In the current study, using DTT, we investigated the characteristics of the APT in comparison with the PT. We found the following three results. First, the APT existed in 18.3% of the hemispheres of the normal human brain. Second, although all of the PTs were found to originate from SM1, 26.5% of the APTs were found to originate from the primary somatosensory cortex without a primary motor cortex origin. Third, values of FA and tract volume for the APT were lower than those of the PT, with no difference in MD value. The FA value represents the degree of directionality of microstructures (e.g., axons, myelin, and microtubules), and the MD value indicates the magnitude of water diffusion [15–17]. In contrast, the tract volume was determined by the number of voxels contained within the neural tract . Changes in DTI parameters observed in the APT, that is, decreased FA value and tract volume with unchanged MD value, suggest less directionality and fewer neural fibers than in the PT.
Like this study, several other studies have reported on the incidence and courses of the APT [3, 6, 11, 12]. In 2001, using the modified Bielschowsky stain, Yamashita and Yamamoto  investigated the incidence and details of the course of the APT in 150 consecutive autopsied human brains. They found that all of the 150 brains examined, with the exception of one brain with holoprosencephaly, showed the APT, and reported that the course of the APT left the PT within the cerebral peduncle and then passed into the medial lemniscus of the pons through the upper medulla. In 2009, Hong et al  reported that the APT existed in 5 (17.9%) of the 28 hemispheres of normal subjects and that the APT descended through the medial lemniscus from the midbrain to the pons, and then entered into the PT at the upper medulla. Recently, several studies have suggested that the APT may contribute to motor recovery in stroke [1, 2, 11, 12]. In 2009, Jang SH reported a patient whose motor function appeared to have recovered via an APT following a pontine infarct located in the PT area . In 2010, Lindenberg et al. demonstrated that patients with alternate motor fibers in the brainstem showed better motor outcome among 35 patients with middle cerebral artery infarcts . However, they did not clarify that the alternate motor fibers were APTs. In 2011, two patients were reported with midbrain infarct or corona radiata, respectively, that showed motor recovery via APT [13, 14]. As for the incidence of APT existence, our results coincide with those of Hong's DTT study and the course of the APT was similar with that of all previous studies [3, 6, 11, 12].