFT, but smaller NM, HA, NVM, VA, MT, anticlockwise revolutions (ACRs), CR, TDT, and MD; right rear time (RRT) remained statistically insignificant across all comparisons.

CONCLUSIONS

In summary, l-carnitine caused gender differences to persist for STC, diminish for NST and STT, disappear for LRT and NVM, change in the opposite direction for TDT and MD, appear de novo for HA, VA, NM, MT, and LFT, and remain absent for RRT and ACR. Some indexes of LMA and STA are sexually dimorphic in adult mice, and l-carnitine differentially maintains, diminishes/cancels, inverts, or creates the sexual dimorphism of particular indexes.

Stereologic analysis of cell number and size during postnatal development in the rat substantia nigra

Parkinson's disease is characterized by age-related atrophy and loss of dopaminergic neurons within the compact portion of the substantia nigra (SNpc) projecting to neostriatum. Despite numerous studies using rodent models to examine mechanisms underlying this disorder, the fundamental question of whether development- or age-related changes occur in the rodent SNpc remains unanswered. The present study used a three-level, optical fractionator approach to estimate the number and size of SNpc neurons immunoreactive for tyrosine hydroxylase (TH) in eight young (2-month) and eight older (7-month) Sprague-Dawley rats. Following standard protocols for animal care and tissue harvesting, every eighth 60-microm section from a gapless coronal series was treated immunohistochemically for TH along with a thionin counterstain. Neither the ventral tegmental area nor the lateral part of the SN was included in the analysis. The total bilateral number of SNpc TH+ neurons (approximately 8000) was equivalent between groups, whereas mean TH+ neuronal volume decreased significantly in the older group (approximately 18%). In contrast, volume of the SNpc increased with age by 17%, as did volume of the entire brain (24%). TH+ cells in the SNpc were also significantly larger on the left versus right side of the brain. These data are consistent with the hypothesis that age-related volumetric expansion of the SNpc is accounted for by an increase in the ratio between neuropil and average neuron somal size during intermediate postnatal development.

Distinct patterns of gene expression in the left and right hippocampal formation of developing rats

A central problem in neurobiology is the elucidation of the mechanisms that underlie left-right asymmetries in brain structure and function. Using a transcriptome screening approach, we found asymmetric gene expression patterns in the right when compared with the left hippocampal formation at postnatal days (P) 6, 9, and 60 in the rat. Of those genes that were differentially expressed, most were predominantly expressed in the right hippocampus at P6, whereas most were predominantly expressed in the left at P9 and P60. Real-time PCR analysis of genes associated with synaptic vesicle trafficking confirmed this pattern. At P6, 9 of 13 such genes were more robustly expressed in the right hippocampus, while only 1 gene was predominantly expressed in the left. Conversely, at P9, 5 of the 13 genes were more highly expressed in the left hippocampus and only 1 gene was predominantly expressed in the right. This pattern persisted at P60: eight genes were more robustly expressed in the left hippocampus, and the remaining five showed no hemispheric preference. These data demonstrate a pattern of early lateralized gene expression that is likely to underlie the establishment of functional asymmetry in the adult hippocampus.

Dissociable effects in reaction time performance after unilateral cerebral infarction: A comparison between the left and right frontal cortices in rats

Reaction time performance reflects the speed of information processing, both in humans and lower vertebrates like the rat. The present study compared reaction time performance in rats following unilateral infarction to the frontal cortex. The objective was to model cognitive impairment as it is seen in humans after stroke. Rats were trained in a reaction time paradigm, after which unilateral cortical infarction was induced photochemically. Reaction time performance was differentially affected after unilateral infarction to either the left or right frontal cortex, whereas sham operation did not result in a significant alteration in reactivity. An overall increase in reaction time of about 10% was present at 4 weeks after frontal infarction. In addition, a lateralized reaction time deficit occurred very early after right frontal infarction as an increase of 10-15% in trials directed towards the contralesional side. Additional analyses showed that these reaction time deficits can be explained differently: the former as a gradual and general decrease in the speed of information processing, whereas the latter shows specific impairment to initiate a contralateral motor response. The former matches well with the mental slowing observed in stroke patients, whereas the latter resembles a neglect phenomenon. We conclude that measuring reaction time performance after frontal cortical infarction in rats could offer a useful tool to model particular human cognitive impairments following cerebral infarction.

