Callosal agenesis affects consistency of laterality in a paw preference task in BALB/cCF mice

Unilateral hemispherectomy at adulthood asymmetrically affects motor performance of male Swiss mice

Evidence exists indicating that cerebral lateralization is a fundamental feature of all vertebrates. In humans, a series of studies demonstrated that the left hemisphere plays a major role in controlling movement. No such asymmetries have been identified in rodents, in spite of the fact that these animals have been frequently used in studies assessing motor behavior. In this regard, here, we used unilateral hemispherectomy to study the relative importance of each hemisphere in controlling movement. Adult Swiss mice were submitted to right unilateral hemispherectomy (RH), left unilateral hemispherectomy (LH) or sham surgery. Fifteen days after surgery, motor performance was assessed in the accelerating rotarod test and in the foot-fault test (in which performance depends on skilled limb use) and in the elevated body swing test (in which performance depends on trunk movements). The surgical removal of the right hemisphere caused a more pronounced impairment in performance than the removal of the left hemisphere both in the rotarod and in the foot-fault tests. In the rotarod, the RH group presented smaller latencies to fall than both LH and sham groups. In the foot-fault test, while both the sham and the LH groups showed no differences between left and right hind limbs, the RH group showed significantly worse performance with the left hind limb than with the right one. The elevated body swing test revealed a similar impairment in the two hemispherectomized groups. Our data suggest a major role of the right hemisphere in controlling skilled limb movements in mice.

Neonatal transection of the corpus callosum affects rotational side preference in adult Swiss mice

In order to test the hypothesis that the ontogenetic development of the corpus callosum (CC) contributes to the establishment of behavioral lateralization, the rotatory behavior in the free-swimming test was studied in male Swiss mice that were subjected to mid-sagittal transection of the CC during the first postnatal day. At adulthood, 16 Acallosal and 30 Sham-operated mice were submitted to 3 sessions of the free-swimming rotatory test (diameter of the recipient=21 cm; session duration=5 min; inter-test interval=48 h). The number and direction of 30 degrees turns were recorded. Our results indicate that transected animals became progressively more lateralized than Sham ones from the first to the third session. This difference between groups was explained mainly by the more pronounced reduction in turning activity to the non-preferred side along the sessions that was observed in Acallosal mice. Our results give further support to the hypothesis that the normal development of the CC is related to the establishment of brain asymmetries in general and behavioral lateralization in particular.

Agenesis of the corpus callosum and the establishment of handedness

The goal of this study was to check whether an isolated agenesis of the corpus callosum, detected in utero with ultrasound recording, would impair the early development of unimanual and bimanual handedness. Twelve infants with isolated agenesis of the corpus callosum, either total (TACC) or partial (PACC) were tested for handedness at the end of their first year, and were compared to infants with typical development (TD), matched for age and sex. A majority of infants showed right-handedness at the unimanual grasping tasks, with no significant difference between the TD and ACC groups. When the object was presented to the left, the TACC infants were more likely to grasp the object with their right hand (with or without the left hand) than both the TD and the PACC infants who used mostly the ipsilateral left hand. The only significant difference between TD and ACC infants concerned bimanual coordination, as less ACC infants (especially TACC) succeeded at the bimanual task, compared with TD infants. In addition, the strategy of the former tended to be less right-handed than that of the latter. Our results confirm the role of the CC in bimanual coordination, indicating that the early emergence of bimanual coordination and, if confirmed, bimanual handedness, are likely to be delayed in the absence of corpus callosum, especially if agenesis is total. They do not support the idea that the CC is necessary for the early onset of handedness.

Sex and handedness effects on corpus callosum morphology in chimpanzees (Pan troglodytes)

Findings suggest that in humans, sex and hand preference may be associated with the size of the corpus callosum (CC). The authors measured CC morphology from MRIs in 67 chimpanzees (Pan troglodytes) to see whether similar effects were present in this species. Hand preference was assessed by performance on 4 tasks, and chimpanzees were classified as left-handed, right-handed, or ambidextrous. In a subsequent analysis, the chimpanzees were reclassified into 2 groups: right-handed and left-handed. The results revealed no sex difference in CC area, but significant effects of hand preference were found for several CC regions (rostrum body, anterior midbody, posterior midbody, isthmus, and splenium) and for overall CC size, with left-handed chimpanzees exhibiting significantly smaller CC measurements than right-handed chimpanzees. The results indicate that lateralized hand use in chimpanzees, as in humans, is associated with variation in CC size.

The Association between handedness, brain asymmetries, and corpus callosum size in chimpanzees (Pan troglodytes)

It has been suggested from studies in human subjects that sex, handedness, and brain asymmetries influence variation in corpus callosum (CC) size and these differences reflect the degree of connectivity between homotopic regions of the left and right cerebral hemispheres. Here we report that handedness is associated with variation in the size of the CC in chimpanzees. We further report that variation in brain asymmetries in a cortical region homologous to Broca's area is associated with the size of the CC but differs for right- and left-handed individuals. Collectively, the results suggest that individual differences in functional and neuroanatomical asymmetries are associated with CC variation not just in humans but also in chimpanzees and therefore may reflect a common neural basis for laterality in these 2 species.

Early callosal absence disrupts the establishment of normal neocortical structure in Swiss mice

In the present study, we tested the hypothesis that the ontogenetic development of the corpus callosum is relevant for the establishment of a normal neocortical structure. To that effect, neocortical morphology (thickness and neuronal density) was analyzed in adult Swiss mice rendered acallosal by midline transection at the first postnatal day (Acallosal group) and in non-manipulated mice. The neocortical thicknesses and neuronal densities of layers II+III through VI were measured in area 6 and at the 17/18a border, both of which present abundant callosal inputs, and in the relatively acallosal area 17. For the thickness measure, significant differences between Non-manipulated and Acallosal groups were only found in the areas that receive massive callosal connections. In area 6, Acallosal mice presented a reduced thickness of layer V, while at the 17/18a border, these mice presented a reduced thickness of layers II+III when compared to non-manipulated ones. No statistical difference between acallosal and non-manipulated mice was found regarding the neuronal density measure. The reduced cortical thickness associated with a comparatively normal neuronal density in neocortical regions which normally have abundant callosal connections suggest a reduction in the number of cortical neurons in acallosal mice. Altogether, the present data indicate that the input provided by callosal axons is necessary for the normal development of the neocortex.

Increased lateralization in rotational side preference in male mice rendered acallosal by prenatal gamma irradiation

In order to test the hypothesis that the ontogenetic development of the corpus callosum is related to the establishment of behavioral laterality, the rotatory behavior in the free-swimming test was studied in male Swiss mice with callosal defects induced by exposure to gamma irradiations at the 16th embryonic day (total dose of 3 Gy). At adulthood, 43 irradiated and 56 non-irradiated mice were submitted to 3 sessions of the free-swimming rotatory test (diameter of the recipient=21 cm; session duration=5 min; inter-test interval=48 h). The number and direction of 30 degrees and 360 degrees turns were recorded. Animals were classified as side-consistent turners (to the right or to the left) when they did not change their preferred side of rotation in all three sessions and in both turning units. In general our results suggested that irradiated animals present more pronounced laterality than non-irradiated ones. In the irradiated group, the percentage of consistent turners was significantly higher than that of non-consistent turners. In the first session, the percentage of animals that presented strong turning preferences in the acallosal group was higher than in the normal group. In first session, the acallosal group presented a higher average number of turns to preferred side than the normal group. Taken together, our results constitute an endorsement to the hypothesis that the normal development of the corpus callosum is related to the establishment of cerebral laterality.