J Neurosurg 2026;145(5):1276–1286
Meyer’s loop turns sharply beside the temporal horn, where its anterior extent varies between patients. In 33 adults undergoing temporal lobe epilepsy surgery, probabilistic tractography depicted the loop in every patient. Deterministic diffusion tensor imaging produced only one partial reconstruction. The difference mattered on visual testing. Patients whose resection cavities overlapped more of the probabilistically mapped tract had greater loss of sensitivity in the opposite upper visual quadrant.
Study and key findings
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Teams in Melbourne and Nashville applied both methods to each patient’s preoperative diffusion MRI using the same anatomical regions of interest. Probabilistic tracking with constrained spherical deconvolution can follow several fiber orientations within a voxel, an advantage where fibers cross and bend in the temporal stem. The investigators registered each postoperative cavity to the preoperative scan and compared the volume of overlap with automated perimetry.
Greater overlap correlated with a lower quadrant-specific visual field score for probabilistic tracking (*r* = −0.63). The association persisted after adjustment for resection volume and surgical technique (*p* = 0.002). The corresponding correlation for deterministic tracking was weaker (*r* = −0.31, *p* = 0.08).
Interpretation and limitations
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The comparison used each patient as their own imaging control. Its main limitation for operative planning is that the injury estimate required the final resection cavity. Whether changing a surgical corridor in response to the preoperative map preserves vision remains an open question. The small cohort included three types of resection, and the centers differed in MRI acquisition and timing of postoperative perimetry.
Clinical takeaway
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When planning entry into the temporal horn, review the anterior turn of the optic radiation on an expert-checked probabilistic reconstruction. If deterministic tracking stops short of the loop, the position of the fibers remains uncertain. That uncertainty belongs in the assessment of the proposed corridor and in counseling about superior quadrantanopia.