Diencephalic Asymmetries

HARRIS, J. A., V. GUGLIELMOTTI AND M. BENTIVOGLIO. Diencephalic asymmetries. NEUROSCI BIOBEHAV REV 20(4) 637–643, 1996.—Structural asymmetry in diencephalic regions has been reported in a number of studies since the pioneering observations by Kemali and Braitenberg, Atlas of the frog's brain. Springer Verlag: 1969. Anatomical differences between the left and right habenulae have been identified in many lower vertebrate species. While there are few reports of structural asymmetry in the dorsal thalamus, there is evidence that asymmetrical thalamofugal projections can be induced in the visual system of chicks by lateralized sensory stimulation prior to hatching. Finally, there have been consistent reports of differences between left and right sides of the hypothalamus in their sensitivity to the effects of circulating gonadal hormones in rats. In most cases, these asymmetries are sex-linked and correspond to a lateralization of function. Although the significance of many of these diencephalic asymmetries is still enigmatic, their existence indicates that asymmetry is not a phylogenetically recent feature of the brain, and that left-right differences in the brain may be mediated by a common ontogenetic mechanism and may underlie the development of highly specialized functions. Copyright © 1996 Elsevier Science Ltd.     

Lateralized courtship in a parasitic wasp

Lateralization (i.e. left-right asymmetries in the brain and behaviour) of courtship displays has been examined in a growing number vertebrate species, while evidence for invertebrates is limited. In this study, we investigated lateralization of courtship and mating displays in the parasitic wasp Leptomastidea abnormis. Results showed a population-level lateralization of male courtship displays. Male antennal tapping on the female's head was right-biased. However, right-biased male courtship acts were not characterized by higher male antennal tapping frequencies, nor success in mating although antennal tapping frequency was higher in males with mating success with respect to unsuccessful males. Overall, our results add basic knowledge to the behavioural ecology of insect parasitoids. To the best of our knowledge, this is the first report of behavioural lateralization in parasitic Hymenoptera.     

Developmental Asymmetries in Experimental Animals

POLANI, P.E. Developmental asymmetries in experimental animals. NEUROSCI BIOBEHAV REV 20(4) 645–649, 1996.—The early embryo orients to the antero-posterior axis and differentiates along this, and the dorso-ventral and lateral axes. From Drosophila melanogaster, detailed knowledge has accrued of how segmentation and dorso-ventral differentiation proceed, and of their genic control, mostly by selector and homeobox (Hox) genes. The study of the control of lateral differentiation, instead, has been largely neglected. Yet handed asymmetry (the “obvious” asymmetries of, for example, heart, lung, anatomical features of the nervous system, etc.) is basic and, possibly, universal. In the mouse, two genes control this: the iv gene which, when mutated, leads to random, in the place of biased, asymmetry and so to random situs inversus viscerum; and the inv mutation which, by contrast, results in 100% situs inversus. Both mutants act as autosomal recessives. Human situs inversus is heterogeneous and may be akin to that produced by the murine iv gene. In spite of situs inversus, there is no shift of hand preference; but there is no information on other lateralization, e.g. of language or of dermatoglyphic patterns.

Handed asymmetry is known in Drosophila, but there is no information on its control. In the experimental nematode, Caenorhabditis elegans, asymmetry arises when differently programmed cells arrange themselves to the two body sides, and is present already at the six-cell stage; and even the major sensory neurons chains along the body axis are distributed unequally on the two sides of the worm. Experimentally, by embryonic micro-manipulation or the use of chemical mutagens, the normal and invariate direction of handed asymmetry can be reversed.     

