Important white fiber tracts surround the MTL and it is important that they should be considered when selecting a surgical approach. These tracts include the visual pathways, uncinate fasciculus, superior longitudinal fasciculus, and inferior longitudinal fasciculus. While the location of Wernicke’s area has been well documented and can be determined intraoperatively through awake mapping, considerable attention has been paid to elucidating the location of white fiber tracts that cannot be found via intraoperative mapping.

Considered a part of the limbic system and temporal stem, the uncinate fasciculus, which lies in close relation to the amygdala and limen insulae, is an often a neglected entity in temporal lobe surgery. This white fiber tract consists of three parts: a ventral (frontal) extension, an intermediary segment within the limen insulae, and a temporal segment [18]. This fiber tract is asymmetrical in size between both hemispheres (27 % larger in the right hemisphere), suggesting its importance even in non-dominant hemispheres [18]. Interconnecting the anterotemporal lobe with the orbitofrontal area, the uncinate fasciculus is involved in linking emotions to cognition [6, 7, 20]. It may also play a role in the retrieval of episodic memories [25], and surgical resection of lesions in this area can result in long-term difficulty with face recognition and object naming [17]. Disruption of the uncinate fasciculus with anteromedial temporal lobectomy or trans-sylvian transinsular approaches may be associated with the psychosocial and cognitive changes seen postoperatively [1, 2]. As part of the temporal stem with its origin in the hippocampal formation and the amygdala, the uncinate fasciculus may be the preferential pathway for seizure spread. This must be recognized when the goal of surgery is to control tumor-induced seizures [12].

Superior and superficial to the uncinate fasciculus, the inferior occipitofrontal fasciculus (IFOF) is believed to be involved in semantic processing [11]. The IFOF consists of two components – a dorsal subcomponent connecting the frontal, superior parietal lobe and occipital gyri, and a deeper ventral portion communicating with the visual association areas: inferior occipital gyrus and posterior temporal-basal area (fusiform gyrus, temporo-occipital sulcus, and basal inferior temporal gyrus). Intraoperative stimulation studies have shown that IFOF stimulation induces semantic paraphasias during picture-naming tasks [3, 4].

Advances in diffusion tensor imaging have further elucidated the anatomic projections of the inferior longitudinal fasciculus (ILF), which joins the posterior occipital temporal regions with the temporal lobe, where it further interacts with the uncinate fasciculus to interact with the basal frontal region. Within the dominant hemisphere, the ILF is proposed to be one of the parallel pathways of the “semantic ventral stream” that constitutes the language circuitry. Intraoperative mapping studies by Mandonnet et al. elicited semantic paraphasias when white fiber tracts underneath the superior temporal sulcus immediately above the roof of the temporal horn were stimulated [10].

Numerous recent studies utilizing Klingler dissection and diffusion tractography have helped to elucidate the course of the optic radiations as they project from the lateral geniculate body (LGB) to the calcarine cortex [6, 24]. As they leave the thalamus, the fibers can be classified into three bundles: posterior, central, and anterior – which progressively take a more curved route to the calcarine cortex. The central group of fibers follows a direct path from the LGB to the visual cortex without any anterior curve, while the central bundle takes a partial anterior curve over the superior surface of the temporal horn (just at the level of the LGB) prior to extending posteriorly. The bundle of most concern during surgical approaches to the MTL is the anterior bundle (Meyer’s Loop) that passes entirely around the lateral half of the tip of the temporal horn before coursing posteriorly within the sagittal stratum to the calcarine cortex. Debate exists with regard to the precise relationship of the anterior bundle to the tip of the temporal horn [2, 5, 9, 19, 24]. Some authors have reported that Meyer’s loop does not reach the anterior tip, while other studies have noted that these fibers not only reach the tip but may also extend up to 5 mm further anteriorly [5, 19, 24]. The risk of quadrantanopsia due to the location of these fibers along the lateral ependymal wall of the ventricle is one of the primary risks of transcortical approaches to the MTL.