Lateralization of Spatial Learning in the Avian Hippocampal Formation

The authors investigated lateralization of spatial learning within the avian hippocampal formation (HF). In Experiment 1, homing pigeons (Columba livia) with unilateral lesions of the right or left HF were trained to locate a goal in a square room containing local landmarks and global room cues. All groups learned the task. During probe trials, when landmarks were rotated or removed, intact pigeons and left HF-lesioned pigeons relied exclusively on global room cues to locate the food goal. Pigeons with right HF lesions were the only group to demonstrably use the landmarks. The results suggest that the right HF is preferentially involved in the representation of global environmental space, whereas only the left HF may be sensitive to local landmarks for navigation.

Visual lateralization in the bottlenose dolphin (Tursiops truncatus): evidence for a population asymmetry?

A previous behavioural study with a single bottlenose dolphin had reported a right eye superiority in visual discrimination tasks, indicating a left hemisphere dominance for visual object processing. The presence of a functional asymmetry demonstrated with one individual shows that this function can be lateralized in this single animal, but cannot reveal if this represents a population asymmetry. Therefore, we conducted a series of visual discrimination experiments with three individuals of Tursiops truncatus under monocular conditions. The tested animals had to distinguish between simultaneously presented stimulus pairs of different patterns, whereby one stimulus was always defined to be correct. Additionally, the animals were observed for their free eye use during training and introduction of new items. The present data set revealed a right eye advantage (left hemisphere dominance) for all tested animals and a predominance of right eye use during daily activities. These results make it possible that bottlenose dolphins are lateralized for visual pattern discrimination at the level of a population asymmetry. Against the background of similar data in other vertebrates, a left hemisphere dominance for pattern discrimination points to the possibility that dolphins exploit local visual details instead of global configurational features to recognize and memorize visual stimuli.

The origin of brain asymmetry and its psychotic reversal

Ontogenetic brain-asymmetry and its reversal in schizophrenia constitute special cases of a more fundamental principle of sensory-motor integration. Transmitted through an immature optical system, asymmetric inputs from the left visual field induce the infant's right hemispheric preference for lower spatial frequencies during early mother-child interaction. The emerging classical features of hemispheric specialisation later in life can be accounted for by a transformation law of the neuronal reference frames based on relativistic non-linear information processing. Accordingly, the asymmetric distributions of the cannabinoid receptor CB1 in the right basal ganglia and the left area of Wernicke reflect the preferences for lateralised posture, positioning, and speech. Epigenetic development of brain asymmetry thus unifies the different aspects related to cradling and breast-feeding, speech- and visuospatial processing, the dimensional conversion of spatiotemporal information and, in the case of a dysbalanced cannabinoid system, its psychotic reversal. The predicted right hemispheric shift and the inverse relationship between Kolmogorov entropy and its dimensional embedding (Shannon entropy) has ultimately been confirmed by non-linear EEG analysis of a fluoro-methyl-anadamide induced model psychosis splitting conscious from unconscious mental processes.

Lateralization of visuospatial processing in the bottlenose dolphin (Tursiops truncatus)

Two adult female bottlenose dolphins were tested for cerebral asymmetries in the visuospatial domain. The animals learned under binocular conditions a three-choice spatial discrimination task with three hoops positioned along a line in the middle of the tank. During a correct trial the dolphins had to swim from a starting position at the tanks wall through one of the hoops, come back to the starting position, choose another hoop, swim back to start and finally swim through the third hoop. For such a trial to be correct, the animals had to swim through all three hoops in any sequence without omitting or re-using one of them. After reaching criterion binocularly, monocular trials (one eye covered with an adherent suction cup) were introduced where the dolphins carried out the same task alternatingly under left or right eye seeing conditions. For both animals, the right eye performance was clearly superior to that of the left eye. Binocular and right eye performances were similar. As a result of the complete decussation at the optic nerve, this right eye superiority suggests a left-hemispheric dominance for the processing of visuospatial information. This is a remarkable deviation from the usual right hemisphere advantage for these kind of tasks found in different species of mammals and birds.

Visual lateralization of pattern discrimination in the bottlenose dolphin (Tursiops truncatus)

The aim of the present study was to investigate whether bottlenose dolphins have cerebral asymmetries of visual processing. The monocular performance of the adult dolphin Goliath was tested using a large number of simultaneous multiple pattern discrimination tasks. The experiments revealed a clear right eye advantage in the acquisition and the retention of pattern discriminations as well as asymmetries in the interhemispheric transfer of visual information. As a result of a complete decussation at the optic nerve, this right eye superiority is probably related to a left hemisphere dominance in visual processing.