Lateralized temporal order judgement in dyslexia

Temporal and spatial attentional deficits in dyslexia were investigated using a lateralized visual temporal order judgment (TOJ) paradigm that allowed both sensitivity to temporal order and spatial attentional bias to be measured. Findings indicate that adult participants with a positive screen for dyslexia were significantly less sensitive to the temporal order of the stimuli than control participants, but did not show a significantly different lateral bias. However, the data indicated that performance on the TOJ task dissociated into at least three factors. One loaded on trials with long Stimulus Onset Asynchronies (SOA) and was strongly correlated with full-scale IQ (FSIQ), and, while also correlated with both poor reading and with symptoms of attentional deficit disorder, was not specific to these. The second factor loaded on trials with short SOAs in which the left stimulus was presented first. Low scores on this factor were associated with poor non-word reading accuracy, and factor scores accounted for variance in non-word reading accuracy that was not accounted for by either FSIQ or the presence of a phonological deficit. This suggests that a “left mini-neglect” syndrome, also reported in attention deficit and hyperactivity disorder, may directly contribute to poor non-word reading. However, attentional deficit symptoms loaded not only on this factor, but also on a third factor, representing on trials at short SOAs in which the first stimulus was presented in left hemifield. This suggests that attentional deficit symptoms impaired temporal processing at short SOAs, regardless of the hemifield in with the stimuli were presented. We conclude that people with attentional deficits find a visual TOJ task difficult when the stimuli are presented rapidly, regardless of FSIQ, and that where a rightward attentional bias is present, non-word reading accuracy may be directly impaired, even in the absence of a phonological deficit.     

Asymmetries in musical aesthetic judgments

Musicians and non-musicians, male and females, were presented with different selections of classical music, both mono- and poly-instrumental, under stereophonic conditions which allowed them to reverse the left-right distribution of the auditory material. The musicians preferred the leading (usually the high) voice to be heard by the left ear; the effect was noted with both sexes but it was significant only with the males. The results are examined in relation to other laterality patterns and an analogy to orchestral arrangements in both the West and the East is noted.     

Similarities Between Sylvian Fissure Asymmetries in Cat Brain and Planum Temporale Asymmetries in Human Brain

Galaburda et al. (1987) have suggested that asymmetry of planum temporale in men would result from an asymmetrical neuronal loss; symmetry would implicate a failure of asymmetrical cell loss. There are several reports indicating a similarity between men and animal in cerebral lateralization. In the present work, a morphological analysis was performed in sylvian fissure (SF) of cats. Asymmetry coefficients (AC) were distributed symmetrically and normally with a mean of zero. However, female cats exhibited a relatively stronger leftward asymmetry (left SF > right SF), and male cats a relatively stronger rightward asymmetry in SF. The results generally supported those of Galaburda and his coworkers. Deviations from their results depended mainly on paw preference. The similarity between human brain and animal brain was accentuated once more concerning cerebral lateralization. The results were not appropriate to conclude about the mechanisms of cerebral lateralization. Testosterone in males and estrogen in females were suggested to be the main hormones contributing to the emergence of cerebral lateralization.     

Lateralized anomalous perceptual experiences in schizophrenia.

Four cases of schizophrenia are described, in which subjects reported anomalous perceptual experiences confined to one visual field, in all cases the left visual field. The pattern, content and laterality of such anomalous perceptions point to the presence of right hemisphere dysfunction in schizophrenia. Further evidence to this effect comes from examining the type of anomalous perceptual experiences reported by subjects with focal lesions in either hemisphere. Those with right hemisphere damage but not those with left hemisphere damage report almost identical experiences to those seen in the 4 schizophrenics described here.     

Visual Field Asymmetries in Schematic Persistence

A modified partial-report technique was used as a tool to study hemispheric asymmetries in schematic persistence. Two types of stimuli were presented: (1) verbal stimuli consisting offunction words and nonsense words, and (2) pictures of faces and nonfaces. The stimulus items were arranged bilaterally in a circular array around the fixation point and the participants' task was to decide whether the probed item was a correct or distorted version of each stimulus type. Delay time between stimulus set and marker ranged from 0ms to 360ms. Calculating a sensitivity measure of accuracy, distinctive visual field differences emerged which depended on the nature of the stimuli; that is, a left field advantage for facial stimuli and a right field advantage for lexical stimuli. In addition, performance decreased linearly as delay time between probe and marker increased. The present results indicate that on a level of schematic persistence perceptual hemispheric asymmetries are already strongly present.     

