It has been extensively believed that handiness is one of the prominent markers of functional asymmetry of human brain. A lateralized population means that more than 50% of the individuals are lateralized in the same direction. It is true that approximately 90% of human population shows a right hand preference in the United States [12
]. However in population-level, several lines of evidences have demonstrated that lateralization in humans is not unique either in nature or extent. There is now accumulating evidence for population-level asymmetries in animals including rats and other rodent animals [13
]. In rats, population-level right-handedness was reported early in1930. The distribution of hand preference in rats is similar to human hand preference. Therefore the uniqueness of men in population-level right-handedness is rejects [7
]. On the contrary, a lot of research reports especially in the relatively old literature indicated no population-level right-handedness in the rat. These seemingly inconsistent results in the literature can be explained in the terms of the differences among testing methods according to Tang and Verstynen [7
The result of paw preference in rats using the modified computerized food-reaching test by Tang and Verstynen [7
] showed that 99.5% of the right handed rats first used their right paw to reach the food, meanwhile only 0.5% of these using the left paw to reach the food. Similar to the right handed ones, 98.6% of the left-handed rats first used their left paw to reach the food, meanwhile only 1.4% first used their right paw in food reaching. In the present study we selected the right-handed rats using the modified food-reaching test and found that about 80% of rats were right-handed and nearly 20% of rats were left-handed. In order to get enough number of rats for the present study we chose the majority (the right-handed adult rats). Also we used only male adult rats to avoid the estrogenic hormones interferences on the paw preference and cerebral infarction.
The asymmetry in the rat brain also influenced the chemical asymmetry including dopamine and norepinephrine in normal adult rats or in the rat with cerebral infarction involving cortex and of frontal cortex in particular [14
]. As expected, hand preference in rats is controlled by the contralateral primary motor cortex, since handiness was reversed after ablation of this region [15
]. In the right-handed rats the left hemisphere could play a more important role in the process of new visual-motor learning [16
]. Regarding the role of left-brain in cognitive function, it was also shown that the latency of visual evoked responses was significantly shorter from the left brain than the right brain [17
]. The asymmetric cognitive control in an animal model may have a major impact in many aspects of biology in respect to normal functioning, superior talents, and diseases. The insular cortex is involved in almost half of patients with nonlacunar ischemic MCA territory strokes. Major insular involvement is associated with large MCA territory infarcts, proximal MCA occlusions, and greater stroke severity [18
]. The right-handed rats with their left hemisphere of the brain as the dominant hemisphere were selected in the present study. The total cerebral infarction volume produced by the left MCAO was larger than that in the right MCAO, and a significantly more severe and prolonged neurological deficit was demonstrated in adult rats following MCAO. So our present study was consistent with the concept of asymmetry of rat brain on the neurological function and pathological observation following MCAO.
Therefore it might be speculated that it is urgent to pay more attention to the asymmetry in the rat brain when using the MCAO rat model. The limitation of the present study includes the number of animals used in each group, and therefore the inability to perform more extensive statistical analysis of all variables such as body weight in terms of their relations with cerebral infarction volumes. It is hoped that further studies will be directed toward this goal.