Time perception as a complex functional system: neuropsychological approach

Ninety-two brain-damaged patients and 111 normal control subjects were tested on their orientation to date; sequencing of events; discrimination of simultaneity and succession; conditioning to time; comparison, production and reproduction of durations; conservation of velocity in clocks; construction of time units; and psychological time. Temporal disorientation was related to advanced age, low educational level, amnesia, dementia and limbic or diffuse brain lesions. Only multifocal-diffuse lesions accompanied by dementia could disintegrate the concept of metric time, while sparing psychological time, discrimination of durations and sequencing of canonically recurring events. Reproduction of durations and verbally mediated temporal conditioning were impaired in frontal and temporal-limbic lesions, which left intact the concept of time. The results support the hypothesis that temporal perception is accomplished by a complex functional system, regarding both its psychological structure and cerebral organization.

Reimpressed selective breeding for lateralization of handedness in mice

Eleven generations of bidirectional selection for lateralization produced 2 lines of mice that differ markedly in degree of asymmetry for hand preference. The foundation population was derived from 6 distantly related inbred strains and 2 stocks of wild mice, M. castaneus. HI line matings were made using mice that exhibited consistent right or left paw use in a food reaching task and LO line matings were made using mice with little overall paw preference. All matings were made without regard to the expressed directions of asymmetry. Line differences emerged at the third generation and increased thereafter. Selection was relaxed at generation 12 and the lines were maintained by random within-line mating. At generation 28 selective breeding was reimpressed for 3 generations. Results indicated that between-line divergence in degree of lateralization had remained high during 17 generations of relaxed selection. Mice of the HI line are more strongly lateralized than mice of the unselected HET population. Mice of the LO line are more weakly lateralized than controls. The selected lines may provide a useful mammalian genetic resource for studying the neurobiology of cerebral lateralization.

Effect of cerebral hemisphere decortication on the cytotoxic activity of natural killer and natural cytotoxic lymphocytes in the mouse

A comparison was made of the effects of left and right cerebral decortication on cytotoxic activity of natural killer and natural cytotoxic lymphocytes in the mouse. Natural killer cytotoxic activity was significantly reduced after right decortication, whereas left decortication led to a less pronounced, though still significant fall. The cytotoxic activity of natural cytotoxic cells, on the other hand, was significantly increased, particularly 15 days after left decortication. These findings mirror the results of previously published personal findings following electrothermocoagulation of the hypothalamus. The suggestion is made that the cortex and the hypothalamus form an integrated system for the control of certain aspects of natural immunity.

Light experience during development affects asymmetry of forebrain function in chickens

The chicken brain has asymmetry of forebrain function of attack and copulation behaviors. This study reports that this asymmetry, revealed by intracranial injections of glutamate, is affected by light exposure. Eggs were incubated in 3 forced-draught incubators matched for noise, humidity and temperature control, so that light and laboratory noise were the only systematic variables. All eggs were incubated in darkness for the first 17 days of incubation after which they were allocated to one of 3 incubating conditions, L+S+, L-S+, L+S-, where + or - indicates the presence or absence of light (L) or laboratory noise (S). Light exposure was necessary to establish lateralization of attack and copulation, sound was not since only the L+S+ and L+S- groups were lateralized. The lateralization of auditory habituation was not affected by these incubating conditions. Further experiments established that on day 19 of incubation exposure for 4 h to a light of constant intensity produced lateralization. 1 h exposure to a light of constant intensity or to a flickering light (1.7 Hz) failed to produce lateralization. Examination of the data for individuals suggests that light exposure synchronizes the direction of lateralization rather than generates its presence. Environmental and genetic factors interact to produce the lateralization of attack and copulation.

Hemisphere specialization of the animal brain for information processing principles

The experiments were performed on white rats. Different techniques for forming conditioned reflexes were used. Hemisphere cortex inactivation was carried out by means of spreading depression. The right hemisphere was shown to dominate for space perception, simultaneous information processing, concrete characteristics perception, and deductive processing; the left hemisphere was shown to dominate for time perception, successive information processing, abstract characteristics perception, and inductive processing. A hypothesis of induction and deduction being the main factors determining function lateralization in the hemispheres of the animal brain was put forward.