Lateralized periodic activity and cerebral infarct

A series of EEG recordings shows the evolution of a periodic activity (PA) in a 57-year-old man with acute cerebral infarction. The complexes ranged from 100 to 150 microV, the duration was 750 msec; the periodic occurrence repeated itself every 2 sec. PA is obliterated during the onset of electroclinical seizures. The CT scan shows temporal brain infarction.     

Lateralized reward-related visual discrimination in the avian entopallium

In humans and many other animals, the two cerebral hemispheres are partly specialized for different functions. However, knowledge about the neuronal basis of lateralization is mostly lacking. The visual system of birds is an excellent model in which to investigate hemispheric asymmetries as birds show a pronounced left hemispheric advantage in the discrimination of various visual objects. In addition, visual input crosses at the optic chiasm and thus testing of each hemisphere is easily accomplished. We aimed to find a neuronal correlate for three hallmarks of visual lateralization in pigeons: first, the animals learn faster with the right eye-left hemisphere; second, they reach higher performance levels under this condition; third, visually guided behavior is mostly under left hemisphere control. To this end, we recorded from the left and right forebrain entopallium while the animals performed a colour discrimination task. We found that, even before learning, left entopallial neurons were more responsive to visual stimulation. Subsequent discrimination acquisition recruited more neuronal responses in the left entopallium and these cells showed a higher degree of differentiation between the rewarded and the unrewarded stimulus. Thus, differential left-right responses are already present, albeit to a modest degree, before learning. As soon as some cues are associated with reward, however, this asymmetry increases substantially and the higher discrimination ratio of the left hemispheric tectofugal pathway would not only contribute to a higher performance of this hemisphere but could thereby also result in a left hemispheric dominance over downstream motor structures via reward-associated feedback systems.     

Asymmetries in Cerebellar Plasticity and Motor Learning

Synaptic plasticity at the parallel fiber to Purkinje cell synapse has long been considered a cellular correlate for cerebellar motor learning. Functionally, long-term depression and long-term potentiation at these synapses seem to be the reverse of each other, with both pre- and post-synaptic expression occurring in both. However, different cerebellar motor learning paradigms have been shown to be asymmetric and not equally reversible. Here, we discuss the asymmetric reversibility shown in the vestibulo-ocular reflex and eyeblink conditioning and suggest that different cellular plasticity mechanisms might be recruited under different conditions leading to unequal reversibility.     

Lateralized cognitive dysfunction and psychotic symptoms in schizophrenia

Our understanding of hemispheric asymmetries in schizophrenia can be attributed to extensive neuropsychological and neuroimaging research on this topic; however, it has yet to be determined whether lateralized cognitive dysfunction represents a single core trait in schizophrenia or whether the lateralized impairments are domain specific. To test whether lateralized deficits are core features in schizophrenia we examined performance across a wide range of lateralized cognitive domains including attention, fluency, recognition memory, perception, and arousal. We also examined the relationship between lateralized impairments and psychotic and affective symptoms to determine whether abnormal hemispheric asymmetries were possibly state-related. The sample consisted of 43 subjects with schizophrenia and 66 normal healthy comparison subjects without psychiatric illness. Schizophrenia subjects exhibited abnormal right hemisphere performance on a test of recognition memory and abnormal left hemisphere performance on a measure of arousal. These findings suggest that lateralized cognitive disturbances in schizophrenia do not represent a single core lateralized deficit. Regarding the symptom analyses, severity of positive symptoms was related to right hemisphere cognitive impairment (including fluency and recognition memory), whereas severity of negative symptoms was related to left hemisphere cognitive impairment (including fluency). Overall, our findings suggest that lateralized dysfunction can occur in both hemispheres in schizophrenia, and that the positive psychotic symptoms may relate more to right hemisphere impairment, whereas negative psychotic symptoms may related more to left hemisphere impairment.     

Lateralized cognitive deficits in children following cerebellar lesions

The aim of this preliminary study was to examine the developing cognitive profiles of children with cerebellar tumours in a consecutive series of clinical patients. MRI and longitudinal intellectual profiles were obtained on seven children (two females, five males; mean age 3 years at diagnosis; mean age 7 years at first assessment). Tumours in three of the children were astrocytomas; of the remaining tumours, two were medulloblastomas, one low-grade glioma, and one ependymoma. In right-handed children, we observed an association between greater damage to right cerebellar structures and a plateauing in verbal and/or literacy skills. In contrast, greater damage to left cerebellar structures was associated with delayed or impaired non-verbal/spatial skills. Long-term cognitive development of the children studied tentatively supports a role for the cerebellum in learning/development. These findings suggest that lateralized cerebellar damage may selectively impair the development of cognitive functions subserved by the contralateral cerebral hemisphere and, in addition, that all children with cerebellar lesions in early childhood should routinely undergo long-term monitoring of their intellectual development.     

Asymmetries in hemispheric control of attention in schizophrenia

Investigators have long suggested that schizophrenia might be related to an impairment in the regulation of attention. In this report, the performance of schizophrenic patients was compared with nonschizophrenic control subjects in their ability to direct visual attention. In the first experiment, patients were distinguished from controls by a slower response to a target in the right visual field than to a target in the left visual field when attention was not first directed to the target location. In the second experiment, patients were distinguished from controls by a stronger bias in favor of symbolic information over language information about spatial direction. In both experiments, the patients demonstrated deficits in attention similar to patients from previous studies who had unilateral lesions of the left hemisphere. The identification of performance abnormalities using tasks that are simple, have dissectable cognitive components, have been related to discrete neural systems, and control for nonspecific variables provide the basis for constructing reasonable hypotheses about the cognitive psychology and functional neuroanatomy of schizophrenia.     

Lateral Asymmetries of Pupillary Responses

We wanted to learn if pupillary changes induced by looking and attending to stimuli on the right and left are asymmetrical. In humans, there are hemispheric asymmetries in the control of attention-arousal systems. Because attention and arousal may influence pupil size, asymmetric pupillary responses may be seen when looking and attending in different directions. Twelve right-handed, healthy volunteers served as subjects. Using infrared pupillography, we recorded changes of pupillary diameter while subjects were looking and attending to the stimuli on the right and left sides of space. For the one second following a saccade, there are three phasic pupillary responses, an initial constriction (C1) then a dilation (D1), followed by constriction (C2). Evaluation of these three responses revealed right-left asymmetries with more pupil dilation (D1) when looking to the stimulus on the right. Our results suggest that subjects are more aroused when looking to the right than when looking to the left.     

Lateralized attentional deficits in drug-free and medicated schizophrenic patients

Performance on a cued reaction time (RT) task, theoretically linked to posterior and anterior neuroanatomical systems in the brain (Posner, M. I. et al., Science, 1988, 210, 1627–1631; Archives of General Psychiatry, 1988, 15, 811–821), was used to assess sensory orienting and maintenance of attention. In schizophrenic patients, Posner et al. found a lateralized abnormality in RT (longer RTs to uncued targets in the right visual field than in the left visual field), as did Maruff et al. (Neuropsychologia, 1995, 33, 1205–1223), but Strauss et al. (Journal of Psychiatric Research, 1991, 37, 139–146), among others, did not replicate this effect. However, the subjects in these studies differed in the percentage of schizophrenic patients taking neuroleptic medication at the time of testing and in the chronicity of the illness. In the present study, we used two groups of schizophrenic subjects to control for the use of neuroleptic medication. The lateralized abnormality in RT was observed in the drug-free group of schizophrenic subjects, but not in the group of drug-treated schizophrenic subjects.     

Lateralized cerebral impairment in schizophrenia. A critical review

The authors review the more recent areas of research in the field of cerebral lateralized dysfunction in schizophrenia. The different approach consistently suggest a definite degree of abnormality in hemispheric functioning in schizophrenic patients: nevertheless, the heterogeneity of patients' populations, as well as some inherent characteristic of the disease (i.e. duration of the illness, length of hospitalization, duration of drug treatment and psychopathological symptoms) should be taken into the account in order to obtain a more accurate definition of the cerebral impairment in schizophrenia.